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Posts by pmillett

 I haven't tried to get the buffer to oscillate, but it might be unstable with very low DC resistance across the inputs.  This would apply only to the input buffers - the output buffers have a high resistance on the gate (the Rp of the Nutube is pretty large) so they should be fine. To be safe, you can either change to 10k resistors on the source terminals of the 2N7000s - that seems to lower the gm enough that there's no oscillation - or change to 2N3904 + 10k like on the...
 I think the biggest risk would be noise pickup.  Try and keep the wires as short as you can. There is no exposed metal on the Nutube package other than the pins, so they are all that needs to be insulated. Pete
OK, I made a discovery - the microphonic "ping" at power-up has nothing to do with the relay engagement.  It is noise induced from flipping the power switch.   I tried leaving the power switch turned on, and instead turned the amp on and off by switching the AC to the adapter.  The relay pulls in after 10 seconds. No "ping" noise (well, none in HD600s - I'm sure there is still some noise in efficient headphones or IEMs).   I also tried moving the power switch very...
 It may also be that you got a particularly noisy relay... With IEMs, I can also hear noise if I run my finger along the knob or case.  I wouldn't use this amp with IEMs, more because of the tube noise than the microphonics.  The same may apply to very sensitive headphones. With less sensitive headphones like the HD600, I don't hear noise at all, and just a small "tink" sound when the muting relay activates. Pete
 I've seen a little tube-to-tube variation, but not a lot.  Certainly not 2x from one to another... Pete
 That seems extreme.  Even with sensitive IEMs I only hear ringing for ~10 seconds after the relay pulls in.  At that point the ringing is below the noise level. Did you use the adhesive foam to stick the tube onto the PCB?  Just curious, as I'm not sure if it helps or hurts... Pete
 Keep watching this thread.  Maybe strangecargo will want to sell his :) Seriously, I'm sure some builders will decide it's not to their liking.  That is always the case, especially with tube amps. Pete
 That is the normal microphonic behavior of the Nutube.  Unfortunately there is nothing to debug... The filament of the tube oscillates like a piano string when subject to shock or vibration.  When the muting relay activates, it puts enough of a mechanical vibration into the PCB that it excites the filament ringing. Due to it's low gm, it is also not the quietest device around.  The Fostex are quite efficient, making the noise and microphonics more noticeable. Pete
 Yes, use the updated resistor values, even with the 2N7000.  I just tried it, and actually the oscillation goes away with 10k resistors on the followers.  So maybe no need to change to 2N3904 at all. I'll do some more measurements tomorrow to confirm that everything is OK. Pete
OK...   I've made an update to the design to eliminate instability in the 2N7000 buffers when the volume control is near zero, and to reduce the power-on noise in the headphones.   Q2 & Q3 are changed to 2N3904 transistors.  R3 & R4 are now 1k, and R21 & R24 are 10k.  The resistor quantities in the BOM remain unchanged, basically two pairs were just swapped around.  Q1 remains 2N7000 FET.  So, the only new components are Q2 and Q3.   I measure no differences between...
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