Author Topic: Attenuation vs. 'Gain Control'  (Read 561 times)

Offline Nidri

  • Trade Count: (+36)
  • AVForums Veteran
  • *****
  • Posts: 1,043
  • Don't panic.
Attenuation vs. 'Gain Control'
« on: November 10, 2017, 08:14:37 PM »
Over the years I have occasionally read about certain amps that control volume by controlling gain, as opposed to attenuation (which I understand as signal reduction).
E.g. the PS Audio Gain Cell products from a couple of years ago, as well as more recent Ayre models.
(I'm sure there are other examples.)

As described in Stereophile (bear with me):

* "Hansen's other remarkable idea for the AX-5 is a circuit innovation called variable-gain transconductance or VGT, first seen in Ayre's top-of-the-line KX-R preamplifier of 2008 ($18,500). As Hansen explains, most active preamplifiers work by applying to the input signal a certain amount of voltage gain, so the signal can effectively drive a power amplifier. But in order for there to be a reasonable volume range—and to simply keep the playback level from being too loud—the voltage-gain stage is preceded by a potentiometer, which attenuates the signal. The drawback of this is that such a preamp will exhibit its maximal signal/noise ratio only at its maximal (unattenuated) volume. As Hansen puts it, "Since most preamps are used anywhere between –10dB and –40dB for an average listening level, this means the S/N ratio in actual use will be 10–40dB worse than on the spec sheet. As implemented in the AX-5—which doesn't incorporate a preamplifier stage per se—Ayre's VGT circuit allows the user to determine how much gain is generated by the amplifier's input stage, which itself comprises a total of four complementary-differential JFETs. The volume knob on the AX-5's front panel controls a pair of enormous, motor-driven, Shallco silver-contact rotary switches, each of which contains dozens of hand-selected, low-noise resistors. Every volume-level adjustment made by the user has the effect of switching into the AX-5's input-stage circuit a different set of resistors, the values of which alter the transconductance of those JFETs—and thus calls into play a specific level of gain corresponding with that setting. The volume system has 46 steps of 1.5dB each, over a range of 67.5dB. (I'm told that, by changing a single resistor in each of the AX-5's channels, one can adjust the overall gain range to accommodate, say, speakers that are significantly more or less sensitive than average.) Thus the AX-5 doesn't use signal attenuation at all, but rather creates variable input-circuit gain, on demand, to suit the desired volume level."

Sounds sensible to my non-engineering brain.
Or is it unnecessary over-engineering - a solution in search of a problem?

[ DETOUR ]
Then, in the Stereophile review of the Rotel RB-1090, I read that the amp had lower distortion at 2/3 power than at 1/3 power. +
Why would this be?
Do amps sound better when driven harder?
Does attenuation 'throttle the beast'?
Let's say you have a high-gain preamp with a high-gain power amp. Somewhere along the chain you're going to need an attenuator to drastically reduce the signal strength.
Is that not like driving a car flat-out but with one foot on the brakes at the same time?

Would the purest system of all be one without any volume control at all, i.e. a set amount of gain?
Not practical, of course, but if you were a crazy audio guy and always listened to the same album at the same level (like I said, crazy) then such a system would surely do the trick.

Is there just too much gain in audio systems in general?
I'm dumb-founded that, in my bedroom system, music plays (softly) even with the volume on the preamp turned all the way down, i.e. at 'zero'. Preamp has 0dB (single-ended) / 6dB (balanced) Gain and power amplifier 35.1dB/29.1dB Gain, 0.5Vrms/1.0Vrms Input Sensitivity. 
For that matter, do volume control pots have a lesser effect higher up in their range?

Not looking to start any kind of debate, just curious as to why certain approaches are chosen/preferred over others.

* https://www.stereophile.com/content/ayre-acoustics-ax-5-integrated-amplifier
+ https://www.stereophile.com/content/rotel-rb-1090-power-amplifier-measurements

Online stereosane

  • Trade Count: (+33)
  • AVForums Veteran
  • *****
  • Posts: 1,388
Re: Attenuation vs. 'Gain Control'
« Reply #1 on: November 10, 2017, 11:05:38 PM »
" Is there just too much gain in audio systems in general? "

I have felt like this about some equipment, my Nad M51 & Predator combo felt like an over reving car that was ready to blow your eardrums out at the slightest increase of volume. Since I read the Stereophile review suggestion to set it at -10db for best results, I can happily say it transformed my system and gave me so much more range to play with.

Interesting read, thanks  :dop:

Offline pwatts

  • Trade Count: (+3)
  • AVForums Veteran
  • *****
  • Posts: 1,134
Re: Attenuation vs. 'Gain Control'
« Reply #2 on: November 10, 2017, 11:31:29 PM »
Fascinating topic. I began writing a response but deleted it all after a page since it just got too complicated. Wish there were more questions like this on the forum.
Others feel free to offer input but this is not a subject with clear-cut answers - if there were there would not be so many different products and approaches on the market.
The choice of where gain is to be added depends on all the devices in the chain and where their noise/distortion originates from. That is why from a performance point of view the system synergy counts and why there is something to be said for buying a system in its entirety as opposed to just cobbling together a bunch of 5star bargains.

Offline Hi-Phibian

  • Commercial Member
  • Trade Count: (+38)
  • *****
  • Posts: 11,177
  • I really prefer email, see my banner for address..
    • Croak Audio Exploration
Re: Attenuation vs. 'Gain Control'
« Reply #3 on: November 11, 2017, 07:21:14 AM »
Does changing the gain of an amplification circuit not also change its performance parameters ? I would think the challenge with this approach is to maintain a uniform sonic character at various volume levels. 







[Anyone who thinks that wealth is measured in money is blind and narrow minded.  Wealth is obviously measured in turntables and records.]
Proprietor of Croak Audio Exploration.
Fair, not crazy, cash paid for turntables and tonearms from Rega, Linn and Thorens.          http://www.croak.co.za

Offline Curlycat

  • Forum Moderator
  • Trade Count: (+49)
  • *****
  • Posts: 2,153
Re: Attenuation vs. 'Gain Control'
« Reply #4 on: November 11, 2017, 01:59:14 PM »
Does changing the gain of an amplification circuit not also change its performance parameters ? I would think the challenge with this approach is to maintain a uniform sonic character at various volume levels
[Anyone who thinks that wealth is measured in money is blind and narrow minded.  Wealth is obviously measured in turntables and records.]

Is that not what Nelson Pass wanted to achieve with his Pass B1 buffer preamp?

http://www.enjoythemusic.com/magazine/equipment/0708/first_watt_b1_preamplifier.htm

Offline chrisc

  • Trade Count: (+90)
  • AVForums Veteran
  • *****
  • Posts: 8,416
  • Cape Town Hi-Fi Club - listen with your ears
Re: Attenuation vs. 'Gain Control'
« Reply #5 on: November 11, 2017, 03:42:55 PM »
The gain would be the ratio between the input and output.  To make it easier for consumers to understand, this ratio is seldom specified but instead, the voltage input to achieve a 0db signal at output is shown as a value in mV, typically 100mV

The attenuation can take place after the first amplification stage where any change in impedance will not affect the signal

If you look at the circuit diagram of single chip amps, this is how they arrange the volume control (you could call it attenuation)

Offline Hi-Phibian

  • Commercial Member
  • Trade Count: (+38)
  • *****
  • Posts: 11,177
  • I really prefer email, see my banner for address..
    • Croak Audio Exploration
Re: Attenuation vs. 'Gain Control'
« Reply #6 on: November 11, 2017, 04:42:52 PM »
Typically pre amplifiers are single gain blocks.  The volume control changes how much goes in or comes out of the gain block, attenuation is then “passive”. 

I believe electrocompaniet and I am sure others have played with changing the actual gain of the block attenuating neither input or output but changing the voltage gain of the active circuit. I assume by playing with changing feedback.  Not sure. However, I have heard the same active devices set up for different voltage gain seemingly sound quite different. 







[Anyone who thinks that wealth is measured in money is blind and narrow minded.  Wealth is obviously measured in turntables and records.]
Proprietor of Croak Audio Exploration.
Fair, not crazy, cash paid for turntables and tonearms from Rega, Linn and Thorens.          http://www.croak.co.za

Offline chrisc

  • Trade Count: (+90)
  • AVForums Veteran
  • *****
  • Posts: 8,416
  • Cape Town Hi-Fi Club - listen with your ears
Re: Attenuation vs. 'Gain Control'
« Reply #7 on: November 11, 2017, 05:38:01 PM »
^^^ yes, correct.  However, careful attention must be paid to the input signal such that the varying attenuation does not alter the impedance, however slight, of the incoming signal and cause inefficiencies at certain levels

Offline Hi-Phibian

  • Commercial Member
  • Trade Count: (+38)
  • *****
  • Posts: 11,177
  • I really prefer email, see my banner for address..
    • Croak Audio Exploration
Re: Attenuation vs. 'Gain Control'
« Reply #8 on: November 11, 2017, 05:40:55 PM »
By changing the voltage gain of the active circuit input and output impedances should be affected far less.   







[Anyone who thinks that wealth is measured in money is blind and narrow minded.  Wealth is obviously measured in turntables and records.]
Proprietor of Croak Audio Exploration.
Fair, not crazy, cash paid for turntables and tonearms from Rega, Linn and Thorens.          http://www.croak.co.za

Offline handsome

  • Trade Count: (+1)
  • AVForums Veteran
  • ***
  • Posts: 843
Re: Attenuation vs. 'Gain Control'
« Reply #9 on: November 12, 2017, 03:46:44 PM »
varying the gain of an amplifier as opposed to attenuating the signal passively is a rather old idea first mooted (i think) by peter baxandall in the 50s. many amplifiers have used variable gain instead of passive attenuators since then. passive attenuators reduce the signal to noise ratio. amplifiers (gain) increase the noise. the variable gain concept attempts to reconcile these two negatives by providing the amount of gain required.

All electrical systems produce noise as a result of the physics of electricity. so any amplification stage will produce noise. passive attenuators will reduce signal level but not system noise DEPENDING where they are positioned.

Typically the attenuator is before the gain stage. so if gain stage produces 0.1V of noise and your signal is 1V you have a signal to noise ratio of 10x (20dB). now if you turn down the volume by half your signal is now 0.5V but your noise from the following gain stage is still 0.1V so your SNR is now 5x (13dB).

A solution could be to put the attenuator AFTER the gain block. If the signal coming out the gain block is 1V plus noise of 0.1V reducing yhe volume by half reduces the signal AND the noise so your SNR remains the same. The problem here is that you lose headroom. Say your gain block has a gain of 10x all circuits have a limit to the largest signal they can pass. Your amplifier will then clip on any signal bigger than a 1/10 of this limit. This was more of a problem in the days of analogue where tape and LPs could and did produce very high signal peaks depending on the recordings. Today's digital signals are tightly defined though: 2V RMS is maximum so you could design for a maximum input of 2V and solve that issue.....but we are not out of the woods yet: passive attenuators have an output impedance that is high (compared to an amplifier) and more importantly varies with volume position. You could solve this by putting a buffer after the passive attenuator - but now you have two gain blocks which is more cost, complexity, distortion etc.

Varying the gain of a gain block therefore is quite a logical solution. In days of yore there were many amplifiers that utilised switchable gain and a passive volume control (Audio Research) or even two volume controls (passive attenuators) one before the gain block and one after (Cambridge Audio I think) the first made sure the input signal wasn't too big - you set it once - the second adjusted the loudness.

Offline Nidri

  • Trade Count: (+36)
  • AVForums Veteran
  • *****
  • Posts: 1,043
  • Don't panic.
Re: Attenuation vs. 'Gain Control'
« Reply #10 on: December 06, 2017, 08:35:19 PM »
Some quotes from Harbeth's Alan Shaw*:

"The design of the volume control 'rotation' v. power v. loudness is rightly a wholly engineering matter. The purpose of the volume contol is to match the incoming voltage (from an external source) to the power potential of the amplifier in a graduated and logical way so that the maximum power is, ideally, supplied when the volume control is at maximum, and not before. That's precidely the same consideration of how to graduate the throttle control on a car. Logically, the harder the user presses down on the throttle control, the faster the car should go, with the maximum speed only when the throttle is depressed fully. Imagine the danger and confusion the driver experience if he went from zero to full speed in the first 20% of the throttle peddle range: undrivable. Consider this: the amp volume control should be arranged in exactly the same way as the throttle controls in an aircraft; maximum speed at maximum throttle, fully forward. Full power, the tap fully open, full volume."

"..if your CD player is of the normal type which outputs 2v rms or thereabouts from a fully modulated (fully loud) CD disc, and you find that you cannot turn up the volume control on your amp beyond 10 o'clock or so, then you have a classic case of voltage mismatch. Your CD player is trying to force an unreasonably high voltage into the amplifier, which is incapable of amplifying what is already an 'amplified' signal inside the CD player's electronics. This equipment level-mismatch is a very common problem in home audio, and I'm sure is in part a reason that a proportion of audio enthusiasts find solace in vinyl, despite the poor surface noise, and limited real-world dynamic range: the tiny voltage from the cartridge combined with the necessarily limited dynamic range of vinyl combine to give a much better use of the amplification potential in the audio amp, with a loud sound achieved when the volume control is advanced to 1/2 or more of the rotation, not crammed down in the bottom quarter of the dial as it tends to be with loud sources such as CD."

"Could it be that most audiophiles are in fact, listening to audio that is being clipped in their amplifiers, for a significant proportion of their listening time, and are not consciously aware of it? Could the subconscious awareness that 'something is not right' fuel the restless quest to upgrade equipment?"

"We've identified that amplifier clipping is almost a certainty when typical CD players output their typical 2.1V rms on their audio output sockets when playing a fully loud CD to an amplifier which has, as many current amps seem to do, a ridiculously high input sensitivity. As CD players have been outputting this typical voltage since their inception 30+ years ago, is there any excuse, other than marketing/psychoacoustics, for not properly matching the home audio amplifier's input to handle this extremely high voltage? I don't think that there is."

* http://www.harbeth.co.uk/usergroup/forum/the-science-of-audio/amplifier-matching-mismatching-and-clipping-a-curse/2174-how-loud-versus-how-far-you-turn-the-volume-control

Offline Ampdog

  • Trade Count: (+1)
  • AVForums Veteran
  • *****
  • Posts: 6,824
Re: Attenuation vs. 'Gain Control'
« Reply #11 on: December 13, 2017, 02:56:48 AM »
Pweu!  (or how is that spelt/ does not appear in my dictionary ....)

I will try to remain objective.  In a way, one can simply say that attenuation x %-volume is a constant. Okay, a rather vague statement, but both achieve the same purpose. Both affects the final signal-strengh/amplitude.

No more, no less.

Attenuation is a wider term and thus more often used in technical parlance. It refers to the gain in an electronic circuit. 'Volume' also does, but only makes sense in e.g. amplifiers where hearing plays a role.

And there one can stop!  There are several ways such an effect is achieved, each with its advantages/disadvantageous. Any complete treatment of such falls beyond a post such as this. So just few points, some of which are evident in the Stereophile descriptions.  Filtering off any promotional fodder, one mainly has two conflicting circumstances to deal with.

1.  If attenuation (or the volume control) is at the input of the amplifier(system), gain of the complete amplifier is adjusted for comfort and overload can never occur. As pointed out though, such control does not attenuate noise generated in circuits after the contol. The S/N % is poor for low signals.

2.  If attenuation is at the end of the - in this case pre-amplifier - the signal-noise ratio remains, as pre-amp noise is attenuated with the signal. The drawback is that all stages preceeding the volume control is always under full amplitude, which may or may not overload some stages.

Ideally one could have two ganged volume controls, one at the input and one at the output side of a pre-amp.  If folks would pardon old hat: Regarding how this relates to hearing: Hearing is related logarithmically to electronic output. (Simply put: Output steps of 2W, 4W, 8W, 16W, 32W, 64W, 128W en so on represent equal loudness changes to the ear. In contrast steps of 1W, 2W, 3W, 4W etc. are not perceived as equal changes in volume.

Regarding input sensitivity there might be a problem. CD players still have widely different output signals as will have tuners etc.  'Matching' all these is a job. In my own pre-amp I provide switched attenuation, so that different inputs may be roughly set to have equal effect. Apart from that one usually finds the volume control(s) at the end of pre-amps, perhaps only followed by a unity-gain buffer to isolate the volume control.

This has been somewhat basic.  There are more factors in the picture like practical headroom and such - so much for now. To your questions in the following post Nidri




« Last Edit: December 13, 2017, 02:59:30 AM by Ampdog »
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)