Leach Superamp, round 2
Back in 2005 I attempted to build a Superamp as my second project. I actually started it while I was building my first amp which was a Elliot P101. The reason I started the Superamp was because I bought the wrong transformer for the P101 and the Leach was a good fit for the transformer. ;)
Anyway, I had trouble getting the Leach to work so I put it aside and went on to other amps figuring that someday I would gain enough experience to be able to tackle the Superamp again. As I continued I built KSA50 with TO3 outputs, a Leach Low TIM, another KSA50 with plastic outputs, an Aleph-X and a Symasym. At that time my 12 year old son dragged me kicking and screaming into RC airplanes and I put DIY audio on the back burner. Well my son has grown up and moved on to other things and I have recently retired so I have a lot more time on my hands. A couple of weeks ago I sat down and got out my KSA50 that had quit working and fixed it. That was so much fun that I also got out the ALeph-X that was having problems and fixed that as well so now I am going to try to tackle the Superamp once again. I have removed the output leads from the board so I could test it and see if I can get it working. I am attaching the voltages I got from the test. I'm hopeful that one of you guys who are familiar with this amp will be able to look at them and see if you see any glaring problems. I do have a scope but haven't been able to find the proper leads so I ordered some and should get them shortly. Hopefully one of you can walk me through hooking it up.
So here are some picks.
Hi, looking at your reading there's odd different volt reading...did you match all the resistors and transistors? pcb 2 has most off voltages..I would tend to rework that board and bring the voltage up slowing plus test the volts at the o/p end anything over 100mv would be to high.
the old lamp tester will show if the amp is ok via quickly going bright at power up then off or use a lower supply to check the amp..
also read leach's info on this amplifier on setting/testing...
Thanks for taking time to help me.
I should have mentioned that the mains voltage changed throughout the day. I also used a variac so the rails were a little different from board #1 to board #2. Even with that, I noticed that the main voltage changed a little while I was in the middle of testing one of the boards. I may have confused the ECB's on some of the transistors as I had to take the reading from the bottom of the board and it wasn't always easy to see which pin was which. I am mainly interested in whether there are any numbers that are off enough to indicate any devices that are not in the ballpark. I have read the Leach document many times. Like I said, I don't have my scope available yet so I can't take any other reading at this time.
I do remember taking time to match things as well as I could when I was building this amp.
Per the Leach docs, I have 100ohm resistors between the output and the emitters of Q16 and Q17. I believe I got 17mv on one board and 102mv on the other. Q7 is shorted per the docs so the bias pot is out of the circuit. Keep in mind that don't have the power transistor connected at this time.
Along with using a variac, there is a 100 watt light bulb between the variac and the transformer. There are also 100ohm 1/4W resistors between the power supply and the boards. Believe me, I took it very easy bring these boards up to voltage. I was very pleased that nothing got warm on the board. :)
Here is a pic of the setup I used to test.
Hope you have connected the diodes D1 to D4.
What is the voltage at the points shown in the jpeg.
what i do in my super amp builds is to install 10ohm 1/2 watt resistors in lieu of the fuses, having built up the completed amp...
the reasoning here is so that in case there is something wrong, such as reversed connected bias diodes, these resistors will vaporize instead of the output trannies frying out...
i do this one channel at a time...
if the resistors smoke out, then look for wiring mistakes...
correct any mistake and start again...
now if no smoke is coming out, you can the measure the output offset voltage,
it should not be more than +- 100mV...
next comes the idle current adjustment..
you can connect a dmm to read low volts accross the 10 ohm resistors and measure current thru them...
the output trannies are stacked so that they share the rails equally and you can check that that is the case...
Those two white wires... look like they are connected to the diodes. Please use separate colors for identifying anode and cathode and have to be connected correctly.
Also use three colors for each connection of a power transistor and bunch the wires separately for each transistor.
Edit: The idle current should be adjusted measuring the voltage across R45(R48). Measure across any one resistor and set the current to nearly half of the total specified in each. Both will never be the same.
I had adjusted the idle current for my Leach amp by the method as in post #6, but when I measured across the emitter resistors, there was no(very little) current flowing.
The voltage between the two point you asked about is .817VDC
Yes, that is the same way I will do it after I attach the power transistors and D1-D4. That is basically the same proceedure I used for setting the bias on my Low TIM,
Please explain your method for setting the bias. Sounds interesting.
if you measure across the emitter resistors,
readings will show lower current, that is to be expected...
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