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Kilentra 23rd January 2003 01:52 AM

Symmetrical amplifier conundrum
 
Hello all,

I am trying to get the first channel of my symmetrical amplifier (designed by Anthony Holton) running, but I'm having some trouble setting up the output stage. I've been waiting for an e-mail response from him for a few days but I'm quite eager to fix this problem so I figured I would ask the forum.

The power supply works fine, voltage is a little high at +/- 74-76V, and the voltages on the board all checked out before installing the output stage.

I first plugged the amplifier in, DC offset was a few millivolts, and set the bias as usual, then realized that some of the output Mosfets were getting no bias (there was no voltage across their source resistors). After a few minutes there was a small spark and two of the output stage Mosfets blew (I unplugged it immediately). The rest of those along with Q10, the biasing transistor, checked out OK with multimeter tests. It turns out a few connections were loose since the pins on the Mosfets did not reach fully through the PCB (I'm not sure if this is an error in the PCB's mounting system or if I made a silly mistake). Nothing was burnt, no smoke or anything, just the gate was shorted to drain on those two fets.

Anyway I got two more Mosfets that were matched close enough, soldered small pieces of wire to extend all the pins to ensure good connections, and double checked everything before powering up again. So when I plug it in, I see DC offset varying from 1 to 1.2 volts, and there is no voltage across any of the source resistors in the output stage. Where did the bias go?

My questions would be: what parts of the circuit should I check to verify that the rest of the amplifier has not been damaged? And what would cause a DC offset and lack of bias? I am not familiar enough with the amplifier's design to troubleshoot this myself.

Thanks very much for any help, I really appreciate it.

Michael

Kilentra 23rd January 2003 07:35 PM

Oy vey
 
Hi again,

The above post can be mostly ignored because it turned out that I had switched the base and collector leads of Q10 (wired off-board) after I had taken it out to check it. What a stupid mistake... I fixed this. All the other transistors on the board are working as well. I had some solderpads come off the board in my, but this was easily fixed with a few jumper wires.

Of course however, there is no free lunch.

After looking over the connections yet again, I turned on the amplifier and found 3mv DC offset. I set the bias to 18mv across a source resistor and kept adjusting it because it was decreasing over time (a few minutes).

Then there was a big pop, so loud it scared me. I think the red clipping indicator LED lit momentarily as well, but I'm not sure. Of course I shut the thing down (actually the fuses did for me) and checked everything. There was a little burning smell around the Mosfets, and no visual damage. I removed one of them from the board and it tested OK, thankfully. (In the circuit they all produce the same readings so I think they're probably all intact). Q10 looks like it's still working too. I am quite afraid to turn the amplifier on again and check it in operation (since that could endanger expensive components and my safety), until I have some idea of what is going on.

I've considered power-supply over voltage as a problem... I forget the exact value when I was running the amplifier, but now (30 minutes later) it is +/- 76.8V at no load. This is with a 50V transformer (which is recommended for the circuit), so I would think such a discrepancy would be acceptable.

I had read about N-channel amplifiers going up in flames because of not mounting the thermal sensing device on the same heatsink on the output stage, so I had piggybacked it on Q11 (one of the N-channel Mosfets) with silver thermal paste and held it down with superglue on the edges. So I don't think that's the problem either.

What happened? This time I really thought I had it right. Sorry to bother you guys... and even some moral support would be greatly appreciated. Thanks again.

http://users.erols.com/jdprice1/symamptesting.jpg

halojoy 23rd January 2003 08:19 PM

Not easy to know what happend.

Seems you have found nothing more than a fuse, that has burned.
How much is 18mV - in current?
Is there any other safety in the circuit
beside the fuse?

With +-75 volt
even not so much current, can be a lot of Power.
P=UxI

Have you any other details to tell us, now?

/halo - wants to know - what happend?

Kilentra 24th January 2003 02:03 AM

Halojoy,

The 18 millivolts is across a .22 ohm source resistor, and there are four Mosfets per output bank (N and P channel), setting the bias current at about 330mA. There is no protection anywhere in the circuit except a 5 amp slow blow fuse on each voltage rail.

Another thing I thought of was that maybe because of imperfect matching, the bias on some of the Mosfets was way high. The amplifier uses a 20 turn pot for biasing... it took like 15 turns to get any measurable voltage, and then the next turn took it past 18mv. Possibly the bias on the other Mosfets was very high because of this sensitivity to adjustment... well, I don't know. The resistors are 5% tolerance and the Mosfets were matched within 0.03v if I remember correctly.

The sound reminded me of when I was carrying my other power supply around and the wires touched for a moment... a snap or pop leaving a warm smell. I don't remember seeing a spark or anything. I think I will take out the rest of the Mosfets and make sure they are all in working order, because it would be really something if this failure didn't damage anything.

Well, at this point I may as well just check all the components again, probe around with the circuit off and wait for some more ideas to pop up. I guess I'm just a bit paranoid in not wanting to fire this one up again (or the other channel which is ready and waiting) until I know what the deal is.

And thanks for posting, maybe helpful people like you can help me get somewhere.

Michael

jleaman 24th January 2003 03:02 AM

2 Attachment(s)
my amp is actualy rinning at 77v and works just fine. getts a little warm but heh oh well. here is some pic's. did you put them resisters on the bottem or the top. : O ) if not then that is your problem.

jleaman 24th January 2003 03:03 AM

2 Attachment(s)
and another.

Kilentra 24th January 2003 03:26 AM

Looks very nice Jason, I see that is the N-channel amplifier that seems to be more popular than the one I have? How does it sound?

I have Xicon 0.22 ohm 5W/5% wirewound resistors soldered to the bottom of the board. And I hope heat is not a problem, since the heatsink is at worst 0.28 C/W. I have two channels of Zen V4 at 1.3 amps (~140 watts dissipated all the time) mounted horizontally on one of those and it's at a safe temperature.

Samuel Jayaraj 24th January 2003 05:43 AM

Jason seems to be repeatedly get confused between Anthony Holton's N-channel amp and other designs, in this case his symmetrical amp design wich can be made with L-type mosfets as well as IRF devices.

Kilentra, how closely have you matched the IRF Mosfet banks and what methods did you use for matching. I would suggest that you check all components, particularly SS transistors, driver/Current source transistors and also check for leaking capacitors. The trouble seems to be at a stage behind the output stage for you to be facing new problems, each time you switch on the amp. If in doubt replace parts that you don't have equipment to test - eg., capacitors. Small signal and other transistors should be checked out of the circuit with an hFe tester and check for leakage.

When you have replaced all parts, use another transformer with a lower voltage or a variac and use only one pair of output devices. It would also be wise to replace insulation kits, ground the heatsink, add 10n and 100n supply decoupling caps, remove source resistors and parallel 1 or 2 watt CFR resistors to obtain the required low value resistance and twist +- rails leads. This will eliminate Hf oscillations if any. When all is well, add the paralleled output devices and up the voltage rungs.

Hope it helps.

Kilentra 24th January 2003 05:54 PM

Sam,

On this board the N-channel devices were matched to between 3.94-3.96v and the P-channel between 3.88-3.90v. This was done by simply hooking the Mosfet up with an 18v power supply and 120 ohm resistor in the matching circuit given by Holton, and measuring the voltage across the pins. He does state they should be matched to +/- 100mv, which is a pretty wide tolerance.

Thanks for the suggestions, I will check all the components this afternoon. I don't have another transformer readily available, but I think I can temporarily wire this one's primaries in series to halve the voltage. I can't say I have an hFe tester or oscilloscope either - just analog and digital multimeters. Don't have enough non-inductive power resistors on hand to try that modification, either. I'm afraid I am poorly equipped.

So I will definitely try adding power supply coupling caps and grounding the heatsink... along with checking over everything again and removing all but one pair of Mosfets. I'm low on spare parts but I can borrow from my other channel for the time being.

Is it safe to run this amplifier at full supply voltage using just one pair of output stage Mosfets? Heck, I should rather be asking if it is safe for a person like me to be running this amplifier at all. :)

Anyway, thanks a lot guys. I'll see how my detective work goes today.

naula 24th January 2003 07:16 PM

Kilentra:

I'm going to build same amplifier as you and will buy the components next Monday, except transformers, supply capacitors and output transistors. I will test the amplifier without expensive mosfets with laboratory supply in the school. If it's working properly then I'll buy rest of the components.

Have you read the Anthony's construction notes for the symmetrical amp? On page four there is "Pre-flight test". It says solder 10 ohm resistor across zener diode ZD3. So you can test the differential and voltage amplifiction stages without MOSFETs.

Oscilloscope would be nice but if you haven't got access to one testing have to be done with multimeter. You can get sine wave test signal from CD player by playing test CD. Measure the input signal voltage. Then you multiply this by 32 and should get approximately same value as output voltage reading.


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