"Millett Hybrid" redux: NuHybrid headphone amp, using the Korg Nutube

[gikigill]

So where can I buy one fully built or even semi built?

 

[pmillett]

 
Complete, but I experience a HF ringing/oscillation noise on power on or whenever the NuTube sees any vibration (it seems crazy microphonic) that slowly fades away over time. Sounds kind of like tapping on a wine glass, but the ringing takes an impossibly long time to fade away. Comes back if I tap the NuTube or even touch the amp in any way. Tube noise also seems to be abnormally high (very audible on Fostex TH-X00). I swapped the resistor values but I used the 2N7000 transistors in a the Q positions. It does amplify music though. Where should I start debugging this?

 
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

 

[strangecargo]

   
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

I wasn't expecting ringing this severe, after reading @tomb describe how hard it was to excite the filament. It's very loud (I thought that the plug-in noise was some kind of OS alert noise I hadn't heard before) and even the slightest rub or touch on the casing of the amp (or turning the volume pot) will set off the ringing, which I can hear on HD650s, TH-X00, and K7XX, though the tube noise is not quite as audible on the HD650 and K7XX.

 

[pmillett]

So where can I buy one fully built or even semi built?

 
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

 

[strangecargo]

   
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

Pete, the ringing is still audible ~25 seconds after the power-on relay clicks. Is that really typical? How does Korg handle this in the effects pedals they've been making with the Nutube?

 

[pmillett]

  Pete, the ringing is still audible ~25 seconds after the power-on relay clicks. Is that really typical? How does Korg handle this in the effects pedals they've been making with the Nutube?

 
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

 

[tomb]

 

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

Pete, the ringing is still audible ~25 seconds after the power-on relay clicks. Is that really typical? How does Korg handle this in the effects pedals they've been making with the Nutube?

Maybe there's a variance in microphonics from NuTube to NuTube as with glass tubes?

I can confirm that mine is a bit easier to start ringing than I first reported, but I haven't experienced it going for 25 seconds. However, the ringing is sort of as you say - more of a high-pitched tone that is hard to distinguish as not being electronic in nature.

 

[pmillett]

Maybe there's a variance in microphonics from NuTube to NuTube as with glass tubes?

I can confirm that mine is a bit easier to start ringing than I first reported, but I haven't experienced it going for 25 seconds. However, the ringing is sort of as you say - more of a high-pitched tone that is hard to distinguish as not being electronic in nature.

 
I've seen a little tube-to-tube variation, but not a lot.  Certainly not 2x from one to another...
 
Pete

 

[tomb]

I definitely installed the foam under mine. Maybe that's strangecargo's issue?

 

[strangecargo]

   
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

I used the adhesive foam. Maybe I got a particularly microphonic Nutube? Even lightly brushing the side of the plastic case or carefully setting my finger on one of the OPA551s will set off audible ringing.

 

[pmillett]

  I used the adhesive foam. Maybe I got a particularly microphonic Nutube? Even lightly brushing the side of the plastic case or carefully setting my finger on one of the OPA551s will set off audible ringing.

 
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

 

[strangecargo]

   
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

Hmm. With the HD650, it takes ~20 seconds for the ringing to fade away. I *am* using the Axicom alternative part from the BOM, as the Kemet relay was backordered at Mouser. Perhaps it has a considerably more violent switching mechanism. I can still hear my fingers on the case (and subsequent ringing) with the HD650 though.

 

[tomb]

 

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

Hmm. With the HD650, it takes ~20 seconds for the ringing to fade away. I *am* using the Axicom alternative part from the BOM, as the Kemet relay was backordered at Mouser. Perhaps it has a considerably more violent switching mechanism. I can still hear my fingers on the case (and subsequent ringing) with the HD650 though.

JMHO, but the violence of the relay is reduced to almost zero if you increase the time for it to engage.

I installed the 1K resistors on the output caps, but it had almost no effect. There is probably enough voltage remaining to leave the relay causing noise upon engaging. Again, JMHO, but upsizing the relay cap to 470uf will stop that.

 

[Heffa]

I *am* using the Axicom alternative part from the BOM, as the Kemet relay was backordered at Mouser. Perhaps it has a considerably more violent switching mechanism.

I just ordered the same relay, together with the rest of the BOM from Mouser as well... We'll see if I experience the same ringing in my amp once the parts arrive and I have found the time to actually build it.

 

[tomb]

This may seem like cheating, but a longer relay time (~30 sec) also gives plenty of time for the NuTube to settle down before you can listen.

A longer relay start cures a lot of ills ...