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

[docentore]

I'm building my NuHybrid now but I have few questions about possible upgrades, mostly caps. Can bipolar caps be used (10uF) instead of standard ones?

 

[docentore]

There is always a little chance that an opamp will oscillate driving some headphones. Headphones are complex impedances, and if they present too much capacitance, the circuit can become unstable. I've not heard of it happening on this amp, but have seen a couple of cases with other opamp-based amps.

That's largely why the series resistors R8 and R9 are there, they make the circuit more forgiving. If you do have oscillation you can make them larger to mitigate it. But the resistor will probably change the sound more than the opamp swap...

Pete

Hi Pete

If I were to test the AD8397 in NuHybrid will I need to alter the pinout as AD is 2 channel opamp and OPA551 single with different pinout.
Thanks

 

[Wiljen]

Hi Pete

If I were to test the AD8397 in NuHybrid will I need to alter the pinout as AD is 2 channel opamp and OPA551 single with different pinout.
Thanks

Yes, you'd have to alter the design. They make boards for using two single opamps in place a single double but I've never seen a drop in for using 1/2 a double in a single socket.

 

[tomb]

Yes, you'd have to alter the design. They make boards for using two single opamps in place a single double but I've never seen a drop in for using 1/2 a double in a single socket.

^ This.

Even further, Pete states that the OPA551 is in the circuit as a buffer, not really what you'd think of as an opamp. He suggests the BUF634 would work.* If that's compatible, it may be that the old Intersil buffers sometimes used in the original Millett Hybrid may be compatible (HA3-5002).

Also, one of the most aggressive buffer chips could be compatible, but Pete would need to confirm this. The LMH6321 is a very aggressive buffer chip, reportedly capable of 700ma for short bursts (maybe the buffer version of the AD8397?). Tangent used it in his latest PIMETA design instead of the BUF634. At first glance, it appears to be pin-compatible so it may work.

If you use the BUF634, you can stack them on top of each other and solder the pins together. This will give you more current burst capability. It was an old trick we used to do with both the PIMETA and Millett Hybrid. However, having tried all of them (including the OPA551) in the Millett Hybrid and PIMETA, I can tell you that the LMH6321 just by itself is better than two stacked BUF634s.

At least one individual who used to frequent Head-Fi claimed that the TO-220 version of the BUF634 was even more superior if you heat-sinked it, which is what Pete did on his original Millett Hybrid that was published in AudioXpress magazine. I don't know if that is true, but it gives you more possibilities.

However, I would pursue the LMH6321 if Pete can confirm that it may work. You'll need Brown Dog adapters because that chip is SOIC-8.

* To be clear, Pete states, "You can substitute your favorite opamp or buffer for the OPA551. It needs to support a 24V power supply, and be unity gain stable. Other than that it's up to you." However, if the purpose is for current, some of the more familiar opamps may be fairly limited in that capability. Use of a true current buffer chip, such as those mentioned above, might be more optimum.

 

[docentore]

Ok, so I have managed to get the amp assembled. Used Vishay-Dale resistors for most of the parts (apart from 1Ohm R8 and R9). As electrolityc caps I used Elna Simic - I had to C4 and C7 to the bottom of the PCB.


Got the bias set at 11V but I've noticed that left lamp is dimmed. Anyone had this before? Is there anything I should be worried? I haven't tried connecting headphones, will try tomorrow. I've checked all solder points and they look fine.
EDIT: all fine, stupid me. I was setting bias for one channel probing L&R testpoints, doh!

EDIT2 (don't want to double post): Amp works brilliantly. Drives my T50RP MK2 ArgonMod quite well. Anyone tried different values of R8/R9 for T50RP or other planars? They are 50Ohms.

EDIT3: palyed around with single opamps I have.
OPA604 -meh, nothing special
OPA134 - WOW, winner, to me sounds better than OPA551
AD8065 and OPA1641 - couldn't test them as DIP adapter I got does not fit the socket.

 

[Prabeer Patankar]

Hello Pete

I recently built this amp and experimented with a couple of single opamps I had in my possession.

I tried OPA2209 ( good details and faster than stock but quite edgy and noisy). AD797 ( very quiet background, good details especially in the lower end however it’s not unity gain stable. It’s most probably oscillating as it heats up quite a bit). I got the Sparkos Ss3601 singles after reading a post on this thread and they are quite good. The extension on either side is very good. The amp sounds faster and there more details and separation.
Last week at a Audio meet up I carried this amp along with my Cayin iHA-6. We connected a Audio Note DAC 0.1x to the Nutube and fed the pre out to the iHA-6. Listening on my LCD-X and the demo Meze Empyrean and HD800 available there at the meet up, the synergy was so beautiful. The sweetness of the DAC was just flowing through to the drivers. Amazing micro details and a lush 3D soundscape. When we took the Nutube out of the chain and fed the DAC directly to the Cayin the whole magic disappeared. There was no 3D enveloping sound stage, the vertical stage and layering between the instruments disappeared. The details were still there but the sweetness disappeared. Maybe there was a impedance mismatch between the DAC and the Cayin.

I am not a electronics guy. Just someone who can solder by numbers. Can any of you more experienced guys tell me what is the input impedance of this amp. The guy who had got the Audio Note DAC to the meet is Aernoud Dekker, the India CEO of Audio Note and he has been asking me about this ever since. Such has been the effect of this little guy.

Thanks in advance
( I am trying to figure out how to post pics of the amp here)

Prabeer

 

[ilikepooters]

Hello Pete

I recently built this amp and experimented with a couple of single opamps I had in my possession.

I tried OPA2209 ( good details and faster than stock but quite edgy and noisy). AD797 ( very quiet background, good details especially in the lower end however it’s not unity gain stable. It’s most probably oscillating as it heats up quite a bit). I got the Sparkos Ss3601 singles after reading a post on this thread and they are quite good. The extension on either side is very good. The amp sounds faster and there more details and separation.
Last week at a Audio meet up I carried this amp along with my Cayin iHA-6. We connected a Audio Note DAC 0.1x to the Nutube and fed the pre out to the iHA-6. Listening on my LCD-X and the demo Meze Empyrean and HD800 available there at the meet up, the synergy was so beautiful. The sweetness of the DAC was just flowing through to the drivers. Amazing micro details and a lush 3D soundscape. When we took the Nutube out of the chain and fed the DAC directly to the Cayin the whole magic disappeared. There was no 3D enveloping sound stage, the vertical stage and layering between the instruments disappeared. The details were still there but the sweetness disappeared. Maybe there was a impedance mismatch between the DAC and the Cayin.

I am not a electronics guy. Just someone who can solder by numbers. Can any of you more experienced guys tell me what is the input impedance of this amp. The guy who had got the Audio Note DAC to the meet is Aernoud Dekker, the India CEO of Audio Note and he has been asking me about this ever since. Such has been the effect of this little guy.

Thanks in advance
( I am trying to figure out how to post pics of the amp here)

Prabeer

Nice enclosure!! Where can i get one?

 

[Prabeer Patankar]

Nice enclosure!! Where can i get one?

Hi
A friend helped me source it from a fabricator in Delhi, India. It was custom made for his Nutube build and the fabricator had a spare lying with him.

 

[Prabeer Patankar]

Nice enclosure!! Where can i get one?

Hi
A friend helped me source it from a fabricator in Delhi, India. It was custom made for his Nutube build and the fabricator had a spare lying with him.

 

[Prabeer Patankar]

Sorry for the repeat post

 

[cgnman59]

I've used the NuHybrid as a pre between a Topping D10 DAC and my Millett Engineer's Amp going through Klipsch KG3.5 speakers. I find it is great as a pre and has helped tamed some of my music that has been a bit harsh through the Klipsch. I agree with you on the soundstage as well. Incredible what Pete has accomplished with the Nutube and about $65 in parts. And really like the enclosure. Very sharp.

 

[tomb]

<snip>
I am not a electronics guy. Just someone who can solder by numbers. Can any of you more experienced guys tell me what is the input impedance of this amp. The guy who had got the Audio Note DAC to the meet is Aernoud Dekker, the India CEO of Audio Note and he has been asking me about this ever since. Such has been the effect of this little guy.

Thanks in advance
( I am trying to figure out how to post pics of the amp here)

Prabeer

Except for one notable exception (the O2), almost every single headphone amplifier design places the volume pot in the signal path right after the RCA jacks. Therefore, the input impedance is equal to the impedance of the volume pot. If you used the Alps RK097 volume pot that Pete specified in the BOM, then 10K is the answer.

 

[Prabeer Patankar]

Except for one notable exception (the O2), almost every single headphone amplifier design places the volume pot in the signal path right after the RCA jacks. Therefore, the input impedance is equal to the impedance of the volume pot. If you used the Alps RK097 volume pot that Pete specified in the BOM, then 10K is the answer.

Thanks for the reply Tomb, I am using the TKD pot which is also 10K.

 

[Prabeer Patankar]

I've used the NuHybrid as a pre between a Topping D10 DAC and my Millett Engineer's Amp going through Klipsch KG3.5 speakers. I find it is great as a pre and has helped tamed some of my music that has been a bit harsh through the Klipsch. I agree with you on the soundstage as well. Incredible what Pete has accomplished with the Nutube and about $65 in parts. And really like the enclosure. Very sharp.

Thanks cgnman59.

It’s pairing very well with my Atoll IN200SE in power amp mode too. I am very happy with this build.

 

[Luckbad]

Very nice! Can you share with members what model Hammond you used? I like it much better than the Serpac, but the Serpac is cheap and good for people on a limited budget.

I finished mine over the weekend and have been letting it break in. Initial listening sessions have been very positive. I shielded the wooden box I made with copper tape electrically connected to the front and back aluminum panels.

Not completely done, but close if you're wondering, the brown boxes are closely matched Russian Silver Mica coupling caps. They're huge for 0.1uF, but sound mighty good.

A shout out to TomB for the initial build and parts updates

Is that an enclosure that can be purchased somewhere? I super dig it.