Class A from scrap

For the three and more legged things
brig001
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#31

Post by brig001 »

Yes, one watt per channel. Should be good when I get my new speakers built as they are 98dB/W. Initial tests indicate that it's loud enough not to hear the screams of "turn it down", so that should be enough :D
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colin.hepburn
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#32

Post by colin.hepburn »

brig001 wrote:Yes, one watt per channel. Should be good when I get my new speakers built as they are 98dB/W. Initial tests indicate that it's loud enough not to hear the screams of "turn it down", so that should be enough :D
AH right thanks Chris/brig001
Don't no where I got 4/5 watts from Yep there has been lots of discussions on the subject of speaker sensitivity and Valve Amp output most seem to say 2/3 watt from the likes of a 2A3 Valve needs to be matched with speaker from 98/100db for good results and volume levels
I will be looking at about 8 watt t into 89db but then there's the Xover to help drain power so they may not work to well but advantage is small room and theres my 93db Fostex 126s but still have to build cabinets for them :shock:
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chris661
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#33

Post by chris661 »

Wow... 100dB@1w/1m!???!

Given, say, 2 watts into such a speaker, that's 103dB@1m. Add 6dB for the other speaker and amplifier channel and it's really really really loud!! Like permanent hearing damage in less than the time for a CD to play through.


On an unrelated note...
Anyone else ever heard/felt their ears clipping before?
brig001
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#34

Post by brig001 »

Just thinking out loud...

Can I get rid of the output capacitor if I make an artificial split supply like Mike H has done with his germanium amplifier. I would also reference the input and feedback to this artificial zero.

What do we think? Any benefit?

Thanks, Brian.
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Nick
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#35

Post by Nick »

You would need a servo to keep it at 0v
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#36

Post by brig001 »

Hi Nick, not sure I understand why. There isn't one on here http://www.tcaas.btinternet.co.uk/jlhupdate.htm but that has a real zero volts. Would the artificial zero not hold there with a couple of resistors? Will try and simulate it later.

Brian.
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Nick
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#37

Post by Nick »

Isn't that what r8 and r6 are doing?
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#38

Post by brig001 »

Just look like feedback resistors to me. In the penultimate circuit, C4 reduces the gain to unity at DC and I would probably leave the capacitor in on mine too - certainly for starters.

Thinking about it, I see what you mean about the servo if I referenced the input completely to the artificial zero, but if I left R3 and R4 as they are and left the DC gain at unity, I think it will be OK. The input signal would still be referenced to the artificial zero.

Hmm, that's not very clear, I'll have a go at drawing it later...
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Nick
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#39

Post by Nick »

Yep, by servo all I meant was a feedback loop that operated down to DC
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Mike H
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#40

Post by Mike H »

If that (above linked diagram) was behaving anything like a real op-amp it would make the o/p = the input (0V in that case) to within a small amount (hopefully).

That isn't, 'cause the input isn't a long-tailed pair, ergo there's a Voltage offset between base and emitter of Q4, plus drop across R8, the o/p will be somewhat positive DC.

Re the way I'm thinking of for the germs amp, with the dual caps, the o/p DC (and the point at the join between these two caps) follows the input bias, not the other way around. It settles at wherever it ends up. There should though be zero DC across the speaker. :D

PS: it would be a DC gain of '1'

PPS: the above wouldn't apply to mine 'cause the input bias is isolated from the o/p stage by a transformer, so the o/p stage has to be "self biased" as far as DC goes
 
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Mike H
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#41

Post by Mike H »

PPPS: just noticed Q4-3 are phase splitters (or one or the other) so isn't quite what I was expecting

EDIT: no it isn't cos it's class A :roll:
 
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Mike H
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#42

Post by Mike H »

Back to the original question, can use two caps in series to make a 'virtual 0V' for the other side of the speaker yes.
 
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#43

Post by brig001 »

Thanks, but can I make it a virtual zero for the whole thing?
That way, the output capacitor (and any effects it has on the output signal) should disappear. Hmm, not sure if that is true, see what you think of this.
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Mike H
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#44

Post by Mike H »

Possibly, but I'm wondering if it will be unstable at LF cos of positive feedback.

Is that right R3 & 4 are 6k? Couldn't be larger?
 
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#45

Post by brig001 »

I knew there had to be something wrong. It's the artificial ground. If that moves, it offsets the input in the same direction, giving the positive feedback.

Thanks Mike and Nick, back to the drawing board.

Brian.
PS. Yes R3 and R4 could be larger, but when I was playing, I had a load of them in my hand :D
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