Penultimate Zen transformer

Hi, I bought someones completed amp that needs proper transformers to replace the ones that are causing hum.
I happen to have a new, old stock Toroid #767.302 on hand. Will this one work for me?
I don't know much about amp building but I think I have the nerve to remove the guts from this one and build a nice, new, larger case for this.
Any help with the transformer choice will be very greatly appreciated!
Penultimate Zen

4 shots here...


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Paid Member
2009-06-09 8:55 pm
Toroid #767.302 may not be ideal for this amp because of its 2x30v secondaries. Penultimate Zen has an onboard regulator (Q5 and associated circuitry in the original article) that provides 42 volts to the amp. The normal input to the regulator is 50 volts, but with 30v secondary you can only get 44-45 volts.

The regulator employs an IRFP044, which has the gate threshold voltage of 2-4 volts, depending on the specimen of the transistor. That is, under normal conditions the voltage at the gate of Q5 cannot be lower than 44-46 volts. In your case, it is 46 volts as provided by Z1-Z5 zeners. Add to this the 2-3 volts minimum drop on R17, and you get 50 volts as per the original design.

This amp dissipates about 100W per channel, so you would need a transformer rated at at least twice the total dissipation (400W for two channels) and with a 36 volts secondary.

If you want to use a transformer with 30 volts secondaries, you may try to replace one of Z1-Z5 with a lower voltage zener (e.g. 1N4732). This will reduce the output voltage of the regulator but will keep it regulating even with lower input voltage. It looks like the amplifier itself will not require massive adjustment if it is fed with 37 volts instead of 42.

The regulator provides the amp with some immunity to the poor filtering of unregulated supply, so I would not suspect the filtering, unless there is an obvious problem like a disconnected cap. However, the input wires and volume controls placed close to the E-I core transformers (and, I presume, rectifiers - I don't see them on the photo) may indeed be a source of hum.
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Melon Head

2006-11-10 10:14 am
Since you are from Canada, buy from this guy SumR - Richard Sumner Technology
He makes very nice transformers.

So you want 2 transformers with two pairs of 18V secondaries and at least a 300VA rating.
I would also ask him, what the price difference is between 300VA and 500VA. If it is affordable I would be tempted to get two 500VA transformer with two pairs of 18V secondaries.
The good thing about this, is once you get these transformers you will also be able to build all of the firstwatt amps as well.
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