KGSSHV Carbon Build Thread

[Rinat]

22 hours ago, Helium said:

Did anyone experience how vulnerable DN2540N5 are?

They are damaged so easily that I had to scratch my head.

E.g. desolder it from a donor known good GRHV, actually new (don't ask why, I had to get spares) and it's detected OK in circuit as N-Ch depletion mosfet.

I desolder it, test it and BAM it's no longer depletion mosfet, it turns into kind of Voltage regulator. And doesn't work of course. I spoiled 4 pcs already, despite the fact that I know how to desolder and handle components. And only one out of 5 was OK after desoldering.

Are they from the same batch?

[Helium]

On 1/26/2022 at 10:33 PM, Rinat said:

Are they from the same batch?

Yes, same batch.

Seems that Hakko 808 isn't ESD safe. Lesson learnt.

[simmconn]

I'm glad to report that both the GeneSiC G2R1000MT17D and the Microsemi MSC750SMA170B work well on the KGSSHV Carbon. The G2R1000MT17D works on the GRHV, too. The overall performance of the Carbon is stellar. It's virtually distortion-free to about 200Vrms at the output, and maintains very low distortion up till 600V!!

You want the ±450V power supply for the Carbon. With ±400V I ended up with something like the following. Still not bad but not as brilliant as the curve above. I figured the PZTA42s are not quite in their linear region because of the reduced Vce.

A word of caution is that both SiC MOSFETs are somewhat 'fragile' compared to the Cree/Wolfspeed C2M1000170D, especially the G2R1000MT17D. I killed a few when matching them on my curve tracer. I guess the Left-right switch on my curve tracer doesn't guarantee that S connects first, then G and then D. No more failures after I connected a 10V zener diode between G-S on the test fixture. An expensive lesson learned😅

Edited March 21, 2022 by simmconn
fixed first picture

[ang728]

45 minutes ago, simmconn said:

I'm glad to report that both the GeneSiC G2R1000MT17D and the Microsemi MSC750SMA170B work well on the KGSSHV Carbon. 

 

But I have different results here 

do you have other changes on the board like zener diode that KG mentioned ?

[kevin gilmore]

simmcon, first graph not shown.

are you measuring balanced? and what additional attenuator are you using?

 

[simmconn]

Here is the attenuator I used (R3 and R4 are the AP input resistance, and not part of the attenuator). The R and C are rated at 500V or higher. The trimmers are adjusted with AP analog loop-back (repeat the below till both are satisfied)
1) Amplitude vs frequency response as flat as possible up to 100kHz balanced (I was able to reach ±0.02dB).
2) Set the AP output to balanced-grounded, short the AP +/-  input to GND, one side at a time. Adjust the trimmers such that the response of the two attenuator arms are the same. I could use the common mode test feature on the AP to do this, but I figured the amount of work would probably be the same.

[simmconn]

10 hours ago, ang728 said:

But I have different results here 

do you have other changes on the board like zener diode that KG mentioned ?

I only changed the LT1021 with the cheaper LT1236-10. The latter has the same performance as the LT1021 except the long-term stability which we don't need. Oh, and I used the DN2540 in TO-92 package for lower cost. Nothing is different from the original circuit electrically.

If the output of the GRHV is low, chances are the passing element (SiC MOSFET) is not damaged. A series linear regulator such as the GRHV is probably the easiest circuit to troubleshoot. Jut need some electronics basics and a lot of patience...

[simmconn]

In a previous post I mentioned the less-than-ideal performance when Carbon is powered by a ±400V supply, and I suspected that the lower Vce on the PZTA42 is the culprit. Now it's been proven. The PZTA42 being a high voltage transistor, has a non-linear region at low Vce, as the slanted curves you can see on the upper left side. With 407V on the negative rail, the transistors on my board works at Vce=8.4V and Ic=20mA, right around the knee. The global negative feedback would have a hard time correcting that non-linearity. It also explains why some people prefer setting the Carbon at a lower current when powered with ±400V supply, as it also improves the linearity of the PZTA42, albeit to a lesser degree.

I guess Kevin chose the high voltage PZTA42 to deal with the power-on transients. I have a quick and dirty fix. Just bias the SiC MOSFET a little higher to give the PZTA42 more headroom. The SiC MOSFETs are biased by two 175k and an 20k at the gate. Reducing either 175k or increasing the 20k would do. The goal is to move the PZTA42 operating point to the right, well into the constant-current region (parallel lines).  I would use Vce=14 to 15V. Pushing it even higher would increase the power dissipation on the PZTA42, eat into the max output voltage swing and have diminishing return. What I did was to put a 260k resistor in parallel with one of the 175k resistors. YMMV because it has to do with the operating point of the PZTA42 in your circuit, the Vgs(th) of your SiC MOSFET, etc.

After the quick fix, one of the channels now measures as good as with the ±450V supply. We can see that the max output voltage is slightly less compared to with ±450V supply. The difference is subtle with the log scale, though.

Now I'm continue to work on the other channel and see if I can find something else.

[audiostar]

6 hours ago, simmconn said:

What I did was to put a 260k resistor in parallel with one of the 175k resistors. YMMV because it has to do with the operating point of the PZTA42 in your circuit, the Vgs(th) of your SiC MOSFET, etc.

Great finding and thank you for reporting! 
From what I can see the majority are running their Carbons with 400V PSUs..

[kevin gilmore]

ttc004 might now be a better replacement, different package.

[audiostar]

On 3/23/2022 at 10:43 AM, kevin gilmore said:

ttc004 might now be a better replacement, different package.

Hey Kevin, from all the alternatives, which would be your choice? Just swap the resistor, try to put in the ttc004, or swap transformer and go with 450 instead of 400V PSU? 

[kevin gilmore]

well it was designed for 450v power.  but really we are talking about only a couple db of difference. a minimum of -100db thd+noise. 

if you swap the transformer then you have to go with higher voltage rated power caps.

[audiostar]

1 hour ago, kevin gilmore said:

If you swap the transformer then you have to go with higher voltage rated power caps.

I have those already at max available rating. Would need a new transformer only. But in general yes, don't forget to check the caps.

Thanks for your insight! You are right on the very small difference and hearing that is different animal than measuring it. 

Edited March 24, 2022 by audiostar

[JoaMat]

Another option – try 25K resistors instead of the 20K and tell us if you hear any difference.

[audiostar]

4 minutes ago, JoaMat said:

Another option – try 25K resistors instead of the 20K and tell us if you hear any difference.

Yeah, 2nd violin in the right corner of the stage will get shifted 1.5° to the center and this is no good 🙂

[simmconn]

I was able to bring the other channel to the same THD+N performance. While I was at it, I tried the current production low-noise JFET from Toshiba, the 2SK209. The first 2 from the cut tape matched really well at 5.9mA Idss at Vds=10V. I'm surprised that I was able to do it without using the microscope or magnifying glass (pardon the flux residue though). I guess I no longer need to pay the hefty price to get the LSK389 or the obsolete K170s.

Regarding jacking up the bias of the SiC FET, it has its limits. The original bias resistors set the SiC FET G at 450*20/(175+175+20) = 24.3V to B- with a -450V supply. After adding 260k in parallel with the 175k Ohm resistor, I was able to set the G at 27V to B- with a -407V supply. Note that the VGSmax for the G2R1000MT17D and the C2M1000170D are 25V, and is 23V for the MSC750SMA170B. So I'd better dial it back a little. In normal operating conditions, the voltage on the 20k resistor will never apply entirely to the G-S. But it kind of tells me how intricate the original design was, with all corner cases considered.

[johnwmclean]

Great reporting Simmconn, I’ll be very interested in a follow up of where you end up after dialing back.

One channel of my latest build has little to no drift at all (no servos installed), which is odd behaviour from my experience.
After 45 minutes warmup offset and balance can be trimmed to under 1VDC.
From a cold start everything is now under 1 volt and stays that way. I’m curious as to the reason, do I need fix or leave as is? 

[JoaMat]

On 3/25/2022 at 1:08 AM, simmconn said:

...

...

Good example of how to put a 2sk209 (sot23 device) on a to92 foot. I presume you can do a similar thing with ttc004 or ksc2690 on a pzta46 foot, they have the same pad c/c distance.

[Helium]

10 hours ago, johnwmclean said:

One channel of my latest build has little to no drift at all (no servos installed), which is odd behaviour from my experience.
After 45 minutes warmup offset and balance can be trimmed to under 1VDC.
From a cold start everything is now under 1 volt and stays that way. I’m curious as to the reason, do I need fix or leave as is? 

Wait a minute, are you telling that no drift is something wrong?

From my experience there is some drift (0.5~1VDC without servo, and ~200mVDC with optoservo installed, both balance and offset), and I wish it wasn't there.

[johnwmclean]

“Normal” behaviour from my experience at a cold start offset is around 10 - 20 VDC and drifts down to under a 1VDC after 20 - 40 minutes.

So yeah I thought it’s abnormal.

[mwl168]

4 hours ago, johnwmclean said:

“Normal” behaviour from my experience at a cold start offset is around 10 - 20 VDC and drifts down to under a 1VDC after 20 - 40 minutes.

So yeah I thought it’s abnormal.

This "Normal" behavior has been my experience as well but I also found the magnitude of offset drift is closely related to the case temperature of the output device too. Does the heatsinks of your Carbon get significantly warmer when the amp warms up? 

I am theorizing that if the amp has oversized heatsink that barely gets warm the offset may drift much less. But this is just a guess. 

[simmconn]

@johnwmclean I changed the resistors so that the bias voltage across the 20 K resistor is 24.3V, the same level as the original circuit when powered at ±450V. The VGS of the SiC FETs are about 4.0V at idle (17.2mA) and Vce of the PZTA42 at 14.75V.

@JoaMat I'm building this amp for the fun of trying different alt parts. As long as both channels have close enough performance, I can confidently move on to try something else. I didn't observe power-on Vce excursions, so I guess medium-voltage medium-power NPNs such as ttc004 or ksc2690 should work fine here.

[johnwmclean]

19 hours ago, mwl168 said:

I am theorizing that if the amp has oversized heatsink that barely gets warm the offset may drift much less. But this is just a guess. 

And that makes sense. But doesn't explain in my instance why 2 identical channels on the same sized sink have very different thermal drift outcomes. (1 channel has an offset of under 1 volt and stays there and the other channel is around 13V and slowly drifts down to under a 1V in around 40min.

[audiostar]

3 hours ago, johnwmclean said:

And that makes sense. But doesn't explain in my instance why 2 identical channels on the same sized sink have very different thermal drift outcomes. (1 channel has an offset of under 1 volt and stays there and the other channel is around 13V and slowly drifts down to under a 1V in around 40min.

Must be some difference in the components somewhere between both channels. I would do a diode test on all the sands and check that all resistors have been soldered where they should be (if not measured one by one prior to soldering).

Edited March 28, 2022 by audiostar

[mwl168]

8 hours ago, johnwmclean said:

And that makes sense. But doesn't explain in my instance why 2 identical channels on the same sized sink have very different thermal drift outcomes. (1 channel has an offset of under 1 volt and stays there and the other channel is around 13V and slowly drifts down to under a 1V in around 40min.

Hmm, that is odd. Sorry, did not read carefully that only one of the two channels behaves abnormally.  Did you check if both channels have the same output bias?