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Old 14th Dec 2018, 8:11 am   #45
Diabolical Artificer
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Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,637
Default Re: 120W PP EL34 amplifier re-born.

Well spotted T, what a clanger! Some good point's there also about the design, I can only say that any design faults are due to it being the first amp I designed, I was feeling my way, trying out this or that circuit till I arrived at one that worked. It's too late now to change anything, the thought of having to start again is unthinkable.

My problem at the time of design was getting full whack out of the OP stage. I remember being able to get about 90w out it (before application of NFB), but nothing more. I was advised to stick at this, but decided to ignor that advise as it would have been a waste of 6 OP valves etc. Two things got me over that hurdle, 1) using the 12BH7 as cathode follower's and 2) reducing the OP stage grid R's to 47k from 100/200k ish.

As I said I was a complete novice when I started both in terms of electronics and valves. I mostly used the Valve Wizard site for how to draw loadline's etc, but there are very few sources of "How to build a multi/parallel valve OP stage", I used three main sources of info. 1) GEC An approach to audio frequency amplifier design - especially the 400w amp. 2) This amp - http://www.turneraudio.com.au/100w-monobloc3-2014.html and other info on his site. 3) This amp - http://www.chambonino.com/construct/constwire2.html

A borrowed a bit from each design/source till it worked, in retrospect not the best way of going about things, but the result hasn't been too bad.

Aye Simon, a big Class A tranny amp is on my bucket list, but I'll start with a little one first. I'll have a crack at JLH's design obviously, as well as others.

Will have a look at the Gainclone Andy. You got a good score there as you know, the Mitsubishi amps have some good review's apart from going for oodles of £'s.

"You say you can't hear a difference between a £5 turntable and a £500 one?" No, maybe my hearing is a bit knackered, I havn't looked after it, using power tools with protection, standing in front of PA stack's at Motorhead gigs etc, however I went to Wigwam hifi show where there were numerous setup's of "top end" hifi, I left unimpressed having heard nothing that bought on an epiphany.

Ta for looking out some preamp design's, this one looks about right so far - http://sound.whsites.net/project02.htm - a bit of switching with gain, tone/vol control's.

Andy.
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