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KGSSHV Carbon Build Thread

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Oh my. If this is TMoney's amp, then I heard it at a meet back in, what, 2014 I think? It's fantastic!! Got me on the road to building electrostatic amps and participating here on HC. :D

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Hi, I have a question regarding earspeaker bias voltage on kgsshvpssicfetdual2new.zip gerber (pic attached).

I understand that the bias voltage circuit is essentially a CCS into a zener string. 

I am trying to get to say , 500V, for my custom-made ear-speakers. Do I simply change the zener string to a combination that adds up to 500V? Or do I need to change anything else?

I tried 3 x 130v + 110v, and even 3 x 110v + 130v. The bias voltage (measured from test point to ground) hovers around 540v. Am I missing out something here?

By the way, I am hoping not to use the 580V + trimm pot method + capacitor and ballast resistor method, as I do not plan to use this amp with a pro-bias earspeaker.

 

Any help is welcome. Thanks!

 

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4 hours ago, dingding123 said:

I understand that the bias voltage circuit is essentially a CCS into a zener string. 

did you test whether the css is actually working?

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Hi sorenb, thanks for the suggestion. I dont think the CCS was faulty, but I replaced the 10m90s with a new one from Mouser anyway. No change in bias voltage :(

The main GRHV voltages are normal: +396V and -406V.

Does the 806k and15k resistors have anything to do with setting the bias voltage?

 

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3 hours ago, dingding123 said:

Hi sorenb, thanks for the suggestion. I dont think the CCS was faulty, but I replaced the 10m90s with a new one from Mouser anyway. No change in bias voltage :(

The main GRHV voltages are normal: +396V and -406V.

Does the 806k and15k resistors have anything to do with setting the bias voltage?

 

did you check the zener string is actually seeing ground? and the 3k is actually 3k value, and sees source and gate?

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Are the Zeners 3W or higher. They may have a higher test point if so. The current regulator supplies about 1.3mA. 

Also, are they 10% or 20%?

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Hi sorenb, yeah, the zener string is seeing the ground and the 3k resistor is alright, and sees source and gate.

Hi Kerry, thanks for the help! The zeners are 3W 5%. What do you mean by that they may have a higher test point? 

Could it be that the labeled voltage is just a nominal one at a certain factory specified current, and at 1.3mA supplied by the CCS, the zeners would give a higher voltage?

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Kerry most likely points to the zeners dynamic impedance which varies with current

1.3mA might operate the zeners too close to the knee 

 

 

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ahhhh I see. One question though. Shouldn't the zener spit out a lower voltage (rather than higher, which is my case) when the current is lower?

Anyway, I will replace the zeners with 1.5W ones.

Also, it seems tempting to change the 3k resistor to say, 1.6k for 2mA, instead of replacing the zeners. Is there any reason I should't try that?

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@dingding123

Why don't you measure the voltage drop on each zener diode in the chain, and find out which one is out of tolerance and giving you a higher voltage than it should?

You didn't tell us the part number of the zener diodes you used. Let's just use the On-semi 1N5952B (130V, 3W) as an example. The datasheet has the nominal voltage 130V, +/- 5% specified at 2.9mA: https://www.mouser.com/datasheet/2/308/1N5913B-D-1801544.pdf

image.thumb.png.af538c7dba615e0e9a6c08ab0e2517d6.png

and the curve below shows the zener voltage goes lower as the current drops (1N5952B would be somewhere between the first and the second curve from the left, following the same trend). So you would be expecting lower voltage drop at 1.3mA than you would at 2.9mA. Of course your parts may vary, but the answers should be in the datasheet of the parts you used.

Did you also verify that the current source did give you 1.3mA? Measure the voltage drop at the resistor coming out of 10M90S K terminal, and use ohm's law to calculate the current.

image.png.3af1ab3c8a9812e6328fb119c35f31cf.png

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What is the unregulated voltage to the IXCP10M90S. 

I agree you should check the current coming out of the regulator. 

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Thanks simmconn for the help!

With regards to the CCS:

The unregulated voltage into the CCS is 601v measured at the anode.

Thevoltage drop at the 3k resistor is 3.37v. Not exactly 1.3mA but somewhat close. So I think the CCS is working as designed.

With regards to the zener string:

The zeners I used are the 3ez series with 5% tolerance. Datasheet here, but they didn't publish the Iz to Vz charts https://www.digchip.com/datasheets/download_datasheet.php?id=60504&part-number=3EZ33D5

Currently the zener inplace are as follows (from left to right, ie, from CCS to ground) 110, 110, 110, 130. The voltage (from ground) reads 550, 380, 258, 127. So the voltage drop of each zener is respectively 230, 122, 130 127. Seems like only the 130v zener is working near its nominal value.

 

 

 

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Dang it haha! Thanks for pointing it out.

I'm planning to swap the zeners to some 0.5W ones, which are the only ones available at my local store. My logic is that 0.5w rating would suffice since the targeted voltage drop (around 100v) multiplied by the current (max 1.3mA -ish) would be around 0.13W.

Is my calculations sound? Or did I make a grave mistake here?

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Nice!  Congratulations!!  I am loving the perspex case.  Though for heat dissipation, making the sides of the case heat sinks and using perspex for top, bottom, front, and back would seem better.  You must have a CNC to do all of those drill  holes so perfectly?  

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Thanks @Blueman2 :) 

I would definitely consider making a chassis with heatsink sides for future projects, but for now, I have yet to source for an appropriate L-shaped aluminium thermocoupler.

The transparent case is meant for prototyping only heh. It's cut to order by a seller in China. Just send them a CAD drawing and they will CNC it for you.

I plan to transfer everything into a black chassis after I get a bigger heatsink. Currently it's running at a modest 14mA at 68 deg C.

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Had to replace a bad lt1021 but I got my psu boards up and running without much issue - the bias TP seems to be ~567 though - is this within spec for Stax? (Does Stax even have a spec ?)

Edited by nopants

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That's about 2.2% lower than the 580V Stax pro bias. The slightly lower bias is unlikely cause harm to the earspeakers.

In terms of sound, lower bias voltage would mean that the earspeakers are less sensitive. However at 2.2% lower, I'm not sure how much of a difference you may experience.

My experience is that the bias does increase a little after warming up. I assume that your measurement was done at start-up. Perhaps you might wanna measure the bias again say, 30 minutes after turning on.

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I didn't think it was an issue I was more wondering what Stax does on their side, then don't even try to regulate the bias afaik right? It's been a while so I've forgotten most of this stuff

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I powered on my Carbon without any headphones and I heard music...coming from the amplifier- specifically the potentiometer (4CP-2508S). It also wasn't attenuating at all- turns out I forgot to connect L-/+ and R-/+ common pins together and that fixed everything, but I swear I can still hear something (albeit attenuated compared to before) when nothing's plugged in. Have any of you guys encountered this? 

FWIW I'm using the version with the divider shields

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