Hi, happy new year to you guys (and gals),
I had setup a superleach and test it on Xmas eve. W/o the outputs and did the test on the board (no scope test) as stated on Mr. leach's article. I had 65mV at the speaker output. This seems a bit high. Can anyone kindly suggest how to traced the unsymmetric problem (or just to ease it, I usually have about 30mV for most of my projects)? Or that is OK and I should jump straight to the final calibration with the outputs in position?
I use 2N5415/2N3440 pair as that's what I have at hand right now. Could this be the cause of the problem? I think CDIL still produces these, any way I can buy samples from them? (say about 20-50 pcs of each)
Tks.
I had setup a superleach and test it on Xmas eve. W/o the outputs and did the test on the board (no scope test) as stated on Mr. leach's article. I had 65mV at the speaker output. This seems a bit high. Can anyone kindly suggest how to traced the unsymmetric problem (or just to ease it, I usually have about 30mV for most of my projects)? Or that is OK and I should jump straight to the final calibration with the outputs in position?
I use 2N5415/2N3440 pair as that's what I have at hand right now. Could this be the cause of the problem? I think CDIL still produces these, any way I can buy samples from them? (say about 20-50 pcs of each)
Tks.
That's only a little bit high; the offset may be produced by mismatched transistors in the differential input stage. My practice with building the Leach amp is to match everything, resistors, transistors and even the Zener diodes as closely as possible.
Hi,
Tks for the super fast response (I thought I am not going to get any thing for at least 2-3 days, it's Xmas!!!)
Well, I did match the input stage transistors but not as far as you had (for the resistors??). Maybe I should try this and see what happen. Any information for the 2N5415 / 2N3439 pair? What did you use ?
I am not a perfectionist, I just don't want to blow any stuff up.
Tks
Tks for the super fast response (I thought I am not going to get any thing for at least 2-3 days, it's Xmas!!!)
Well, I did match the input stage transistors but not as far as you had (for the resistors??). Maybe I should try this and see what happen. Any information for the 2N5415 / 2N3439 pair? What did you use ?
I am not a perfectionist, I just don't want to blow any stuff up.
Tks
The Leach design is pretty much a 'mirror' image so I try to match every resistor with its complement. For the 300 ohm resistors, I match them as a group and the diff amps I try to get at least a fair match for the NPNs and then for the PNPs as separate groups, including the cascodes. It's really hard to match NPN beta to PNP beta since 'complementary' transistors so rarely are. And just for the additional challenge, I make sure the zeners are as close to 20 volts apiece as I can get them, or at least insure they're reasonably closely matched; 5% is specified so don't worry if you can't get any closer with what you have.
For the VAS and driver output transistors, just test to verify they're good and the beta isn't radically different. The transistors you have available should work.
What, blow up the amplifier? Isn't that the fun part, fixing our mistakes after the smoke clears and we can send the fire brigade home? 😀
If there's a lot of offset when the whole amplifier is built, we'll troubleshoot it from there.
For the VAS and driver output transistors, just test to verify they're good and the beta isn't radically different. The transistors you have available should work.
What, blow up the amplifier? Isn't that the fun part, fixing our mistakes after the smoke clears and we can send the fire brigade home? 😀
If there's a lot of offset when the whole amplifier is built, we'll troubleshoot it from there.
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Oh yeah, and take a few days off if you do have a serious problem and get frustrated. It's easier to work on unfamiliar and difficult problems with a clear head. 😉
Happy New Year!
Happy New Year!
Some time ago when I built Leach amplifier (non super) , I just matched the diff.transistor pairs and Zener diodes. I use resistors with 1% tolerance. DC output is almost zero.I share the above solutions.
If the zeners don't match, or there is a small mis-match in the diff pairs, I suggest adjusting the value of either R15 or R16 to reduce the DC off-set.
http://users.ece.gatech.edu/~mleach/superamp/circuit.pdf
http://users.ece.gatech.edu/~mleach/superamp/circuit.pdf
Hi,
measure the voltages across R1, R2, R7, R8, R9 & R10.
Tell us your results.
Remember, keep the voltmeter probes oriented so that the black is always towards the most negative part of the PCB.
One of your results will probably come back -ve, all the other should be +ve.
measure the voltages across R1, R2, R7, R8, R9 & R10.
Tell us your results.
Remember, keep the voltmeter probes oriented so that the black is always towards the most negative part of the PCB.
One of your results will probably come back -ve, all the other should be +ve.
Hi guys,
Tks for all the info. input. I did measure the voltage across the 2 zener pairs. They are around 39.5 so I think that is OK. And Damon, you are right, I should leave it for a while and let my head clears up (or reset?) first (well I had working on it for 2years, why not wait for a few more days but the thing is that it is sitting on my working bench that I might need to use for other jobs). I will try out djk's suggestion on return and if it doesn't work, then try on Andrew's (only because djk's suggestion involved less work).
Signing off for a couple of days then....
Tks for all the help I can get from everyone.
BTW for experienced guys, trouble shooting is fun but for me, well.... But the truth is : you learn thru mistakes. I had too many smooth projects so it's time to learn....the hard way.
Tks for all the info. input. I did measure the voltage across the 2 zener pairs. They are around 39.5 so I think that is OK. And Damon, you are right, I should leave it for a while and let my head clears up (or reset?) first (well I had working on it for 2years, why not wait for a few more days but the thing is that it is sitting on my working bench that I might need to use for other jobs). I will try out djk's suggestion on return and if it doesn't work, then try on Andrew's (only because djk's suggestion involved less work).
Signing off for a couple of days then....
Tks for all the help I can get from everyone.
BTW for experienced guys, trouble shooting is fun but for me, well.... But the truth is : you learn thru mistakes. I had too many smooth projects so it's time to learn....the hard way.
Hi bigpanda,
I completed a superamp in early December 09 and found the o/p offset was less than 80mV in both channels when powered, but floated up to +10VDC a few seconds after the AC was shut off. This 10VDC slowly dropped as the supply caps discharged but took so long that my 5" 2W test speakers were roasted after a couple of on/off cycles.
If you have a scope or fairly responsive dc meter then be sure to check for a similar drift issue. The solution was matching both the zeners to less than 50mV difference and also matching the diff pair transistors as stated in the build instructions. A slight offset still happens after the mains are off but the drift is less than 200mV.
Be patient because the results are worth it. The superamp is the only amp I've owned, built, or used that can control my speakers effectively although a Bryston 4B was close. The superamp also seems to have endless headroom.
Best of luck.
I completed a superamp in early December 09 and found the o/p offset was less than 80mV in both channels when powered, but floated up to +10VDC a few seconds after the AC was shut off. This 10VDC slowly dropped as the supply caps discharged but took so long that my 5" 2W test speakers were roasted after a couple of on/off cycles.
If you have a scope or fairly responsive dc meter then be sure to check for a similar drift issue. The solution was matching both the zeners to less than 50mV difference and also matching the diff pair transistors as stated in the build instructions. A slight offset still happens after the mains are off but the drift is less than 200mV.
Be patient because the results are worth it. The superamp is the only amp I've owned, built, or used that can control my speakers effectively although a Bryston 4B was close. The superamp also seems to have endless headroom.
Best of luck.
Hi people,
After a burial for 4 months, I dig it out and tried again. Like you guys suggested, I fiddle around with the 4 zeners by first measuring the individual voltages and swap to make the best couples that has the least difference between the 2 packs. I am able to get about 25mV and am well satisfied.
When I tried to make the second one some other things pop up :
After throwing the power switch, the current limiting test resistors did not burst into flames but just give out a burst of white smoke (not much). The dc offset did not change but so did the voltage on the test resistor, ie. the pot cannot adjust the bias current. I turned it off and after about 20 min, turned it on again. Same thing happened, burst of white smoke again. Check everything but seemed to be OK. Head scratching again. The resistors did not burnt up (measure with a DVM and shows 10ohm). After some thought, I decide to make the pot in the middle range before I power it up (suggestion was to have it in max resistance in the beginning before adjusting). This time there was no smoke and as I adjust it, things come out right although it is quite sensitive and difficult to adjust. I got them both setup and running. I am having 25mV, 10mV dc offset with them.
Shortbreak :
Thanks for the tip. I measure the DC offset during power off as you suggested and there they are, as high as about 1V and last for around 20 sec. I am thinking of a DC protection circuit. Would that help? I know it did some protection at power up but power down........?????
I pulled out another old project and it behaved the same. Seems that this is not just for super leach only. Can anyone thow some thoughts about this?
Tks for the help.
After a burial for 4 months, I dig it out and tried again. Like you guys suggested, I fiddle around with the 4 zeners by first measuring the individual voltages and swap to make the best couples that has the least difference between the 2 packs. I am able to get about 25mV and am well satisfied.
When I tried to make the second one some other things pop up :
After throwing the power switch, the current limiting test resistors did not burst into flames but just give out a burst of white smoke (not much). The dc offset did not change but so did the voltage on the test resistor, ie. the pot cannot adjust the bias current. I turned it off and after about 20 min, turned it on again. Same thing happened, burst of white smoke again. Check everything but seemed to be OK. Head scratching again. The resistors did not burnt up (measure with a DVM and shows 10ohm). After some thought, I decide to make the pot in the middle range before I power it up (suggestion was to have it in max resistance in the beginning before adjusting). This time there was no smoke and as I adjust it, things come out right although it is quite sensitive and difficult to adjust. I got them both setup and running. I am having 25mV, 10mV dc offset with them.
Shortbreak :
Thanks for the tip. I measure the DC offset during power off as you suggested and there they are, as high as about 1V and last for around 20 sec. I am thinking of a DC protection circuit. Would that help? I know it did some protection at power up but power down........?????
I pulled out another old project and it behaved the same. Seems that this is not just for super leach only. Can anyone thow some thoughts about this?
Tks for the help.
a relay type speaker protection circuit is a standard accessory to all my amplifier builds, leach or not.....
although you can get away without one for the leach since an offset voltage of less than +/-100mV is surviveable....offsets during turn-offs should not be a problem since all you are doing is to discharge the rails......
although you can get away without one for the leach since an offset voltage of less than +/-100mV is surviveable....offsets during turn-offs should not be a problem since all you are doing is to discharge the rails......
Hi Tony,
Long time no see.
I do have bleeding resistors (1k 5w) on the rails but it still takes a while for the caps (23000uf on each rail) to discharge which bothers me.
Long time no see.
I do have bleeding resistors (1k 5w) on the rails but it still takes a while for the caps (23000uf on each rail) to discharge which bothers me.
then use a normally closed relay which opens when power is on and disconnects the bleeders and closes soon after power off to reconnect big bleeders.
I would always recomend a speaker protection relay. Without a servo, the Leach is prone to turn on and turn off transients. This a very good amplifier otherwise.
My current (ver 4.5) Leach amplifier has no audible transients at all; they are present of course, but at very low level.
I could attribute that to carefully matching the zeners and all resistors, but I can't recall that of the five or so I built over the years that any of them had turn-on transients. One is unstable during turn-off, but only used for backup so I've never really tried to troubleshoot that issue.
I could attribute that to carefully matching the zeners and all resistors, but I can't recall that of the five or so I built over the years that any of them had turn-on transients. One is unstable during turn-off, but only used for backup so I've never really tried to troubleshoot that issue.
My Leach super amp never lets out any transients during power up or power down. Not a thump or pop- it's great I have no speaker time-delay relay, no servo etc.
Why is everyone having so much DC offset 😕 I matched the input diff-amp transistors and less than 10mV. Unless the bias current is way off?
P.S. I had hassles with the power transformer arcing the on/off switch and that was loud!
Why is everyone having so much DC offset 😕 I matched the input diff-amp transistors and less than 10mV. Unless the bias current is way off?
P.S. I had hassles with the power transformer arcing the on/off switch and that was loud!
"One is unstable during turn-off,"
When mine start making 'squeeg' sounds on turn-off, I know it's time to replace the bypass caps. After replacing the 100µF for the front end, and the 22µF for the outputs the amplifier is quiet again. Then I replace the cap in the feedback loop to ground. Imaging returns, and the bass is tight again.
When mine start making 'squeeg' sounds on turn-off, I know it's time to replace the bypass caps. After replacing the 100µF for the front end, and the 22µF for the outputs the amplifier is quiet again. Then I replace the cap in the feedback loop to ground. Imaging returns, and the bass is tight again.
In my case, I attempted to bring a 2.something board up to almost a 4.x version. It never quite sounded 'right', so I may have made a mistake somewhere. Measures good, but at times it sounds edgy, and there's no mistaking the oscillation at turn-off. I reduced that somewhat by introducing aggressive bleeder resistors to force the power supplies to drain down more symmetrically.
Eventually I ordered new boards from Prof. Leach, carefully built them up, and they sounded wonderful from the first time I listened to them. Much more air and depth. I put the old boards into a new chassis with salvaged parts from my very first Leach amplifier project; not surprisingly, the problem followed the boards. One of these days I really have to try and fix that...
Eventually I ordered new boards from Prof. Leach, carefully built them up, and they sounded wonderful from the first time I listened to them. Much more air and depth. I put the old boards into a new chassis with salvaged parts from my very first Leach amplifier project; not surprisingly, the problem followed the boards. One of these days I really have to try and fix that...
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