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Old 28th October 2012, 07:09 AM   #2581
Puffin is offline Puffin  United Kingdom
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Quote:
Originally Posted by deduikertjes View Post
I'm afraid that you read a little bit more on how to build this thing and how to fire it up, else you'll probably fire it up.

In short, be sure to have low rail voltage (eg 5V or so) and have one of these variable resistors adjusted so that you have minimum bias current. Then adjust DC offset, let it warm up a little more and keep DC offset small.
The start increasing bias current. If everything is still ok, go for higher rails voltage and gradually work your way up to the desired Ig (bias current) while keeping DC-offset close to 0.

succes, MArco
P.S. don't forget to read more ...
I posted this in March with the intention of getting the amp up and running. I am only just about to finish it. With regard to VR1, VR2 and VR3 what is the order for checking bias and dc offset, i.e in which order do I adjust them?

Thanks. Rob.
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Old 28th October 2012, 09:27 AM   #2582
AndrewT is offline AndrewT  Scotland
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you will probably have to go back and forth between them. As you adjust one the amp temperature will change. That will require a further adjustment. 25T pots in parallel with fixed resistors can make fine adjustments much easier once you know roughly what resistances you need.
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Old 28th October 2012, 10:13 AM   #2583
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Ok, thanks Andrew.

Rob.
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Old 28th October 2012, 12:50 PM   #2584
paulb is offline paulb  Canada
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I received this from Geoff Moss when I used his PCB design to build my JLH. You'll have to check whether his VR1 & VR2 match the schematic you're using. The instructions are based on the schematic of Fig. 3 at http://www.tcaas.btinternet.co.uk/jlhupdate.htm:

As for initial setting-up, before power is first applied I would adjust both VR1 and VR2 for maximum resistance. After power is applied, VR1 is adjusted to set the output dc offset to near zero and the quiescent current (and current sharing) checked by measuring the voltages across the 0R1 resistors. If all seems well, VR2 is adjusted to raise the quiescent current to, say, 1A. After a short period to allow the transistors to warm up, the output dc offset is reset to near zero by adjusting VR1. Back to VR2 again and raise the quiescent current to, say, 2A. Wait a while and reset the output offset.

Unfortunately, the adjustments of VR1 and VR2 interact and so the setting of the quiescent current and output dc offset is an iterative process but after about four steps you should be able to get the current and offset somewhere near to the required values. At this point, it is necessary to let the amp idle for an hour or so in order to ensure that thermal equilibrium has been reached and that the amp is at its normal operating temperature. The quiescent current and output dc offset are now trimmed to the required values. These are best rechecked after a further settling period (say about 30 minutes).
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Old 28th October 2012, 02:10 PM   #2585
Bonsai is offline Bonsai  Taiwan
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I am surprised that there is that much interaction. My sx-Amp holds quiescent current at 1.4A +-40ma worst case from ambient to about 65C or 70C which is the final operating temp. My bias arrangement is different to JLH, with good 1st order delta Vbe tracking.
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Old 28th October 2012, 07:29 PM   #2586
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Quote:
Originally Posted by paulb View Post
after about four steps you should be able to get the current and offset somewhere near to the required values[/I]
Required values?

Is VR3 redundant?
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Old 29th October 2012, 06:15 PM   #2587
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bump.
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Old 31st October 2012, 08:57 AM   #2588
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Required values?
DC Offset as close to 0 as you can get. What's acceptable is your decision.

Current is also your decision. Current and voltage together determine the heat dissipation in your output transistors. Your heat sink must be able to cope with that (and of course the transistors must be able to cope to).
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Old 31st October 2012, 09:01 AM   #2589
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Is VR3 redundant?

VR1 and VR2 is DC-ofsett adjust.
VR3 is bias (Current).
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Old 31st October 2012, 02:31 PM   #2590
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Quote:
Originally Posted by deduikertjes View Post
Required values?
DC Offset as close to 0 as you can get. What's acceptable is your decision.

Current is also your decision. Current and voltage together determine the heat dissipation in your output transistors. Your heat sink must be able to cope with that (and of course the transistors must be able to cope to).
Thanks.
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