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Old 30th April 2009, 01:33 PM   #631
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Indeed, with single transformer I recommend a single rectifier board shared between two channels. Only CHG pads connect to chassis, nothing from rectifiers. There shouldn't be any star ground outside the amp boards, see more info here: http://www.diyaudio.com/forums/showt...69#post1518369

Please see this link how to measure for correct PS voltage: http://www.diyaudio.com/forums/showt...88#post1518288

If that checks out fine, the next thing you should do is to measure DC offset from the amp: http://www.diyaudio.com/forums/showt...04#post1521304

When offset measures OK, the chip most likely works fine, if there is still no sound, your wiring may be a problem.
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Old 30th April 2009, 01:40 PM   #632
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Hi ,

I am building two monoblocks out of Peter's kit.
The design is rather complicated and thus the Ps is in a different box underneath the amp box.
The Ps box has its top made out of 1 mm copper plate, its bottom also and one of its side. The rest is wooden.
The amp box at the top has its bottom 1cm thick copper and the rest is wooden. So actually between the ps box and the amp box there some copper and air.

Peter do you think there might be a humming problem in the amp or i need more shielding? Keep in mind i will use good quality components and cables.
I know the question is theoretical but i have to do it since the design of the box is rather unique and difficult and it would be difficult afterwards for changes.


thanx
Nick
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Old 30th April 2009, 02:22 PM   #633
tbrooke is offline tbrooke  United States
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Quote:
Originally posted by Peter Daniel
Indeed, with single transformer I recommend a single rectifier board shared between two channels. Only CHG pads connect to chassis, nothing from rectifiers. There shouldn't be any star ground outside the amp boards, see more info here: http://www.diyaudio.com/forums/showt...69#post1518369

Please see this link how to measure for correct PS voltage: http://www.diyaudio.com/forums/showt...88#post1518288

about 36 volts I have capacitors but this seems high to me

If that checks out fine, the next thing you should do is to measure DC offset from the amp: http://www.diyaudio.com/forums/showt...04#post1521304


quote:
<<< This particular amp (right channel) measures 32mV offset when volume is completely down (input shunted to ground). >>>

I assume this means simply short the center of the RCA jack to ground?


quote:
<<< and 70mV when volume is at the maximum (15K combined input impedance of 50k pot and 22k input shunt resistor) >>>

No pot on mine I can connect preamp or output from soundcard
or do I just short a resistor to ground from the center of the rca jack, if so what resistor?



When offset measures OK, the chip most likely works fine, if there is still no sound, your wiring may be a problem.
n volume is completely down (input shunted to ground).
Quote:
This particular amp (right channel) measures 32mV offset when volume is completely down (input shunted to ground).
Quote
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Old 30th April 2009, 02:46 PM   #634
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Quote:
Originally posted by tiglitosa

Peter do you think there might be a humming problem in the amp or i need more shielding?
The shielding is not always a must. In fact, I'm still using an amp without any shielding at all, boards and transformer simply mounted to an aluminum bar: http://www.diyaudio.com/forums/showt...580#post636580

There is no humming issues present at all. Of course, that will depend on your local conditions.
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Old 30th April 2009, 02:47 PM   #635
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Quote:
Originally posted by tbrooke

n volume is completely down (input shunted to ground).This particular amp (right channel) measures 32mV offset when volume is completely down (input shunted to ground). Quote
So what else is there that it does not play?
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Old 30th April 2009, 03:02 PM   #636
tbrooke is offline tbrooke  United States
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Default Previous Post

Sorry my previous post got garbled with the quotes.


Basically I was unclear about measuring DC offset without a pot.

Tom
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Old 30th April 2009, 03:50 PM   #637
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To measure offset you don't need to shunt the input, as you already have the 22k R2 resistor there. That will show the highest offset, if you shunt the input, the offset will go down.

The average values measured for batch of chips were posted here: http://www.diyaudio.com/forums/showt...77#post1524877
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Old 30th April 2009, 08:49 PM   #638
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Default speaker ptotection

Hi again
Can you guys tell me how many of you are using some kind of speaker protection and does it really need to be used in gainclone?
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Old 30th April 2009, 09:36 PM   #639
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Never used one and based on customers feedback it's not essential.
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Old 1st May 2009, 02:06 AM   #640
tbrooke is offline tbrooke  United States
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Default Troubleshooting revisited (LM3875)

I set up my DC offset measurements as follows

No Pot - Input to ground -
measured at speaker terminals - no load

Right - oscillates between 6 and 7 mV
Left - oscillates between 5 mV 36 mV

I can hear the transformer humming in time with the oscillations

rechecked pg+ and pg- both about 36 volts and fairly constant
rechecked wiring about 10 times
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