[Radio Wrangler]

No. Don't.

There have been several discussions over reducing the gain of the stereo 20 and the other Leak amplifiers. Search around and read them.

These amplifiers are very sensitive compared to most modern ones. This sensitivity is designed into them and its consequences run through all the original design calculations.

12AT7 and 12Au7 are pin compatible with the 12AX7 and have lower gain. BUT all the valves are in a feedback loop. The loop will attempt to restore the designed value of overall gain if you fit one of these lower gain devices. This will change the dynamics of the overall amplifier, its frequency response and its distortion and its noise floor and thee output impedance presented to the speakers.

To get the stereo 20 to what it is, a lot of careful maths was done. Certainly, the amplifier can be rejigged to make it less sensitive without having to change any valves... just resistor and capacitor values. Again, there will be consequential changes to the dynamics, frequency response and all that, but by careful design... about as much work as Leak put into designing the thing in the first place... it should be possible to make it quite good.

No-one has done this... at least no-one has done this properly. The reason being that when you've finished you have modded stereo 20, and they are worth an awful lot less on the open market than a virgin one.

There are plenty of people prepared to do all sorts of mods for you. They all make extensive claims for the improvements rendered. The biggest improvement is to their bank balances. The problem is that they wipe out most of the resale value of your amplifier. You can never get back originality, at least in the eyes of the people who pay the big bucks for them.

The best ploy is to leave it alone and just fit an attenuator in each channel's feed from your preamp. This way the sound of your amplifier is not disturbed.

It's quite common that the channel to channel matching of stereo potentiometers is poor at lower levels. The attenuators will mean your volume pot needs to be higher up where it's better matched.

One thing, having a system with too much gain and only needing to open the volume pot a crack has a psychological effect. It makes people think, 'wow that amplifier must be powerful!' once you've put attenuators in and got smoother action in the volume control, you might feel the amp is less dramatic. This is just your perceptions tricking you, the amp will be the same.

When their power amplifiers were designed, Leak made a big splash about 0.1% total harmonic distortion. THey were only just able to do this on some of their amps. To do it, they needed to use stronger feedback with more loop gain than other designers and to keep the amplifiers stable with that amount of loop gain took quite some doing. So the design is a bit like walking on a knife edge. It can be changed, but you have to know just what you're doing, and it will be a fresh design at the end. No longer a real Stereo 20. No longer having the reputation. If done properly, it will be OK, but no-one will believe you.

David