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RE: UPDATE, 5/22

Posted by triode3 (M) on May 23, 2025 at 02:50:14

In Reply to: RE: UPDATE, 5/22 posted by Tre' on May 22, 2025 at 20:10:48:

I did not want to start getting into the weeds too deeply, but whatever entity converted this amp to balanced not only removed the entire PI, but modified the input:

On the schematic, the input has a low pass filter combined with gain control to give the requisite input impedance (and gain control). This was removed and replaced with a voltage divider on each leg of the balanced input, cutting the voltage down substantially.

During my first test (which was only a few days ago), I thought the preamp my friend gave me was broken/bad/had low output, as both amps were incredibly low volume. (e.g. full throttle on the balanced preamp gives maybe five (5!) or eight (8!) watts of output from these 300W monsters. Truly, I felt the preamp was bad. After inspecting the amplifiers input circuit, it is obvious the "input filter" is killing the signal.

Sometimes wondering is fun: did some previous owner have high efficiency horns and wanted a pair of 300W monoblock heaters that put out around 10W from a balanced preamp? Well, they got it. :)

Input transformers, balanced preamps, these things will not work in the present state. I have a fair number of values of audiophile grade resistors on hand, but I will need to do some circuit analysis before changing resistors. I have a sneaking suspicion this problem is two fold:

- The voltage divider is reducing the balanced input too much
- The resistor network is too low of an impedance for the gates of the first transistors, and loading them down.

The schematic is reading 60k input impedance, this circuit would put that at more like 12k. This is in parallel with the input impedance of the first stage.

Truth be told, this would be a lot easier if these amps were 50lbs each and not 110lbs each. :)

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