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supernet 20th April 2002 02:18 PM

I need some help with super Leach amp!
I just finished my circuit boards and connect them to a power supply of +/-30V and everything seems fine.The current is fine, but then I connected my signal generator and osciloscope. I started at 100kHz and I saw nice clean sinus signal, but then I came down to 300Hz and there was a DISTORTED signal ( seems like a tooth saw). Then I checked the other board and the same problem: Clean signal down to 300Hz and then distorsion. I made 2 stereo Low tim amps and everything was fine, but with my first Super Leach amp is something wrong.

I double check TWICE all transistors and ... Is there a possibility that there was too low test voltage +/- 30V.

Thank you for help!

nar 20th April 2002 05:26 PM

Sure you were not too high voltage input?Check if OK then mesure V out,is the gain OK?Did you expect that gain?
How much does the amp draws.Out of the wall.
What is the design of this AMP?
Perhaps your load impedance is too low.

supernet 20th April 2002 05:45 PM

Voltage gain is about 20. The input voltage is 1 V peak and the current is about 10mA (via safety 100 ohm resistor) and identical to +and - rail.

I just wondering, but the amp is made for +/- 85 to 93 supply and think that +/- 30V are too low voltages.(this is the maximum of my bench supply).


nar 21st April 2002 02:33 PM

MMMMMMhhhhhhh. Supernet,I think your design is OK but a current draw of 10mA on the supply rails is strangely low for me.What is the supposed power output of the amp?I think your amp is not biased high enough,for a reason I ignore,but might be current limitation on your lab-benchtest supply,or,what I think more realistic,the supply voltage is too low and the amp has probably a threshold under which certain voltage potential supply,if too low,is unable to get proper bias for the amp.Especially if it is a complex bipolar design with many stages,the front end will not conduct well under certain amount feed voltage.Moreover,the output stage certainly has not its own points to work properly,and the bias of output transistors is certainly too low.

Check with the good voltage for supply rails.If problem is still there,assure that the front end is correctly polarized,using if necessary measure of the running bias for it,then looking at the specs.on transistor datasheet you use.

Then if seems OK you should be able to pick your test signal not distorted at the out of front_end circuitry.

Do the same for all stages.Then if problem persists,check out for feedback loop and correct value of this loop ( and components values associated.) Maybe you've done a mistake at this precise point.

Keep me informed of your evolutions.


djk 21st April 2002 02:48 PM

I'm surprised you can get it to work at all on +/- 30V, Prof. Leach says the minimum test voltage should be +/- 50V. The front end is cascoded at +/- 40V.

supernet 22nd April 2002 10:27 PM

Thanks for help!!

Now I tested boards with +/- 60V and it works superb!!!!

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