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KGSSHV Carbon Build Thread

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Having embarked on a KGSSHV Carbon build, I have one question that I can’t find a full answer to on the forum. 

Because I have both balanced and unbalanced sources, I will be including input switching on the build. For the SE input the simplest option would be to merely connect the SE ground to the -ve balanced input and connect to the source with a pseudo balanced cable.Another option would be to add a couple of input transformers, which would apparently improve noise and hum rejection. The SE cables from the source are by necessity 17’ long so my guess is that the transformer option is likely the preferable option, would anyone have experience hooking up a SE input using both methods and care to comment?

BTW thanks to the folks who have helped with advice and sourcing components.

Regards

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gepardcv's solution is the easiest and cheapest, but if you really want RCA connectors also, you could wire a DPDT switch in (or 4PDT if you also want to disconnect the signal) in to short the -signal to ground when using the RCAs. The amp board itself will handle the rest.

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Some success, some failure! - One of the GRHV boards behaved, output slightly high at -408v as I hadn't matched resistors (will be swapped out once the batch of resistors arrive). The other GRHV voltage ramped up as I increased the variac to around 80v then shut down. Checked all solder joints, tried again and pop. Examination showed that I had accidentally placed a 50 ohm resistor in place of the correct 50k that is adjacent to the LT1021. Not sure what else may have blown, no visible damage, but as a precaution the plan is to put in a new LT1021 as well as replace the errant resistor, then power up again slowly using the Variac. I have spares for all the other semiconductors so those would be the next items to swap out if needed.

Being very cautious as although I have but together quite a few amps, this is my first foray into HV stuff. (even using 1200v gloves). 

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I would try replacing the correct value resistor first and test the board again before swapping out the LT1021.

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Fired the board up again via the variac, got to around 350v this time and the 50k resistor at the same location as the original 50ohm one smoked. Checked the resistor type and its the correct 1/2W KOA Speer. I am guessing that the original failure must have taken out nearby transistors, perhaps the KSC5026 or BC557 pairs or the 10M90.

 

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You can try do a simple diode test of those transistors and compare the results between the two boards to see if one or more are out of ordinary. 

Make sure the capacitors are completely drained before you perform the tests though. 

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Thank you mwl. Pulled the transistors and a couple do show dead shorts and quite different values from pristine versions. I am pretty sure which items are toast and will replace this week. Hopefully that will get the build back on track. Currently checking the KGCCHV Carbon boards to make certain all the resistor values are correct.

Being very cautious with the board and checking that all caps are fully drained before working on them  - thanks for reinforcing the warning!

 

 

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Sorry guys, another question. Using the LV supply to adjust the current by measuring the drain on the SIC's works, however I have an inbalance between the two devices, adjusting one for 18mA results in the other showing 16mA. Is this normal? (both boards show the same response).

Did fix the HV supply by changing out all the semiconductors that could have been affected by the initial 50k resistor error. Also matched resistors for the voltage set and 408v and -407v

 

Regards

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Hmm, I am confused by your question. What do you mean by “using the LV supply to adjust the current”?

The bias current of the amp is set by the CCS. You use the trim pots in the CCS while measuring the voltage drop of the test points to set the bias current. 

Glad to know that you fixed the HV supply.

Edited by mwl168

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9 hours ago, MartinC700 said:

Sorry guys, another question. Using the LV supply to adjust the current by measuring the drain on the SIC's works, however I have an inbalance between the two devices, adjusting one for 18mA results in the other showing 16mA. Is this normal? (both boards show the same response).

Normal or not – I don’t know. You apply LV cross the current course? This is an old post of mine when I checked current sources of a Carbon.

On 3/29/2017 at 7:13 PM, JoaMat said:

I’ve used the method described by Soren (link here) for a quite a while. Found it reliable and easy to use and it’s an advantage not to do the measurements on a live amplifier.

While I did this on my latest board I got some funny readings on one current source. Increasing the voltage (I use lab power supplies) the current increased to a much higher value then expected. It turned out that when increasing voltage above some 15 volts the other current source start to provide current. At about 70 V it provided full current. Didn’t understand how this could happen unless I did something wrong. Checked my board layout for more errors, changing components etc.etc., but I didn’t found anything explaining this issue. Finally, after two days of investigations, I decided to just power it up and hoping for the best. OK flipped feedbacks. When corrected the board worked all right.

I still don't understand....

Edited by JoaMat

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Ok. This is making more sense now.

@MartinC700 : It is a good idea if you pre-adjust the CCS using what JoaMat and steefdebrijn described, there are other variables will come into play when you actually power up the amp. You still need to fine-tune the CCS once you actually power up the amp and allow it to warm up. 

Again, be careful when you make adjustments with the amp on. These HV applications are not forgiving and it’s easy to accidentally short something or get electrocuted. Many of us had that unpleasant experience, myself included.

Edited by mwl168

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Now at a wiring up stage  and one  thing confuses me, and that is the grounding scheme. Numerous versions seem to be posted (two examples attached). Using the GRHV boards with the LV supply on one, there will be an additional ground to I presume the star ground.

Does anyone have an opinion as the best grounding scheme?

Regards

Wiring Example 2.JPG

wiring example.jpg

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Pretty sure you'll have major noise problems with the handwritten scheme.

The other scheme is, I believe, @johnwmclean's, and it works in my experience, with just one change: I do not connect the pot grounds to anything. The diagram suggests connecting the pot to the star ground, and I found that causes noise. With regard to split PSU boards: yes, run each one to the star ground. Connect each amp board to whichever PSU board is closer. Also: some XLR sockets have extra pins connected to their bodies specifically to make pin 1 to chassis connections easier.

Don't forget to file off anodizing at the star ground, as noted in that diagram. I also file off some anodizing on the (invisible) inside of every panel of the case to improve continuity everywhere, and always double-check continuity between all panels and earth.

When you have finished adjusting the amp and like its balance and offset numbers, connect a headphone while not playing any audio. Spin the volume pot. With correct wiring, correct adjustment, and a good volume pot, you should have dead silence across its entire range.

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Thanks for the info, but surely the pot ground must be connected to ground somewhere otherwise it is just acting as a resistor in series with the input?

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I posted a question about this on the advice thread two years ago, and here’s a helpful reply: 

 

Edited by gepardcv

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Hi,

I am new to this forum and I would need help regarding the power supply for KGSSHV Carbon.

I have built the positive and negative power supply on single boards version 1.8. The positive supply runs without any issue but the negative does not start. I spent a few days on this already and I am not sure what to try anymore.

The problem is that the output voltage sits at 18.56 V and remains there with only 0.46 V across the 20k sensing resitor. Both current sources are running since I am measuring 10 V on LT1021-10 and 12 V across the zener diode. The problem seems to be related to STN9360 and the circuit around it. The emiter to base voltage is approx. 8.56 V with base being positive. The input current source provides approx. 0.76 mA (checked with 20 V DC source, R1=2.8k as on the silk screen). Drain of the SiC MOSFET sees full 460 V as it should but the Gate Source voltage is only 2.58 V.

I am not sure what to check anymore. Maybe someone has an idea where the problem could be? I replaced almost all the semiconductors in the meantime but nothing helped. The strange thing is that the positive supply is populated in exactly the same way (same component values) and it runs without any issue from the beginning.

Regards,

Martin

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Post a clear picture of the board will help to diagnose the problem.

Did you check and make sure you have the correct resistors populated? Maybe start with the current limit resistor and the ones that determine the output voltage?

Edited by mwl168

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7 hours ago, Superberg said:

Hi,

I am new to this forum and I would need help regarding the power supply for KGSSHV Carbon.

I have built the positive and negative power supply on single boards version 1.8. The positive supply runs without any issue but the negative does not start. I spent a few days on this already and I am not sure what to try anymore.

The problem is that the output voltage sits at 18.56 V and remains there with only 0.46 V across the 20k sensing resitor. Both current sources are running since I am measuring 10 V on LT1021-10 and 12 V across the zener diode. The problem seems to be related to STN9360 and the circuit around it. The emiter to base voltage is approx. 8.56 V with base being positive. The input current source provides approx. 0.76 mA (checked with 20 V DC source, R1=2.8k as on the silk screen). Drain of the SiC MOSFET sees full 460 V as it should but the Gate Source voltage is only 2.58 V.

I am not sure what to check anymore. Maybe someone has an idea where the problem could be? I replaced almost all the semiconductors in the meantime but nothing helped. The strange thing is that the positive supply is populated in exactly the same way (same component values) and it runs without any issue from the beginning.

Regards,

Martin

did you populate the CPC1117 ...either remove the CPC1117  or applie +2V across the pins
If you have ~18V on the output, with the CPC1117 populated you probably have to reduce the cascoded CCS current....are you using 2k8 or 4k7?

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Hi,

Yes, I did populate the CPC since at the beginning I didn't have have enough time to think what was what in the circuit.

I am using 2k8 resistor since that was on the solder mask! I also tried with 4k7 at some point but it didn't seem to help. What would be your advice? Remove the CPC change the resistor? To what value?

It is very strange though that the positive supply works and it is built in exactly the same way!

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Either remove the CPC if you are not gonna use it, or apply +2V across 
As said, if you have +18V with cpc1117 shorting, the cascoded CSS most likely needs to be reduced, 4k7 usually works 

48 minutes ago, Superberg said:

It is very strange though that the positive supply works and it is built in exactly the same way!

bad soldering?

Edited by sorenb

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