Saturday, November 21, 2015

I love digital volume controls

Digital volume controls are one of the things Most Misunderstood by subjectivist audiophiles (and I'm one of those partly--but one of the rare ones who have come to embrace digital volume controls).

I was where many audiophiles are now in 1979.  That was when I began surgery on my Marantz 2270 trying different opamps to replace the line amp board in that unit, and finally going to a "passive preamp" consisting of a 32 position digital switched attenuator (same as used in G.A.S. Thedra...I had friends at G.A.S, and I installed that in a ModuLine box that had been used as the original prototype of the design which was ultimately sold to Dennessen as the Dennessen Preamp...designed by other friends of mine with my help...but I fried the prototype Dennessen board I inherited by connecting the unterminated 9V battery wires I ended up with nothing more than a passive attenuator box that I upgraded to the G.A.S. stepped pot, which IIRC was 25K).

I could see no reason to do anything other than a passive attenuator for two decades.  (I built a tube-based phone stage for turntables.)  I certainly believed it was the cleanest approach.  But with my passive attenuator box, I couldn't easily adjust balance, and I never could get the imaging right on my modified speakers.  And I didn't think I could set balance any other way than by remote control.  And that locked up my imagination for about two decades.  I cursed my lack of good center image, but I never did anything about it.

I could have added balance control in various ways, it's not wonderful to do this without a line amplifier which provides buffering, but it can be done.  But balance really has to be adjusted from the perspective of the listening position.  Any kind of balance control that you have to adjust while standing up and away from the listening position won't work.  You have to pop up and down making little balance adjustments and then checking out how they sound.  This turns out to be impossible because each time you sit down the position may be slightly different making the required balance control adjustment different.  And remember you have to do this every time you change the volume level with most stereo pots.  The worst of all are preamps with separate analog level controls for each channel.  By the time you get the balance and level right for listening to the recording it will be sometime the next day.

Finally around 1998 I bought my dream, a preamp with remote volume and balance controls.  I also at the time thought it to be about the cleanest possible preamp, an Aragon 28k.  So while it wasn't passive, I figured the added distortion and noise were sufficiently low.  It has a low distortion discrete transistor amplifier (essentially a discrete differential op amp with feedback) powered by a discrete transistor regulator.   (Actually, it's very hard to beat the best chip op amps--if you really have the best.  It's not hard to beat the early 741 op amp.  It is extremely difficult to beat the best chip op amp made, an OPA 211.)

Anyway, other than the all discrete part, which is debatable, it is straight forward no-nonsense, just a good circuit with good parts (except perhaps that motorized pot...).

It has very low distortion, always at the residual of my measurements (the last one was 0.004% or better).  And it has remote control balance, and quality jacks as I had been using for quite awhile on my prototype unit.

Now I tried adjusting the balance on every recording.  Still easier said than done with my very wide angled speakers.  And I noticed right away that it varied a lot.  Finally I figured out the #1 variable.  The position of the volume control.

Something else I became aware of at that time.  I was usually only turning up the volume about 9 to 10 o'clock.  Barely cracked.  It occurred to me (1) that it had poorly tracking channel balance in that part of the control, and (2) what a waste!  internally the signal goes through a pot reducing the level 100 times, then gets re-amplified by the line amplifier back up 10 times.  Would have been much simpler just to have a passive attenuator...well, as I had been doing.

So now that I had balanced control (instead of the fixed attenuators) I really did have to use it as a correction all the time.

I became aware of the Classe CP-35 (with digital volume and balance) sometime around 2000.  I should have bought it at that lovely store.  But I didn't actually buy one until around 2005 and on eBay.  Not only did I love the perfect volume tracking (now discovering I didn't need to use it anyway, now having perfectly matched high end Revel M20 loudspeaker) I just alway thought it sounded good.

Quite contrary to my long standing technical belief in passive preamps, the CP-35 wasn't just liberation from having to reset balance with each volume adjustment, or the limited range or size of adjustments with stepped attenuators which I had used before.  It Sounded Better To Me.  This rarely happens with so much clarity.   All the passive attenuators, and even the re-amplified potentiometer of the Aragon 28K, sounded dark.  The CP-35 seemed to lighten the music in the same way the volume display lightened the front panel.  It brought the music more to life, and I decided I liked this.

And then in occurred to me I should not have been ignoring impedances, gain reductions, re-amplifications and so on.  Every time you have a big R followed by some C you have a RC network which is rolling off the highs.  All the rolloffs are additive, and we sense not so much the higher frequencies as the slower rise times when nearly any kind of rolloff is added to the rolloff there already is.  So figuring bandwidth of 20k being "good enough" is wrong.  A reasonable analysis suggests that every electronic component should have bandwidth to 200kHz or better, to achieve sufficiently flat time delay.

Anyway, never even having a big RC in the middle is good.  There is nothing magic about a potentiometer, pots are horrible, though some less horrible.  That's just for starters.  People somehow believe that pure digital gain reduction (which, btw, is not what most preamps simply with digital volume do, and even fairly ordinary home theater receivers have "direct" modes that bypass conversion of analog inputs to digital) looses resolution.  That may not be true, if reducing no more than 48dB when attenuating a 16 bit digital source in a 24 bit digital pathway, as SPDIF automatically is, particularly with sources that do attenuation, and most things with SPDIF input these days for sure.  At least not in digital domain and output.

But meanwhile, even a simple pair of resistors to attenuate introduces lots of losses, among them automatically being resolution.  No matter how you figure resolution, it is lost in the process of resistive attenuation.  Relative to noise, absolute volts, absolute quanta, every way you look at it resolution is lost when you reduce level with a passive pair of resistors, even a fixed pair (though other kinds, especially pots, add other problems).  Plus whatever distortion from the resistors themselves, which is always plenty btw.  But even if the resistors were perfect, information is inherently lost (though you could say, not enough lost to make any difference, but there we are back in compromise land again, not the world of perfection that no-information-loss implies).  And this is especially true if you take the most hard headed Shannon's Laws approach which considers noise a limiting factor.  In addition to adding noise, the resistor bridge reduces signal.  It's inherently a signal to noise ratio loser.  Plus it looses bandwidth because of the R being added to the signal path with must always be followed by some capacitance, the worst case being a long hi capacitance cable to the power amplifier.

(Some would argue the pot or resistors also reduce noise, and that is true.  They reduce the incoming noise by exactly the same proportion as the signal is reduced.  But there no increase in S/N from that. To that, the resistors will always add a little noise.  So overall, S/N is decreased, though possibly only by a small amount, depending on the series resistance in the pot being used.  It's worst, of course, with a large value pot.  Old timey tube equipment often used 250K and 500K pots.  Even if followed by gain circuitry, those have to be understood as information loss devices.  With something like 10K of overall resistance achieved with high quality resistors, as in a device like the Placette passive preamp, there is far less information loss.  But still there is loss.)

It is better, at least in principle, best to avoid gain reduction.  If gain is being increased overall, it's pure stupidity to reduce the amplitude level first, then apply a fixed amount of gain.  And yet that's the standard analog preamplifier design, all the classics to now pretty much I can think of.  Some do switch in varying amounts of gain during the attenuation, such as reducing gain by 6 and 12 dB when possible, thus, the worse case attenuation down to 18dB is only 6dB, and the average less.  But that's usually only done at the very high end, like the top Mark Levinson.

Another approach is to go variable gain all the way, sometimes varying it far negative.  That may be what some or all chips do.  Only practical limitations apply to this approach, in principle it could be perfect, though of course the gain reduction loses information as all analog gain reductions do, it could theoretically be the most perfect and therefore lose the least information.  I think this is what many gain reduction chips do, though some may combine resistive switching with that for performance or hype.

Anyway, I'm not quite sure what the Classe did, I think basically it used a audio attenuation chip, which were widely available in 2000 or even 1995.  Some of the better ones are quite good now (though becoming harder to find).  Mark Levinson in their first preamp with digital gain controls, the 38, from the mid 1990's, said they varied the reference level to a DAC, in effect making an analog amplifier with digital volume control, from some of industries most refined parts.  But are DAC's really the way to do this, and not custom engineered parts, engineered as volume control, but with very high quality?  I believe the top parts now are quite good, at least since the top now discontinued Burr Brown unit, which I think was available in 2006, then discontinued and reappeared in a Finnish part.

I concluded in previous article that any weakness in the Emotiva XSP-1 (Gen 2) in unbalanced outputs is probably related to my guess that it makes everything balanced then unbalanced.  Nothing to do with the digital volume per se, which is clearly balanced and shows no weakness--no distortion down to invisible above a SOTA noise floor--in full balanced mode.  Then a SOTA preamp has to have both balanced and unbalanced preamps in parallel, depending on nature of inputs and outputs, so as to go through as few conversions as necessary for a given kind of inputs and outputs.  I don't think emotive did that, not even sure about Levinson et al.

 Anyway, with all that irrelevant personal anecdote covered, I have to think deeply about digital volume controls themselves, and not just my background story...

First, there are two kinds of digital volume controls.  One type operates in the digital domain.  Almost always when this is done, the signal is already in digital form for some other reason, such as it was recorded and distributed that way.

When starting with 16 bit, and reducing less than 48dB (the equivalent of 16 bit), I have heard it said and I believe that no resolution is lost (with proper dithering, which turns out to be necessary in maintaining resolution) when outputting and inputting 24 bit, and spdif is automatically 24 bit, it maintains that many bits whether they are used or not.

So when my Tact Audio RCS 2.0 reduces level in 0.1dB steps, it computes the needed reduction and applies dither with 48bit resolution (or something like that, it's done in a DSP) then outputs 24 bit SPDIF, there is no loss to deep reduction, way below anything I do, especially now in the Living Room where I can use as little as no reduction at all thanks to low gain.  So I can sometimes run right at 0dB and even wish for a bit more (which Tact delivers, up to 6dB more, though I don't like the idea of using digital multiplication, but technically it should be fine as long as you don't get to digital clipping, which can be seen and heard).

So, repeat after me: analog volume inherently loses information, can't be avoided, digital volume loses no information down to volume reduction you never seriously use.  Plus digital volume is accurate, has no channel imbalance so no endless worry with each volume adjustment, resolution to imperceptible-difference 0.1dB available in some equipment I have, and above average resolution in others.  Plus it does not essentially induce bandwidth loss, another kind of information loss.  Distortion is possible, but can be far below that of most other active circuits.

Ahah, you could say there's one catch.  Your fair DAC's aren't truly perfect to 24 bit.  Ahah!  But the output of the best DAC's, in my opinion, are far superior to most if not all preamps.  DACs are the most perfected thing.  That's why I power my power amplifiers now that I can direct from DAC's (though it limits my level capabilities a tad, the 2.5V output of the Audio GD DAC doesn't come close to driving the Krell either to rated power or actual clipping (though, reasonably speaking, one would think it to be enough).

It turns out, in electronic circuits, the hardest thing to do extremely well is drive a load.  So it is that analog to digital converters are the most perfect of all things, per dollar spent.  Digital to analog converters are next, up to generally limited outputs of 2 volts into high impedance load.  Preamps nowadays put out 8V or more, and can drive 100 ohm loads if not 10 ohm loads.  That is great, but makes it a difficult thing to accomplish with low distortion.

So in my view, it makes sense, if you are starting with digital sources anyway, of doing the attenuation if any in digital also, and then only driving power amplifiers with the outputs of DAC's.  That's sort of what I've been doing since 2006 or so, but the "DAC" has really been the output of a Behringer 2496 DCX, which isn't terrible at 2V output but isn't great above 3.5V.  I ran the Tact through a DEQ then a DCX all in digital mode.  So digital never left 24 bit digital domain until the DACs.

But anyway, as I've sad before, most audiophile stuff that takes analog inputs and outputs (with the exception of a fully loaded Tact or something like it) and reduces volume does so never "sampling" the input into the digital domain.  There are fancy parts that do that with the equivalent of varying the gain of an amplifier, or precisely switching in resistive elements when needed too,  And that's probably the way to go if you do actually have analog inputs and want analog outputs.  I now eschew devices that don't give you at least 0.5dB resolution down to -30dB, and I think 0.1dB is best.  Most audiophile solutions based on using relays to switch fixed resistors, such as the early Krell preamps or the Placette passive or active preamps, don't have enough resolution in gain adjustment.  And it may well be, the chips actually outperform them, except in the audiophile imagination.  (Mind you, I've lusted over these relay and fixed resistor solutions for a long long time.  They've either been unaffordable, or I've decided they weren't the best way to go, which is where I am now.)

As for me, I started using digital crossovers and room correction in 2006 and believing in their most important advantages (such as time alignment, and room mode attenuation) I haven't looked back.  Digital gain is the way to go when you do that, when you're already in the digital domain, level adjustment can be done effectively without loss of information in most cases.

But I still need analog domain digital volume controls in limited applications, such as the preamp setting the gain for my Masterlink digital recorder...that's the #1 job my Emotiva XSP-1 does now and might do for a long time, because it's hard to beat it's balanced outputs, though it would be nicer to have the 0.1dB resolution, balance, and polarity of a Mark Levinson 38/38S/380/380S/32/320/etc, so if I ever get one of those it could replace the Emotiva.  In the end (until it died) that is what I had used the Classe CP 35 for, except I also ran the main outputs in parallel to the Masterlink and Sonos, I figured it to be important to send the same 2V output to both.  (I've subsequently found that generally to be unnecessary, particularly with the high gain output (!) of the Emotive phono stage (at Processor Output).

The #2 job of the Emotiva, though actually the #1 in regularity of use, is as a selector switch for which analog devices feeds Sonos, as well as the into the aforementioned preamp-to-Masterlink.  This is fine and perhaps good enough.  Though I haven't seen measurements of the Emotiva Processor Output, I'd guess they are significantly better looking then the Unbalanced Main Outputs, which has a tiny tiny bit of tiny distortion spikes poking up above the very low noise floor.  If it did have those same distortions, or worse, I wouldn't want to keep the Emotiva in this selector-switch role.  And just because of the uncertainty I imagine myself ultimately getting (if not the Levinson) simply an unbuffered high end selector switch.  Which I could make out of my old passive pre (but it's too ugly by my modern standards).  So anyway I imagine making or buying a nice unbalanced selector...I use the upgraded (Teflon Jack) version from dB Systems to select the inputs to the Lavry AD 10 which allows me to run high quality analog into my digital system.  And that wasn't expensive at all.  I did chose to re-work it with some dampening material inside the chassis.  But that would be one acceptable solution, and there are no doubt others (and why wouldn't I use a higher end one than the dB systems in the living room, and move the dB systems to the bedroom).

But as far as the phono stage itself, if I continue using the Emotiva in that role, there's no much advantage in bypassing the Emotive-as-selector because, essentially, I can't bypass it.  I can't take anything other than the Processor output as the input to that hypothetical switch box, and then run it to the box with more cables.  Right now, the output of the Processor Output, which I'd have to use anyway, goes through a short low cap cable to the Sonos Line Input.  It Could Not Be Better, assuming I were using the Emotiva Phono Stage.  The only improvement that an external selector could provide would be for the other inputs, and then at the slight expense of the Phono Stage.

And as far as eliminating the Emotiva Phono Stage, that's easier said than done.  I could easily modify the dB systems to have the optimal loading for my cartridge.  That's the easy part.  The hard part is that it doesn't have quite as much gain.  And then it would be uncomfortable to use at the same Sonos Line Input level setting.  The phono would always be too soft, and I'd have to boost the gain...with a line amplifier of some kind.  So then, is my conglomeration of phono stage and line amplifier still better than the Emotiva?  My initial impression is that the Emotiva may be overall better sounding than the dB, and significantly quieter as well as having more gain.  So really to replace the Emotiva I might be talking about something much higher end.  Probably the only way to get what I'd want here would be to make it myself, perhaps with OPA 211's.  Short of that, and until I make that, the Emotiva may be good enough.  High End phonos rapidly become very very expensive these days. and mostly not worth it in my opinion.  The Emotiva is probably as low noise and distortion as most of them, only lacking in qualities perhaps now not measured, if they even exist.  Though mind you, I'd love to have such if it wasn't too bad technically and sounded good, and there are a lot of lust worthy ones in the $20k range.  But most audiophile preamps likely have far more distortion to the outlets having sufficient gain to make LP and CD levels comparable, 2V peak.


A discussion of the legendary Burr Brown audio volume control and its successor.

A discussion of audiophile superstitions against digital level reduction (digital domain digital control is the only kind mentioned, somehow even these geeky people don't seem aware of digitally controlled analog devices--which is the vast majority of digitally controlled preamps--but otherwise their analysis is devastating to audiophile superstitions, basically everything lost by digital reduction is also lost by analog reduction--which then loses more, but analog-o-philes can't see that, but of course an analysis that entirely presumes Shannon's Laws, as I don't, but similar principles still apply indifferent to the belief or disbelief in unmeasurable resolution).

The successor to the Burr Brown audio control made by Finnish company.

Clueless megabuck high enders rant and rave.

Glossy gloss.  Still don't get that there are digitally controlled analog devices.

No comments:

Post a Comment