JC-80 eBay PCBs & Power Train

[ticknpop]

He's in Los Angeles today and Montreal next weekend so no DIY for a while.

[Wolfsin]

The 33mH common mode choke arrived and I built the loading jig today. The jig provides a dummy load after the regulator (one 10 watt 150r resistor from each rail to ground). At 30vDC this allows 200ma to flow. Without load the voltage across the caps soars. Even with 1k bleeder resistors my caps reach 45v so hot plugging a loaded regulator might not be pretty.

The dropout for the Kubota is 4.9-5v. I have not yet assembled a tracking prereg but already know to expect a dropout >6vDC. While I want to keep as much dissipated heat as possible in the external enclosure, I also want to be able to swap regulators easily without altering the inductance. That means I want the loaded voltage at the output of the rectifier and caps board to be at least 30v + 6.5v (the dropout of the regulator with the larger dropout). Leaving a little extra will allow for error at a cost of a little more heat dissipation from the regulator in the same case as the preamp.

[Wolfsin]

1 r-core 2x37vAC secondary
8 FFP20UP20DN Soft recovery rectifiers
2 thermistors
1 39mH CM choke your inductance TBD
6 2200ufd 50v
4 6800ufd 50v
2 1k 5w bleeder resistor
1 Dual Rail Linear Power Supply PCB
1 fuse 250ma

Notes:
1) Regulator out voltage depends on regulator dropout; select transformer first then similate TBD's. if 30v secondaries, CM choke not possible
2) TBDs include choke inductance, capacitor voltage
3) pcb comes with add'l flex & written docs
4) fuse each channel separately
5) ebay advanced search 'by seller' audiosector for pcb and ssrecord for NOS talema common mode chokes

[hags]

Is there any schematic available for the boards that are being sold on ebay? I have searched but have found nothing for either the original or the copy.

I would like to take a peek at the schematic, if just a partial.

[Wolfsin]

Originally Posted by hags

Is there any schematic available for the boards that are being sold on ebay? I have searched but have found nothing for either the original or the copy.

If you are talking about the Kubota regulator, no. In the case of the JC-80, a BOMless partial is toward the end of *Èý×êÐÅÓþ*Dennesen JC-80ȫƽºâÇ°¼¶¶Æ½ðPCB°åÿÉùµÀ-ÌÔ±¦Íø. The schematic of the rectifier pcb is distributed in hard copy with that board. Which one interests you?

[hags]

Originally Posted by Wolfsin

If you are talking about the Kubota regulator, no. In the case of the JC-80, a BOMless partial is toward the end of *Èý×êÐÅÓþ*Dennesen JC-80ȫƽºâÇ°¼¶¶Æ½ðPCB°åÿÉùµÀ-ÌÔ±¦Íø. The schematic of the rectifier pcb is distributed in hard copy with that board. Which one interests you?

Oh, sorry about that. The JC-80 schematic is what I was looking for.

I have schematics and boards for a couple of different versions of the Kubota regulator.

I also have a couple of Borbely super shunts in use. It seems that regardless of what I monkey with they are the defacto reference for uses under 200ma.

The JC-80 looks like a jfet complementary, differential to mosfet source follower with dc servo??? Hard to tell.

[Wolfsin]

Originally Posted by hags

The JC-80 looks like a jfet complementary, differential to mosfet source follower with dc servo???

Certainly there are complementary jfets, it is differential, the outputs are mosfets, and it employs servo circuitry. John Curl calls it a transconductance amplifier. There are two contributors who've completed and are listening and appreciating thier units but since neither has responded to your question I am doing my best.

[pinnocchio]

For the schematics, it is another story... It is a John Curl IP and the only way you can get them is if you ask John or buy the PCBs from the Ebay resellers.

We all have the Chinese PCBs and schematics that are distributed with them but without author approval I do not feel comfortable handing them out...

Hope you understand

Thanks
Do

[Wolfsin]

Thanks Do. I think hags got what he needed from the schematic posted on the web but maybe not. I agree we should not divulge more than is already 'out there'.

The eBay cm choke vendor agreed to a matched pair of 39mH cm chokes and Wolfsin izAbout to test and measure incircuit, under load. I finished the long cut required along the edge of the main chassis mount. As soon as I invent a reliable banana holder I'll look for a peel to slip on!

[pinnocchio]

Here's the latest news from my friend's PSpice/Orcad simulation...

Setting the gain

"If you reduce the value of R17, R18, R19 and R20 to 330 ohms,
it will drop the gain to about 5.7x (15.2dB). This would
cause the overall bias to increase somewhat, so you'll need
to reduce the R41 trimpot adjustment a bit to get back to
230mA current draw."

Using IRF610/IRF9610

"Running at the lower gain also increases the frequency response
to -3dB at almost 15MHz. This is in part also due to the
IRF610/9610's lower gate capacitance compared to IRF510/9510.

Anyway, using the IRF610/9610 does not otherwise change the
operating points or the gain of the amp."

Asking if in simulation, IRF610/9610 was worse than IRF510/9510

"No, IRF610/9610 is not worse than IRF510/9510. It has lower gate
capacitance so the bandwidth is wider. Reducing the gain also
makes the bandwidth wider because an amp usually has a fairly
constant gain-bandwidth product, that is, as you decrease the
gain, the bandwidth naturally increases.

Having a wide-band amp is good, 2MHz is superb, but 15MHz is
not 7.5x better. We could only hear signals to 20KHz (if
that much), but usually 10x to 100x that frequency is more than
enough to ensure optimum performance within the audio band,
such as minimum phase shift and distortion, etc. Too much
bandwidth may make the amp act as a radio receiver in certain
circumstances.

Decreasing the gain should not degrade the sound. If anything,
it should lower the noise floor significantly."

The 2MHz was when running the simulation with 15.3x gain and IRF510/IRF9510...

I will follow up with some graphs shortly!

Thanks
Do