If Preamp Gain is Set to High-What Consequences?

Hi Newbee,

Depending on the specific design, the volume control can be located at or near the input of the preamp, or somewhere in the middle of its internal signal path (following an active input stage that receives the input signal), or even at the output in at least one case that I know of. See some of the preamp schematics shown at the Bryston site for examples of designs having the volume control function somewhere in the middle.

That is really a side issue, though. Regardless of where the control is located, if the preamp's gain is specified as 15 db it means that the output voltage is 15 db greater than the input voltage when the control is at max. Reducing the setting of the volume control from its maximum position will reduce that number correspondingly, most likely to negative numbers (corresponding to the output voltage being less than the input voltage) in the range of positions Foster_9 is using. That is very common these days, especially with digital sources.

Personally I think there are too potential many tradeoff's trade offs to bother unless one is trying to minimize pre-amp noise, typically seen in a tubed unit.

I agree, as I indicated at the end of my earlier post. Unless, that is, it is readily possible for Foster_9 to change the internal gain settings himself, in which case he may want to experiment a little. A consultation with Joule would also seem to be in order, as you suggested, to see if they feel that with this specific design any particular part of the volume control's range would be significantly more optimal to be using than other parts.

My pre-amp has noise as seen by the amp. The line stage has 26db (a lot, I think, for old amps with more than 1.5 input sensitivity.) This noise is constant and doesn't change, with VC at 0 or max setting. It only changes when I put an in line attenuator between it an the amp. It doesn't change when I put an inline attenuator between a source and the pre-amp input.

That would simply say that the noise is being introduced at some point in the preamp's internal signal path that is "downstream" of wherever the volume control is located, assuming it is not being introduced in the interconnect cable.

Best regards,
-- Al

Almarg, Question that occurs to me arrising out of the use of tubed pre-amps and your measurements of the gain of pre-amp in your post which suggests that the gain of a pre-amp is contingent on the rotation of the VC.

My pre-amp has noise as seen by the amp. The line stage has 26db (a lot, I think, for old amps with more than 1.5 input sensitivity.) This noise is constant and doesn't change, with VC at 0 or max setting. It only changes when I put an in line attenuator between it an the amp. It doesn't change when I put an inline attenuator between a source and the pre-amp input.

Doesn't this confrim my assertion that the pre-amps amplifying stage is fixed? Do I need more coffee? :-)

Foster 9, Oh, the value of a re-read of your original post. :-)

To answer your specific question, I would not change the pre-amp gain from 15db unless Joule feels the changes it would make in the optimal position of your volume control (as suggested as possible in my last post) would make it worth while. It seems from your description of your VC's position that you only use it from about 9 to 11 o'clock. If your pre-amps gain as set by Joule were further reduced you would get more use of the VC, presumable a finer gradiation of volume changes and perhaps impedence changes. Al is much smarter than I on these matters, so he can advise or Joule can help.

FWIW. :-)

Foster 9 and Almarg, Obviously I had a brain fart - posted before coffee! :-)

But, changing the measured fixed gain of the pre-amp from voltage to db I think is the extent of my error ( :-)) unless I do not understand how most pre-amps work.

In pre-amps with which I am familar, in the basic circuitry the attenuator is placed before the amplifing stage of the pre-amp. Rotating the VC only changes the strength of the signal from the source to the pre-amp. It does not change the strength of the pre-amps amplifying stage which remains as it was set by Joule at 15db.

Am I wrong? How so?

Foster 9, FWIW and I don't know why it should be worth anything after my first comments, many pre-amp manufacturers suggest that the preferred VC position rests between 10 and 2 o'clock. Something to do with impedences I think, depending on the design of the VC, which makes some sense to me. If there is any validity to that it seems to me that it can be obtained by controlling the pre-amps fixed gain by reducing it internally, by reducing the gain as seen by the amp by attaching incased fixed value attenuators, reducing the strength of the signal of the source, either by initial selection or addition of inline attenuators (incased fixed value resistors). Personally I think there are too potential many tradeoff's trade offs to bother unless one is trying to minimize pre-amp noise, typically seen in a tubed unit.

Al, I always respect your comments and I'll look forward to you edification! :-)

Thank you Al, as always.

Newbee, I don't see that Foster_9 has confused anything, given the clarification provided in his second post. He is saying that the gain of the preamp has been set internally to 15 db. Nothing has been set to 15v or 15 volts. Which means that when the volume control of the preamp is turned all the way up, its output voltage will be 15 db greater than its input voltage. That corresponds to a ratio of about 5.62x, meaning that the output voltage at any instant of time will be about 5.62 times as great as the input voltage at that instant, with the volume control at max.

And, yes, reducing the volume control setting from its max position will reduce the actual gain provided by the preamp from the specified number, since the specified number is based on having the control at max.

Concerning whether or not reducing the internal gain setting would result in improved sonics, IMO it's anyone's guess, as there are many preamp-dependent variables that are involved. Anecdotal evidence seems to suggest, though, that having a gain structure which results in running the volume control in the upper part of its range often (but not always) provides best results. Personally, though, my instinct under the circumstances would be to leave it as is.

Best regards,
-- Al

Thanks for the information Newbee, but I'm not confusing anything. The gain of the Joule can be set internally from lower to higher by changing the position of jumpers on a circuit board. The Joule technician set "gain select" at the highest setting, a measured 15db. This leaves me with much less play in the volume control (Joule calls it "gain" on the faceplate). I'm wondering if the gain-select jumpers had been set to a lower setting and I had more play in the volume control than I currently have, if that would improve the sonics or make no difference. For example if I had the ability to turn the voulume as high as the 50% point rather than being limited to 25-40%.

You are confusing the 'gain' of your pre-amp and the strength of the signal from the source. The gain of your pre-amp line stage as seen by your amp is constant 15v (?). Your volume control does not change this.

The strength of the signal from your source varies, but for example CD sources typically average about 2v but can vary from 1v to 6v.

Now the part misunderstood by many.....The volume control on your pre-amp does not change the 'gain' from your pre-amp's gain (line stage) section it controls (attenuates) the strength of the signal from your source. So when you rotate the VC you are reducing the strength of the signal from the source.

Hope that is clear. Anyway, no problem. What you describe is normal.

If I was not clear in my original post, the internal gain setting was set by a Joule technician to its highest setting (measured at 15db). The gain control on the front (the volume control) is what I can turn up 25-30% on 2 of my sources and up to 40% on my other sources before the speakers get too loud.