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Class A single-ended requires a transformer or coupling capacitor.

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In Reply to: RE: Class A SS or chip amp tda2040 posted by rcollege on October 22, 2010 at 01:02:17

"Class A" means the output device never shuts off. There is substantial DC current flowing through the output device all the time. In a single-ended amp, there is no way to balance the bias current, and the speaker cannot tolerate this. Single-ended amps made of tubes require specialized output transformers that can tolerate the DC, and these are wound to match the high tube plate impedance to the low speaker impedance.

I don't know if anyone makes a specialized output transformer that matches low transistor impedance to the speaker, while tolerating the bias current.

An alternative would be to couple the speaker with a huge capacitor, but this would thoroughly degrade the sound.

Follow Ups:

Really? - Ric Schultz 16:35:07 10/22/10 (1)
Single ended transistor circuits can be designed using plus and minus supplies. The Pass Labs Aleph series is like this. You have a differential front end and a single ended output that has a current source for the other half. You bias it as high as you like and with a cap in the feedback to ground you get around 40mv offset. Amps like this can sound fantastic. The Aleph series has a sliding bias output stage to give it more current as the load demands. While this makes for more power into lower impedances it messes up the sound. A pure fixed bias output stage sounds best. I have done it both ways.

The trouble with single ended class A circuits is that they are so inefficient. I was biasing my amp at 3 amps bias and so I was using 150 watts a channel to get 25 watts.....ridiculous. Tons of heat and tons of heat-sinks and tons of capacitance in the power supply to get rid of the hum.

I would go with the Tripath. I heard a Vinnie amp that ran off batteries and with a few mods was guite good. I am done with class A single ended (at least for now).
- Thanks. I did not know this had been done. - Al Sekela 15:04:55 10/23/10 (0)
  Agree about the heat issue. There is no point in listening to class A if the air conditioning has to run to keep the room temperature under control.

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