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Old 9th July 2013, 05:18 PM   #201
amptech is offline amptech  Scotland
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Default leach superamp

Quote:
Originally Posted by still4given View Post
I don't have anything other than what is called for in the BOM and schematic. What's weird is I can't find anything different between the two boards yet one works perfectly and one oscillates. The one that oscillates has stopped oscillating a couple of times. There are some slight voltage differences between the two as noted in post #192.

I'm not opposed to adding some things if they will work.

Thanks
Hi Terry, some amplifiers have slight voltage differences between them due to parts used such as the resistors being high or low in there values and this can have and effect on volt reading's.. if possible try one amp module by it'self and take readings.
One thing to do is earth the large heat-sinks as they can act like antennas thus picking up stray fields and the amplifier under test can start to oscillate as it pick's this up and amplifies it.. use shielded leads for the i/p put cables and keep o/p leads away from psu's...

regards A.
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Old 9th July 2013, 10:00 PM   #202
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Hi Anthony,

I grounded the sinks and moved the PS as far as I could without making new wires. (About a foot away) I have had a shielded cable on the input from the beginning.

Without a load on it, the output looks good. There has to be some reason that the oscillation is only on the lower part of the sound wave. The other channel looks the same with a load as without.

Thanks.

PS. I bought the "The Audiophile's Project Sourcebook" Good reading so far.
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Old 9th July 2013, 10:19 PM   #203
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There's plenty to read within that book,from in stages to o/p's plus protection circuits plus all the terms used vas..long tail dif input,pole comp caps even power amp circuits to build.

I've build a few of mosfet designs all with good very good performance for them selves..

ps check out the chapter on ways of biasing the o/p stage via voltage drops across the emitters.

Regards A.
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Old 9th July 2013, 11:08 PM   #204
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Yes, I'm looking forward to reading through it. I'm sure I will learn a lot.

The bias thing has been a little weird on the Leach. All of the emitter resistors read different. They are not far off from each other but they are off.
If nothing else, I am learning. This may end up being a mono amp for my sub.
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Old 9th July 2013, 11:25 PM   #205
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Emitter dropper resistors can be from .22 -.33 or.47 ohms leaches amp should be .47 say 5watt.
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Old 9th July 2013, 11:58 PM   #206
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Mine are .33R per the BOM.
I see from 15mv to 19mv across them when set to 125ma at F1.
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Old 10th July 2013, 02:42 AM   #207
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Quote:
Originally Posted by still4given View Post
The bias thing has been a little weird on the Leach. All of the emitter resistors read different.
Watch for fake transistors!!! Inconsistent bias readings between parallel units is a big RED FLAG. That could explain why one channel is oscillating and the other not. Got a close-up of the markings on the outputs? And where did you get them?
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Old 10th July 2013, 05:25 AM   #208
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I got the O/P's from On-semi as samples back when they still did that. Everything else came from Mouser. Here's a shot of them in the channel. I can take a better one of the devices is you need but like I said, they came directly from ON.
Click the image to open in full size.

I also just replaced all of them on the bad channel and got the exact same readings so I don't think it is the O/P's.
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Old 15th July 2013, 10:48 AM   #209
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Default leach superamp

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Originally Posted by still4given View Post
Mine are .33R per the BOM.
I see from 15mv to 19mv across them when set to 125ma at F1.
One way to set bias up is to check readings over emitter resisters and let the amplifier warm up and re-trim where need and aim for equal measurements.
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Old 15th July 2013, 10:56 AM   #210
AndrewT is offline AndrewT  Scotland
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Vre with a single pair will show a difference top to bottom due to tolerance of resistor values (Re).
Vre with multiple pairs will show a difference top to bottom and between adjacent due to resistor tolerance ands due to transistor tolerance.

Matching of resistors is very easy. That should never be ignored.
Matching of Transistors is much more difficult, but good Vre agreement can be achieved with careful Amateur Methods of Matching, especially of Vbe @ bias current.
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