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TU-882R Headphone Amp - Possible Mods

First may I say thanks to Victor for being a pleasure to deal with.

I'm about to build this amp. I have a fair bit of experience designing and building valve hifi but I've never done a headphone amp and the TU-882 seems a good place to start.

I plan to carry out some mods as I build; I was thinking of some component upgrades as follows:
. a better volume control
. omit C1 and C2; and having done that, R1 and R2 can be omitted
. use Takman resistors in all important positions
. coupling caps C5 and C6 to be either Jupiter copper foil or Mundorf silver/gold/oil; I use the latter and like them a lot (after *months* of running in), but an expert friend suggests the Jupiters.

But I'm now considering some mods to the circuit.

1. R3 / R6 grid stoppers ... 3.3K looks excessive; I generally use about 100 ohms, and some folks say that much more than that degrades the sound. The 2C51 is not an extreme valve and I have not seen any mention of a tendency to oscillate that might need a large grid stopper. So I plan to reduce the value to a few hundred ohms.

2. Stage 2 does not have grid stoppers; I'll see if it's possible to add them even though this may mean cutting the PCB strip.

3. Stage 2 grid resistors R13 /R14 are 1 megohm; this seems large, and a spec for one of the 2C51 types suggest 500K max grid resistor so I'm considering using this value.
However, the low output 'power' of stage 1 might make 1 meg work better so I'm undecided.

4. Elna Silmic caps for cathode bypass, with appropriate values. I use these and like them.

5. There is some global feedback via R9 and R7. The arguments run and run as to benefits of feedback. I'm considering putting in a removeable link so as to try it with and without.

6. A more fundamental change; V1 runs at only about half a mA; way too low in my view. (V2 runs at about 9mA at a good looking operating point).
So I'm considering increasing the current in V1 a bit. The supply voltage means the operating point can't be changed that much, but about 1.3 mA looks possible at about 80V on the anode and about 1.7V bias. R11 anode load resistor changes from 220k to about 80k, and R5 bias resistor changes to about 1300 ohms.
R11 will then still be 'large' compared to the valve internal resistance Ri.
Increasing the current should reduce the noise floor and give better drive to stage 2, so improving dynamics and subjective 'power'. But basically I'd expect quite simply an improved sound. Perhaps this is the reason for the slight lack of ultimate resolution that I've seen mentioned.
The best solution would be to provide a much higher voltage to stage 1 but of course that means a separate power supply; I may well try that later.

7. C7 caps, 220pF; these give a bit of high frequency rolloff. Possibly to remove a bit of noise, perhaps resulting from stage 1 running at such a low current? Or perhaps to remove HF resonance from the grid of V2 which doesn't have grid stoppers? I just don't know!
If stage 1 can be improved, C7 may not be needed; I'm tempted to reduce its value a bit and/or put in a removeable link to try with/without it.

In conclusion, I think there's a lot of potential for improvement, but even so I'm a touch confused as I don't know why some things were designed as they were, and evidently the unit works OK as standard.

I have enough confidence to try my mods, but any comments are welcomed.

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Topic - TU-882R Headphone Amp - Possible Mods - Mike C 16:11:32 01/04/16 (16)


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