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UGS-UP-Mini: A 25W SUSY Amp
UGS-UP-Mini: A 25W SUSY Amp
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Old 7th August 2018, 08:12 PM   #1
needtubes is offline needtubes  United States
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Default UGS-UP-Mini: A 25W SUSY Amp

I've wanted to try a SUSY amp for years now... So, I decided to finally make one this summer to break the monotony.

Like the original UGS power amplifier, this is a compilation of several different circuit ideas. It is, of course, based on the original UGS-UP amplifier, which is itself based on Pass Labs' X-series of amplifiers. My goals were the following:
  1. Up to 25W output, since we've seen quite often that this is sufficient for many listeners (including me).
  2. First few watts in Class A.
  3. Option to scale up the power output or Class A envelope by using larger MOSFETs or heatsinks (or both).
  4. Option for no source degeneration on output stage, as discussed in recent threads on the XA25, F4 Beast, DEF, etc.
  5. Option for local or global feedback loop.
  6. Option for resistor-based single-ended bias in any of the four quadrants, as used in Pass Labs XA*.5 series.
  7. Option to load one-half of the driver stage to tailor the distortion spectrum.
  8. Uses TL431 plus a thermistor to establish bias instead of a typical Vbe multiplier.
  9. Uses "current feedback" as implemented in the Sony VFET2 amp and many of Pass' recent offerings.
  10. Only two trimpots used to zero absolute and relative offset.
  11. Small-sized PCB to allow compact builds.
  12. Option to use the PCB to build a stereo amplifier instead of an X-amp.

Attached are images of my boards and temporary test build. Circuit description to follow...
Attached Images
File Type: jpg ugs-up-mini-pcb.jpg (320.6 KB, 1037 views)
File Type: jpg ugs-up-mini-pcb-stuffed.JPG (517.0 KB, 1008 views)
File Type: jpg ugs-up-mini-test.JPG (449.7 KB, 992 views)

Last edited by needtubes; 14th September 2018 at 09:11 PM.
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Old 7th August 2018, 08:24 PM   #2
needtubes is offline needtubes  United States
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Schematic for the UGS-UP-mini and SPICE predictions (frequency response, FFT for circuit as pictured, and FFT with 10k resistor on negative phase driver stage) are attached. My measurement rig doesn't do balanced output, so I cannot measure the true performance of the amp. Some notes:
  • R35 is used for SUSY operation.
  • R35a and R35b are used instead of R35 to create two separate channels on one board.
  • R33 and R34 are option front-end load resistors to alter the THD spectrum.
  • GR grade JFETs with IDSS around 5mA were chosen since I have these in stock. BL can be used with some changes.
  • The thermistors should be in contact with the heatsink (mount on bottom of PCB).

My heatsinks are around 5" tall by 9" long with 1.25" fins. This gives just enough dissipation for 1A bias per channel (so, 0.5A per phase) on ~16V rails with a 25C rise. One could, of course, scale up the heatsink size and increase the bias. Bias is rock solid, even with no source degeneration (as pictured).

And, lastly, the amp sounds great. Very detailed, clean sound from top to bottom, nice soundstage, although a little forward in its current configuration. Now to experiment with SE bias and different loads on the driver stage...

My thanks to Nelson Pass, CheffDeGaar, and others for inspiring the various pieces of this project.
Attached Images
File Type: jpg ugs-up-mini-schematic.jpg (263.3 KB, 1009 views)
File Type: jpg ugs-up-mini-frequency-response.jpg (190.3 KB, 1036 views)
File Type: jpg ugs-up-mini-fft.jpg (176.0 KB, 134 views)
File Type: jpg ugs-up-mini-fft-FE-load.jpg (176.6 KB, 142 views)

Last edited by needtubes; 7th August 2018 at 08:32 PM.
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Old 8th August 2018, 01:27 AM   #3
Dennis Hui is online now Dennis Hui  Canada
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UGS-UP-Mini: A 25W SUSY Amp
Great stuff! Always nice to see a SUSY.
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Old 8th August 2018, 02:06 AM   #4
aspringv is offline aspringv  Australia
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UGS-UP-Mini: A 25W SUSY Amp
Nice project! I've been considering doing something similar, but... All I've done is thought about it!

Subscribed.

May I ask why the choice of the bias circuit? As opposed to any of the other options?
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Old 8th August 2018, 04:54 AM   #5
Luke is offline Luke  New Zealand
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UGS-UP-Mini: A 25W SUSY Amp Send a message via AIM to Luke
nice project. Would it be easy to allow for a separate front end regulator as in cheffs ups amp?
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Old 8th August 2018, 05:15 AM   #6
EUVL is offline EUVL  Europe
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The bias circuit comes from F4 but you can just as well use a Vbe multiplier.
It is not all FET anyhow, unless you change the current mirror BJTs to MOSFETs.

If you are not using NFB, then making R13,14 trimable will allow you to have slightlay different Vgs for P- and N-MOS.


Patrick
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Old 8th August 2018, 05:34 AM   #7
EUVL is offline EUVL  Europe
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> My measurement rig doesn't do balanced output, so ....

You may need a differential probe.
If you use AD8421 in unity gain, you can get -125dB THD even at 10V p-p.
And it will take +/-18V.


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Old 8th August 2018, 05:40 AM   #8
EUVL is offline EUVL  Europe
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See also (single-ended, simplified) :
Pass HPA-1, what do we know?
(Post #10)


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Old 8th August 2018, 07:55 AM   #9
aspringv is offline aspringv  Australia
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UGS-UP-Mini: A 25W SUSY Amp
Default What prompted the query...

Quote:
Originally Posted by EUVL View Post
The bias circuit comes from F4 but you can just as well use a Vbe multiplier.
It is not all FET anyhow, unless you change the current mirror BJTs to MOSFETs.

If you are not using NFB, then making R13,14 trimable will allow you to have slightlay different Vgs for P- and N-MOS.


Patrick
I was just idly wondering really. I've seen quite a few variations in something close to this exact circuit from the gr25 double bjt, to the usual Vbe multiplier, to the f4 tl based option to Zen Mod's new hall effect/M2 bias. Needtubes called out the TL in particular and I was intrigued as to why?

ZM's recent effort is pretty cool.

Last edited by aspringv; 8th August 2018 at 07:57 AM. Reason: clarity
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Old 8th August 2018, 08:14 AM   #10
EUVL is offline EUVL  Europe
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The M2 bias is self biasing. No adjustment required.
But you have to have a certain voltage across the source resistors for the optocoupler to work.
It might be a limitation (design constraint) that you do not want.

The TL431 is stable and does not drift with temperature, as a Vbe multiplier does.
But for my taste it is rather noisy, unless you use the On Semi version and bypass one of the resistors.

The right BJT can be very low noise, and it has a negative tempco.
Since most FETs has a positive tempco, so the reduced bias voltage of a Vbe multiplier might not be a bad thing.

Or you just use two resistors and trim them both for bias AND offset.
This is especially useful with lateral FETs as they have a negative tempco and hence self stabilise.
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But, just my personal opinion.


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