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GB for DC coupled B1 buffer with shunt PSUs

go to Borbely's jFET 2part article.
It shows how to measure parameters of jFETs and lots of information on how to use them.
 

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Old style Mesmerize B-1 buffer

This pcb and bits came with some Hacker items I wanted quite a few years ago. I rediscovered recently it and decided to try to put it together. I don't know if this thread is still active.



I have added the LEDs which are matched at 1.82V with 5mA. So that adds up to 9.1V, good enough?
The K170s that came with the pcb are grouped as 4 "tight for B1" (I assume this is the middle section) and 6 for PSU (presumably the end with the LEDs).
I have IRFP9140NPBF and IRFP140NPBF pairs (which I think are OK on the old BOM) a volume control and selector.



Dropbox - Mes with LEDs.JPG


I now need to spend some money (maybe £50) to complete this and the question is am I on track to make this worthwhile? I need a suitable transformer (12v 50VA), decent capacitors and resistors. I only need a single input so I could omit the 6 relays on the right hand end and the selector (I would need to understand how to do this).


Blore Bowron A3074 Chassis Transformer 2x115V 50VA 12V+12V | Rapid Online



Should I proceed? Or can someone point me in the right direction.


Ralph
 
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Proceed, its 100% worthwhile. The group of four tightly selected for Idss Jfets go to the middle section. Wire incoming signal directly under the Alps pot to its input pins when skipping the selection section. Are you going to use sinks or the chassis floor for the Mosfets? If yes, you can use 2x18R (2W) where it says 2x68R (1W). That move will hot-rod it to near 200mA PSU bias. Better sound. You can also use 15V-0-15V transformer for little better PSU performance when on sinks or chassis for heat exchange. Don't forget to insulate the Mosfets with pads of silicon or mica if they are going to be attached for sinking. If you will not sink them stick to 12V-0-12V and 68R current setting resistors so the Mosfets will generate lower heat that they can self support.
 
DCB1 "10 Years After" Black "Hot Rod"

Dropbox - Mes with LEDs.JPG

I am almost at the completion and test stage, I think I know how to test the PSU and DC offset. Also I can see how to wire the volume pot and just a single input. I want to check a few items before I proceed. I will be using 15v-0-15v and I have installed x4 18R 2W as specified.
1. BOM specifies 1R x2 in the centre of the board near the psu test points. On the black board they are 10R. Am I right to have used 1R?
2. I have used Mosfet alternatives specified on the old BOM. IRFP140NPBF and IRFP9140NPBF. Are these OK on my "Hot Rod"?
3. The pink heatsink pads, do I use these dry or with paste?

I have really enjoyed this and learned a lot, hope it works.
Thanks to all. Ralph
 
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1. Yes you are right to have used 1R. Those are Vref current test point resistors that I settled to 1R in the latest boards. Each mV across 1 Ohm indicates 1 mA passes through it. It takes a reliable DMM in mV range but its straightforward to read.
2. Yes they are great alternatives.
3. Dry

Good luck and let us know
 
The testing stage

This is where I am now, looks good, but may not be. I need help. Pictures:-
Dropbox - P1000572.JPG
Dropbox - P1000573.JPG
Dropbox - P1000574.JPG

Lights up fine, and a click from relay, one side slightly brighter as expected.
+10.25V & -9.75V seems fine from the PSU.
Output DC offset is R 29-30mV & L 39-40mV which is not OK.
1. Any ideas? I used the 200mV range on my meter (which is old and cheap so may not be reliable?) The volume control is set at minimum.
2. Heatsink is warm 18x14cm 0.5cm thick aluminum plate. Is this enough? I am also unsure how tight the Mosfets should be screwed down to it.
3. Solutions and suggestions need to be pretty basic, rather than numbering a resistor to test the position and value would help.
Thanks in advance. Ralph
 
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Your realization looks really good. Very nice pictures. Your 'heatsinks' are big enough. It's very possible that your four in line JFETS (signal pairs) aren't well matched for IDSS. That can create unacceptable offset. (https://www.diyaudio.com/forums/solid-state/23546-matching-jfets.html#post274893)
What to do is remove them and verify their IDSS one by one. You would need 0.1mA matches by pair for a very good result, but 0.2mA could be also acceptable. If they are hopelessly unmatched ask the good man Tea-Bag if he can help you with a new well selected quad for those.

P.S.
Regarding your maybe unreliable meter readings: A very cheap but accurate for mV small multimeter that I have between others and I can recommend is Aneng AN8008. Easily found online on ebay etc. Not rugged or safe for electricians work like a Fluke, but it will solve inaccuracy issues for low voltage electronics work. Its auto ranging with back light, so more modern and practical than the yellow one in your second picture. Useful at least even if your old one proves that it wasn't reading that wrong after all by comparison.
 
I am waiting for my new multimeter to arrive. I am then going to check the board carefully before taking off the jfets to test them. Even with heatsinking, solder sucker and braid I am going to risk damage to the components and board I think.
Can I just check to be sure I am wiring this transformer for a centre tap 15v-0-15v correctly?
Dropbox - Photo Dec 07, 09 25 33.jpg
Finally should I continue this thread or is there another I should be using?
Thanks for the advice. Ralph
 
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P.S. Your board is very old issue, not double sided. Did you add jumper wires where white lines are showing for? A long line in the vicinity of AC input and a short one at the end of PSU in the vicinity of two 220R resistors. Double sided boards actually have copper on the top side under those lines. But not the single sided ones.
 
previously I was using dcb1 to drive FW clone and behringer nu3000dsp, now i got another step into 4 ways with additional minidsp 2x4HD and LXmini board.

if my memory still good, behringer has around 20+ kOhm input impedance, while i dont find any details on minidsp but maybe the same around 20's because it must be using opamp input buffer. With 25k LXmini potentio, it means dcb1 might see around 5k load.

will this be OK? i still have alot of SK170 pairs hence I can build another 4 buffer but no space on my chassis :D
 

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I wouldn't recommend below 10K loading due to the limited current a small JFET like the 2SK170 can provide. The load limit mostly depends on the interconnect wires length and their per meter (or foot) capacitance spec. The higher the frequency and the higher the signal amplitude more current is needed to charge the load capacitance faster. Fortunately in music the HF energy is usually lower than in the power range 200Hz-800Hz. Bottom line is, if you won't hear HF smear or distortion in your setup its still sufficient. Use low capacitance cables in your situation.

If there were no interconnects and no other load capacitance, or much available drive current, the minimum would be 2.5k loading due to the DCB1's 250Ω Zout (satisfying 10xZo loading rule of thumb).
 
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DCB1 has around 30Ω Zo from the JFET itself + 220Ω from the output resistor (anti-oscillation guard). The resistor's value can be lowered when there is fixed low capacitance loading. If square wave is still good on the oscilloscope without ringing edges and no whistle noises are born in the listening then the lowered value is enough.