KGSSHV CARBON

Adjusting Constant Current Source

To set the Constant Current Source (CCS), use the two current adjust trimmer pots. Measure across the test points. The calculation to set the current from the constant current sources is: .

You should measure 1V for 20mA Bias.

Note: 120Ω resistor installed instead of 182Ω to achieve 20mA bias.

Note: The trimmer will end up being ~25mΩ

Adjusting CCS using only Low voltage

Alternatively you can trim the Constant Current Sources using only the Low voltage supply. The set current can be measured either by the voltage across the test points, or using a Digital Multimeter (DMM) in mA mode (assuming the amplifier board is not connected to anything):

Connect LV+ -> AMP B+

Connect LV- -> DMM (Common)

Connect DMM.(+) -> the probe

Place the probe to the tail of the CSS (the Drain of the SiCs (middle leg), or the 100Ω resistor adjacent to the DN2540 away from the heat sink)

1. Now you can read CSS current on the meter if measuring current directly, or 
2. Measure voltage between the test points. Adjust until measurement is 1V for 20mA. (Calculation is ).

Note: one might shoot a mA lower as the current grows a bit at higher voltage and heat.

Figure 1: Connections to adjust CCS Bias with LV supply only.

Adjusting Offset and Balance

Adjusting Offset and Balance will likely have to be done simultaneously, as changing one has an effect on the other. These should be adjusted 5 minutes after switching on, and again when the amp is completely warmed up (approx. 1hr.)

Figure 2: Pro Bias Stax Socket Pinout.

Adjusting Balance

To set the balance, measure the voltage between L+ and L- on the Stax jack. The voltmeter should be on the 1000V DC scale. Adjust the left channel balance pot to as close to 0V as possible.

Adjusting Offset

To set the offset, measure the voltage between L+ on the Stax jack and GND (On the case or test point). The voltmeter should be on the 1000V DC scale. Adjust the left channel offset pot to as close to 0V as possible.

Operating with the Servo

Adjusting the amplifier without the servo, you need to adjust the offset and balance to 0V. If you wish to operate the Opto Servo, you need to adjust the Offset between +15V to +20V.

Let the amp warm up for 20min - 1hr, and adjust the balance.This will also improve the offset automatically.

Figure 3: Sennheiser HE60 and Stax Plug and Socket Pinout.

Figure 3: KGSSHV Carbon amplifier channel Schematics.

Improvement Modifications

From the Head-case forums, the chain for improving the 400V Carbon.

“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.

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.

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.

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“I dialled back the bias from 20mA to 17.2mA, installed a 562k resistor in parallel over one of the 175k resistors and achieved 24.8V bias voltage across the 20k resistor and Vce of the PZTA42 is 13.25V.”