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Old 27th October 2006, 11:51 PM   #1
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Join Date: May 2006
Default An all DHT push pull amp.

I'm on my way to building my first all DHT amp - time to use DHTs all the way through. It will be all balanced. I've fixed the tubes for the first experiment - 3A5 diff pair cap coupled to the volume control, 1J6 driver and either cap or transformer coupled into 2a3 output stage. Danbury output transformers in the first instance - they're pretty good and I've used them before. All on a stereo chassis. The ultimate amp will be two monos, and more like 30 into 26 into 2a3 with O-netics iron. This is a dry run. Rationale:
a) 3A5 is surprisingly good and very clear, 1J6 is a decent driver and between them they should have enough gain.
b) volume control is after the first stage since I intend to drive the amp straight out of my DAC digital board, which has DC superimposed on it. I do this at present and it works very nicely.

Definitely DC current sources and Morgan Jones CCSs for the first two stages. I thought of using AC for the 2A3s.

I'm quite excited about this - I've been building up to it for 6 months with various experiments. I'll try and write out a circuit diagram when I have it working.

Any comments so far? Andy
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Old 28th October 2006, 08:06 AM   #2
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Any comments so far?
Only to say it sounds like a recipe for success, so long as your speakers are satisfied with the modest output from PP 2A3s (but I'm sure you have this in mind). An all-balanced design is THE answer for low noise. I doubt if you really need a CCS in the tail of the second stage but it won't hurt.

Good luck with it.
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Old 28th October 2006, 09:41 AM   #3
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I doubt if you really need a CCS in the tail of the second stage but it won't hurt. >>

It's not direct coupled, because the volume control is between stages one and two, since it has to cope with DC on the signal to the first stage. I made the volume control 100k in the first instance - may need tweaking. Andy
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Old 28th October 2006, 12:37 PM   #4
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Location: Zürich
Hi Eusebius

Sounds pretty nice to mee! Will you be using the shunt kind of pot for volume? I mean the one with the two resistors of same value going into the extremes of the pot, and the wiper connected to either side of it? I have never seen it done, but shall work, although I don't know how heavy a load it will represent for the 3A5...but then, I don't know a lot of things..

I am curious how this turns out. Any chance of a schematic of what you have in mind (also to clarify the connection of the volume control). It just makes understanding easier!

Good luck with the project - winter has just begun!

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