Planned PYE Amp Bridging Mod Need help

[joewhite903]

Hi All I have a very old PYE Amplifier , Radio died years ago. Soo was planning if possible to convert the amp block to Mono Bridged and use it for a small Subwoofer.
I was told ages ago that it was possible, But as im still in the getting to grips on amps Stage as im a Lampie By Trade.

What i can Tell you about the amp is its a ( HIFI Sound Project 6691 )
The outputs are TIP42C and TIP41C.
Power Caps are 35v 4700uf
There is no markings on the transformer but there 2 supplies one (ac - ov -ac ) and a (ac - ac Which Powered the 12v Back-light Fuse bulbs)

After having a look at the PCB it looks as though the two amps are completely separate other than shared power and shared PCB. but the inputs and outputs are each to there own board.

Hope Somebody can help

p.s Also I want to make a Pre amp Board, With Volume Control and Low pass Active Crossover

Thanks Joe

[Steve Conner]

You can bridge almost any stereo amp with no modifications. You just need to make up a little circuit that inverts the phase of your mono signal.

You feed the normal signal into one channel, the inverted signal into the other channel, and hook the speaker between the positive terminals of the two channels.

This is really easy to do with professional amps that have balanced inputs, you just make up a lead that swaps the hot and cold. You can't do that with unbalanced inputs, so you need this inverter thing. Here's an example on Rod Elliott's site:

http://sound.westhost.com/project14.htm

(you'll also find schematics for preamps, crossovers, etc, on there)

Each channel sees half of the speaker's impedance, so if the amp was rated to drive a 4 ohm load before, it won't handle less than 8 ohms bridged.

[J M Fahey]

*IF* your amp can drive 4 ohms each channel (many do), you can bridge it to provide ChA+ChB power into 8 ohms bridged; if it can't, don't bother, specially with somewhat puny TIP41/42 outputs.
For a thread on this including PCB and Vero board layouts, and a universal circuit (works with 12V single to +/- 50V or more supplies) check:
Need to drive 100w-ish into a 16ohm load, bridged LM4780?
Comment #6 says:
>>>

6) You can turn any old junked stereo home amplifier (which can drive 4 ohm loads, most do) into a fire breathing dragon driving full power into an 8 ohm Guitar speaker. (well, sort of, definitely *much* more than you thought possible)
Have fun.

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