Power op amp driving valve in A2

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Mike H
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#31

Post by Mike H »

Oh-kay, how's my idea of what is effectively a power op-amp grab you? :D
 
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#32

Post by Andrew »

Nick wrote:
Andrew wrote:
Mike H wrote:Off-topic suggestion for Nick ~ as regards use of an op-amp to regulate a supply rail (low impedance negative as well as positive)
I did this a while back, with Nick's help I managed to stabilize it, so it didn't continually blow up op amps, was proving a bit expensive up until then.

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If I remember correctly, we got through a lot of desoldering braid that day :-)
Yes, we did, didn't we? I learnt a lot tho', I may have another go at this again sometime. I think the idea is fundamentally sound.
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#33

Post by pre65 »

Mike H wrote:Oh-kay, how's my idea of what is effectively a power op-amp grab you? :D
No grabs ? :lol:

Looks like the project will be downgraded and the patient moved off the operating table. :cry: :cry: :cry:
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#34

Post by Dave the bass »

Nick wrote: If I remember correctly, we got through a lot of desoldering braid that day :-)
Tell me you weren't using the braid as fuse wire.............! :-)

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#35

Post by andrew Ivimey »

A solder sucker is an even better fuse, too, honest!
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#36

Post by pre65 »

I don't really want to give up on this idea.

I tried e-mailing P Millett some while ago but no reply.

Trawling the interweb has not helped at all, so how about putting an interstage transformer between the driver amp and the 1626 grid ?
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#37

Post by pre65 »

Perhaps I should be trying to understand the theory a bit more. :)

Thing is, how does an op amp driver deal with the grid current that flows in A2 ? :?
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Paul Barker
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#38

Post by Paul Barker »

Very well and that is the opamps puropse in life. To provide a high input impedance and a low output impedance so that it can drive a power hungry stage. It is particularly suited to the task you have for it.

I think Jack Elliano does a kit of this nature.
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#39

Post by pre65 »

Thanks Paul.

I had a look at his web site, and it seems he uses a custom interstage (A2LD-7),23-30 ohms primary and 20K ohms grid load.

http://www.electra-print.com/specialuse.php
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#40

Post by Nick »

pre65 wrote:Perhaps I should be trying to understand the theory a bit more. :)

Thing is, how does an op amp driver deal with the grid current that flows in A2 ? :?
You need to read the spec for the amp in question and see what current it can sink and source. Low output impedance is one thing, current drive is a different thing.
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#41

Post by pre65 »

Well, I was thinking of a couple of mono amp modules using the TDA2050 chip, but the data sheet does not mention current sinking. Or if it does I can't find it.

http://www.ebay.co.uk/itm/110712864225? ... 2530wt_957


http://www.datasheetcatalog.org/datashe ... s/1461.pdf
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#42

Post by Mike H »

Can be worked out from the power figures on the TDA2050 datasheet:

E.g., 5% distortion, supply = +/- 22V, load = 8R, 25W r.m.s.

= 50W peak, x 8 = 400

Square root of 400 = 20V peak

20V / 8R = 2.5 Amperes (peak)

10% distortion, supply = +/- 22V, load = 8R, 32W r.m.s.

= 62W peak, x 8 = 496

square root of 496 = 22.27V peak

22.27 / 8 = 2.78 Amperes (peak)

So in other words, more than enough - for a valve grid anyway :D

Shouldn't be bothered by the bulk of the current mostly going in one direction only. And is wired up just like a op-amp. + in, - in, + supply, - supply, out.
 
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#43

Post by Nick »

Shouldn't be bothered by the bulk of the current mostly going in one direction only. And is wired up just like a op-amp. + in, - in, + supply, - supply, out.
Shouldn't be I agree, but I have my doubts, simple to check though, Simulate the valve grid with a resistor and diode in series.
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#44

Post by Mike H »

Yep.
 
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#45

Post by Nick »

10% distortion, supply = +/- 22V, load = 8R, 32W r.m.s.

= 62W peak, x 8 = 496

square root of 496 = 22.27V peak
Err, you seem to be swinging beyond the supply rails. Mind you 10% distortion.

You would need to be using a high µ valve if 20v was the peak drive on the grid.
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