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Commercial Gainclone kit- building instructions

Hi£¬Peter

everything is ok.
the transformer secondary volt is 27.7VAC, rectifier output is 38VDC,after connect amp board the V+/V- down to 35.6V.
no signal input and dummy load,the DC offset is 0.042V.
 

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There is nothing to worry about grounds. Some guidance was given here: http://www.diyaudio.com/forums/showthread.php?postid=1108820#post1108820

The RCAs need to be isolated from a chassis. You are using what looks like Cardas RCAs, they should come with plastic isolation washers.

I would suggest rotating transformers to minimize wire lengths.

If you are adding source selector and potentiometer, please see this diagram: http://www.diyaudio.com/forums/showthread.php?postid=1516297#post1516297

The ground does not go through selector, but does through potentiometer: RCAs --> pot --> SG on amp board
 
Peter Daniel said:
I would suggest rotating transformers to minimize wire lengths.
I would suggest rotating the transformers to minimise hum and buzz injected into the amplifying circuits. Then cut the connecting cables to length.

Remember to twist the flow and return wire pairs. This applies to mains and LV power and audio cables.

If there is a third wire to a pair, (eg. +ve, -ve, Zero Volts) then all three need to be twisted together.
 
troubleshooting PD LM3875 Kit

My amp gets extremely hot and the transformer seems to vibrate a little and of course no sound. I rechecked the wiring and looked for shorts and things seem ok

I measured the voltage on PG+ V+ PG- V- and I think I found my problem since the voltage was about 36 volts and the transformers is 250 VA and is supposed to put out 25 volts.

My problem (I think) is I have 0V -> AC1 and 25V -> AC1 and the same for AC2 on my rectifier board but it should be 0V -> AC1 0V -> AC1

25 V - AC2 25V -> AC2

Is this right I wanted to make sure before I changed it and blew something up

Tom
 
Peter Daniel said:
This link shows how to connect dual secondaries transformer to rectifier board: http://www.diyaudio.com/forums/attachment.php?s=&postid=1508798&stamp=1210694958

On AC1/AC1 and AC2/AC2 you will be able to measure AC voltage only.

If transformer produces 25V AC, you will get approx 36V DC between PG+ and V+

OK so I think I am ok coming out of the rectifier. My kit had two rectifier boards but from the forum it seems that one might work better so I am using 1

Another thought I had was that I ran a wire from the CHG hole on each amp board to my star ground. I do not have anything from my rectifier to star ground since I thought it would go through the amp boards.

So my problem must be in the amp boards. I used the continuity check on my multimeter to look for shorts. I thought the chips may have been shorted to the heatsink but I checked the pins to ground and nothing except for the 2 pins with the resister shows a connection to ground.

I have been using an old 8 ohm 1 watt speaker as a load

Any thoughts on where to look next?
 
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/showthread.php?postid=1518369#post1518369

Please see this link how to measure for correct PS voltage: http://www.diyaudio.com/forums/showthread.php?postid=1518288#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/showthread.php?postid=1521304#post1521304

When offset measures OK, the chip most likely works fine, if there is still no sound, your wiring may be a problem.
 
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
 
Peter Daniel said:
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/showthread.php?postid=1518369#post1518369

Please see this link how to measure for correct PS voltage: http://www.diyaudio.com/forums/showthread.php?postid=1518288#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/showthread.php?postid=1521304#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).
This particular amp (right channel) measures 32mV offset when volume is completely down (input shunted to ground).
Quote
 
tiglitosa said:

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/showthread.php?postid=636580#post636580

There is no humming issues present at all. Of course, that will depend on your local conditions.
 
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