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Old 29th Jun 2017, 5:35 am   #217
Diabolical Artificer
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Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,637
Default Re: 807 (maybe) amplifier build. Now EL34

All is well. had a play yesterday by starting out swapping over the OP's off the cathode followers. AHHHG! Hideous OP sine (video on way) What the? So I sat down with a rough schematic adding the sine phase at each stage. After some thought I sussed, as Howard suggested it comes down to the phase splitter. One grid is grounded, it is this grid's relationship, or rather the same triodes anode to the grounded OPT secondary that is causing the grief.

To save difficult unsoldering, for test purposes I swapped the OPT sec taps around, IE black 0v became signal out positive, 8 ohm tap became 0v/ground. Cutting a long story short applied NFB no bother. What is more encouraging is that there is is no instability at all with NFB applied.

There's more to do though. I noticed that THD wasn't greatly reduced for quite a bit of a reduction in power OP. THD is less than 1% with open loop gain at normal listening levels rising to 2% THD at 105w OP, not bad. I realise these are only rough figures. Frequency response is pretty flat from 100hz to 12khz and squarewave reproduction is good.

Stability is actually very good with NFB applied. I'm afraid the maths approach is beyond me at present, try as I might I can't understand it David. I'd need to take 0 or A level maths before getting to grips with those equations. Talking to someone about this, they said finding the transfer function of an amplifier is a can of worms in itself.

Andy.
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