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Old 19th February 2006, 09:07 AM   #1
frckid is offline frckid  United States
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Default Im testing my leach amp and its acting funny

Hi everyone,

I have constructed two channels, both with the same problemss.

Testing conditions: short C12, connect signal and non-signal grounds, and wire the two resistors between either side of R36 and the output

Rails: +/- 8V

Input voltage: 1Vpp sine wave at 1kHz

I end up getting a huge current draw (nearly 1A) from the negative voltage supply and about 40mA from the positive supply with rails of +/- 8V.

If I remove the short across C12, i get balanced current draw from each supply (about 48mA at +/- 8V rails), I get a sine wave centered at 3.5V on the output that is not amplified at all, actually slightly attenuated.

If I ground the feedback path and remove the short across c12, I can get a small sine wave on the output with a +3.0V offset.

What could be going on? Has anyone else had any trouble getting their leach amp to work?

THanks,
Jon
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Old 19th February 2006, 10:42 AM   #2
MikeB is offline MikeB  Germany
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If i remember the leach schematic correct, it uses zeners to set the reference for the cascodes in input-ltp having a voltage bigger than 8v.
This means, the Leach can't work yet at +/- 8v. About shorting caps, this could result in big dc-offset, showing such behaviour. Maybe your Leach is fully functional, just the test setup not ?

Mike
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Old 19th February 2006, 10:56 AM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
Power up your bare PSU and check supply voltages. Switch off.
Connect to the amplifier.
Fit 100r resistors to the two main fuses.
Set bias pot to maximum resistance (minimum bias current).
Fit a mains light bulb in line with the primary windings of the mains transformer.

Stand back and switch on.

Now go measure.
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regards Andrew T.
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Old 19th February 2006, 11:04 AM   #4
Eva is offline Eva  Spain
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Follow AndrewT advice (I don't want to sound redundant ).
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