super leach with 65mV at speaker output

[bigpanda]

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.

[Damon Hill]

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.

[bigpanda]

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

[Damon Hill]

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.

[Damon Hill]

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!

[osscar]

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.

[djk]

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/...mp/circuit.pdf

[AndrewT]

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.

[bigpanda]

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.

[shortbreak]

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.