UGS-UP-Mini: A 25W SUSY Amp

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Oh hey, I had no intention of jumping in ahead there. Happy to wait as well - I'm thinking I can try a latfets in there with some leads to the board and do a lower bias version easily enough.

I've plenty of the k246 matching j parts, so it'll be a somewhat franken-build of your original build, but hopefully fun nonetheless.
 
I have about 10 boards that I can spare, so no shortage yet. I've been down and out with a nasty cold, and still can't hear out of my right ear because of the sinus pressure, but will PM both of you shortly for shipping details. Sorry for disappearing.

Before the sinus pressure became unbearable, I did have a listen to the amp with a 10k resistor loading only the positive phase of the front end (R33 installed). The effect was to alleviate the forwardness of the original presentation and push the soundstage back a foot or two so that it is more suspended between the two speakers. Ears were fuzzy from the cold, so I can't say more yet. But, those effects were noticeable.
 
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I have about 10 boards that I can spare, so no shortage yet. I've been down and out with a nasty cold, and still can't hear out of my right ear because of the sinus pressure, but will PM both of you shortly for shipping details. Sorry for disappearing.

Before the sinus pressure became unbearable, I did have a listen to the amp with a 10k resistor loading only the positive phase of the front end (R33 installed). The effect was to alleviate the forwardness of the original presentation and push the soundstage back a foot or two so that it is more suspended between the two speakers. Ears were fuzzy from the cold, so I can't say more yet. But, those effects were noticeable.

A hearing related illness, with this sort of hobby is doubly unkind. I wish you a speedy recovery. :)

I’d assume that the above gives you some second harmonic (positive or negative?) and that sense is the end result. Is there a difference in terms of offset changing or gain on that side?

...and is the susy attempting to correct for that distortion? I guess I need to reread the susy paper. :)
 
Yes, the added load on the positive phase affects the relative amount of second harmonic to third harmonic. With no load (and assuming perfectly matched halves of the bridge), we have only odd harmonics (basically just third at low output levels). Increasing the load yields higher levels of 2nd harmonic. With relatively high values for R33 (larger than ~22k), SPICE predicts third dominates second. Below that, second dominates. With R33 at 10k, SPICE says I should have ~6dB more second harmonic than third, and that the second harmonic should be of negative phase. Generg did some experiments with the second-to-third harmonic ratio over in the F4 Beast thread. I'll have to re-read his comments about the differences in sound.

Adding R33 into the mix did not affect offset very much. Actually, I haven't yet bothered to zero the offset entirely since I had planned to test some different configurations out. I ballparked the relative and absolute offset to <10mV and called it good. Gain is a touch lower on the positive phase with R33 in place (according to SPICE).

I hope to finally be mobile tomorrow so that I can get some padded envelopes and a shipping quote for you and Itsmee.
 
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Yes, the added load on the positive phase affects the relative amount of second harmonic to third harmonic. With no load (and assuming perfectly matched halves of the bridge), we have only odd harmonics (basically just third at low output levels). Increasing the load yields higher levels of 2nd harmonic. With relatively high values for R33 (larger than ~22k), SPICE predicts third dominates second. Below that, second dominates. With R33 at 10k, SPICE says I should have ~6dB more second harmonic than third, and that the second harmonic should be of negative phase. Generg did some experiments with the second-to-third harmonic ratio over in the F4 Beast thread. I'll have to re-read his comments about the differences in sound.

Adding R33 into the mix did not affect offset very much. Actually, I haven't yet bothered to zero the offset entirely since I had planned to test some different configurations out. I ballparked the relative and absolute offset to <10mV and called it good. Gain is a touch lower on the positive phase with R33 in place (according to SPICE).

I hope to finally be mobile tomorrow so that I can get some padded envelopes and a shipping quote for you and Itsmee.

OK, thanks for the explanation! Why the choice of negative second harmonic distortion? I seem to recall that Pass talks about single ended amplification as an intrinsically single ended medium and I took the impression away that it was a positive distortion (although, that is an impression, I'll have to re read)

At the end of the day though, if it sounds good, then happy days!

Cheers again.
 
Why the choice of negative second harmonic distortion?

This, again, stems from some discussions have taken place in the XA.8 Current Source thread, F4 Beast thread, and elsewhere. This interview at Stereophile with Mr. Pass sums everything up nicely:

Nelson Pass said:
There was a consistent subjective observation that there was a difference not only with the level of second harmonic, but phase also. Negative-phase second harmonic tends to expand the perception of front-to-back space in the soundstage, separating instruments a bit. Positive phase does the opposite, putting things subjectively closer and "in your face." I have heard this sort of comment from people who were not in a position to have expectation bias, so I treat it seriously.
 
The trick is to figure out how to get the negative phase second harmonic to appear. Many ideas have been tossed around in the threads I mentioned in the previous post. It looks like Pass Labs uses single-ended bias to get the job done (and Mr. Pass has stated that he prefers to get his second harmonics from the output stage), but I found in simulations that R33 has a similar effect. I may play around with SE bias on a future build with larger heatsinks to compare the sound.
 
Attached is a parts list for the boards, for those of you who have boards headed your way. For the initial startup, I recommend not having the MOSFETs mounted with jumper set for local feedback. Test voltages at the MOSFET gate pads to make sure the bias is not too high, and adjust P1-P4 as needed to match gate voltages for N channel and P channel parts. When satisfied, change jumper to global feedback (if desired), solder the MOSFETs in, and test again.

I still have a few boards left, if anyone else is interested in building this amp.
 

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That's good to hear! I hope you enjoy the build.

I've had a couple more listening sessions with R33 in place now that my hearing is mostly back to normal after the cold. The sound is definitely less forward, but perhaps it is now too polite. I may try a slightly larger resistor value next to see how I like that instead.
 
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