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Amplifier Troubleshooting.
Amplifier Troubleshooting.
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Old 15th May 2008, 01:01 AM   #21
lepomis is offline lepomis  United States
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Quote:
Originally posted by 5th element


Interesting as the schematic I posted clearly says 6.8k? You did click to enlarge it right? Otherwise the numbers and stuff get blurred, the effect depending on your screen resolution.

I'll take your word for it, I did not want to register. I need to find my glasses I guess I wish I could help you more with your troubles, but I do not know enough about trouble shooting circuits to build that amplifier.
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Old 15th May 2008, 02:34 AM   #22
5th element is offline 5th element  United Kingdom
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Quote:
Originally posted by lepomis



I'll take your word for it, I did not want to register. I need to find my glasses I guess I wish I could help you more with your troubles, but I do not know enough about trouble shooting circuits to build that amplifier.
You need to register to view the image at full size? That's the last time I use them >.< When I load through the page, at the top of the image there is a yellow border, on the far left there is some text that reads "Original Image Size." You don't get that without registering?
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Old 15th May 2008, 06:24 AM   #23
Mooly is offline Mooly  United Kingdom
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Amplifier Troubleshooting.
Re post 12, IRF FET's will never work in this design. Two main problems, 1 Thermal Runaway. 2. They need a much higher Vgs-hence no bias current.
Offset voltages in the range you are getting suggest poor circuit design IMO rather than a fault in construction. Offsets of " up to quarter of a volt " were considered acceptable at one time, and to lower it the circuit had to be tweaked on an individual basis, usually by swapping transistors around.
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Old 15th May 2008, 07:47 AM   #24
AndrewT is offline AndrewT  Scotland
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but those exicon FETs are Laterals.


5th,
have you posted one schematic and built something different?
You need a slap, not help!!!!!
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Old 15th May 2008, 08:07 AM   #25
Greg Erskine is offline Greg Erskine  Australia
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Amplifier Troubleshooting.
Hi 5th element,

Has someone mentioned R2 looks to be the the wrong value?

I think it should be 47k not 470k, just a guess.

EDIT: I just notice Randy has used 470k in a couple of his schematics, so it might be right afterall.

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Old 15th May 2008, 08:17 AM   #26
Greg Erskine is offline Greg Erskine  Australia
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Amplifier Troubleshooting.
Quote:
Originally posted by AndrewT
but those exicon FETs are Laterals.

5th,
have you posted one schematic and built something different?
You need a slap, not help!!!!!
The original Slone schematic uses 2SJ162 /2SK1058, so I think 5th just grabbed the wrong models for the simulation, his schematic (and build?) are using the right substitutes.

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Old 15th May 2008, 08:17 AM   #27
AndrewT is offline AndrewT  Scotland
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yes, post10.
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Old 15th May 2008, 08:24 AM   #28
Greg Erskine is offline Greg Erskine  Australia
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Amplifier Troubleshooting.
Quote:
Originally posted by AndrewT
yes, post10.
Arh, I did search (Ctrl F) for R2 but couldn't find it. I see why.
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Old 15th May 2008, 02:01 PM   #29
5th element is offline 5th element  United Kingdom
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Yes I am unable to import the correct spice models into Circuitmaker so I chose something in the available parts. But now, to simplify matters, I have removed the output stage. I have built the design using the Exicon FETs.

I tried altering the value of the 470k resistor

Quote:
Has someone mentioned R2 looks to be the the wrong value?
(yes Andrew mentioned the time constant of the NFB and the input filter were miles apart) and tried 47k. Result? I've got less then 2mV offset in both amps. BUT they still clip prematurely. Now I have tried the adjustments Slone recommends in the book for altering the protection circuitry (increasing R24/25), but it has very little effect.

Do you think that the values in the protection circuitry are decent? Or is something wrong here?

I also tried deactivating the protection circuitry, rail per rail, both were activating prematurely this time.
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Old 15th May 2008, 03:23 PM   #30
gain is offline gain  United States
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the way the protection circuitry appears to work is the source current flowing out of Q13 (and its compliment) drops a voltage across R26. This voltage is then applied as the input to the voltage divider consisting of R24 and R21. When the output of this voltage divider (voltage dropped across R21) hits ~0.7V, it turns on Q9 which then sucks gate drive away from Q13, limiting its source current.

if my understanding of the circuit is correct, and with the values shown, this should happen at ~2.33A source current out of Q13. this trip point should be able to be adjusted by alteration of the voltage divider R24 + R21.
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