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Old 7th December 2013, 03:28 PM   #1
JFace is offline JFace  Canada
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Default Variable Gain Control Topology

I've always wondered if a variable gain topology exists for tubes. I'm not talking about a gain stage followed by a voltage divider (potentiometer), but rather an active type of control that uses feedback (like an op amp).

Check out the attached schematic. Does this have any merit? If I understand properly, it has large input impedance, small output impedance, variable gain, and no noisy potentiometers in the audio path.
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Old 7th December 2013, 03:47 PM   #2
DF96 is offline DF96  England
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It has a 'noisy potentiometer' in the feedback path. This will have a similar effect to a 'noisy potentiometer' in the 'signal path'.

The snag with this sort of circuit is that you can't reduce the volume to zero (or even very small) without risking instability in the feedback loop. I suspect that is why almost all systems use a potential divider to control volume.
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Old 7th December 2013, 04:04 PM   #3
teemuk is offline teemuk  Finland
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Quote:
I've always wondered if a variable gain topology exists for tubes.
There are several.

Yours is almost as original as making bias variable.
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Old 7th December 2013, 04:13 PM   #4
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IMO, voltage dividers are a technically superior solution. Changing the gain of active stages is almost always a bad idea because you can't change the gain without changing other parameters that you'd really like to remain constant.
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Old 7th December 2013, 04:21 PM   #5
JFace is offline JFace  Canada
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Quote:
Originally Posted by Conrad Hoffman View Post
IMO, voltage dividers are a technically superior solution. Changing the gain of active stages is almost always a bad idea because you can't change the gain without changing other parameters that you'd really like to remain constant.
Let's say we designed for the worst case scenario for those changing parameters:
  • The lowest amount of input impedance does not load the preceding stage
  • The highest amount of output impedance is still low enough to drive the next stage
  • The frequency response at it's worst is still acceptable

My motivation behind this idea is to fine tune the gain without taking apart the amp, and without degrading the S/N ratio (while possibly even improving it). This wouldn't be a volume control per se, but as the thread title suggests, a gain control. Similar to the input some mixing boards, the gain control only adds to the volume (that is, gain), it does not reduce. Control the volume on the final stage prior to the amplifier where the signal is highest in the preamp.
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Old 7th December 2013, 06:25 PM   #6
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Ah ha, so now we add more conditions and even an application! I'll still say that unless one is interested in a limited range of gain, it's still a bad idea to change the gain of active stages. If you can keep the max/min range low, then the change in BW and such will be manageable. IMO, a divider followed by a buffer/gain stage is still the better way to go. If you don't want to attenuate, don't turn it down all the way. Another possibility that might appeal, and I know very little about them due to expense, would be a transformer type control. In theory that could give you the best noise performance (a reactance doesn't generate noise) and allow a fixed gain stage.
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Old 8th December 2013, 01:32 AM   #7
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What you want is a beam deflection tube like 6JH8 or 6AR8 or 6ME8. You use a dual bias pot to control two deflectors, which deflect the plate current sheet beam across two target plates in the tube to set the output level. The pots only set a DC voltage, so they can be cap filtered to remove any noise. Search on the forum, its been done before.
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Last edited by smoking-amp; 8th December 2013 at 01:36 AM.
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Old 8th December 2013, 10:58 AM   #8
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Here's a schematic of an amp whose gain can be varied by a single (non-noisy) resistor:

http://www.ianbell.ukfsn.org/EzTubeM...rdCCTsht3.jpeg


Gain is variable from 6 to 40dB. Changing closed loop gain also changes open loop gain by changing gain of first stage (a technique first used over 70 years ago). Amount of NFB is therefore almost constant. This plus NFB down to dc ensure stability at all gains.

Cheers

Ian
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Old 8th December 2013, 12:39 PM   #9
Ketje is offline Ketje  Belgium
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You could try something like a multiplier
Mona
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Old 8th December 2013, 01:21 PM   #10
teemuk is offline teemuk  Finland
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- Bias control
- Screen voltage control (e.g. pentodes)
- Differential with an adjustable current source
- Variable resistor adjusting negative feedback ratio
- Adjustable gain amplifier in feedback loop (e.g OTA)
- using semiconductors, tubes or photoresistive devices as adjustable resistors in various suitable places
- pentagrid converters and beam deflectors

Are we really just going to start listing all the possible alternatives without any further discussion?
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