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In Reply to: Re: I worship the DHT electric posted by Thomas Mayer on March 9, 2007 at 03:15:14:
When you say DAC goes directly into the DHT linestage, does the DAC still have it's own outputstage, or do you go directly from the DA-Converter ICs to the DHT ?>>Directly from DA converters to grids of line stage, or first stage of integrated amp, which is in both cases the first of my 3 amplification stages. The volume control is after the first stage.
Would you share the schematic of the balanced linestage ?>>
It's quite banal, and balanced all through - differential pair with anode resistors appropriate to DHT (27k to 47k, whatever) and CCS under it. Published designs for a CCS include Allen Wrights, on his site, and Morgan Jones on page 134 of his book. You can cap couple this to the next stage or preferably use a transformer in push pull - I have some Lundahl LL1660, LL1635, LL1671 for example. Then the volume after the first stage and another driver stage as above, cap coupled or ideally transformer in Push Pull. Then the output stage.
I attempted a differential circuit with DHTs, which had some noise problems. This came through the differential nature, which means the filaments have a high impedance connection to ground.>>
??? Not sure how balanced can be worse for noise - my amp's entirely silent.
Follow Ups:
Hi!I had some noise coupling between filament supply and B+ supply of the circuit. Since they did not share the same grounds, I got some noise. How is your filament supply connected to the B+ ground ? I assume since you use a CCS in the cathode (filament) to ground connection it's floating ?
I had some noise coupling between filament supply and B+ supply of the circuit. Since they did not share the same grounds, I got some noise. How is your filament supply connected to the B+ ground ? I assume since you use a CCS in the cathode (filament) to ground connection it's floating ?>Yes, that's right. The filament supply is floating in all cases. I run each diff pair off one filament supply, in series or parallel - doesn't matter with a current source since if you take one valve out in the first case it goes open circuit and in the second the current stays the same. As you say, the CCS sets the current in the diff pair and that's earthed to the common audio earth, with -15vDC supply.
Hi!Thanks for sharing. I need to look into diff DHT topologies again...
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