Katz's Corner Episode 8: My Current Affair

Editor's Note: As I read and edited this article an ever-increasing grin spread across my face as I realized it was Bob's geekspeak love note to Ti Kan and Morsel for their ledgend M3 DIY headphone amp. I'm sure they're both ticked pink at your praise, Bob.

It seems to me Bob's message here is: If you're a fairly accomplished DIYer, this is surely an amp to consider buildings.

This episode is full of ideas on what makes audio sound better (or worse). If you're like me, constantly searching for scientific reasons, you'll enjoy this episode. Join me in trying to make audible sense of it all.

My Current Affair
AMB M3 headphone amp. Nicely designed. Let's start with a feature I can speak of on pretty firm ground: It's DC-coupled. That was a very intentional choice on the part of Ti Kan and Morsel, designers extraordinaire, and probably the first buzz word in their description that caught my interest when I was choosing which amp to build.

DC coupled means that its frequency response extends down to the nether regions, 1Hz or below. But hey, we've been told we can only perceive down to 20Hz, right? The rest is subsonics, and we can't feel subsonics in headphones anyway, right? Even though both of those statements are pretty true, I still believe that having extreme low frequency response makes a serious difference to the sound of an amp, and how that low frequency response is extended makes a difference as well. I think that the sound in the officially-audible region (20Hz and above) is definitely affected by the response in the subsonic region. In this case it's not the quantity of the bass that results, but rather the quality.

What makes DC coupled amps sound better? Is it lack of phase shift? Is it lack of distortion by eliminating distortion-causing capacitors? If so, why not extend the response of every amp to DC, that shouldn't hurt, should it? But DC coupling might hurt...because DC offset (common in DACs) can cause clicks and pops, it can cause noises and distortion in potentiometers, and designers have been taught to try to avoid distortion, pops and clicks by employing coupling capacitors in their circuits sometimes to the point of overkill—sometimes you see one at every stage.

My philosophy is if you can replace two coupling caps with only 1, then do it. And if you can replace an output coupling cap with a DC servo circuit, then do it! If not, then no cap often sounds better than any cap, in my experience. But please don't go trying to take out caps from your amps willy-nilly, because there are good reasons for the majority of them, in a well-designed amp.

Over the years I've compared the sound of preamps that I've built or modified using big, high quality output capacitors versus those that don't use output caps at all. Every time, without exception, the output coupling cap blurs the bass definition. The cap never seems to help the sound; at best the cap is neutral or undetectable. A well-designed DC servo (which is an electronic replacement for a capacitor) can, however, maintain tight, deep bass without the sonic cost of a capacitor. Or, simply avoid coupling caps, if you accept the potential risk. So, I applaud Ti and Morsel's choice and I believe that DC coupling is a contributor to the sonics of the M3. I measured DC offset with no load, inputs shorted, as an insignificant 0.03 to 0.06 mV, a remarkable result for an amp without any DC servo, testament to the quality of the parts.

Another contribution to the sound of this amp is the Sigma 11 power supply. Power supplies can make or break the sound quality of an amp, so don't skimp on them. In my build, the Sigma 11 supplies 36 volts to the Mosfets, far more than necessary to sustain the desired output swing and current into a load, but for me, too much ain't enough! Using an oversize power transformer, this supply begins to stop regulating only when the line voltage sags to 97.8 volts. Again, overkill. The Sigma's paralleled MOSFETs keep the heat down, so there's no danger of burning up. Its output impedance is far lower than that of typical IC-based regulators, which results in a very tight bass. I believe the reason some "boutique" amplifiers sound spongey is that their power supply regulators do not have a low enough impedance.

Next, the designers provided a secondary regulation on the audio board to the opamps. This isolates the opamps from the outputs far more effectively than simple bypass capacitors. The secondary regulator drops the voltage to 34 volts, still permitting a significantly-high voltage swing. Even though we would not need that swing for ordinary, average musical passages, I believe that the additional headroom translates to greater impact of instantaneous transients and improved macrodynamics. Headroom. Potential energy. Overkill. Good design. Good for the ears.

Let's Be Discrete?
Speaking of opamps, "discrete" is currently in vogue, but in my opinion it is not necessarily the be-all, end-all that boutique amplifier designers would like you to think. Certainly AMB could have chosen to use some discrete opamp instead of a "run of the mill" integrated circuit. But it can be effectively argued that the OPA 627 IC which I chose is not your father's IC opamp. The 627 is a premium opamp with extremely high slew rate, open loop bandwidth, extremely low noise and other factors that mean things to geeks like me. It sounds very transparent; in fact it has little or no sound of its own.

It can be argued that this IC's lower distortion (lower than most discrete opamps) is what makes it sound so transparent. Some discrete opamps are so close to neutral that the touch of color they provide is what the doctor ordered, but based on my past listening and experimenting with several discrete opamps and shooting them out against the 627, I'm not convinced that "discrete" is a recipe for better sound all the time; the choice of opamp has to be considered in the context of the entire amplifier and signal chain. So skip the advertising buzz word, take a look at the whole circuit. Especially when one of the most important audiophile facts is that the combination of DAC, preamp, amplifier, interconnects and loudspeaker or headphone is far more important than the choice of any single component, no matter how superior a review it has gotten. If you pick a bright-sounding component to go with your bright speakers, you're in for trouble!

This point of view that discrete is not necessarily better could possibly offend some of my best friends, who have designed some of the best-sounding discrete opamps, but in my experience many of the discrete opamps I've tried are somewhat colored. Often due to their low open-loop bandwidth which does not permit much feedback and adds a certain "distortion du jour" effect (a nice color, hopefully).

Speaking of feedback, the M3 implements global negative feedback from the Mosfet output back to the opamp's negative input. Global negative feedback is often cited as an evil to be avoided at all costs, causing problems like a dry, closed-in, undimensional sound. Certainly not the characteristics of the M3! So I stand on the fence on the issue of feedback, which for many has become a religious concept at this point. I don't know enough to be able to say if it's always good or bad, but I do subscribe to Fred Forssell's assertion that you have to start with low distortion to begin with before you employ negative feedback. And given the known extra sweet harmonic character of Mosfets, perhaps the M3's global feedback tames enough of it to produce a wonderful, transparent, pure-sounding amp, in conjunction with the 627 opamp. I also think this amp sounds best to me with lots of feedback, 100% feedback at unity gain. As I mentioned in the previous episode, I think the amp gets a bit shallow at +10dB gain, which is accomplished by using 10dB less feedback! Don't ask me: I don't design 'em, I just listen and speculate.

If you want to investigate this point of view about opamp distortion, I suggest you read the impressive research paper by the talented designer and audiophile Samuel Groner: "Operational Amplifier Distortion". It's a very technical article meant for designers, but contains much light for DIYers.

Impedance, Polarity and Frequency Response
I calculated output impedance to about 0.2-0.3 Ohms. Given that my Fluke Ohmmeter sometimes tells me my 20 Ohm load is 20.5 and sometimes 20 Ohms depending on how tightly I squeeze the probes, there is some leeway in the impedance calculation. Polarity is non-inverting.

Next on order is flatness of response and high frequency bandwidth. I used my Prism Lyra 2 interface for all measurements at 192kHz sampling, to permit accurate measurements to almost 96kHz. Flat response is an attribute which we expect nowadays from solid state gear and the M3 is no exception. Measuring the preamp output, Room EQ Wizard reveals that the right channel of my build is an insignificant 0.02dB higher in level than the left. Low frequency response probably extends below the 10Hz point where I began to measure. In Room EQ Wizard, both channels of the preamp show a probably inaudible high frequency rise of less than 0.02dB from 10k to 20k, with response down less than 1dB to 50kHz.


M3 preamp frequency response, left and right channel, using Room EQ Wizard.

Another measurement of the preamp, using Spectrafoo, reveals the high frequency extension, and if you look closely you'll see the tiniest visible increase at high frequencies—doubtfully audible. This is the left channel at +10dBu output. Notice that the Muses and TKD attenuators have identical response and extension at their 0dB position. It's flat from at least 16Hz through to being down about 0.3dB at 32kHz, and extends to -3dB at 64kHz. The phase response mirrors the frequency response.


M3 preamp frequency response and phase, left channel, using Spectrafoo. TKD (blue) and Muses (red) attenuators match in frequency response in the 0dB position.

Noise and Distortion
You can never clip this amp at unity gain with a consumer DAC. Even the +18dBu (6.1 volt) maximum output of my Prism interface will not clip it. This is about 18dB higher than the output of a standard CD player! In order to clip the amp I had to engage the +10dB gain switch.

To evaluate distortion and noise, we have to conquer interface issues. An amplifier with a driven headphone common (active "ground") is especially difficult to measure without inducing hum or high frequency coupling from nearby sources, because we must not connect the input ground of the measurement interface to the amplifier or it could short and blow up an output device. I've got a capacitor on the shield, but still I am not entirely sure how much of the measured noise is caused by interfacing anomalies and how much is in the amp itself. Regardless, the amplitudes of noise and distortion being measured are probably so low as to ensure their inaudibility.

The next display shows the residual noise of the Prism interface (one channel) connecting its DAC output directly to its ADC input (in green) with a 1kHz tone at 0dBu. As you can see, it is very clean, just a little 2nd harmonic at -119dBu. Notice that the Prism interface noise rises from about 50kHz upward, which is caused by the noise shaping of the digital filters in the Prism's converters. This noise does not affect the frequency response measurement.

Overlaid is the headphone output of the M3 measured through the same interface, 1kHz at -14.8dBu, which would produce an SPL of 103dB in the Audeze LCD-X. At that level there is no detectable second harmonic from the M3. Notice the small hum at 60 and 180Hz and the high frequency anomalies almost certainly from RFI or other noise getting into the interface because I must not connect the ground of the Prism input to the driven "ground" of the M3. The shield is grounded through a capacitor which helps but not completely. The hum anomalies are at vanishingly low levels below -104dBu (5 microvolts), which mostly displays the resolving power of Spectrafoo and this interface, not anything which is the least bit audible. I'm pretty sure the hum is from my floating connection and not from the amp. Wideband signal to noise ratio (unweighted) of the preamp measures an excellent 86.9dB per channel at full gain (unity), or 78.7dB at the +10dB gain setting.


Prism interface 1kHz 0dBu DAC connected to ADC (green). M3 headphone output (red) 1kHz @ -14.8dBu, equivalent to LCD-X producing 103dB SPL. Scale is indBu.

In Episode #7 I mentioned that the Muses digitally-controlled analog attenuator appears to be sonically invisible/indistinguishable from a physical switched potentiometer made by TKD. The measurements seem to hold this out. It's mostly interesting that I can squeeze out some measurable differences at this incredible level of performance.

For example, in the next image, preamp output (left channel) displaying 19 and 20kHz mixed each at +10dBu. In blue, the preamp using the Muses digital pot, and in red, the TKD. Apparently in this image the TKD has more distortion than the Muses! But I blame the slight increase on the phase of the amp's distortion products combining with the distortion of the Prism. Note that the Prism itself produces measurable 1kHz difference product (-98dBu) and near 19kHz, so it's hard to say how much to blame on the amp. I suspect the amp contributes less distortion than the Prism itself. Never mind, I have nothing up my sleeve.


M3 preamp, 19 & 20kHz each at +10dBu. Muses digitally-controlled analog potentiometer, blue, TKD in Red. Ironically, the TKD appears to have more distortion than the Muses.

We can make the Muses attenuator give away its secrets by measuring both attenuators at extreme levels. Here is the preamp with a 1kHz sine wave at an extremely high output level of +18dBu (6.156 volts, a little below clipping). We can see that the Muses must be an active attenuator and the TKD is passive as the Muses distorts quite a bit more at these extreme levels.


Preamp output (left channel), 1kHz at +18dBu, Muses attenuator in green, TKD in blue. Scale indBu.

We assume the TKD has zero noise floor for practical purposes and that the Muses is an active attenuator. We can deduce the noise floor of the Muses to some extent by playing with the attenuators and observing the output noise of the preamp. The noise with the Muses is constant at all attenuations. However, when I turn the TKD down all the way I can see about a 1dB reduction in noise, which must be coming from the Prism DAC. So the Muses noise is enough to obscure the noise of the Prism DAC. Mainly this curiosity indicates that the M3 preamp's noise is about the same as the Prism's state of the art DAC.

Next, these two distortion plots at an extreme +15dBu (4.3 volts) demonstrate that measurably the TKD attenuator is "cleaner" than the Muses. In other words, we can tell that the M3 preamp itself has very low midrange distortion only by using the TKD attenuator, but the Muses pot masks the preamp's distortion. Even so, we're talking about measurable differences near -90 and below -100dB below the fundamental!

For what it's worth, the preamp THD with the TKD measures 0.001% in the midrange while with the Muses, it's 0.002%. I don't think I can hear that difference, provided that the harmonic structure is benign. The squiggles above 10kHz are probably anomalies in Room EQ Wizard, which is really not designed to measure electronics, it's designed for speakers, which have much higher distortion. I wouldn't trust this measurement above 10kHz; besides, harmonics of fundamentals above 10kHz are in the supersonic range (20kHz and up). Foo says the preamp has a THD+N at 1kHz at 0dBu, 0.006 to 0.008%.


Preamp (right channel) at 15dBu, TKD attenuator set to 0dB. Midband distortion circa 0.001%.


Preamp (right channel) at 15dBu, Muses attenuator set to 0dB. Midband distortion circa 0.002%.

The headphone output into a 20 Ohm load is as exemplary as the preamp. I have available distortion measurements at many frequencies and levels. Here is a sample of the M3 producing the equivalent of 100dB SPL into an Audeze LCD-X (I used a dummy load, don't worry!). Muses in green indistinguishably overlays the TKD in blue. The legend mistakenly says "103dB".


M3 headphone output noise at the equivalent of 100dB SPL at 1kHz, LCD-X into 20 Ohm load. Muses and TKD attenuators are overlaid but there is no measurable difference. Vertical scale is in equivalent SPL.

I have evaluated another digitally-controlled analog attenuator chip with similar measured rating (in the 0.006% THD range) that I consider sonically unacceptable. It sounds harsh to the ear. So the mystery lingers on and the only thing we do know is that an overall THD number is not a useful criterion for sonics. Much more important to the ear is the distribution of the harmonics and how they are masked by the noise and the signal.

Class In Session
I personally believe that Class A operation makes a sonic difference and improves purity of tone. This M3 is biased to be in Class A operation for a large part of its operating range. While the maximum output voltage just before clipping is nominally 7Vrms (sine wave), which is 2.45 watts, it would operate in Class AB at that level into a load. But with 105mA bias it still can operate Class A up to 0.882 watts into 20 Ohms, or 4.2 volts per channel, which is still quite respectable as that would produce 132.4dB peak SPL with the Audeze LCD-X phones!

Don't Get Cross
Next up is crosstalk (stereo separation). The preamp has remarkable crosstalk (stereo separation), left channel driven, right measured, of a remarkable -111dB at 1kHz, reducing to a still fantastic -86dB at 20kHz, into a high impedance load. This means that the opposite channel's signal is down in the noise floor—under normal circumstances it is completely inaudible and masked by the main channel.

The measured performance is nearly as good as a dual mono block preamp with independent power supplies and chassis. But I was puzzled when measuring the headphone output with a 20 Ohm load, that the crosstalk reduced to only -42dB at 1kHz! When I raised this point with my engineering expert, John Chester said, "of course, 46dB makes perfect sense because of the output impedance of the driven ground, which you calculated as 0.1 Ohm." Aha: Ohm's law! The lower the impedance of the headphones, the worse the crosstalk, because this is a driven ground system with a common "ground" that has a measurable output impedance. I then tested the headphone output without a load, and the stereo separation returned to about 100dB, confirming the laws of physics, as pictured here:


Four simultaneous test tones (20Hz, 1kHz, 10kHz, and 20kHz) are in the source channel, each at -4dBu (not shown). The opposite channel crosstalk is shown in blue when loaded by 20 Ohms. But when the amp is unloaded, the crosstalk at 1kHz (red trace) reduces to a remarkable -97dBu. See text.

But let's put these numbers in perspective. 46dB is still excellent stereo separation. It's probable that no one on earth can hear the difference between, say 40dB separation and 100dB. When phono cartridges were first introduced, they had a separation of 15dB, and eventually cartridges matured to around 20dB, which was considered excellent. 20dB is the difference between a shout and a whisper, so this M3 amplifier has over two shout-to-whisper ratios greater stereo separation than even the finest phono cartridge! I think it's not to worry. How much stereo crosstalk is acceptable in an amplifier? Probably anything better than about -30dB is audibly insignificant.

The only way that an amp with a driven ground could have better measured separation would be to have yet another ground channel (including another opamp and another pair of Mosfets) and to use a 4 pin "balanced" connector for the headphone output. I would be curious if such an amp sounds better than the M3, all other things being equal.

In Conclusion
And so we've come to the end of our story. Clearly, each of these sonic parts contributes a subtle improvement to the sound. What I think makes the M3 even more special is that the whole is greater than the sum of its parts.

In our next episode, I planned on evaluating some midpriced headphones, but instead I'm going to write about my fascinating experience with the Harman compensation curve. I have a number of bombs to drop that will blow your collective minds.

tony's picture

You've been doing that from day 1!

Just this year, 2015, I came to realize that I may never quite know what any of my music actually sounds like. I learned this as I read the "Katz's Corner" series.

2). I've also learned that my hearing has the same types of deficiencies as my eyes, both need and can be corrected.

3). I've learned that transducers ( headphones ) can be electrically and mechanically 'adjusted', there may even be a 'range of adjustability' in each design. I think that I've learned that I can make nearly any headphone sound any way I choose, I can even make an 'open' headphone perform as a 'closed' headphone. ( closed headphones are more difficult to work with )

4). I've learned that purchased headphones can be inconsistent in performance but the Sennheiser range is remarkably consistent. ( some folks in Vienna assure me the AKG headphones are in the consistent category ), the Beyer stuff may also be.

5). In my last years of travel I've seen the little white wires from people's ears change colors! This year I'm seeing folks, everywhere, owning aftermarket IEMs.

combined with:

6). Institutions like my Detroit Symphony Orchestra Broadcasting Live over the Internet making all their Performance series available to a Global Audience. Only two weeks ago I took some guests to a DSO Orchestra Hall performance ( Company seats - Front & Center ), I returned home to listen to the same performance thru my iMac and base Sennheiser/Schiit system to realize the internet Sound was higher quality than actually being there!

7). Big Sound 2015 taught us that we no longer need to dedicate major funding to have an outstanding music system.
( $100,000 will purchase nearly all of the systems Tyll had vs. the $100,000+ it would take to duplicate a recommended High-End System from a Glossy Mag's list of admired pieces ).
A frugal and careful person can do quite nicely with less than $1,000! HighEnd is becoming accessable to 'Everyman'.

8). With Katz's Corner #8, we have an exciting Amplifier possibility to consider along with AtomicBob's recent discovery of the Project Sunrise 111( Garage1217) and Cavalli ( x20 in price ).
We have wonderful options, all are easily affordable to anyone that doesn't have 5 children to educate or anyone with better than Taco Bell employment.

9). I recall, wayyyyyy back in 2011, Tyll-Jude-Steve G. at RMAF doing a seminar where I was recommended Schiit & Sennheiser HD600s. As look back at all my years in the Audio Industry, that Schiit/Sennheiser recommendation was the best I've every gotten or given. Thank you Tyll.

10). Guys like Bob Katz are 'raising the Bar' for music reproduction, the CDs I've been buying lately ( 2014 releases) are all superb, I attribute this to Mastering Studio's meticulous attention to detail and the quality of electronics in use. The M3 Amp design is probably an example of what every Pro-Studio is working with and towards.

I'm watching consumer Audio becoming an 'On-Fire' Industry.

These next 3 years are gonna be exciting!

Tony in Michigan

Bob Katz's picture

Dear Tony: Thanks for your love letter! But seriously, I'm not sure yet if "any" headphone can be made to sound like any other. It's not all frequency response. For sure I believe the large, coherent diaphragms of the Audeze and Stax create a big, spacious image with depth. I doubt that correcting the frequency response of a small diaphragm headphone can come close. There are also issues of headroom and distortion which I could not imagine. In the next couple of weeks I plan on evaluating the mid-priced headphones which Tyll sent me. I'll try some EQ on each (adjusted by ear) and let you all know how close they may come to the Nirvana high priced spread.

I heard the LCD-4's at the AES Convention the other day. Sweet, extended, excellent. Maybe a little bass light, which is unusual for an Audeze. So, adding bass, was it a problem? Not at all. It made up for that little deficiency and produced a headphone that to my ears may surpass my equalized LCD-X, even when I equalized them using Tyll's scientifically-derived measurements. So let's summarize by saying that I think EQ can take a headphone a long way towards perfection, but if you start with a headphone that's much closer to perfection, you get even more perfection :-).

tony's picture

Quite right.
I'm kinda tweaking and fooling around, all basic workbench fussing, certainly not precise work and I'm rather confident that my 'claim' is disprovable by any experienced person. I sort of feel like the MaryKay rep telling old girls they can look 13.

Earlier on, you mentioned making a DAC and an Amp, something any semi-capable soldering person can consider doing. So, now, with stretches of time on my hands, I'm beginning to live a life as a DIYer and lov'n it.

I'm even look'n at soldering station gear and mentally day-dreaming about making some headphone cups outa various materials.

I'm coming to feel like a kid in the 1940s building a Ham Radio Station and actually having a QSO with another person in Washington State ( or Europe ).

My old drinking friends are all patiently waiting for me at Oakwood Cemetery, I have no idea when I'll be able to join them ( my Cardiologist will be looking at me to give me clues ). Till that time arrives I have only one Work related task: supporting the guy from Vermont in the 2016 election, then I'm on my own, doing anything I like ( wife permitting, of course ).

I'll buy a Mac laptop ( at least 1 TB of solid memory ) to use as my Record Player. I'll load up that program you recommended and every piece of music I get my hands on. I'll name it "Chopin-Liszt"
and I'll continue this adventure. I might even start shaving once per week, or less.

Audeze at AES, I suspect we're in for a whole lot of this sort of thing. Our Audio Industry numbers reflect a ground-swell of interest in the General Population ( even Globally ). We are still in recovery from the financial Bubble bursting, ( events like this take a solid Decade to run their course ), 2017-18 is the break-out year. I expect all these headphone outfits to be prepared to benefit and be ready to launch ever greater levels of performance. I anticipate faithful music reproduction from the iPhone & $100 IEMs all the way up the escalator to $50,000 plus levels to be common-place in the near future, people won't leave home without them.

I'm already seen'n the Recording-Mixing-Mastering people ready for it and leading the charge!

Carnagy Hall on your Bar Stool, on your Park Bench or any dam place you are at any dam minute you want it : Carnagy Hall on-demand!

Oh, by the way, the Phonak people are already out in front with their top system ( Bravo, I think ) .

I hope to be spending my winters down there in Gater land, next year. fingers crossed.

Hope you are well.

Tony in Michigan

logscool's picture

Great write up really cleared up a lot of questions I had about different aspects of the topology of the amp. With what you have said about discrete vs. IC what do you think about the AMB B22 built up as either a 3 channel "active ground" like the M3 or a 4 channel fully balanced amp?

Also with what you said about the crosstalk on the M3 bein lowered by going fully balanced and also other potential advantages of going to a balanced amp, what do you think about the different options for making a balanced M3 with two boards?

Bob Katz's picture

Thanks, logscool. Good questions. I have no idea about how the B22 balanced or the M3 balanced would sound without trying them. My greatest instincts, however, are to stick with the Mosfet and opamp topology they pioneered in the B3. I really like the sweetness and effortless power of the Mosfets. Does the B22 use the same Mosfet topology as the M3?

However, I'm bulidlng a bipolar headphone amplifier right now with the Kirkwood circuit and we'll see how it sounds. It does NOT use overall negative feedback and has an output impedance of about 2-3 ohms. So it will be an interesting comparison.

castleofargh's picture

for those who want to check the influence of crosstalk(or how the difference felt in going balance is not the better crosstalk!) I suggest to play around with the crosstalk VST available here http://bedroomproducersblog.com/2014/09/20/sleepy-time-dsp/

and yeah from what I've seen, in general, low impedance IEMs/headphones don't help an amp in getting the most amazing specs(lower load=more current flowing, blahblahblah, science!). but as always what matters is audibility. crosstalk is really the least significant measurement for me, unless it really is crazy bad. but even then, it's not something that sounds bad, in fact I suspect many people would enjoy some added crosstalk.

TheAudioGuild's picture

If crosstalk is such an evil, loudspeakers would have to be virtually unlistenable.

I can perhaps see it being an issue with binaural recordings, but...


Jim Tavegia's picture

I must also tell you that I am enjoying the heck out of my new Tascam DR-680 mk2 and being able to track and then mix has really upped my sound quality. New mic pres in this unit are not bad at all as they worked to improve the circuitry. Nothing less than native 2496 will do.

zobel's picture

If that unit had individual pots for each channel, it would be better, and actually function as a recorder/mixer. I would buy one it had that capability since it apparently has good pre-amps, and decent sound. Drat.

Jim Tavegia's picture

Download the manual and you will see how it is done internally. The DR-680 MK2 is great fun for under $600 if you shop and could also just make a great needle-drop recorder for those inclined to do such things. If you are a musician, YOU need one of these.

I look forward to the next unit that I hope does 6 tracks at 24/192.

zobel's picture

I've been using a Tascam DP-01 FX/CD 8 track, and have been having lots of fun with it for years now. I use some Rodes (NT1) some MXL, and a couple CAD M179, a pair of monoprice 600700,(don't snicker, they sound uber good for cheap) and they get everything covered fine for my needs. It's fun to get some quick location sound on the Tascam DR-05. Your set up sounds like it is darn good, and no doubt you get some great recordings with your gear! My "studio" takes up about half of the basement of our house, not fancy, but it is enough to do some tracking and get some mixes mastered on cd.
I enjoy reading your posts here about your recording hobby, after all, there is no better way to appreciate audio than to produce your own! I find that true of music too. Keep recording, and posting here!

xnor's picture

why amb uses this active ground nonsense. It just lowers performance...

Oh that reminds me of NwAv's article about active grounds and how he was banned from amb's forum for this and the Mini3 fiasco.
That's why I don't consider them good designers... but it looks like the M3 performs better due to more power.

Bob Katz's picture

I am familiar with NWAV guy's rant on active grounds. There are far more important factors to the success of the M3 amp. I doubt that the active ground topology contributes either negatively or positively to its sound quality. Measurements indicate that the biggest compromise is crosstalk with low impedance cans. You don't have to love the M3 but you don't have to drool over everything NWAV guy says either. I'm not impressed anymore with his O2 amplifier :-(.

xnor's picture

Of course the M3 has (much) more power than the Mini3 so I guess you don't get into this area where the effects of the active ground start to really show (crosstalk being an exception to that). I am of course talking about measurements here, which depending on the circumstances can be pretty horrible until they impair perceived sound quality.

But I am still wondering: why? ;)
The important part of NwAv's article is not the occasional rant-y sentence but a summary of facts, sound engineering advice and audiophile myth busting.
Btw, can the M3 be built without an active ground? (Not gonna build one, not impressed with amb, just curios.)

You gotta admit that O2 is not bad for its age, price, size, DIY-friendliness and considering that it can also run on batteries... agdr also has developed some cool "mods" on diyaudio.
But I'm not using one myself.

TheAudioGuild's picture

Why? There is no good reason. It became obvious on HeadFi some years ago that the people who got the whole "active ground" thing started simply didn't understand basic circuit theory or that current only flows in closed loops.

That's how so much of this stuff gets started. People with a rather naive understanding of something reach erroneous conclusions and come up with some rather silly ideas about how to "fix" it.


zobel's picture

Xnor, do you find some amp designs that have no caps in the signal path (DC coupled) to be smoother sounding and less fatiguing than most? se, what do you think? I have an old Creek integrated amp (DC coupled) that, while not a powerhouse, is sweet.

TheAudioGuild's picture

Depends on the cap.

I don't have any general capophobia. I haven't had any issues with a good quality, properly-sized film cap on inputs. But I'm not a fan of using electrolytics on outputs where they're only blocking a small DC voltage compared to the maximum signal voltage. In those cases, I'd go DC-coupled either by manual adjustment or servo.

However on proper single-ended designs where the electrolytic is biased by half the power supply voltage, as long as the cap was properly sized for low frequency response, I haven't encountered any problems. But such designs are pretty rare.


xnor's picture

Hmm, not really.Ideally and for optimal performance a headphone should never see any DC.
You don't have to use caps to ensure that though..

zobel's picture

the more important quality determining factors of a solid state amplifier among these?
1) fewest gain stages, (such as a passive pre-amp to power amp)?
2) no caps in signal path? (Alternate dc block)
3) class of operation?
4) IC op amps vs discrete components (assuming the best of each)?
5) amount of negative feedback employed?
6) type of power transistors (mosfet, jfet, etc.)
7) current reserve and speed of power supply (headroom)
8) balanced vs single ended?
9) all of the above, each are important.
10) none of the above, the design matters most
11) all of the above and their use in the design is crucial.
12) a proper blind test of the amp comparing it to other amps.

xnor's picture

Well, my answer would be similar to 11) in that most of these points can play an important role depending on the design.

So most points require a response on their own, but I will keep it short.
Gain: I think optimizing gain is something that is neglected too much. There are people with headphone setups that cannot go beyond 9-10 o'clock on the volume control...

Caps: see previous answers.

Class of operation: If nicely implemented AB, D, A all work fine. Exceptions are e.g. simple, discrete, low-feedback designs that would produce lots of crossover distortion if they didn't run in class A. But better designs can achieve better performance at higher efficiency..

IC vs. discrete: There are quite a few discrete headphone amp designs out there that perform worse than IC designs and cost more...
To me it's primarily a question of requirements (load impedance, output power, distortion, noise etc.)

Which parts? I don't like the approach of picking some must-have parts first and then trying to design the amp around it. Instead, look at the requirements and take the best parts for the job.

Negative feedback: Too little NFB can do more harm than good. Ironically, some audiophile companies tout their products to be better by using little NFB - what a clown show.
But NFB applied properly is awesome.

Bridged ("balanced"): Again, typically not needed for headphones. Just get an amp with a proper ground.

Blind tests: Everyone should have made this ear-opening experience.

ultrabike's picture

Saw some of that, and indeed active grounds may not be w/o issues. However, one possible reason to have active grounds may be to remove caps from the output path. Not sayin it's perfect, but I don't think NwAvGuy discussed that (or don't remember seeing it).

Main problem would be that one may not be able to use the output of the headphone out as a line-out. Here are some articles that sort of discuss this:


http://www.ti.com/lit/ds/symlink/tpa6132a2.pdf (skip to section 8.3.1)

Those issues may more specific for DACs, but again, some folk may use the headphone out as a line-out for convinience and that may or may not be a problem depending.

As far as the O2. I believe it is not a capless design. I think capacitors C13 and C14, located between the gain stage and the current buffers, block DC. The corner frequency can be readily be found to be 1.8 Hz, NwAvGuy has this in one of his articles as well.

tony's picture

That thing has taken on a life of it's own.

It's kinda like Corn Flakes!, everybody seems to have nice memories of the darn thing.

Hardly ever hear anyone mentioning the Cmoy any-longer.

Tony in Michigan

tony's picture

I think that I can see how this tool can be helpful to a recording engineer, it's something you can count-on!, it's a calibrated, high standards and high performance link in a chain.
I'm glad you have it! I hope it helps in creating recordings.
I might even hire one of those 3 builders.

Lately, I'm exploring transducers. i.e. Noontec drivers, both the 40mm in the Zoro and the 50mm which resonates down to 5 hz.

No tell'n where this work will take us but I don't feel like I'm spinning my wheels. I know there's always unintended consequences from stuff like this. If I'm lucky I may find a couple of missing puzzle pieces which may result in an important sum.

I applaud you,

Tony in Michigan

xnor's picture

Sorry, but I have to make a last comment.

In the Mini3 fiasco it was criticized that amb's measurements were incompetently done (they seem to be unloaded despite being clearly labeled as loaded) and misleading. So I've checked his website ... he's still got those same measurements online! Nothing changed in years.

But now to the M3: his measurements show -95 dB crosstalk with a 33 ohm load (!) and -94 dB crosstalk with an 8 ohm (!!!) load.

Mr. Katz, please go to his forum and ask why he still keeps these misleading and incompetently done measurements online...

tony's picture

Am I to think that these AMB design discussion points in some way diminish the performance benefits of these designs to the point of making them unworthy? or are these tiny runs in the stockings that could've been addressed and corrected but weren't!

I think that I'm seeing ( in you ) a competent electronics designer.

Compromises and preferences prevail in completed designs.

I'd ask for your summary of these AMB designs and the Garage1217 and the Asgard2 ( that I own and seem to love ).

I'm not understanding your position on all this.

Tony in Michigan

xnor's picture

My position is that years ago it has been pointed out to amb that his specs are wrong (based on trivial calculations that even a non EE can do - so if he cannot do that I'm seriously questioning anything he produces) and his measurements are wrong and so he is advertising stuff that is false/misleading.

A few years later nothing has changed, so I have to assume that he is doing this deliberately ... probably so he can sell more stuff.

tony's picture

Is that why Bob Katz did all those measurements?

In my business we trust and we verify, which I suppose is why I'm not building electronics DIY, I haven't the tools to verify.
I could buy one of these Amps from one of three builders available but I'm so darn happy with Schiit I don't feel the need.
AtomicBob is showing-off a Garage1217 project Sunrise tube amp that looks like a fun tube roller.
Alex Cavalli is offering a range of $4,000 Amps that everyone seems delighted with, Chord has some nice stuff too.

I've know a few guys ( only a few ) that seem able to take a box of little electronic parts and create a masterpiece. I had one as a customer in my Retail Audio Business. I rather suspect Bob Katz to be of that group. I understand M.Moffat at Schiit to be another such person. Perhaps AtomicBob falls into that group.

I'll admire their work and in Bob Katz's case I'll buy his Mastered Recordings.

From my vantage point, I'm taking Bob Katz's word that the amp performs well, I think Tyll admires Katz's M3.

I'm not at all certain that I would have the same results if I attempted building one.

False and misleading is a standard of the world we live in, I wonder if Katz's measurements establish anything important.

I conclude that a product able to impress a Mastering Engineer is probably a impressive design.

Ok, maybe I'm a Katz fan-boy.

Tony in Michigan

JIGF's picture

Thanks bob, amazing read!

Also, still looking forward to the expanded system for amp reviewing Tyll is developing.

tony's picture

Sennheiser is stating for the new 1060 headphone, it's said to be the lowest distortion of any transducer ever produced.

Well, that's one hundredth of a percent or 1/100.
Human eye can resolve 0.001 % which is one minute of an angle and also a one inch square at 100 yards.

Can this vanishingly low distortion level have relevance in the context of Amplifier performance?

I guess we'll find out soon as Sennheiser is showcasing this new technology for all our reporters.

Tony in Michigan

Jim Tavegia's picture

There is alot going on between slew rate, various caps, and Freq response. What I do know is that headphones that I use to tolererate like the Sony 7506 ( I have 2 pair that I have used for years) I cannot use any more now that I have my AKG K701s, 2 pair of K271s and a pair of Focal Spirit Pros that I will use for mixing that are warmer, but I love the AKGs. I will buy 2 more pairs of AKG K271 and use those for performers (total of 4) who come to my home to record. Amazing when you really start comparing where you end up.

I am more concerned about owning great cans and less about which amps, but it all does matter.