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Old 20th June 2013, 01:05 PM   #1561
sng001 is offline sng001  Australia
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Quote:
Originally Posted by Siberia View Post
I've got only one DMM at hand.

Have anyone succeded biasing it with one instrument? Is it possbile?
Hi Siberia,

I only used one multimeter to adjust the Bias of VSSA, the first channel was a bit hits and misses.

The current thru the VAS increase with heat sink temperature, in my case it was 110mV (cold) to 165mV (hot heat sink).

I used the first channel as a reference,
1. Adjust TR1, measure the effective resistance of R9 to get 200R.
2. Adjust TR2, measure the effective resistance of R10 to get 200R.
3. Adjust TR3 so that the effective resistance about TR3 is about 530R.
4. Connect the VSSA to power supply, I also connect a light bulb in series to the primary of the transformer to limit the AC current - the light bulb should give a very dim red glow only.
5. Measure the voltage across TP1 and TP2, turn TR1 clockwise to increase the voltage - target is about 120mV.
6. Measure the output offset, connect the positive lead to output quik-on, Adjust TR2 until you get close to zero offset - turn TR2 clockwise to make the output more positive.
7. Disconnect the AC and light bulb.
8. If your meter can measure mA, connect the meter in series with the positive supply & the quik-on, turn TR3 clockwise to increase the MOSFET bias - the target is 182mA. Use LC's procedure as detailed in his supplement PDF in post #1547 if you cannot measure mA.

The VAS current will increase with heat sink temperature, so measure the voltage across TP1 & TP2 after the Heatsink is warmed up - I measure 165mV for HS temp ~ 55C.
Voltage across R7 = 15.5V (ccs current = 2.28mA)
Voltage across R8 = 16.0V (ccs current = 2.35mA)

LC may add with his comment for acceptable range for VAS and ccs current.

Happy building & enjoy the result.
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Old 20th June 2013, 01:27 PM   #1562
Siberia is offline Siberia  Lithuania
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Aaaah, that might be very useful when I get home tonight. Huge thank you lot for the write up, sng001.
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Old 20th June 2013, 09:57 PM   #1563
Rick G is offline Rick G  United States
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Default 2 Done

Got my 2 built. Now just need the rest of it.!
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Old 20th June 2013, 10:15 PM   #1564
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by sng001 View Post
...it was 110mV (cold) to 165mV (hot heat sink).
if the heatsink gets really hot, I say you might have a problem..IMO

should only get very hot if you drive it really hard
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Old 21st June 2013, 02:21 AM   #1565
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Quote:
Originally Posted by Lazy Cat View Post
Guys, today's capacitors topic was the reason not to include capacitors in VSSA PCB set. Impressions which brand to use are so subjective that I simply left the decision to each individual.
Yes, those long hours to listen and compare, and feel a solution sound better with this record, and worse with this other one :-)
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Old 21st June 2013, 02:26 AM   #1566
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Quote:
Originally Posted by Joachim Gerhard View Post
When i get it working i will also try this :
Augmented Feedback Error Correction (AFEC)
Good luck :-)
Warning: get the fastest OPA you can find.
I am curious to see if your conclusion will correlate mines.
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Old 21st June 2013, 02:34 AM   #1567
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Quote:
Originally Posted by AKN View Post
Sorry, I cannot beleave this is true. Schematic has been changed and we are not informed.
With listening time, you can try several changes and find some improvements. This don't mean the original schematic was wrong in any way. You can consider this as 'tuning'.
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Old 21st June 2013, 02:58 AM   #1568
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Thanks, this AFEC project has zero priority but i found it interesting.
Bonsai himself sais that this technology can affect the stability of the amp and more testing is needed.
I know that ultra low distortion is not necessary to make a great sounding amp.
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Old 21st June 2013, 03:16 AM   #1569
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Old 21st June 2013, 09:00 AM   #1570
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Quote:
Originally Posted by Lazy Cat View Post
Schematic connections were not changed, only values of installed gate resistors are zero, again not by mistake.
The very fact that circuit can be stable without gate resistors speaks a lot about quality of LC's circuit and pcb design. Jumper is far more linear than any resistor, including bulk metal foil. I think group buyers should be more than satisfied that instead of 100R and 68R gate resistors they have zero ohm jumpers on the board. This project is absolute success.
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