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VCA fader consoles
Old 26th November 2009
  #1
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shangoe's Avatar
 
🎧 15 years
VCA fader consoles

hallo,

i just like to get some opinions about the impact of vca faders on the sound in analog consoles. is this something that degrades the sonic quality? and in case yes, is there a way to mod them to make it acceptable?

thanks!
Old 26th November 2009
  #2
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Ben B's Avatar
 
🎧 10 years
VCA faders are noisy. Get a moving fader automation system if the noise is a problem.

Many VCA consoles allow you to swap paths between the small and large faders. For example, "VCA's to Monitor" mode on the SSL switches the small faders into the main channel path in Record Status, so that you don't have to record through the VCA's.

-Ben B
Old 26th November 2009 | Show parent
  #3
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JohnRoberts's Avatar
 
🎧 10 years
VCA technology has improved over the years so older VCA based consoles had slightly higher distortion. While newer VCAs are also somewhat lower noise the noise floor in recording with proper gain structure is typically limited by microphone self noise or room noise.

It is possible to upgrade older consoles with newer VCAs for a marginal improvement.

JR
Old 27th November 2009 | Show parent
  #4
Lives for gear
 
🎧 15 years
Hi
As John said VCA technology has improved considerably.
Many of the pieces of equipment using VCAs in the past were not tested and re engineered with proper understanding of the requirements for a decent sounding VCA, they were treated as 'just a chip' whereas in reality their needs are far more stringent than the common op amps.
Simply swapping for the latest equivalents can be an improvement but really their 'environment' needs reappraising too.
Matt S
Old 27th November 2009 | Show parent
  #5
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JohnRoberts's Avatar
 
🎧 10 years
+1.. not to make this sound more mysterious than it is, modern VCAs have gotten cleaner to the point that sonic effects from the control voltage path can now be perceived in the audio output with SOTA bench equipment testing.

Again, not a night and day difference, but if you are in pursuit of "straight wire with gain" perfection, it requires more than just dropping in the newer VCA. The complete circuit should be evaluated in light of newer part application advice, which has a few subtle differences.

For perspective, just because you can "see" a noise modulation beneath high level sine waves in THD+N tests doesn't mean it's audible in use with program material. Likewise a lower distortion figure, inside a dynamics processor VCA will likely be swamped out by the normal artifacts created from side chain attack\release time constants modulating the CV. I wouldn't lose too much sleep over this.

Hardware purists, fire up your soldering irons.. (or call Jim Williams).

JR
Old 27th November 2009 | Show parent
  #6
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🎧 10 years
Quote:
Originally Posted by JohnRoberts ➑️
+1.. not to make this sound more mysterious than it is, modern VCAs have gotten cleaner to the point that sonic effects from the control voltage path can now be perceived in the audio output with SOTA bench equipment testing.

Again, not a night and day difference, but if you are in pursuit of "straight wire with gain" perfection, it requires more than just dropping in the newer VCA. The complete circuit should be evaluated in light of newer part application advice, which has a few subtle differences.

For perspective, just because you can "see" a noise modulation beneath high level sine waves in THD+N tests doesn't mean it's audible in use with program material. Likewise a lower distortion figure, inside a dynamics processor VCA will likely be swamped out by the normal artifacts created from side chain attack\release time constants modulating the CV. I wouldn't lose too much sleep over this.

Hardware purists, fire up your soldering irons.. (or call Jim Williams).

JR
IME VCA faders affect the sound of your tracks considerably...there seems to be a lot of distortion there...and mostly the kind I don't want...the best way to hear for yourself...rent an SSL room for four hours somewhere...bring up a session on the main faders.. listen...then...bring it up on the small faders (don't assign VCA to these faders)...listen...huge difference...don't pan either for this experiment....there's something with panning on some SSL's while on the small faders that does something weird too...this has to be a larger format SSL...the 900 doesn't have small faders.
Old 27th November 2009 | Show parent
  #7
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🎧 15 years
Hi
You are starting to compare apples and bananas here. There is nothing to say that SSL have got the best out of the VCAs, although their implementation is probably better than most but you have also to contend with a whole pile of other amplifiers switches and whatever which are more likely to be messing the sound up than the 'VCA' itself.
If you were interested enough you could actually link out the 'VCA' chip (module) leaving all the other parts in place. You may well find the audio is compromised without involving the VCA itself.
To find out what can be achieved you have to strip the circuitry down to the essentials. input amp, VCA and post amp plus some sidechain control volts stuff. Now you can evaluate the VCA. Putting your now perfectly honed VCA subsystem into a box of writhing snakes will now compromise your 'perfection'.
Anyone can assemble a mic or line amp or two and get it to sound decent. Putting 400 of them into a box with common power supplies is a different matter altogether.
Matt S
Old 27th November 2009 | Show parent
  #8
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🎧 10 years
Quote:
Originally Posted by Matt Syson ➑️
Hi
You are starting to compare apples and bananas here. There is nothing to say that SSL have got the best out of the VCAs, although their implementation is probably better than most but you have also to contend with a whole pile of other amplifiers switches and whatever which are more likely to be messing the sound up than the 'VCA' itself.
If you were interested enough you could actually link out the 'VCA' chip (module) leaving all the other parts in place. You may well find the audio is compromised without involving the VCA itself.
To find out what can be achieved you have to strip the circuitry down to the essentials. input amp, VCA and post amp plus some sidechain control volts stuff. Now you can evaluate the VCA. Putting your now perfectly honed VCA subsystem into a box of writhing snakes will now compromise your 'perfection'.
Anyone can assemble a mic or line amp or two and get it to sound decent. Putting 400 of them into a box with common power supplies is a different matter altogether.
Matt S
sure OK... but as far as examples go, flipping your session from the VCA faders to the small faders on an SSL can give you a pretty good idea. Same console different faders...one's VCA one's not... that doesn't seem like apples to bananas...seems about as good a comparison as you are gonna get without "stripping"
Old 27th November 2009 | Show parent
  #9
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🎧 15 years
Hi
I am afraid not. There is a lot of 'junk' in a SSL and so much of audio is to do with what is physically next to other things. There is also the issue of how many FET switches the two paths go through.
Please don't get the impression I have a 'downer' on SSL, I would put the same challenge to any desk.
I didn't say getting a true comparison would be easy but to 'blame' a VCA as being a bad boy is unfair if it is surrounded by so many other compromises.
In the same way as trying to compare any of the state of the art gear while monitoring on a pair of Radio Shack '10 Buck special' speakers.
Matt S
Old 27th November 2009 | Show parent
  #10
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JohnRoberts's Avatar
 
🎧 10 years
Yup, I don't want to throw stones at any consoles out there. Many of them do have a sound for better or worse.

I don't think I'd rent a few hours of a SSL room to test your premise, but next time you do, have some fun with a simple null test. Take two supposedly similar audio paths, and them null them against each other into some other mixer, or perhaps the same console using some other bus.

Just flip the polarity of one, and see how much is left over when you try to subtract one signal from the other "identical" signal. Note: several consoles don't do so well on this test, while some of the deviation is harmless, band limit filtering and associated phase shift.

Getting back on topic, the new VCAs can reduce only the coloration associated with the VCA section. As has been noted there are other ways to diminish the integrity of any audio path besides VCAs.

JR
Old 27th November 2009 | Show parent
  #11
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Musiclab's Avatar
In my old console I had a Megamix system that used Aphex VCA's . It was a pretty transparent system
Old 27th November 2009 | Show parent
  #12
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🎧 10 years
Quote:
Originally Posted by JohnRoberts ➑️
Yup, I don't want to throw stones at any consoles out there. Many of them do have a sound for better or worse.

I don't think I'd rent a few hours of a SSL room to test your premise, but next time you do, have some fun with a simple null test. Take two supposedly similar audio paths, and them null them against each other into some other mixer, or perhaps the same console using some other bus.

Just flip the polarity of one, and see how much is left over when you try to subtract one signal from the other "identical" signal. Note: several consoles don't do so well on this test, while some of the deviation is harmless, band limit filtering and associated phase shift.

Getting back on topic, the new VCAs can reduce only the coloration associated with the VCA section. As has been noted there are other ways to diminish the integrity of any audio path besides VCAs.

JR

Umm this IS on topic...however the previous post makes a good case for not "blaming" the VCA...I can't argue with that logic it makes sense, however, in my experience with VCA faders in a console vs console without IN GENERAL is that there IS audible distortion...( that I don't prefer ) and less apparent headroom...I use consoles all the time..although I can't technically argue that that's not the circuitry beside and around the fader or whatnot, I'm pretty sure I don't like what VCA faders impart....and that seems to be what the OP was looking for...do VCA's cause the sound of stuff to change....yes.
Old 27th November 2009 | Show parent
  #13
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🎧 15 years
Hi
A VCA in itself does not, or at least need not, affect the headroom.
I am not defending consoles that are badly designed and I suppose this is where the VCA's bad rap comes from. OK the distortion from a VCA is usually second harmonic which is often attributed to some 'warmth', indeed a certain designer used a VCA as such a control.
Matt S
Old 27th November 2009 | Show parent
  #14
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🎧 10 years
Quote:
Originally Posted by Matt Syson ➑️
Hi
A VCA in itself does not, or at least need not, affect the headroom.
I am not defending consoles that are badly designed and I suppose this is where the VCA's bad rap comes from. OK the distortion from a VCA is usually second harmonic which is often attributed to some 'warmth', indeed a certain designer used a VCA as such a control.
Matt S
Thanks Matt! I've been wondering that for a while...second harmonic = warmth? or is it haze?
Old 28th November 2009 | Show parent
  #15
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JohnRoberts's Avatar
 
🎧 10 years
Quote:
Originally Posted by Matt Syson ➑️
Hi
A VCA in itself does not, or at least need not, affect the headroom.
I am not defending consoles that are badly designed and I suppose this is where the VCA's bad rap comes from. OK the distortion from a VCA is usually second harmonic which is often attributed to some 'warmth', indeed a certain designer used a VCA as such a control.
Matt S
From Gary Hebert's (THAT engineer) AES paper
Quote:
The major sources of distortion in log-antilog VCAs are errors in the logarithmic characteristic
of the base-emitter voltage versus collector current in the core transistors due to parasitic base
and emitter resistances, mismatches among the core transistors, and non-linearity in the input
amplifier used to drive the core transistors. Each of these error sources is examined in the
following sections, and the techniques used in the current design to minimize distortion from these
sources are described.
Indeed the distortion caused by the first two mechanisms are low order even harmonic. Very familiar to anyone who has misadjusted the symmetry trim on old school VCAs. Now with better device matching at wafer level, and laser trims at the wafer, we can see better < .01%THD+N untrimmed.

The third mechanism is related to diminishing loop gain margin in the front end opamp and is not simple low order even harmonic. Now that the devices are so much better matched this third mechanism could become dominant, but they finessed that by using a transconductance amp topology here for it's better slew/HF gain margin.

The AES paper contains more discussion and pages of equations detailing all the distortion mechanisms. I'd be lying if I said I read and understand every word, but I can read the performance graphs, and these newer parts are so much better than the old.

JR

NOTE: I am not pimping for THAT.. I just appreciate good engineering when I see it. I was granted one patent for a crude class A VCA years ago, so I have an interest in VCA technology.
Old 28th November 2009 | Show parent
  #16
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🎧 10 years
Just a few quick remarks:

So called "Blackmer" VCAs (dBX and THAT devices) are not very tolerant regarding high impedance on control port. That means that anything higher than few tens of ohms on control port will degrade THD.

PSRR is not stellar either. VCA core is usually biased by resistor connected to negative supply, so ample local PSU bypass is mandatory (IMH & completely unscientific opinion, when someone describes degradation as "haze" my first suspect is PSU and bypassing).

Control path must be verrrry clean. Noise, local crosstalk and crosstalk from other channels will directly modulate signal passing through VCA.

So, with modern THAT devices, surrounding elements have much bigger influence on audio degradation in VCA than VCA itself.

cheerz
Urosh
Old 28th November 2009 | Show parent
  #17
Lives for gear
 
🎧 15 years
Hi
Thank you for the above observations.
A VCA 'chip' has 4? very sensitive 'input' ports ALL of which must see low impedance, clean 'ground' and stable supplies for the unit to perform well as a whole. These points were often ignored by either circuit or layout design and as a result 'less than sparkling' performance results.
Yes a VCA will always be more distorted than a 'simple' amplifier stage but with care it need not be the 'achilles heel' of a complete system.
Matt S
Old 28th November 2009 | Show parent
  #18
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JohnRoberts's Avatar
 
🎧 10 years
Quote:
Originally Posted by recnsci ➑️
Just a few quick remarks:

So called "Blackmer" VCAs (dBX and THAT devices) are not very tolerant regarding high impedance on control port. That means that anything higher than few tens of ohms on control port will degrade THD.

PSRR is not stellar either. VCA core is usually biased by resistor connected to negative supply, so ample local PSU bypass is mandatory (IMH & completely unscientific opinion, when someone describes degradation as "haze" my first suspect is PSU and bypassing).

Control path must be verrrry clean. Noise, local crosstalk and crosstalk from other channels will directly modulate signal passing through VCA.

So, with modern THAT devices, surrounding elements have much bigger influence on audio degradation in VCA than VCA itself.

cheerz
Urosh
+1 mostly..

My take on the control port issue is like archaeology, as the VCA noise floor dirt gets cleared away, and core distortion (due to device matching) gets reduced, the external effects become more apparent and measurable.

Yes there is a distortion component related to control port termination impedance (due to mismatched beta of VCA device pairs), and a noise modulation due to multiplication of the audio signal by the control port voltage. In that same white paper Hebert shows a comparison of the new VCA with low noise (AD797) opamp and more general purpose (TL081) driving the control port. The 4uV noise contribution of the GP opamp was clearly visible on the THD+N plot. This is also a consideration when driving the control ports differentially since noise at these two ports will sum (+3dB). FWIW, the white paper drove only one control port to show the part in the best light. For console fader applications commanding large amounts of attenuation, it is often useful to drive both control ports.
------
RE: PSRR, indeed the early VCAs used a simple resistor to the rail to set operating current density for the VCA. (IIRC it could be tweaked in to trade S/N for distortion). The modern part uses internal current sources, so PSRR is apparently not enough of an issue for them to even mention in the white paper. Of course any audio circuit should have clean PS rails.

JR
Old 28th November 2009 | Show parent
  #19
Lives for gear
 
🎧 15 years
Hi
Following on in agreement with John it is obvious that the control ports must be very low noise which, with the range of new very quiet op amps capable of driving low impedance loads excellent performance can be obtained. This may unfortunately need the PCB to be redesigned to make best use of them. Lots of power decoupling, low impedance ground planes and 'star' grounding as appropriate are often tricky to 're-engineer' neatly into a 20 year old PCB!
Dual differential using both ports could be interesting using a pair of VCA chips with balanced audio signal path. This technique may also help with control voltage 'feedthrough' for 'comp / lim' duties.
Built using SMT parts on a small multilayer board should be able to produce outstanding results.
Matt S
Old 29th November 2009 | Show parent
  #20
Gear Guru
Very low noise precision opamps will lower noise modulating the VCA THD. dbx designs benefit from that as well as mondo psu decoupling and treatment.

What has been missed here are the THD trims for VCA's. Most are set during manufacturing and sometimes are rushed or the devices are not allowed enough settling time to stabilize their temperature. Many were done on older test gear that may have also skewed results with their self generated THD residuals.

Any VCA that was built years ago is in need of a re-trim from age and drift. Careful use of modern analyzers will usually offer better THD than the original specs.

Jim Williams
Audio Upgrades
Old 29th November 2009 | Show parent
  #21
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JohnRoberts's Avatar
 
🎧 10 years
+1.. revisit symmetry trims on older VCA consoles, its good studio maintenance.

You can non-invasively make spot distortion measurements of the channels in place to determine if they are in need of a tune up.

JR
Old 29th November 2009 | Show parent
  #22
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roginator's Avatar
 
🎧 15 years
I use Neutrik TP401 in my studio for VCA calibration
I recalibrated 3 MCI 500 desks (all VCA) and its big difference!!!!!!!
Old 30th November 2009 | Show parent
  #23
Lives for gear
 
🎧 15 years
Hi
VCA distortion trimming must be done with the desk in a 'operational' state. Pulling modules out to 'tweak' them allows them to cool down which for many implementations allows them to drift.
You really should have distortion trims available from the front panel (even the AMEK BIG has this) so that the strips do not need to be removed. It is possible, but not easy, to arrange for distortion drift to be cancelled out by extra circuitry but I am not aware of any that actually do this. Compensation for LEVEL drift due to temperature is quite often included as this is somewhat easier.
Matt S
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