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Old 25th April 2005, 01:59 PM   #11
rabbitz is offline rabbitz  Australia
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Default Re: Need help quick please! P101 bias related

Quote:
Originally posted by ShinOBIWAN
Hi all!

Rod says to set resistance to minimum on the 20k pot, I have installed it in the same orientation as shown below:

Do I turn fully clockwise or anticlockwise as you look at the board in the picture above for min resistance?

Help please, and thanks.
You probably already have the answer but turn VR1 CW (clockwise) to get minimum resistance. Check with a DMM from the single pin to the 2 joined pins to be sure. BTW, you've got a 20K in your post.... typo? as should be 2K.

I'd be interested how the final bias voltages come up as I didn't get close to the documented values across the 100R test resistor.... a lot of others haven't either. With min resistance on VR1 I was getting 0.68V-0.85V and on the final 2.68V-2.87V. Amp works well without any problems.

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Old 25th April 2005, 04:00 PM   #12
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Well managed to get sound with the 15v PSU as a test setup.

However I'm having trouble with the 56v PSU now

Connected it all up and the resistors R21 & R22 as shown here (green ones closest to the faston connectors):

Click the image to open in full size.


Details on the PSU here:
http://www.bmm-electronics.com/Produ...roduct_ID=3310

I'm using a 40VAC 12.5A 1000VA transformer.

When a multimeter is connected to the output I get a reading of ~114v so something is wrong since it should be ~56v. Again no amps connected, just the PSU and transformer.

Why doesn't anything go smoothly with this project, the only things that work properly are the amps and I built those, these PSU's are supposed to built by proffessionals yet I've had nothing but trouble.

Could someone please take a look over the PSU data and the reading I took.
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Old 25th April 2005, 04:19 PM   #13
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Quote:
When a multimeter is connected to the output I get a reading of ~114v so something is wrong since it should be ~56v. Again no amps connected, just the PSU and transformer.
If you measure between the positive and negative rail , you will find 56 X2 = 112 Volts.

You must measure between the positive rail and ground + 56 Volts and between the negative rail and ground - 56 volts...
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Old 25th April 2005, 04:47 PM   #14
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Quote:
Originally posted by Tube_Dude


If you measure between the positive and negative rail , you will find 56 X2 = 112 Volts.

You must measure between the positive rail and ground + 56 Volts and between the negative rail and ground - 56 volts...
Oh bugg*r

Can you tell I haven't done this before?

What about the resistors R21 and R22 in the schematic, these get extremely hot.

Measured from + to ground and its reading ~58.5v DC. Mac the PSU is rated for 57v. Could this cause R21 amd R22 to heat, they are so hot as uncomfotable to touch. All other resistors are cool.

Is this happening because I have no load on the PSU?
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Old 25th April 2005, 06:36 PM   #15
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Don't suppose anyone can help with this, its getting to the stage were I just daren't leave it on for more than a minute, those resistors are getting very hot, even in this short space of time.

I've now connected one amp to the board, playing music and they're still getting hot. Everything is cool.

I'll draw up a connection diagram and you folk can take a look over it to see if I've gone wrong somewhere.
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Old 25th April 2005, 07:12 PM   #16
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Okay here's my layout:

Its a large image but only ~90k in file size.

Click the image to open in full size.

Anything wrong with that to cause the two resistors to heat like mad after a very small space of time?

PS. I have two of these boards and both exhibit the same problem.
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Old 25th April 2005, 07:20 PM   #17
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What value are they?

:edit: I have had to remove the BMM schematic as it is noted as copyright to them.
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Old 25th April 2005, 07:24 PM   #18
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Quote:
Originally posted by ShinOBIWAN




What about the resistors R21 and R22 in the schematic, these get extremely hot.
They are bleeder resistors.
What is the value (in Ohms) of R21 / 22 resistors?
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Old 25th April 2005, 07:55 PM   #19
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You can calculate the power drop across them using Ohm's Law, P = V^2/R. If that is less than they are rated for, then they are within tolerance. These will get warm!
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Old 25th April 2005, 10:43 PM   #20
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Yep sorted it.

Emailed BMM and its completely normal, resistor is 1K2R/5W which using your formula PM, gives around 3W. Looks safe enough to me.

Rigged another up to the PSU so I have stereo now. All is great but I've got an audible hum from around 2 meters away when the room is quiet, obviously a grounding issue that's probably due to less than ideal wiring.

Hopefully I can sort that out, for now though I'm going to test/tweak the active XO for the project I'm working on
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