I did want to keep the PAS intact or reasonably so..
The Sound Valves VTP100 and VTP101 are Cathode Follower designs and I believe are based on the PAS circuit.
I did want to keep the PAS intact or reasonably so..
Understood.The author wanted to stay out of "improvements" and keep the PAS as intended.
That is exactly what I did with one of my PAS preamps. I added output jacks and a FET input opamp buffer to each channel. When driving lower impedance loads I used the buffered (opamp) outputs and when driving high impedance tube amplifiers, I used the original (tube) outputs. Best of both worlds. I used the filament supply to power the opamps and I added opamp buffers to the tape monitor outputs also.It may be blasphemous but my first thought was an OP Amp of some kind, though I wouldn't know exactly how to go about that either
My preferred solution is to use open-loop 'diamond buffer' ICs such as BUF634P. Power comes directly from the existing +/-11VDC tube heater rails. It's a good idea to buffer the tape outputs as well, if they will ever be placed in service. Here's one way to do it without drilling any new holes:
View attachment 1023254
Yes, by all means, use a socket for the opamp. You can then try other opamps to see if they change (improve) the sound. Some opamps will oscillate under the right (wrong) conditions. That is one reason I added the resistor at the output of the opamp. It reduces the effect of the load on the operation of the opamp and enhances the stability of the system.I really know next to nothing about OP amps.
I have read going to one to another can open other issues. That is, oscillation and other things .
So , I would stick to a working model for my application.
That said if you put a socket in you really could play . They're cheap enough.
Without the proper gear and knowing what to look for (and possibly knowing what to try to get it to misbehave). For me rolling would be pissing in the wind.
Also I've been wandering in directions I never thought I'd travel so maybe I'll end up on this path one day.
The main outputs (going to the power amp) are the ones that normally need buffering, so that's where I start. The tape outputs often go unused in modern stereo systems but I think it's reasonable to buffer them anyway, while I'm in there, because it adds little effort to the job and buffering offers a huge benefit in many possible usage scenarios. The input circuitry of solid-state audio gear commonly adds major distortion to any signals that happen to be connected when device power is off.Do you just use the buffered out jacks for everything?
I mean do you have both buffered and unbuffered outs?