DC offset problems with leach amp

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#41
remove a fuse and put your ammeter in line; or, as Leach indicated, put 100ohm resistors in place of fuses to limit current, and measure voltage across resistor to determine current. You may want to add a fuse in line if you do the ammeter thang. You don't want to smoke your ammeter if it's not self protecting!
 
#42
i see you got the ver 4.5 there, if iwere you this is how i would proceed:
1. do not connect the bases of Q16,17 just yet, make a temporary jumper connection between pts. A and B, set p1 to maximum.
2. connect the juction of R34,35 to output,
3. connect a 1k ohm 1W resistor to +/-Vcc on the pcb and to power supply, this way if anything this will limit the current to 50mA on either rails,
4. apply full voltage, and check the voltages on the bases of Q6,Q5 they should be identical,
5. check the voltage drops accross R11,12 they too shoud be identical..
6. any deaviation from 4,and 5 should give you the problem...

way i understand it leach amp pcb's are made out of double sided fr4 glass epoxy material with one side as ground plane where all circuit grounding is done and the other side as solder side. i must admit that i am not very familliar with ver4.5 as i have done only super leach amps ver1.....your problem could very well be a component nat connected to ground such as zeners, that is why above steps are needed to confirm....your pcb look neat from where i sit....
 
#44
Sadly no updates yet. I have everything laid out and ready to go, so hopefully I will be able to get everything up and running this comming week. (lousy real job getting in the way of things that really matter ... speakers) 😉

Thanks for all of your input though.

BTW, I have sent Professor Leach 3 e-mails now, and he hasn't responded to any of them. Are you just using the address posted on the leach amp web sight?


Thanks,

Wes
 
#46
Joan2, should I do these steps before I connect the power transistors?

Also, Professor Leach did e-mail me a few days ago. He said the he suspects my problem was as follows:

"The problem you described sounds like you don't have the common terminal on the power supply connected to circuit board ground. This leaves the circuit ground floating and the board doesn't clip symetrically."

I guess this might be the problem, but since I connected both boards identically, (and several times) and still got the same problem I am a little skeptical. It could be a problem with my traces though.

Finally, I have a newbie question: Are the two input jacks one for each channel, or are they one for signal ground and one for signal input? (i.e. does one jack have two wires going to one board, or does one jack have two wires going each one to a seperate board?)

I will try your suggestions once I hear back from you about the output transistors. Everything else is ready to go.


Thanks,
Wes
 
#47
just as i suspected, mr leach thinks you may have a missing ground connection, so check and double check....the input wires have two strands, one is for ground, the other is for the signal which goes into the center terminal of the rca jack, the other to the outer shell of the rca jack, usually thru a solder lug and fiber washers....there are two (2) ground points that you have to connect to the psu ground, one near the input, the other near the speaker out...it is important that you ground both..if you ground only one, then you have problems....good uck!!!
cheers!!!
tony
 
#48
test on boards

Ok, first the good news. I didn't kill myself poking around in the amp. Yay for still being alive. I plugged it in and no smoke or flames or big explosions.

Now the bad news: The voltage at the base of Q6 was 31.47 V. The voltage at the base of Q5 was -29.29 V.

The voltage drop across R12 was 2.8V while the drop across R11 was 2.9V.

So, now I suppose that we know whats wrong right? Only problem is that I have no idea what these differences imply. :bigeyes: Here is where the expertise of many people more knowledgable than myself comes in. So... Do your thing guys.

I assume this means that I am missing a ground connection somewhere. But does it show where? If not, I suppose I'll go back and check them all. (just a continuity check at all of the place where the circuit schematic shows a ground connection right?)


Thanks all for your help,

Wes
 
#49
wes,
there's your clue right there, you should be getting equal voltages on bases of Q5Q6, strange, but you should be getting positive voltage at the base pof Q5, negative at the base of Q6, are you sure you got the negative prod on the ground of the psu? btw, what is the reading of your +/- Vcc's?
 
#52
Hmmmmm

Ok, this is kinda wierd. For a while +-VCC's were equal after the resistors in the fuse ports and the voltages I measured earlier were equal as well. During this time I was getting a DC at the output of 700 mV. Not good, but not as bad as what started happening then.

Maybe I accidentally shorted something I don't know, but now the DC at output is 10 V 😱 and the voltages are the same as I measured earlier.

I am completely clueless as to what this means, all my connections still look good so maybe it is a failed component?

Thanks,
Wes
 
#53
Once again, (I gotta stop doing this...) I was doing more measuring around D15 and D16 and heard a small "pop". Well now the DC offset at output was 24 V. So, I figured I did something to the diodes (can these be burned out?)

Anyway, I changed D15 and D16 for new ones and now the DC at output is back to 10 V.

Hmm, maybe I'll get two more diodes and replace D13 and D14.

Any suggestions based on the last 4 posts (I know...I've been busy)


Thanks guys,
Wes
 
#54
sweet equality (not quite)

OK, I changed the rest of the zeners and...

all of the voltages you reccomended that I measure were equal.
I now only have a 700 mV offset at output.

Then things started going wrong again. I got back up to a 10 V offset after measuring. So, I started playing around with the zeners again.

At the end of the night I have about a 5 V difference between +-VCC after the resistor in the fuse spots and only -140 mV of offset at the output. This means that the different sides are pulling different current right? Now that the offset is much lower; is this normal, or should they be pulling the same amount of current? I haven't checked the output waveform but I suspect that since when I varied the +- VCC relationship when I tested before the waveform evened out that this will happen also.

I am really confused right now.



Thanks,
Wes
 
#55
Last post I promise

Ok, scratch that last paragraph. I noticed that I had a solder bridge on one of the diodes. Now things are back to being equal with the 700 mV offset at output.

So... can I expect this to dissappear when I reconnect Q16,17 and the output transistors, or is this a problem?


Thanks for putting up with all my shenanigans

Wes
 
#56
breath in, breath out

see you got the ver 4.5 there, if iwere you this is how i would proceed:
1. do not connect the bases of Q16,17 just yet, make a temporary jumper connection between pts. A and B, set p1 to maximum.
2. connect the juction of R34,35 to output,
3. connect a 1k ohm 1W resistor to +/-Vcc on the pcb and to power supply, this way if anything this will limit the current to 50mA on either rails,
4. apply full voltage, and check the voltages on the bases of Q6,Q5 they should be identical,
5. check the voltage drops accross R11,12 they too shoud be identical..
6. any deaviation from 4,and 5 should give you the problem...

way i understand it leach amp pcb's are made out of double sided fr4 glass epoxy material with one side as ground plane where all circuit grounding is done and the other side as solder side. i must admit that i am not very familliar with ver4.5 as i have done only super leach amps ver1.....your problem could very well be a component nat connected to ground such as zeners, that is why above steps are needed to confirm....
Click to expand...

step 1 and 2 is a temporary connection, to complete the circuit somewhat.
step 3 is for safety, also temporary.
you are lucky to encounter this problem, after solving this you have earned your wings and can do more challenging problems!!!!
 
#58
Ok, this is making me a little angry. I touched nothing since I last measured. NOTHING. This morning I plugged it back in and measured the +- VCC and it was off. Also the 10 V offset at output is back.

I don't know what to think except that maybe there is some failed component that comes online sparadically.

Any thoughts?

Thanks,
Wes
 
#60
Success (the long way)

Alright, first off I have my amp up and running. No problems. I must say that the sound quality is outstanding. It seems to have lots of power on reserve while still reproducing all of the subtle details. (OK, I may be a little biased because this is the first amp I have ever built) But WOW. I can't wait to finish my electrostatic panels and hook them up to it.

But I must say that I didn't manage to fix the problem with the old board. Instead I shyed away in frustration and populated a new board with all new transistors. But although I never managed to resolve the problem with the old board, I wanted to thank all of you for your help. (especially joan2) You guys made this a much more enjoyable and much less frustrating experience.



Cheers,

Wes
 
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