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Old 13th May 2008, 09:28 PM   #1
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Default Amplifier Troubleshooting.

I've been here before and with the very same amplifier, Slones design 11.8 in the high power audio amplifier construction manual, although I have never actually had them working completely as intended.

The problems can be summed up by two things.

1) High DC offset.
2) Protection circuitry tripping way before it should.

I have contacted Slone on a few occasions trying to solve this, but he failed to respond when I replied with the results of his tests.

First he asked me to make sure all the zenners were of the correct wattage, which they were.

He then asked me to check the voltage across the zenners and also the voltage across R16 and R18. Everything checked out normal, to which I responded, but I never got a reply, and even after replying for second time a while later, still no response.

I have also tried simulating the design in Circuit Maker 2000, but ran into a brick wall when trying to import individual spice models, I cannot figure out how to do this in a simple fashion, the program seems to be able to handle .lib files, but not .mod. Nevertheless I tried using the FETs used in the passlab amps and other transistors for pre drivers, but only got mixed results.

The DC offset and the biasing seem to be somewhat linked, I do not know if they should be, but in my amps they are. If I alter the value of P1 to one extreme the DC offset reaches about -200mV(this is as low as it goes), but I cannot get anything above about 5mV bias across the output FETs. Swinging the value of P1 around a bit, increasing the DC offset to around -280mV, allows the biasing to function where I can get the specified 33mV.

A while ago I tried disconnecting the protection circuitry, one rail at a time and from memory the side that's causing the over protection is the negative one, coincidence with the -ve DC offset? I don't know.

Slone says these amps are capable of delivering in excess of 250 watts into a 4ohm load, which my pair don't even come close to. Driving a pair of Peerless XLS at 55hz (where peerless says each are at 8ohms) the amps protection decides to start working at around 11 Volts RMS.

The XLS are run Open baffle and obviously the amps clip way before I run out of xmax when playing normal music. If I disable the protection circuitry I can hit Xmax easily, removing the protection circuitry isn't the answer.

Aside from the high DC offset and limited output power the amps sound just fine and I do use them, however I have always used them with the notion of getting them properly working one day.

Having talked about this a while ago on DIYaudio, most of which discussed had little effect and with getting nowhere with Slone, I've almost given up expecting them to function. I have heard of Slone posting a couple of faulty designs in the past, but this amp he's actually built, there's a picture of the damned thing in the book, so it obviously functioned for him. (Why build something and never test it?)

I would obviously prefer one final attempt at getting these working, over buying/making another design, but I almost get the impression that something really isn't right.

So here I am asking one final time, to see if any of you electrical whizzes can come up with anything.

Many thanks,

Matt.

P.S. - If I cannot get these to work, what other designs are there that would work well with a 60V power supply? Preferably with the output devices mounted on either side of the PCB, that way I could just recycle the cases and PSU.
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Old 14th May 2008, 09:25 AM   #2
gain is offline gain  United States
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Hi 5th,

If you can, please post the schematic of the amp you built, will be a big help.

If the circuit has a differential amp for its input stage (i believe most of Mr. Slone's designs do) then the dc offset may be due to a current imbalance in the differential stage (LTP). try beta-matching the LTP transistors, especially the ones for the current mirror, if applicable. you could also try replacing the emitter degeneration resistors in the LTP with a trimmer. say you have two 100R for emitter resistors, replace with a 200R trimpot with the wiper connected to the 'tail' (toward the constant current source). set the trimpot in the middle (100R on each side of the wiper) then power up with a voltmeter tied across the output, then use the trimpot to 'tune' away any dc offset.

at any rate post schematic and i'm sure one of the guys here will have it figured out in 2 seconds.
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Old 14th May 2008, 03:19 PM   #3
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Alright here's the schematic, bare in mind accuracy is dependant on my accuracy in copying it into circuit maker I've checked everything thought and it appears to be correct.

In copying the schematic into paint to create a gif, some of the black blobs showing, what connects to what, were removed, I painted them back in, but there is of course the chance I missed one or two.

The Zenners at D1 and D2 are 18 Volt 1/2 watts. D3 and D4 are 3.9v 1/3 watt.

The first trimpot (500ohm 10% R13) is P1. The second trimpot (500ohm 60% R20) is P2 in Slones schematic and is quite obviously the one used for setting the bias. The % simply refers to how far around the trimmers are set, I assume 50% being midway.

http://www.turboimagehost.com/p/325123/amp.GIF.html
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Old 14th May 2008, 03:40 PM   #4
DRC is offline DRC  United Kingdom
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the 2 diff ip stages appear to have different gain due to the different Re resistors, ie 250 (500/2) vs 100. A drafting mistake ?
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Old 14th May 2008, 03:57 PM   #5
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Quote:
Originally posted by DRC
the 2 diff ip stages appear to have different gain due to the different Re resistors, ie 250 (500/2) vs 100. A drafting mistake ?
That's the way Slone has it in two of his designs, I thought this seemed odd too, but not having a large enough understanding of all of this I am not one to make any kind of real judgement.
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Old 14th May 2008, 05:38 PM   #6
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measure the voltage drop across each of the 4 LTP collector resistors R9, R10, R11, and R12 with the amp idling, no signal applied. this will give an idea of how well the LTPs are balanced.

are you using 1% resistors for these?
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Old 14th May 2008, 05:51 PM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by gain
measure the voltage drop across each of the 4 LTP collector resistors R9, R10, R11, and R12 with the amp idling, no signal applied. this will give an idea of how well the LTPs are balanced.

are you using 1% resistors for these?
it isn't balanced!!!
6k8 is far too large.

C8 & C9 do not complement each other.
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Old 14th May 2008, 06:20 PM   #8
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Quote:
Originally posted by gain
measure the voltage drop across each of the 4 LTP collector resistors R9, R10, R11, and R12 with the amp idling, no signal applied. this will give an idea of how well the LTPs are balanced.

are you using 1% resistors for these?
R9 = 1.18V
R10 = 3.06V
R11 = 1.16V
R12 = 2.84V
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Old 14th May 2008, 06:21 PM   #9
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Quote:
Originally posted by AndrewT
it isn't balanced!!!
6k8 is far too large.

C8 & C9 do not complement each other.
Hi Andrew, what would you recommend?

C8 in Slones schematic is two 10uf polar electrolytics back to back, Circuit maker didn't like this for some reason, so I substituted a 5uf non polar.
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Old 14th May 2008, 07:01 PM   #10
AndrewT is offline AndrewT  Scotland
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Hi 5th,
allowing 2mA through Q8 gives 300mV across R19 and it's complement.
Allowing 600mVbe across Q8, gives a total voltage of 900mV across R11.
Allowing 1mA through each half of the long tail pair requires 910r for R11 and it's complement.
R4 & R5 have 18V and pass 2mA therefore to match 910r they need to be 9k1.

Time constant of NFB loop is 120mS (1k2 & 100uF).
Time constant of Input Filter is 2350mS (470k & 5uF).
This reversal and mismatch could lead to a possible LF instabilty.
50k & 2u2F at the input is a possible alternative, RC=110mS.
If the source also has a 10uF at it's output then RC drops to 90mS.
Now set the NFB>=1.4*110mS>=150mS.
R=1k2, C>=130uF, use 150uF.

Note, these example values are not fixed, but the method shows where you have choices and how your selection cascades through the various stages of the design.
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