Symmetrical amplifier conundrum

[Kilentra]

Naula,

Good to see someone else is taking on this project. Hopefully with good testing equipment and more experience you will not have the same problems as I did.

I have read the construction manual and did the pre-flight test... all the voltages were right on the mark. It was only when I tried to bias the output stage that I started having problems. The first problems were due to my stupid mis-wiring and now I have damaged some things because of that. Just be careful with your own amp... I am a good example of what not to do.

So anyway, I removed the output stage and performed the pre-flight test again. This was the result (correct values in parentheses):
R3: (1.6V) 0.60V
R5: (1.6V) 0V
R15: (1.0V) 0.06V
R12: (0.5V) 0V
R13: (0.5V) 0V
R8: (14.6V) 14.61V
ZD1: (15V) 15.14V
R11 (offset): (0-0.1V) 51.5V... wow.

I lost another P-channel Mosfet and two N-channel Mosfets from the output stage. Let's hope I don't destroy any more.

Judging from these results it appears as if the input stage current source is OK, but everything else is screwed up. It looks like I will be replacing all of the transistors. Does anyone know of a U.S. source for these small signal devices that does not charge high shipping and handling?

What I'll do is pre-flight test the other channel... re-drill the PCB's mounting holes so the Mosfet pins are long enough... and be extra careful. It would be nice to get one channel running to boost my confidence. Thanks again for the help so far.

[halojoy]

I am sure you will get it right in the end.

It only needs one little wrong connection
somewhere in the circuit
and nothing will work alright, at the output.

Thinking of all the mass of connections and components
there is in a Power Amp,
it is a wonder they will work at all.

/halo

[richeros]

Hi!

Im in the starting fase of building the symmetrical amp, I have the Heatsink/transformer/PCB and all the MOS-FETs. The problem I have is regarding some parts value....

So I thought Id ask you what you use.... (Anthony has not answered on my e-mail)

* C3 - schematic = 100nF, part list = 10nF, what is right?

* Do you know a good replacement for ZD3 and ZD4 or would any simular
work?

* What is the rating of C12? - ,47 !? Is that 0,47uF?

* Is R42 1Mega ohm?

Hope anyone can help me as these questions are the last barrier...
Thanks!

[Kilentra]

Hi richeros,

For C3 I am using 100nf. I did not receive a parts list. In fact I made the "mistake" of ordering high-voltage polypropylene caps (notice how on my board there are some large red/brown caps instead of the blue rectangular MKT caps normally used). I am not sure if this will improve the sound, but it did cost a little more and was a bit of trouble fitting some of them into the PCB.

ZD3 and ZD4 are specified as 1N4737, I assume any old 7.5V zener diode will work. C12 I assume is 0.47uf. At least that's what I used. I also used R42 as 1Mohm. I just used all the values from the schematic in the online construction manual.

I recommend you order some extra small signal transistors (MJE340s, BC546s, etc), because if anything goes wrong you'll need some. They only cost maybe 50 cents each. Also see if you can get some extra Mosfets so you can match them more closely, and so you can replace some when they get destroyed.

I'll have to ask again if anyone knows where I can get a few of those transistors in the U.S. without the usual handling costs. Why can't these places just put them in an envelope and mail it?

Anyway, I used a drill to extend the Mosfet mounting holes closer to the pins. This made them close enough so I could solder them directly into the board and they would still fit. The second channel passed the pre-flight test and is just waiting to be fired up... I'm not at home now but I will post a picture soon.

I have one last question. Long after I unplugged the second channel the power supply wires touched somewhere and the clipping indicator LED lit briefly. I think the supply was down to just a few volts, and there are diodes on the rails onboard to prevent switching polarity, but is there any chance this damaged something on the board? I would rather not have a similar "conundrum" with the second channel! (And I cannot easily do the pre-flight test again, for the Mosfets are all soldered and firmly mounted.) Thanks again guys.

[Kilentra]

Hi again,

I have completed the second (first one working) channel of the amplifier and it works, which to me is quite a relief. However, I connected the output to the large solderpad on the PCB next to ground (which I assumed was the right place for the - output) and after a few minutes of music at moderate levels, R9 (10 ohm resistor to ground; I used 0.6W version) started smoking. It's quite burnt as you can see, it used to be light blue.

Duh! Well, of course if you connect 10 ohms between the speaker and ground you will pass a lot of current through the 10 ohm resistor. This is the kind of brain-fart I'm trying to avoid.



The lesson here: if you are building the symmetrical amplifier, don't solder your output wire into the spot on the PCB where you might think it should go - connect it directly to ground.

So I replaced the resistor, reconnected the output and it works again. But who knows, this could have damaged something else. The replacement parts for the first channel are on the way. Again, thanks for the help.

[Kilentra]

Hi, I have one last question.

How am I supposed to mount Q10, the Vbe multiplier device, on top of the output stage Mosfets?

I used a dab of silver thermal paste in the middle and applied superglue to the edges to hold it down, which I assumed would create a good enough thermal coupling. However, the tops of the Mosfet cases are noticeably warmer than the top of the BD139 casing... they are around 45C I think (fairly hot) whereas Q10 is barely warm, which I assume cannot be a good thing for the stability of the amp. I am wondering how others who built the N-channel amp mounted the similar device from that project?

I read the thread where Arne K's amp went up in smoke from improper thermal stability and I've been reluctant to turn up the volume on this one because of those concerns.

Is the thermal coupling I have adequate, and if not, what do I do with this thing?

[richeros]

Hi!

Have been woundering how to do that myself..... "piggyback"

Use thermal paste or a silicon pad between??!!

Wouldnt it be possible to mount Q10 on top of the O/P device with the same screw?? or wouldnt Q10 come close enough to the "core" of the O/P?

Richard
NORWAY

[Cobra2]

My amp did have a small dose of magic smoke...most probably because of too low watt-rating on the deg. resistors, combined with high volt psu (+/-75V) and tough speaker-load.

Now cured, and I'm very happy with it.

Arne K
NORWAY

[Kilentra]

As you can see the mounting hole on the BD139 is far smaller than that of the IRFP240 and I would be afraid of damaging the die by drilling to enlarge it. Electrical isolation is not a problem with the plastic case of the Mosfet.

Arne, I have an overvoltage (as high as 77V) power supply and difficult speakers as well. Degenerative resistor power should not be a problem... at 500 watts output into 4 ohms there would be 11 amps RMS current, spaced over eight 0.22 ohm resistors so each would have to dissipate less than one watt.

BTW, how does it sound?

Did Mr. Holton ever tell you just how closely coupled Q10 should be? Maybe I could rip it off the Mosfet, put in longer wires and bolt it to the aluminum bracket there? (Actually, that would probably be an improvement over what I have now). I am having trouble reaching him by e-mail.

[richeros]

The IRFP240 has Ø-3.6mm and the BD139 has Ø-3.1.

I used a M3 bolt to mount the BD139 on top of the IRFP240, worked out well, but is it good enough?

(This was an older version of "Symmetrical amp" - didnt work that well)

Richard