Russ Andrews "Zapperator" circuit

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Not specifically, general question, but that's interesting

I tried this many years ago and was amazed at just what did happen. Testing into a dummy load and all was good up to a couple of watts output and then anything above that and the distortion suddenly set in.

Its frequency dependent and the distortion turns a nice sine into a horribly distorted triangular wave (from memory... its a long time ago) and with what also looks like gallons of crossover distortion thrown in.
 
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No don't get me wrong I'm not interested in just producing an exact clone of the product, just taking inspiration from the idea and producing something that works in a similar way.

I am actually quite keen to try that out - and if I find a value that works better then I can always make some with that fixed resistor value (although again difficult without a scope)

Well step one is work out what the problem is that needs solving. Whilst there were many hair shirt amplifier designs in the 80s that would oscillate as soon as you looked at them, thankfully this is less so now (Naim owners are still being fleeced for inductive speaker cables of course).

What could a doodad connected at your speaker actually do? It could act as a conjugate to make the speaker an easier load to drive, but that is speaker dependent. It might terminate RF pickup, but you amplifer should have an output coil to stop that getting back in. And if that was the issue you would be better off with something that actually does something like this. Ben Duncan's Pure Henry (note this is not saying that would help).

So so far I can't think of something it might be doing in order to work out what it would look like.
 
A CR at the speaker end of the cable can roughly ensure that the cable is terminated for RF. This stops the cable resonating, which may reduce the pickup at that frequency - which may matter if there is a signal at that frequency and if the amp is poor at rejecting RF coming in at the speaker terminals. It may be noted that most systems omit this cable termination, and most systems works just fine.
 
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