Electronics

Peter F · Joined: 10 Dec 2005 Posts: 4

I recently acquired a Guitar combo.
The power amp had been removed. Consequently the power to the preamp board was also missing.
I have bought a hy2003 to replace the power amp ( with dedicated psu and transformer.
I have built a 15v power supply + regulator for the preamp.
Unfortunately the only signal that comes through the preamp is intermitant and seemingly when the input is at its strongest.
Is my 15v transformer too low a current ? (rated at 6VA) or is it something more serious ? Question

Arfa Daily · Guest

Are you sure that the preamp only needs +15v ? In my experience with guitar
amps, and I do repair a lot of them, it is usual for the preamp to require a
negative supply as well, as the circuitry is usually based on 4558 or
similar opamps.

Also, the supplies for the preamps are very often derived on-board, from the
main +/- 50v or whatever rails that feed the power amp. So +/- 50v goes
across to the preamp, where it is zener'd down to +/- 15v, with a couple of
wirewound resistors and 5 watt zeners.

The clues you should be looking for are does the preamp employ opamps ?
Where the power comes onto the board, are there two electrolytic caps for
decoupling ? Where the power comes into the board, are there any signs of
wirewound resistors ?

The sound problems you are describing could well point to only one rail on
an opamp stage.

If it's a ' famous ' make, then the schematics for it are almost certainly
available, probably for free, from one of the many excellent guitar amp
schematic sites on the 'net.

Arfa

Peter F · Joined: 10 Dec 2005 Posts: 4

Thanks for your advice AFRA
I've got hold of a schematic and the preamp is definitely 15v although there appears to be a 5v to power a multi effects processor.
4558's are used but are clearly shown at 15v.
Any other ideas ?

neon · Joined: 25 Feb 2006 Posts: 570

you got a 15 volts AC transormer and you are pushing a 15 volts regualtor regulators should have a buffer of at least 3v to oporate i think your power design is not good the regulator is starving for volts to regulate you might try installing a monster cap on the input it might work. take care

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