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Remove negative feedback loop from HR50?

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6/17/1999 0:59 AM

Remove negative feedback loop from HR50?

Ken or Trace or Mook or anybody - I'm still trying to figure out what to do with this Soldano HR50. The only thing I have decided is that no one else is going to do anything. One tech said he would remove the negative feedback loop and that he knew how to stabilize the preamp after he had done so. Anyone know anything about this? What's the idea? I know Dr. Z. advertises that some of (or maybe all) his amps have no negative feedback loop. Try to enlighten me, people. Thanks very much for the technical references. I've been poring over books and have learned a lot already. I really appreciate the help.

6/17/1999 3:30 AM

I dunno what the guy means by "stabilize the pre-amp".. the -fb loop goes around the power amp.. The basic idea is that -fb levels off your freq response and also sharpens the transition from clean to mean. Removing the -fb loop will do a couple of obvious (and not so obvious) things:

1. Tonal response will tend to get more midrangey

2. Output stage gain increases (amp may seem louder too)

some old amps (notable Magnatone) sometimes had "stage/studio" switches which were -fb on/off switches to take advantage of this.

3. The amp will distort more gradually/less suddenly

4. The low end will get "looser" and the high end **may** get more strident, although depending on the amp (esp the particular output transformer) it may get clearer.

In any event it's easy to unhook the wire, play a bit and see if you like it.. Amps w/o -fb have been designed and voiced for it.. it's not really something you add/subtract without making adjustments in general although you can.

6/17/1999 1:15 PM

SPEED SAID ---> I dunno what the guy means by "stabilize the pre-amp"..

@@@ I'm with Speed on this. This isn't as simple as this tech is making it sound from what you stated.

I don't have a schematic (please send me one of you have one) but after looking at his Marshall mods, the SLO, and playing on one for 2 seconds ... I would say he has some 220K plate resistors somewhere in the preamp section.

You could change these to 100K's and this should brighten your day up considerably! (ha, ha)

SPEED SAID ---> Amps w/o -fb have been designed and voiced for it.. it's not really something you add/subtract without making adjustments in general although you can.

@@@ THIS IS A VERY, VERY GOOD POINT! The entire amp is voiced with feedback so if you remove it most likely all hell will break loose AND the tone will change dramatically.

Trace

6/22/1999 2:33 PM
Graywater	Folks, As to the "stabilization," negative feedback can be used to offset positive feedback, i.e., oscillation, and very high gain stages sometimes depend on negative feedback to prevent oscillation at specific frequencies. Now, whether the above applies to the amp in question is anybody's guess. GW

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