I finally fired up my Leach Amp!!!! - diyAudio
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Old 14th May 2002, 06:19 AM   #1
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Talking I finally fired up my Leach Amp!!!!

I finally got around to testing my Leach amp. It seems to work, as in that it didn't blow up or oscillate (at least that I could see on my sub-cheapo scope). It clips about 2/3 of the signal off at 1.15V input and stops clipping at about 0.4V input. I cannot get the output voltage above about 20V. This is with a 40W bulb in series with my power transformer. If I run it normally (no current limiting), it acts similarly, but not clipping quite as early, still really bad though. I am using a test CD and a small CD player as a freq. generator. It cannot go above about 0.4V @1kHz, 1.125V @60Hz.

I have yet to set the bias current.

What do you suppose the problem might be?
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Old 14th May 2002, 06:29 AM   #2
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by the way,
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Old 14th May 2002, 07:09 AM   #3
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A 40W globe will probably drop the voltage far to much to allow for correct operation. A 10ohm power resistor usual does the trick for testing or a far higher rated globe.

Sorry but i cant help you regards why the amp is clipping early when directly mains powered. I can only suggest the usual:- check device placement / values, check transistors and diodes for correct function and so on. Is the power supply outputing the correct voltage?
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Old 14th May 2002, 07:13 AM   #4
jleaman is offline jleaman  Belgium
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Default Dont use a disc man

Dude your probably getting the clipping because of the disc man it is not a good source for audio you need something with mour stable out put id try something diff mabey feed the audio from your disc man into a preamp..
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Old 14th May 2002, 07:32 AM   #5
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Thanks, I'll try it when I get a good signal source down in my basement lair (my stereo system is upstairs). I'll just build a simple 741 preamp, how does that sound? I just want to do a basic test on the amp, I don't need a super clean signal.
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Old 14th May 2002, 07:58 AM   #6
fcel is offline fcel  United States
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Kilowatt: I don't know if this would mean anything in regards to using your portable CD player as a testing source. Long time ago while looking at the spec of the CD owner manual, I noticed that regular home CD player puts out 2V while portable CD player only puts out 1V. For listening, I actually have to turn up the volume control more if a Portable CD player is hook up to the pre-amp with everything else staying the same.
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Old 14th May 2002, 08:48 AM   #7
swede is offline swede  Switzerland
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Hi,

You don't really have to go the whole mile to build a pre-amp. Just build an attenuator, as mentioned in the thread "Passive Preamp" and hook your regular CD-player to it.

I'm running my Leach Amp using a Pioneer DVD-player + an attenuator. Works like a charm.

You should also try and run it with a 100 ohm resistor instead of your lightbulb...

Good luck,
//magnus
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Old 14th May 2002, 09:55 AM   #8
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Quote:
Originally posted by swede
Hi,

You don't really have to go the whole mile to build a pre-amp. Just build an attenuator, as mentioned in the thread "Passive Preamp" and hook your regular CD-player to it.

I'm running my Leach Amp using a Pioneer DVD-player + an attenuator. Works like a charm.

You should also try and run it with a 100 ohm resistor instead of your lightbulb...

Good luck,
//magnus
Swede,

While the above solution MAY surfice for normal use, a Discman is a poor source for testing purposes and as suggested above should at very least be fed via an active preamp or preferably totally excluded from the test bed.

Also, I agree that a suitably rated 100ohm resistor should do the job well in most cases.

Kilowatt,

If you really want to use a light bulb, use ~ 120V 150W kind.
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Old 14th May 2002, 10:21 AM   #9
swede is offline swede  Switzerland
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Sorry if I'm confusing you all...

I did not recommend to use a discman, though the translation from my head to english to the keyboard and over the cyberspace might have indicated so.

What I meant was to use a regular, off the shelf CD-player (not portable).

If you follow Prof. Leach's recommendations on how to test your board, you'll probably find the source of your clipping.

http://users.ece.gatech.edu/~mleach/lowtim/part2.html

As I said before, use two standard 1/4 watt 100 ohm resistors (one for each powerline). Most diy:ers have them laying around in big piles. You are probably not an exception. ;=) That'll do the trick. I've blown a whole bunch of transistors in my earlier tries to get my Leach Amp to work, before I started using resistors instead of fuses when I tested my amps.

And! I think that if the amp doesn't blow up when you have the resistors hooked up, you can go ahead and disconnect them. The amp will behave strange if you still have them there. Try and disconnect your lamp! It might work.

//magnus

//magnus
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Old 14th May 2002, 11:49 AM   #10
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My first testing was done with Sony Discman(0.7Vmax!).Now i own Marantz CD player.Problem with discman is his weak and low quality signal,so amp cant play loud.Discman in my case added some noise to signal.
100Ohm resistors works fine for me and shoud do same for you.

P.S.I have my share of problems with transformers.At least output transistors are still first
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