can you alter the gain of an amplifier?


I never use active preamps and prefer sources with built-in volume controls over passive preamps when possible.  I've had great success using cd players by Consonance, Theta, & OPPO directly in my amp(s) but find that sometimes, even with the volume full up, the sound level is just not loud enough.  With my big, solid state amps, I know I'm not out of power, just not enough gain (this was particularly problematic with an Aleph clone).  I was wondering if it's possible/feasible to alter the gain of an amp to achieve better results using a passive volume control or the on-board volume control in the source?  I won't be doing the work myself.  I just want to know if a decent repair/mod shop could make the alterations.  Thanks!
lcherepkai
Perhaps in some cases, but I wouldn’t recommend it without first giving the designer/manufacturer some idea of how much you want to increase the gain, and getting his or her blessing.

For example, if the solid state amp uses a considerable amount of feedback, it may be possible to increase the gain simply by changing a resistor in each channel. However, doing that would to some degree increase total harmonic distortion, lower damping factor, increase output impedance, and affect sonics for those and other reasons.

In extreme cases increasing the gain of an amp might also change it into an oscillator, possibly resulting in damage to it and/or the speakers.  The possibility of oscillations would depend in part on the impedance characteristics of the particular speakers that are being used, so the modified amp could conceivably work fine for the tech but blow up in your application.

Basically, increasing the gain amounts to incorporating a design change, which should not be done without complete information and full understanding of the specifics of the existing design.

Regards,
-- Al

I agree with Almarg.
Now, I do have a Crown amp that does have a 3-position switch that can change the gain of the amp to 3 different settings - 0.7V for max output power (which has gain=32dB), 1.4V for max output power (which has gain=34dB) & a fixed 26dB gain (which has a sensitivity of 3.1V for max output power).
In this case I was lucky that i can change the gain without much fuss.
If such a switch is not provided then changing the gain is much more complicated & will have effects on many performance parameters like Almarg cited.
Well... you have made decisions which, imo, make it more difficult to achieve superior sound. By insisting upon using volume control in sources, i.e. CDP, you are locking yourself out of potentially far superior systems. You may have had success in using CDP as source and preamp function, but I found in many iterations with very fine CDP's that one can obtain far better sound by doing two things; use the CDP as transport and get an outboard DAC, and use a passive or active preamp with it. You are in no way assured to have superior sound simply because you are using the volume control of the CDP and the CDP's DAC. 

You may lose a touch of the most extreme refinement/cleanness going this route, but you will have an overall far higher end result in terms of most other parameters of fine playback. Trying to futz with amps is not a good solution, imo, and you will still have a largely compromised (not fully realized) system.  :) 

Adjustable gain in amp is very helpful to synergise with preamp or any input source with volume control.
Well, here's a trick you may really like.  Try using a 1:2 signal transformer. You can double the output voltage, while halving the input impedance though. I actually really like the sound quality, and it isolates the inputs.

Jensen Transformers is the usual benchmark for these.

Best,


Erik
Interesting suggestion by Erik. A number of people have reported here and elsewhere that they have found Jensen transformers of various models to have essentially no sonic downside, while a few have reported a slight loss of transparency.

A minor correction, though: If the particular transformer doubles the signal voltage (which would be a 6 db increase in gain) it would result in the preamp seeing the amp's input impedance divided by four, not two.

Regards,
-- Al
Now, these are some fine responses.  Al, your advice is ever highly informative and always a pleasure to read (in my posts and many others).  Doug, you turned me on to bridging the Pathos Classic amps I have a Cambridge Audio 840W which I'll probably do the same with as funds allow but the desire to NOT have an active preamp is largely based on money.  The expense of a really good preamp is a deal breaker for me.  I've had a few decent ones from Celeste (by SIM Audio), Perreaux, and Antique Sound Labs and all were easily bettered by not having them in the audio chain.  I believe I had a Theta Miles and then a Consonance Droplet 5.0 with these 3 preamps.  I do pay attention to cabling and had quality cables/cords on hand so the preamps were handicapped in this way.  The sound was just obviously more open and detailed without the preamps.  And I didn't hear a reduction of dynamics or bass weight without the preamps.  Both of these CDP's had plenty on their own.  Perhaps a top notch preamp would take my system(s) to the next level but I just can't afford it.  And Erik,  I like your advice.  I'll look into the Jensen Transformers.  Thank you all very much!
Whoops! Almarg is right, I was wrong. :)  It's been a while since I had to do transformer math, but you got the idea. :)

Best,


Erik

It may be safer easier to up the gain on the output stage buffer of your source/s.

What amps do you have at the moment that you want to have more gain with?

Cheers George

Whoops! Almarg is right, I was wrong. :) It's been a while since I had to do transformer math, but you got the idea. :)

Best,


Erik
yup! everything changes (up or down) by the square of the turns ratio....
George, amps on hand include a Cambridge Audio 840W, a pair of Xindak XA8800MNU monoblocks, and a nice gainclone.  Sources include a Cambridge Audio 851C, and both Modwright and Upgrade Company modded Oppo BDP-83SE's  with the Modwright having the outboard tubed power supply.  It seems that the Modwright plays less loudly with the volume full up than the Upgrade Co Oppo.  Thanks.

Unfortunately there are no circuits online for any of these, but a good tech will be able to give you 6db of gain on any of these for 1hr’ish labour.

I would look at the source to increase the gain on (less feedback), as many are just opamp output and all that’s need is one small feedback resistor changed on each channel.

Doing the amps could result in sound change and or oscillations, if not monitored on the bench afterwards, saying that amps usually increase in stability with an increase in gain (less feedback)


Cheers George

I appreciate the input George.  I have a good tech whose done repairs on my hi-fi for several years only an hours drive away.  I'll check with him.  Would you advise sending him the sources or the amps for modification?  Thanks again.

The sources shouldn’t change as much the sound as the amps may, and they should be easier for him to do, if he knows his way around output buffer opamps on cdp’s. The tube output cdp will be a different animal,, but should also be doable. 

Cheers George

Thanks to all for your fine advice and the benefits of your experience. 
What's the sensitivity of your loudspeakers and in dBs how loud is loud enough?
I have a few pairs of loudspeakers that are in use in various rooms.  The Meadowlark Audio Heron hot rods are a 4ohm load with a sensitivity of 90dB while the Bohlender Graebener 520's are a 4 ohm load rated at 88dB.  I have a pair of large DIY Legacy Focus clones (not really clones but similar in appearance using Eton drivers with Peerless 12" woofers).  The sensitivity and ohm are unknown but were significantly more sensitive than the Von Schweikert VR4's they replaced.  I'm picking up a pair of Triangle Stratos Lunas tomorrow with a sensitivity of 93dB. 

With regards to how loud is loud enough?  Oh, on occasion, I do like some music quite loud.  Can you really listen to Rage Against the Machine any other way?  Or the Red Hot Chile Peppers?  Though I usually listen to jazz, both classic and modern and chill music or house music.  These are all listened to at levels well above background but would not preclude conversation.  Also note that different speakers reproduce sound in different ways.  The BG's are dipole ribbon hybrids and close up, they sound like they aren't producing much sound but step back and the sound fills in and fills up the room.   Thanks.