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Old 13th Sep 2017, 7:06 am   #236
Diabolical Artificer
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Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,633
Default Re: 807 (maybe) amplifier build. Now EL34

Aye, it weighs a bl**dy ton, but to be honest it's not that loud. I managed to crank it up to 10 as it were, without killing the guinea pigs who share my workshop and it was grand. I do need to put earplugs in for signwave testing though. This was with my lashed together test speaker, which is probably inefficiant.

I forgot to say I have been using the amp to listen to music and it sounds well. However it's a bit bass heavy, which is easily solved. It's tricky to appreciate any difference though, to other amps, as it's a mono amp at present, so I stuck to old Beatle's stuff, a bit of Donovan before he went all trippy and Charlie Mingus sounds especially good as does Pentangle - double bass heaven.

Lastly for a while, I had to reduce NFB a tad, increasing the NFB R from 4 to 5k. This means NFB is about 1/50th. I'm still not completely au fait with the math's and have no idea how to apply the result to reduce oscillation. I intend to build a simple circuit using an opamp to drive a single ended EL84 in an effort to nail the math's etc at some point soon.

Finally after all the changes THD is 0.04% @ 1/2W, 0.08% @ 12.5W, and 0.18 @ 50 and 112W using 1khz. F response is flat from 50hz to 6khz where it starts to roll off, down by 3dB @ 20khz. Square wave looks good at 1khz with a bit of rounding of the leading edges, but at 10khz it looks more like Grandad's blunt old #3 saw used for cutting breeze blocks. Meh, it'll do.

Thank's for your interest Eddie. Actually, the peripheral circuit's have been the most interesting and fun bit on this amp, I've learned a lot building them. I nicked most of them, but modified them somewhat, spending hours bread boarding circuits, good craic. I'll do a complete schematic when i get a mo and post.

A.
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