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Old 2nd April 2011, 08:49 PM   #21
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Originally Posted by aparatusonitus View Post
You have over-biased the output stage with R19=215ohm...bypass it with a piece of wire for a moment and measure output bias again.
Still ~95.0mV
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Old 2nd April 2011, 10:21 PM   #22
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Still ~95.0mV
OK, at this point I would suggest you to up the bias a bit (yes, up, to around 312,5mV over emitter resistors, so each output transistor will dissipate 200mW at idle. Now, your headamp will stay in class A most of the time and will last forever. Just replace R19/49 with, say 22R resistor and go higher until you get desired 312,5mV over emitter resistor.
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Old 3rd April 2011, 09:27 PM   #23
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Originally Posted by aparatusonitus View Post
OK, at this point I would suggest you to up the bias a bit (yes, up, to around 312,5mV over emitter resistors, so each output transistor will dissipate 200mW at idle. Now, your headamp will stay in class A most of the time and will last forever. Just replace R19/49 with, say 22R resistor and go higher until you get desired 312,5mV over emitter resistor.
I think R11 and R15 (plus trimpots) are meant for biasing the output stage, Although i could aways be wrong, when you got me to short the resistor the voltage drop across the output resistors never changed. Third, I don't think it's safe to increase the heat dissipation on the output transistors, they already idle at ~85c, I could add TO-220 heatsinks to the tops of them, two transistors per a heatsink.

What about overcompensating the the feedback circuit?
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Old 3rd April 2011, 10:35 PM   #24
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Quote:
Originally Posted by KnightofAwesome View Post
I think R11 and R15 (plus trimpots) are meant for biasing the output stage.
Directly no...indirectly yes...circuit DC voltages/currents will be influenced a somewhat by voltage drop over R1/2/8/9 if you change value of R11/15.

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Although i could aways be wrong, when you got me to short the resistor the voltage drop across the output resistors never changed.
That is simply impossible...few post above you sad you have measured ~95mV and I would call it a massive drop from a previous ~960mV.

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Originally Posted by KnightofAwesome View Post
Third, I don't think it's safe to increase the heat dissipation on the output transistors, they already idle at ~85c, I could add TO-220 heatsinks to the tops of them, two transistors per a heatsink.
With +/-16VDC supply, Pd of 200mW for TO-92 and 12,5mA of current they are perfectly safe...read the datasheet.
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Old 8th April 2011, 12:04 AM   #25
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There are two 1N4841 diodes in series with R19, I should have measured the voltage drop across them, This whole thing has been going down hill and I apologize for my poor performance. I've still been fighting with this amp, and just about on the verge of giving up.

I still have a over biasing problem, the transistors are ~90-100c, I'll handmake some simple heatsinks made out of basic aluminum, using thermal epoxy to bond them to the output transistors.

As for biasing the output stage, I have two options:
1: I can remove D3 and D4 and replace R19 with something like a ~500 ohm resistor.
2: I can use aparatusonits's method and replace R19 with a >22 ohm resistor
(On top of this, I might spend some time to find some 1.6volt LED's, since 1.65v is too much, i think...)

I must also deal with a injection of noise from the mains courtesy of my computer's PSU. I don't know if i should simply add a simple ~0.01uF cap across neutral and hot, or go with a full blown HF filter.

I'm almost considering to just remake the whole board in eagle, but i think that would be a little rash. I just feel as though i regret this.
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Old 8th April 2011, 12:27 AM   #26
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Hey...come on...you have some troubles with it, so what...but it plays music so you are 75% done

Do not replace biasing diodes D3/4 + R19 with one single 499 Ohm resistor...Diodes are there to provide some temperature compensation for output stage.

Could you measure DC voltage over R3/4/5/6 and R17/21 for as please?

Forgot to say...while taking measurement short input (signal to signal ground) and without load connected to output.
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Last edited by aparatusonitus; 8th April 2011 at 12:31 AM.
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Old 8th April 2011, 03:11 AM   #27
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Sure thing!, I found one of the pots to be broken, these measurements are of the right channel.

R3= 0.14v
R4= 0.11v
R5= 0.17v
R6= 0.12v

R7= 14.54v
R21= -14.56v

I attempted to jumper R19 again, my wire was hard to keep in place, but i managed!

Routput(J)= 0.2v
Routput= 0.912v

22ohm resistor it is!

Out of curiosity (don't spoil this), using your values from above; I "estimated" what would the temperature would be if the amp was down biased to 0.35v.
I estimate 29.4c, I won't need a heatsink if I'm correct

I can't seem to get a recoding of the PSU noise, nor do i know if it's there at the moment, It's clearly above 60hz, but i have no clue how to eliminate it.

Edit, Post #21 is wrong

Last edited by KnightofAwesome; 8th April 2011 at 03:23 AM.
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Old 8th April 2011, 03:53 AM   #28
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From your measurement I can tell you that you have under-bias input stage and over-bias VAS stage. I think, if your resistors in circuit are all correct in value, that you have problems with R10/14 100k trimers...take them out of the circuit, you don't need them.

Disconnect one end of R61/1Meg resistor, pull servo opamp out of circuit (you should use J-fet opamp for servo opamp e.g. LF411 or similar, not BJT input such as OP27),
pull lower end of R12 and upper end of R13 resistors of the pcb and temporally solder both ends together in "air".

Short the input signal to signal gnd, no load at output and measure voltage again over all resistors.
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Old 8th April 2011, 04:48 AM   #29
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Quote:
Originally Posted by aparatusonitus View Post
From your measurement I can tell you that you have under-bias input stage and over-bias VAS stage. I think, if your resistors in circuit are all correct in value, that you have problems with R10/14 100k trimers...take them out of the circuit, you don't need them.

Disconnect one end of R61/1Meg resistor, pull servo opamp out of circuit (you should use J-fet opamp for servo opamp e.g. LF411 or similar, not BJT input such as OP27),
pull lower end of R12 and upper end of R13 resistors of the pcb and temporally solder both ends together in "air".

Short the input signal to signal gnd, no load at output and measure voltage again over all resistors.
I'll measure all the voltages tomorrow and (completely?) remove the DCservo circuit.

I was using ~600ohm resistors for R11 and R15, I need to use the 5K trim pots that I installed. So here are the measurements of having R11=499ohms and R15=499ohm (pot and resistors).

R3= 0.213v
R4= 0.183v
R5= 0.220v
R6= 0.178v
R17= 14.50v
R21= 14.48v

Here are the measurements of having a zero DCoffset

R3= 0.206v
R4= 0.182v
R5= 0.235v
R6= 0.187v
R17= 14.52v
R21= 14.46v

This is good? I'm using OPA134PA.
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Old 9th April 2011, 02:28 AM   #30
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I don't know how much more of this i can take, fix some problems, more problems arise! I've never had this much problem with electronics ever.

OK, here we go again!

Decreasing R19 to 24.9ohms makes everything a lot more sensitive, the DC servo works much better now (or it appears to), The amp is a lot more sensitive to the power supply voltages, The LM3xx regulators won't be good enough in this operation.
Adjusting the volume pot will move the offset move a good -100mV (rightchannel) left channel moves less (+/-20mV)

So, I provide myself with a ultimatum
A. I rework the entire PSU to include more regulation, more noise filter ect ect. While also completely redoing the entire boards, buying a bunch of new parts and trying again from from scratch (almost).
B. Giving up on this project, selling all that i have for ~$100CAD and loosing ~$70CAD in the processes and starting something else.

I've been saying this too much, but quite frankly... I don't know what to do...

Cheers
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