One thing that I never really understood is why Vox derived amps use a 2 stage filtering of the power supply versus everyone else's 3 or more stages.
Can anyone explain why Vox designs do it that way?
I also notice that the Rocket uses the Fender 3 or more stages in series approach. Is that a significant contributor to the difference between the AC30 and the Rocket?
The question has come about because I have a few amps that all use a VVR supply to regulate the PA & PI and I have duplicated the number of stages in the power supply to match the number in the normal non-VVR'd version of the amp.
E.g. In the attached schematic, could the last 8uF cap and 2k2 resistor stage be removed without affecting the performance and sound of the amp? I accept that the previous resistor should be increased to maintain the voltage.
The question is relevant because saving the space for that extra 8uF cap and resistor would be most useful.
Pre amp power filtering - 2 or 3 stage?
Moderators: pompeiisneaks, Colossal
Pre amp power filtering - 2 or 3 stage?
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Re: Pre amp power filtering - 2 or 3 stage?
The AC4 had two filters; every other Vox had at least three: the AC15 had a pi filter for plates and screens (choke in front of the plate); separate filter for PI; another for the EF86.
In your case, I think you'd wind up injecting way too much ripple into the preamp if you omit that filter stage (especially as there's no choke). You probably could eliminate that filter if you took the PI off the VVR (in effect adding a filter stage back in prior to the preamp filter), but then you'd need some kind of post-PI master to keep the PA from overloading when the VVR is turned down. Since space is an issue, you may be able to do it with a dual-gang pot, but the signal to power ratios may be less than ideal.
In your case, I think you'd wind up injecting way too much ripple into the preamp if you omit that filter stage (especially as there's no choke). You probably could eliminate that filter if you took the PI off the VVR (in effect adding a filter stage back in prior to the preamp filter), but then you'd need some kind of post-PI master to keep the PA from overloading when the VVR is turned down. Since space is an issue, you may be able to do it with a dual-gang pot, but the signal to power ratios may be less than ideal.
Re: Pre amp power filtering - 2 or 3 stage?
Hey, the exact same discussion was raised at the 18Watt forum by the guy that drew that schematic mod. When we were testing out the idea of PI/PA VVR on 18 watters it was a simple test to try it without the extra filter stage on the preamp supply. It added a bit of 100Hz hum. As you say the intent was to duplicate the number of stages the amp would normally have without VVR. But if you've got amps with PI/PA VVR it's an easy enough test for you to bypass that last stage (move the preamp supply wire) and hear it for yourself. The thing I didn't try out was using a much bigger cap to see if that could remove the hum added by only having one stage instead of the two shown.