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resistors and Bias
resistors and Bias
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Old 11th July 2019, 03:33 AM   #11
nigelwright7557 is offline nigelwright7557  United Kingdom
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I always design my amps to work on clockwise to increase bias.
I always write on pcb next to pot "Start ACW" for start anti clockwise.

I put a resistor in series with the pot so the bias can never go infinite.
Added safety is a Zener across the bias circuit to stop high bias voltages.
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Old 11th July 2019, 04:10 AM   #12
JMFahey is offline JMFahey  Argentina
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I would assume nothing unless the actual Bias circuit is drawn and posted here , period.
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I assume its all the same for all technologies and it has to be set to 0 ohm.
Donīt.
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A well designed amplifier will 'fail safe' if the adjustment pot opens up.
true, but does that apply here?
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If the pot is 100r I should increase it to 100r for 0 bias? Is that right for both channel?
Not enough data to answer.
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I can't measure current but can resistance.
Then you can NOT adjust bias, period.
Please post amp schematic so we can suggest how to measure idle current .

If not possible, then do NOT mess with that amplifier.

You donīt even know what idle current you should set it to, nor any means to measure it, so only thing you can do is damage.
Expensive damage.
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Old 11th July 2019, 04:36 AM   #13
Ian Finch is offline Ian Finch  Australia
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Originally Posted by dj_holmes View Post
I dont. I thought it was a general rule for all bias adjustments.....
If you don't have a circuit but you thought all Vbe multiplier circuits were the same, then I assume it's a commercial product that you are tinkering with so most likely, the direction is CW for increase and this would be specified in its service manual somewhere. So, what is the make/model?

Alternatively, you could simply power up as presently set if you haven't changed every component in the amplifier already. Attach your multimeter leads securely across one of the large emitter (or source for mosfets) resistors connected to the output transistors. Using the lowest voltage range, adjust the pot. setting slightly while watching the change in the small voltage of only 5-30mV usually. Polarity is unimportant. This is the basic procedure for all bias current setting. You would then be certain which direction, either CW or CCW, was correct for minimum setting.

As the procedure will be detailed in the service manual, there is no cause for concern about reading mV instead of mA since the small voltage drop across the fixed resistor(s) is analogous to current, by Ohm's law.
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Old 11th July 2019, 10:57 PM   #14
dj_holmes is offline dj_holmes  United Kingdom
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Assume it Nad c300.

Its mostfet and the service manual is easily available but I cannot seem to attach it.
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Old 12th July 2019, 01:18 AM   #15
dj_holmes is offline dj_holmes  United Kingdom
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Ok looking at Vr402 circuit diagram which way would it be adjusted so the bias is at minimum?
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Old 12th July 2019, 02:25 AM   #16
Ian Finch is offline Ian Finch  Australia
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It's just as well that you told us the model because C300 is a mosfet type and this one has no source resistors from which to monitor the bias current anyway. The bias current is controlled by a separate, current regulated power supply and you'll have to assume that its adjustment is CW for increase, facing the top or front face of the pot. as shown on the larger attached diagram.

If you had no cause to adjust it, leave it set as it was previously, since this type of bias controller does not operate like the classic Vbe multiplier. Power up and the bias current will likely be close enough for safe operation as is. Then consider whether any adjustment is necessary.
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Old 12th July 2019, 02:36 AM   #17
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by Ian Finch View Post
It's just as well that you told us the model because C300 is a mosfet type and this one has no source resistors from which to monitor the bias current anyway. The bias current is controlled by a separate, current regulated power supply and you'll have to assume that its adjustment is CW for increase, facing the top or front face of the pot. as shown on the larger attached diagram.
.
When amps don't have emitter/source resistors they sometimes remove a fuse and measure the current across the fuse holder in one rail.
If I remember correctly the Maplin lateral mosfet amp was like as it had no source resistors.
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Old 12th July 2019, 03:27 AM   #18
Ian Finch is offline Ian Finch  Australia
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That's true, Nigel. Here, NAD used a schottky/resistor combination in the positive supply rail for the purposes of both current sensing and providing test points. Attached is a snip of the amplifier output stage schematic for those with keener eyesight.
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Old 14th July 2019, 04:29 PM   #19
dj_holmes is offline dj_holmes  United Kingdom
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Do I set the boas to maximum resistance or lowest then to get minimum bias?
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Old 14th July 2019, 04:32 PM   #20
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by dj_holmes View Post
Do I set the boas to maximum resistance or lowest then to get minimum bias?
You need to draw out the circuit around the bias pot and then it will be obvious which way the pot works.

A possibly dangerous way to do it is to set it half way and measure bias current and then turn pot each way to find which way is correct way to set it right.
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