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7th Jan 2022, 6:38 pm | #1 |
Pentode
Join Date: Dec 2021
Location: Willenhall, West Midlands, UK.
Posts: 161
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jlh 1969 voltage mismatch
quick q so i built this circuit a while back,
but got it back out to have a mess with it. if you look a my crap drawing i have a 2v diff between the l and r channel, one is 0.8 the other 2.8. every part between the 2 boards is within .1 tolerance. everything is the same voltage on each board apart from where i have marked on the drawing. this is making the one board with 0.8 run about 10 % hotter. any ideas. |
7th Jan 2022, 6:43 pm | #2 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,951
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Re: jlh 1969 voltage mismatch
If there's a static voltage across the speaker-terminals it suggests that the output-coupling capacitor [C2 in your diagram] is leaky.
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7th Jan 2022, 6:57 pm | #3 |
Heptode
Join Date: Feb 2006
Location: Marlborough, Wiltshire, UK.
Posts: 915
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Re: jlh 1969 voltage mismatch
0.8v represents 0.08mA through the 10k resistor; this can only come from leakage through the output cap. This is high with the other channel even more but given that the standing current in the output transistors is over 1A, most unlikely to be the cause of overheating unless there is instability going on.
The standing current is set by R1, R2 and the current gain of TR2. These may have shifted from when first built. You need to check this, just the current on the V+ supply to the whole amp will do. To check for instability, you really need a scope but a judicious finger on various points may give a clue. There was a later article covering some mods to the design, to add compensation components and tame its bandwidth though I don’t think it was particularly sensitive originally. |
7th Jan 2022, 7:11 pm | #4 |
Nonode
Join Date: Aug 2010
Location: Resolfen, Wales; and Bristol, England
Posts: 2,588
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Re: jlh 1969 voltage mismatch
JLH 1969 amplifiers run very hot - mine achieves a heatsink temperature of 80º in normal use. I used 105º components for the output capacitors to give them an even chance. If you have a different DC voltage on the speaker outputs it must be a leaky capacitor.
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7th Jan 2022, 7:16 pm | #5 |
Pentode
Join Date: Dec 2021
Location: Willenhall, West Midlands, UK.
Posts: 161
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Re: jlh 1969 voltage mismatch
just pulled the 2500 caps one is 3476 the other 3347.
they where brand new never would have thought they would be so off. so out of all the components on the board i never tested these. live and learn lol, thanks guys. i just wasted about an hour pulling every other part lmao. |
7th Jan 2022, 7:22 pm | #6 | |
Pentode
Join Date: Dec 2021
Location: Willenhall, West Midlands, UK.
Posts: 161
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Re: jlh 1969 voltage mismatch
Quote:
i have an old rack mount case i am temped to put it in its massive with larghe heatsinks. the heatsinks are small for now so i changes a couple of things to keep the temp under control. but when i just put my hand on to test the temp i come close to getting 3rd degree burns off the one channel lol. |
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7th Jan 2022, 9:30 pm | #7 |
Nonode
Join Date: Jun 2007
Location: Coulsdon, London, UK.
Posts: 2,152
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Re: jlh 1969 voltage mismatch
If the output capacitors are electrolytic then some leakage through them is to be expected.
The capacity tolerance can also be large. Put an 8 Ohm resistor across the output terminals and then measure the voltage across it. |