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Old 8th July 2013, 08:22 PM   #221
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Join Date: Aug 2005
I finally got it working. Had some issues with grounding, as my relay based attenuator is not built dual mono. Everything is sorted now.
The same feeling as with Paradise phono preamplifier: In the beginning, the sound is quite mediocre, in one hour is definitely improved and after 2 hours is really something on the top.
i have the transformers, rectifiers and first stage of filtering in a separate box. A special cable is built to carry Two differential sets of voltages and the control voltage. i have the second set of filter caps in the main box, followed by Kubota regulators and then one ebay kit board per channel.
Active devices are 10mA Idss dual fets (2SJ109, 2SK389) followed by matched pairs of 2SK214/2SJ77 in the output stage. The cap multipliers and the active devices in the Kubota regulators are matched 2SJ313/2SK2013.
Everything runs quite hot, but bearable. The case is huge and vented and it seems to have enough passive cooling power.
Many thanks to John, for publishing the correct schematic. i understand that the transistors used in the past were different.
Cheers!
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Old 9th July 2013, 10:43 PM   #222
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Well, it's delightful! After one more day it really shows itself like a great preamp! Maybe the idea of cascoding through us into the blowtorch area, but I think it's just fabulous.
Anyway, JKoch, if you will design the whole thing again, please use the case as a heatsink, for devices sake. You will have a great preamp even in the original state, if you have Toshiba's and Hitachi's in original format. Probably you can get better results with cascoding the Hitachi's and using VN's, but this is a new experimental field, if you dare to approach I didn't as I had the PCB and no VN's, but even this way is a success!

Cheers!
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Old 13th July 2013, 08:42 PM   #223
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Fellows!
It's not a joke with thermal dissipation!! Two 2SK2013 just went to Valhalla after few days of continuous running. I had to reopen the case and drill some holes around the JC80 PCBs. i reduced also the Ids through the final MOSFETs to 80 mA (which is the lower limit regarding symmetrical voltages and also considering the sound). I measured the temperature after replacing the dead MOSFETS through the vents in the upper side of the case, just at the 2SK214 case, and after 2 hours, the temperature is 63 degrees Celsius, which I guess will be safe. Probably I can raise the Ids with few mA, but the sound is the same as with 100mA, and I would not take any chance to increase it further.
As some people asked me in PM's to take some pictures of my build, I did it now. Please check the attachments.
The blue pcbs are JC80 line amps, the red ones are Kubota regulators and the green one is the relay based atternuator. I moved the relays for source selection on a test board because the original has the unused inputs left open, while I wanted to have them connected to ground. Also, the power button of the case it's used now as a "Tape Monitor". You can also see the preamp integrated in my home-built audio furniture that's using cheap IKEA parts
Cheers!
Attached Images
File Type: jpg DSC_4362.jpg (661.7 KB, 346 views)
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File Type: jpg DSC_4375.jpg (340.0 KB, 82 views)
File Type: jpg DSC_4376.jpg (283.0 KB, 86 views)
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Old 15th July 2013, 04:14 AM   #224
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Hi metallicus69,

Very nice work!

Very happy you like it. I am very satified of mine as well. I use the AMB Delta 1 and Delta 2 for source selector and volume control.

Hope more will join and try this build, it is really worth the efforts!

Ciao!
Do
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PREAMPs : AMB α10 + JC-80 clone | Balanced DCB1 | LDR3x | AMPs: AMB β24 | Aleph 5 | FirstOne | ML-2 | TSSA | Telos 390 Clone | My_Ref FE
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Old 15th July 2013, 05:22 AM   #225
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Thanks Do!
I used the attenuator from diygene.com which matches the case from audiophonics. There is no way I can build such cases with my home equipment so I prefer ready-made solutions.
I also hope more will join, but that is why I posted details and my notes about thermal issues. This really should me regarded as a small power amp rather than a preamp when considering power dissipation of the devices.
Cheers!
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Old 17th July 2013, 10:09 PM   #226
JKoch is offline JKoch  Poland
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Hi metallicus69

Great work. The simple preamplifier looks very complicated. You packaged many circuits in this case. It looks very nice. Congratulations!
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Old 18th July 2013, 08:11 AM   #227
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Thanks JKoch! There are not too many circuits;JC-80 (one for each channel) Kubota regulators (one for each channel) - these are the heat sources - one relay based attenuator (unfortunately not dual mono) with relay based source selection and the control module for this, mounted behind the front plate, that provides also the convenience of remote control.
What I also did was moving the selection of sources relays near the inputs and modifying the switching circuit to ground every unused input. Probably an overkill, but in my opinion this is the way input selection should be done.
All the primary sources (+/-40V for each Kubota and +15V for the atenuators) are located in a separate box to be able to cope with the space and electromagnetic field issues. It could not be any simpler considering my goals (remote control for example).

How is your design going? Did you confirm that your MOSFETS are genuine? Have you tried/will you try the cascoding of the outputs proposed by John? I am very interested in your results too

Cheers!
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Old 18th July 2013, 06:01 PM   #228
JKoch is offline JKoch  Poland
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metallicus69

In any way there are a lot of circuits. As I see you separated main PSU. This is a great idea. In my project all trafos and I stage of audio PSU will be in separate case (and PSU for control circuits too). It seems that main task of the JC-80 is heating the case. :-)

I am still working on the JC-80 circuits. There will be both channels and output sockets (XLR, RCA and BNC) on one pcb. I will plan to use 2SK214/2SJ76 without cascoding. I am not an electronic so such circuits would be to difficult for me. The JC-80 pcb will take few days else. I will publish all schematics in this thread.

I am not experienced in such circuits like JC-80 and I do not know how to measure bias, in where point? Could you help me, please.

My MOSFETs: I checked them using diode tester only (see the web: Testing a MosFet). Maybe tomorrow or on Monday I will try to check them according to Nelson Pass (see attachment). The first simply test showed that they have proper pins like Hitachi (except one N-type MOSFET. It seems be damaged. If the will be genuine after the Nelson Pass test I will buy more them to chose best matched pairs.

Regards
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File Type: pdf Matching MOSFETs (Nelson Pass).pdf (109.1 KB, 44 views)
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Old 18th July 2013, 07:34 PM   #229
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Hi JKoch,

On each MOSFET there is a source resistor, original 20Ohms, In my case still an acceptable 25 Ohm. You measure the voltage drop over it while adjusting the 200Ohms BIAS trimmer and apply the OHM's law. 2,5v/25Ohms = 100mA for example. If you correctly match the transistors and passive components, you will see very close values in all 4 source resistors. In my case I have 80mA and works like a charm. If you go higher it's better, as long as the heat has a way to run out . No need to go higher than 200mA I suppose. 70mA was too low in my circuit.

Good luck and keep us posted!
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Old 18th July 2013, 08:04 PM   #230
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Nice work guys!

Keep up the good work!

I really want more people to try this build, it sounds really good.

Please also check this thread which we first started. I believe JC added some info in there as well.
JC-80 eBay PCBs

Ciao!
Do
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PREAMPs : AMB α10 + JC-80 clone | Balanced DCB1 | LDR3x | AMPs: AMB β24 | Aleph 5 | FirstOne | ML-2 | TSSA | Telos 390 Clone | My_Ref FE
DACs : TPA Buffalo II + OPC's NTD1 I/V | TPA OPUS + IVY III | Moon 380D DSD Speakers : Jim Holtz Statements | Astasia SD | Tubasson Mark 2

Last edited by pinnocchio; 18th July 2013 at 08:06 PM.
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