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.
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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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.
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.
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.
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.
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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Here's a schematic of an amp whose gain can be varied by a single (non-noisy) resistor:
http://www.ianbell.ukfsn.org/EzTubeMixer/docs/EzTubeMixer/EurocardCCTsht3.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
http://www.ianbell.ukfsn.org/EzTubeMixer/docs/EzTubeMixer/EurocardCCTsht3.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
- 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?
- 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?
Limiting amplifiers used pentagrid tubes for their variable gain stages.
They worked quite well.
They worked quite well.
Hi Ian
Maybe I'm wrong and this be silly, but in the SRPP topology no need for grid stopper in the upper triode, but in the bottom triode can be more useful.(?)
To be honest, I have never really found the need for grid stoppers with the 6922. The top one is really a bit of a left over from an alternative design that uses a mu follower stage instead of an SRPP.
Cheers
Ian
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hmmm, trying to understand how this works. It looks like variable cathode degeneration on the first tube. Which is changing the local NFB - if you don't consider degeneration to be feedback then I would agree with you but if you do consider degeneration to be feedback then clearly this local NFB is not constant.
Changing the pot's value to lower Ohms also attenuates the global N feedback signal from the 47K resistor, while it simultaneously increases the gain of the 12AX7 (by reducing local N feedback). Net result is the amount of global N feedback signal stays near the same. (additional gain and additional attenuation in the loop)
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ah! thats pretty clever. I guess it trades local for global feedback though so the nature of it is shifting quite significantly but the concept is neat.
You are right, there is local and global feedback operating. The point of the circuit is that a single component alters both.
One of the earliest amplifiers to use this technique was the V76 - check out the first two pentodes:
http://www.wagner-microphones.com/images/v76.gif
The technique is sometimes called a TRANSAMP and has been used in some modified op amps to inprove stability in virtual earth mixing systems.
Cheers
Ian
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