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Old 27th April 2013, 04:47 PM   #1
rongon is offline rongon  United States
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Default tube preamp vol ctrl -- before or after?

I have a question relating to line stage with tall bias, such as a 6V6 preamp with a bias of -10V or so.

I understand one rationale behind putting the volume control in front of the line stage is that the stage will then never see anything close to an overload condition, so distortion will be minimized. In the typical preamp using something like a 6SN7 or 5687 tube with a bias of maybe -4V, a 2Vrms signal applied at its input can come close to overloading the stage, causing distortion to increase.

However, if I have a 6V6 line stage (like Salas') with a grid bias of around -10V or more, even that 2Vrms signal shouldn't come close to overloading it. Does it now make sense to consider putting the volume pot (or stepped attenuator) on the output of the preamp, instead of at the input? Or does that cause frequency response errors due to a changing resistance on the output of the preamp stage?

What if the tube used for that preamp has a plate resistance of well less than 1000 ohms (and a mu below 10), and I use a 50k ohm pot on its output?

I don't know enough to identify all the issues that might be involved, and I'm hoping to gain a better understanding. Thanks for any advice you can send this way.

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Old 27th April 2013, 05:00 PM   #2
Loren42 is offline Loren42  United States
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well, it is easy to try both ways and will not cause harm.

Most people use the input as the place for the volume control.

Some like the output as it lowers the hum and noise a little.
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Old 27th April 2013, 05:02 PM   #3
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Hi!

If you want to use a resistive volume control, like a 50k pot, don't put it at the output. It will cause a rather high output impedance (depending on the volume setting) Which kinda defeats the purpose of such a beefy line stage driver.

I use transformer volume controls in my preamps and put them at the output. In this case the output impedance also changes with volume setting, but gets lower as the volume is turned down and at setting like -12dB is in extremely low territory.

Best regards

Thomas
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Old 27th April 2013, 05:04 PM   #4
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In order not to increase the output impedance of the preamplifier you will need to use a low value output pot.
You are placing a heavier load on the output load (the volume control is in parallel with the volume control) which will increase distortion.

I do not see any practical implementation which could achieve an output impedance lower than about 10K, when a well designed valve preamp could achieve output impedances of less than 200ohms.

Really I cannot see any good reason to put the volume pot at the output, unless you buffered it after volume pot - which is not desirable either.

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Old 27th April 2013, 06:25 PM   #5
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Why do you find a buffer after the pot 'not desirable'?
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Old 27th April 2013, 06:40 PM   #6
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by mrkramer View Post
Why do you find a buffer after the pot 'not desirable'?
More noise, hum, and signal degradation.

The fewer components in the path, the better.
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Old 27th April 2013, 06:51 PM   #7
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But with the pot at the output, the signal AND the noise added by the gain stage get attenuated. A good unity gain buffer won't add much.
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Old 27th April 2013, 06:53 PM   #8
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Extra valve, and yes I think it would probably degrade the sound for little overall benefit.

Shoog
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Old 27th April 2013, 07:25 PM   #9
rongon is offline rongon  United States
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I see. What I don't understand is how this is different than having the volume control at the input of your power amp, with the sources feeding it. Is that because the sources (like a CD player) will have solid state output stages?

I whipped up a drawing to illustrate the proposed location of the pot.

Assume the output resistance of V1 is 1k ohms.

The grid bias on V1 is -10V.

Does adjusting the pot (VR1) change the output resistance from V1, as seen by the grid of V2?
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Last edited by rongon; 27th April 2013 at 07:29 PM.
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Old 27th April 2013, 07:34 PM   #10
GoatGuy is offline GoatGuy  United States
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Hmmm... I see that a few folks here need to remember Thévenin's theorem... which states that resistive voltage-dividers have in impedance equal to the pair of resistances in parallel (R1 R2)/(R1 + R2) equivalently in series with a voltage source that is VR2/(R1+R2) volts.

IN other words, the impedance goes down as a tail-attenuator is swept progressively toward R2 = 0 ohms. One just has to remember that there's the pair of "resistances" in the gain stage - the plate-to-power-rail resistor, and the plate resistance itself, these, in parallel with the R1+R2 of the output attenuator pot.

The use of a "tapped transformer attenuator" is wickedly wonderful stuff, but doesn't actually achieve remarkably much more than a resistance divider (AKA "pot") derives.

The only "problem" will be finding a match between the output impedance of the gain stage and the "unattenuated" position of the interstage attenuation pot, AND the input stage of the amplifier that follows.

I've done a fair amount of experimenting with this over the years, and I find that it only "works" if the amplifier input stage has fairly high input impedance, say, 100K+ Then without too much attenuation, a 100K audio-taper pot can be used quite practically. Typically, its insertion at least attenuation causes about a -3 dB overall gain drop. Usually not an issue.

GoatGuy

PS: I also like a 4-pole ganged pot configuration - which attenuates BOTH the input and the output in lockstep. Oh, sure, could be 2 pole (simple stereo), giving the experimenter some control over the relative degree of preamp stage distortion, to "find a sweet spot". But usually once the values are known, the "next amplifier" can have the 4-gang pot. Since "4-gang" pots are rare, I usually resort to stepped attenuator switches. Lots of those around. Just need 4 disks.

GG
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