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Re: MOSFETs work fine too...

Posted by Robert Karl Stonjek on December 14, 2001 at 01:37:54

In Reply to: Re: MOSFETs work fine too... posted by Steve Eddy on December 14, 2001 at 00:46:54:

Understood. Half the disagreements on newsgroups and message boards start when one or both parties do not have enough time to outline their case clearly enough.

Reiterating:- the debate began on the subject of feedback and whether or not it is really necessary. I pointed out that MOSFET amps need feedback to maintain a constant output with frequency when the load changes with frequency.

Someone pointed out that there are several varieties of MOSFETs and not all perform in the manner I outlined. I specified Hitachi MOSFETs in later discussions.

You said that feedback was not necessary for MOSFETs either but later said that it is necessary to correct for "inherent limitations" of some designs, and also maintained that there are other (non-feedback) methods of achieving the same thing.

I also mentioned that MOSFET amps can correct for the errant voltage output of a driver, such as might be generated by an overshooting speaker.

I did perform an additional experiment. I build a subwoofer that uses a compound woofer - basically the compound woofer is two woofers glued together face to face.

So I have some of these lying around. In the experiment I powered one of the woofers. The other was passively driven, simulating an overshoot condition.

The input voltage was 6VRMS at 20Hz. The voltage generated by the passively driven woofer was 2.3V with no load.

I then connected various resistors to act as loads/damping and checked the voltage output of the passively driven woofer:-
No Load.2.3V
10r.....1.2V
1r......220mV
0.033R...12mV
Amp.......9mV (passive woofer connected to power amp; amp input shorted)
Short.....9mV

Note that the amp gave the same result as the short. The error bars are two big to say more than that. Accuracy of around ±2mV and 1.9mV measured noise.

Also, the leads I was using were just regular hookup cables with alligator clips on the ends. To short the woofer, I connected them together, but when a voltage still appeared, I tried shortening the lead and found the voltage dropped (as expected). But this does say something about cables and damping - the two leads were only 300mm in length.

Notably, the amplifier’s feedback circuit showed a 0.5V error voltage passed to the gates of the MOSFETs. This seems to show that the amp was ‘pushing against the voltage’ produced by the speaker.

If I had a couple days spare to set up the test properly I might find something interesting, but is it really worth it? My original point was that feedback is required to keep some (ie Hitatchi) MOSFET amps form varying output voltage with load. You say there is another approach to achieve the same thing.

Anyway, if a person was really serious about sound they would dispense with the passive crossover and drive each speaker with a separate power amplifier. I think there is something to be said for specialist subbass, bass-mid and treble amps, though that is one further step I don’t think I’ll bother taking!! :)

Kind Regards,
Robert Karl Stonjek.

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