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what went wrong with my amp?
what went wrong with my amp?
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Old 24th May 2004, 11:19 PM   #1
Wagener is offline Wagener  South Africa
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Default what went wrong with my amp?

I just finished my leach amplifier. I set the biasing pot until a current flowed.

I then connected a 4-ohm (organ speaker) to the output.

I connected the input of the amp directly to the output of the PC's soundcard. The amp worked but the playback was very soft. Even at full volume the amp played very soft.

I then decided to connect the input of the amp to a set of logitech amplified speakers.

I set all the volumes of the amplified speakers, ect to zero.

When I turned on the amplified speakers (logitech) the organ speaker at my amplifiers output made a very violent thump and then the amp went dead.

I had the ammeter connected between the cap and the +Vcc of the amplifier the whole time. There is now zero current flowing to the amplifier.

I have to finish this amp before wednesday and I don't know where to start looking for the blown component (small signal transistor I suppose)

Also, I had 1.5A fuses connected between the +Vcc and -Vcc. I made it low amps to be safe. Neither of the two blew?!

Please help. What type of signal should I feed the amplifier before it will start to perform? Pre-amp? I thought the soundcard will provide a large enough signal. Why did the logitech speakers blow my amplifier?
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Old 25th May 2004, 08:52 AM   #2
djk is offline djk
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Wire a 100W lightbulb in series with one side of the line cord.

Check R51

Use a 1:1 input transformer until you get your problems sorted out.

Your chances of getting this fixed by Wednesday are ZERO.

At his web site prof. Leach said to allow about four months from start to finish.

Always turn on the signal source well before the amplifier.

Always turn off the amplifer well before the signal source.
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Old 25th May 2004, 09:39 AM   #3
Eva is offline Eva  Spain
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Some basic things to theck :

- Output offset [pretty obvious]

- VAS biasing : Measure votage difference between Q14:B and Q15:B. Measure also voltage drop across R23 and R24

- Input stage biasing : Measure voltage drops across : R7, R8, R9, R10

- Input cascode biasing : Measure voltage between Q5:B and ground, and between Q6:B and ground [obvious : Q5:B stands for 'base of Q5']
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Old 25th May 2004, 10:00 AM   #4
lucpes is offline lucpes  Romania
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Can't be this simple, but it looks like the mains fuse blew if there's no current at VCC+ and VCC-. Remove the two rail fuses. Start the amp supply (rails disconnected). Does it supply current? If not, the problem is there. Replace the mains fuse. Check if it works again. Stop the amp supply. Discharge the big capacitors (if it worked) using a 5W 100R resistor (or a lightbulb with aligator clips). Put the rail fuses back in after you did this check.

The amp should've been ok with sound card input, DO NOT connect it to another amplifier's speaker out.
We should no more let numbers define audio quality than we would let chemical analysis be the arbiter of fine wines. Measurements can provide a measure of insight, but are no substitute for human judgment. Nelson Pass
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Old 25th May 2004, 10:15 AM   #5
Gofer is offline Gofer  Netherlands
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Also check that your ammeter's fuse (if it has one) is still ok.
You can measure the curent through each device by measuring the voltage across the emmittor resistors. This is much safer.
Before energizing the amp make sure nothing is blown, otherwise things will start to burn.
It is a very difficult amp to build, even more to trouble shoot.
I wish you good luck.

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Old 25th May 2004, 09:11 PM   #6
ChocoHolic is offline ChocoHolic  Germany
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My personal estimation is exactly in line with Gofer.
Most Digital Ampere-Meters use fast or ultra fast fuses inside.

You can also measure the rail voltage in front of the current meter and behind it. If the rail is fine in front of the current meter, but down behind, then you can be sure that the fuse in the Ampere-meter has blown. If so, then you should check carefully your power transistors before you power up again...

Measuring the rails is my first standard check, which I typicall do...
and then ....well, how could it be different ...
then I would follow Eva's list for basic checks...

If you are lucky nothing has happened and the only issue is that your
logitech speakers cause heavy thumbing during turn on. (May be some big input caps and a input stage that operates from DC-level different from ground...., cheap practice...)
You could try to turn on your logitech first, before you power up you leach.

Good luck
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