Yet another amp build...

[Thermionics]

It's a sickness...

I just finished another interesting amp build. While the Pass ACAs sound fantastic on my bubba'd Vandersteen IIs, they lack a certain amount of heft when you need that extra push over the cliff (to paraphrase Nigel Tufnel).

Enter the NuClassD amp. This is another Pete Millett Joint (at this point, I might need to start getting a punchcard or frequent-flyer miles for all the Millett-designed gear I've built). It's a MP7770 Class-D amplifier (similar in design to the Hypex Class-D chip) driven by a Korg NuTube 6P1.

What is a NuTube, you might ask?

It's a dual DHT (based on a Vacuum Florescent Display) running at very low voltage that acts as a gain stage in front of the MP7770. When the two are paired together, the amp is good for about 50WPC into 8 ohms or 90WPC into 4 ohms. Lots more grunt.

Please ignore the pine needles on the floor - the dogs went running through a moment before I put down the amp and hooked it up. I used a Chinese chassis from eBay, amplifier and power-supply boards from Pete, and an Antek toroidal transformer.



Photo was taking while listening to a high-bitrate rip of Vampire Weekend's Contra album. Full bass, extended highs, and a ginormous soundstage from the Vandersteens.

I'll post some photos tomorrow of the interior of the amplifier so you have a better sense of how big the power supply is in proportion to the tiny amplifier boards.

Now I'm gonna listen to more music.

-D

 

[Thermionics]

One caveat on the Korg NuTube 6P1 - it's hella-microphonic. I'm having to be very careful how I place the amp and how I mounted the 6P1 tubes on the board and the boards in the amp. That being said, when properly isolated, it sounds glorious.

 

[Opa1]

Interesting amp. Might have to try this one out.

 

[rogerfederer]

I built the bottlehead smash preamp and the tubes it uses are very microphonic. Found that I had to treat it like a finicky turntable and use isolation techniques.

Sounds like a great amp!

 

[Thermionics]

I built the bottlehead smash preamp and the tubes it uses are very microphonic. Found that I had to treat it like a finicky turntable and use isolation techniques.

Sounds like a great amp!

Yeah. I've got some old AudioQuest sorbothane feet (the ones that stick to anything and everything) that I may try underneath this. I also might temporarily remove the circuit boards from the right side (mounted vertically - this seems to help) and lay down some Dynamat to damp resonances on the panel before reaffixing the circuit boards. Not sure if there's anything I can affix to the 6P1 tubes proper as I'm not sure if it is the glass or the structure that is the issue.

But even with the microphony caveat, it sounds way better than any Class D amp has a right to.

 

[rogerfederer]

w/ the smash i ended up moving it to another system; 1st place i tried it was in room w sprung wood floor and was just too much vibration. did better w heavy target stand and room w less foot traffic. i also weighted it down.

i'm also intrigued by this amp. have tried a lot of class D flavors so far (icepower, tpa 3116, 3118, 3250, classDaudio, hypex ncore) and like the sound.

 

[Thermionics]

A few photos...

Top-down showing the beefy power supply / Antek trafo and the teensy signal boards to the right. I'm running 66000uF of capacitance per channel to the NuTube and MP7770, so I'm not going to run out of power any time soon - in fact with no signal, powering off the amp will leave it running for a disconcertingly long period of time (although if playing music, it only continues for about 5 seconds before shutting down the signal).

Note: I'm going to have Front Panel Express mill a new back for this chassis as I ham-handed the drilling (I always seem to do that - I need Nate to become my spirit animal):



Here's a closeup of the boards showing the MP7770 chip (in between the two mustard-colored mica caps) and the Korg 6P1:



Tried to take a shot sans flash, but it came out blurry - however, you can see the heater glow on the 6P1:



Assembly was very easy - Pete does great boards and great instructions.

Other than the aforementioned microphonic tendency (which is a result of the internal structure, not the glass), the amps are very quiet and have been designed with a certain DHT-quality sound signature per Pete.

-D

 

[StevenZ]

Where can one find more info on these boards/design?

 

[Opa1]

Where can one find more info on these boards/design?

Right here. http://pmillett.com/

 

[Thermionics]

Where can one find more info on these boards/design?

Specifically, http://www.pmillett.com/NuClassD.html.

 

[Thermionics]

Found some threaded bumpers (metal core with squishy rubber) for feet and mounted those - they definitely do a better job than the little hard rubber bumpers that came with the case in isolating the amp. As long as you don't bang the amp around while it is playing, there's no microphony to speak of. However, if you do bump it, then you get a clear 5.8kHz tone like the chiming of a bell through both speakers. Not going to cause any damage to the speakers, of course, but a bit annoying.

The more I run this amp through the Vandersteens, the more I like the combination. Plenty of heft in the lower registers (but not sloppy) while smooth mids and highs with crazy imaging / soundstaging.

-D

 

[MrEd]

Nice build, I guess I have been under a rock as I have never heard of this design or topology before.
Glad you shared it with us.
I had a pair of Vandies for several years, they were paired with my Dynaco St70.

 

[Thermionics]

w/ the smash i ended up moving it to another system; 1st place i tried it was in room w sprung wood floor and was just too much vibration. did better w heavy target stand and room w less foot traffic. i also weighted it down.

Thankfully, the basement is floating floor on concrete slab, so very little bounce.

i'm also intrigued by this amp. have tried a lot of class D flavors so far (icepower, tpa 3116, 3118, 3250, classDaudio, hypex ncore) and like the sound.

Apparently the topology of the Monolithic Devices chip is quite similar to the Hypex chip, so they could conceivably have a similar sound.

 

[MrEd]

I came back to this thread while contemplating a chip amp build for my wife's bedroom system.
What are your thoughts and advice after spending some time with it ?

 

[Thermionics]

I came back to this thread while contemplating a chip amp build for my wife's bedroom system.
What are your thoughts and advice after spending some time with it ?

It has been in my main system (driving the Klipsch in-walls in the kitchen) for the last year or so. I have zero complaints - it draws so little power that I just turn it when I power up the system and leave it running, even if I have the secondary pre-outs switched off at the Yamaha C4. Sounds quite musical on the Klipsch with plenty of headroom.

 

[MrEd]

Thanks for the feedback.
That moves them up on my list and they are more diy than just buying a pre made board.
I like the hands on part..

 

[SONDEKNZ]

It has been in my main system (driving the Klipsch in-walls in the kitchen) for the last year or so. I have zero complaints - it draws so little power that I just turn it when I power up the system and leave it running, even if I have the secondary pre-outs switched off at the Yamaha C4. Sounds quite musical on the Klipsch with plenty of headroom.

Interesting project. Thanks for the good write-up, THERMIONICS.

Would you be able to describe how this amp sounds to your ears - compared to a good low-medium power, traditional tube-amp?

 

[Thermionics]

Interesting project. Thanks for the good write-up, THERMIONICS.

Would you be able to describe how this amp sounds to your ears - compared to a good low-medium power, traditional tube-amp?

The 6P1 combined with the Monolithic Devices chip amp gives it a nice triode-y quality that makes it very nice with the Klipsch in-walls. Previously I had been driving them with a Parasound Zamp and I thought it made them a little too aggressive in the upper registers. The NuTube smoothed things out nicely without sounding rolled off.