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Old 6th June 2013, 11:26 PM   #1
00940 is offline 00940  Belgium
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Default Tubes in an opamp feedback loop ?

Disclaimer 1: I'm posting this in the tubes forum because I doubt it would receive much attention in the headphones forum (seems mostly ss over there). Sorry if it was a mistake.

Disclaimer 2: it involves opamps, don't me.

This said, I once had a nice headphones amplifier which made use of the following topology:

Click the image to open in full size.

It was a kit by Helmut Becker from Audiovalve (see details here: PCL805-Roehren-Kopfhoererverstaerker_von_Helmut_Becker).

Of course, I could just clone it, I've got all the information needed. But it was in fact more powerful than needed (especially for the hd650 I've got now) and I was wondering if a simpler version couldn't be made, using ecl84 rather than ecl85.

Here are two ideas (see attached pics). They are determined by what I've got on hand (a good 300Vdc supply, a quad of ecl84) and the schematics are just concepts (lacking dc protection and the like).

The first one uses three supply (-20/+20/+300), is dc-coupled (which is stupid... output caps might be safer) and uses a cascoded cathode follower inside the opamp feedback loop. About 25ma standing current.

The second one uses a White cathode follower output inside an opamp feedback loop. Which means input and output caps and a floating PS for the opamp. Same standing current but can deliver a lot more current into the load.

Any of these looking like a worthwhile summer project ?
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File Type: jpg lunatic1.JPG (52.2 KB, 168 views)
File Type: jpg lunatic2.JPG (57.6 KB, 166 views)
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Old 7th June 2013, 08:51 AM   #2
roline is offline roline  United States
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Default glass and sand

Or you could mix it up differently, tube for the differential input and sand on the output.
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Old 7th June 2013, 09:10 AM   #3
00940 is offline 00940  Belgium
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Neat little circuit. Reminds me of the Borbely hybrid opamp (scrolling down to fig 4: Designing An Opamp Headphone Amplifier | HeadWize ). But more economical by using the heaters as load.

This said, I already have a tube+sand hybrid: 12au7 gain stage on 150v, ccs loaded, followed by a servoed darlington buffer. Sounds nice but I'd rather do something different.
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Old 7th June 2013, 05:07 PM   #4
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the only thing I would do is put a resistor in series to the non inverting input on the op amp (signal in) but it would take some calculation as setting up the gain structure will be critical.
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Old 8th June 2013, 10:06 AM   #5
00940 is offline 00940  Belgium
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Sorry for the question, but I'm a bit confused. Which circuit are you speaking of ? The original RKV, my first proposal, the second one or roline's ?

In mine, tubes provide no gain at all.
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Old 8th June 2013, 10:33 AM   #6
FoMoCo is offline FoMoCo  United States
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Done right, it should work. But... May I ask why?

By the way, compensation will probably be a nightmare. You've got a good chance of making an oscillator with that circuit. That's not to say that it will oscillate, but be sure to check for parasitic oscillations.
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Old 8th June 2013, 11:03 PM   #7
wa2ise is offline wa2ise  United States
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I did something like this:Click the image to open in full size.
when inserting some tubyness (AKA a little bit of 2nd H) into a CD player.
Click the image to open in full size.
More such at Modifying CD player DAC circuits
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