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Old 1st September 2004, 07:37 PM   #1
Wagener is offline Wagener  South Africa
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Location: Paarl
Default please help with my amp!!!

I am still having trouble with my Leach Amplifier. It is only the one channel I am concerned about right now.

This is what happened. At first I got -0.007V when I switch on the amp. This voltage slowely decreases to -0.009V after some

10 minutes. That I suppose is normal because of thermal drift. It seems that the amplifier "warms up" When I connect a

speaker to the amp the heatsink starts heating up and after some time I noticed that I get DC out of the channel.I think this

only happened when I connect the speaker while the amp is still "warming up"

The speaker is a 8-ohm subwoofer with a capacitance of 1.2uF (including the cable which actually causes this capacitance)



At this very moment I have this scenario (no load connected, no input signal) : When I power up the amp I get 4.2V DC at the

output which rises to 16V atfer some time. I also get 10V on the collector of Q12 rising to 19.3V and 15.5V on Q13's

collector rising to 15.5V.

The 16V that I get at the ouput is also measured at the bases of the 4 output transistors (Q18-Q21) So I know that they are

ok. Also tested all the transistors (on the PCB and output trannies) C-E for shorts, could not find any.

Where do I start looking, I suppose it could be the capacitors C6a & c6b but how do I check them?

Leach amp plans can be found at http://users.ece.gatech.edu/~mleach/lowtim/

Please tell me what voltage measurements to send into this newsgroup.

I would appreaciate any help.

Johan Wagener
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Old 1st September 2004, 10:15 PM   #2
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Personaly I don't like class AB differential amps, because they can be hard to stabilize the DC output and DC drift.
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Old 1st September 2004, 10:27 PM   #3
AKSA is offline AKSA  Australia
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Yes, agreed. They have two voltage amplifiers, with drawn pistols across the prairie, always disagreeing. The result: poor DC offset control, no trivial thing on a SS amplifier.

Further, the non-linearities of the full differential mean that even order distortion is minimized, permitting odd order to bubble up through the mix, in turn militating against full musical presentation.

Better to use a single ended voltage amplifier........ Lower parts count, better DC offset, proper H2/H3 distortion spectrum.

Flame suit donned and ready.


Cheers,

Hugh
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Old 3rd September 2004, 02:57 PM   #4
Wagener is offline Wagener  South Africa
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yesterday when I turned on the amp it worked perfectly never supplying dc at output

This morning I switched it on and it started up with DC from the start. Once it starts with its DC state nothing helps. I tried discharging all caps by pressing some foil at the bottom of the PCB. Powering it up again will just do the same - DC.
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Old 3rd September 2004, 04:24 PM   #5
MikeB is offline MikeB  Germany
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To wagener:

Maybe some Fuse blow and the voltagesupply does not work properly ?
Or some cold soldering ?

To cunningham and aksa:

Personaly, i do like fully differential ClassAB, with these i have the lowest
DC-offset and lowest DC-drift. And i don't see lot of odd harmonics,
simply 2nd harmonic and 3rd at 50% of 2nd...
And without any dc-blockingcaps or dc-servos !
Ever tried single diff with bjts and without dc-blockingcap ?
I agree, it uses more parts.
My experience with symdiffamp style: very steady and reliable.
But this is the wrong thread and does not help wagener in any way...
A lot of people like the Leach-Amp.

Mike
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Old 3rd September 2004, 07:34 PM   #6
djk is offline djk
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Check R51

Without R51 the DC will drift over time and the output could go to either rail.
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Old 3rd September 2004, 09:24 PM   #7
ingrast is offline ingrast  Uruguay
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Default DC drift?

Have you scoped it?

Smells suspiciously of parasitic oscilations. If that were the case then check carefully transistor specifications as used vs. nominal, layout etc.
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Old 3rd September 2004, 10:21 PM   #8
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Default Shake all the parts, see if some is broken

And only making bad contact.... re-solder the entire amplifier.... do that, will be fast, will spend less time than try to discover theoretically....if no result, you really will have to replace parts.

When inside maintenance offices...people normally give some small punches to see if some contact problem appear...this is never done when customer is present, and looking...but they did.

Those intermitent defective, if i undertood correctly that have some intermitence, normally are solder problems or components broken....check it.

regards,

Carlos
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