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Old 5th July 2012, 03:55 PM   #11
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yup, it is a problem you shoul'd fix.
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Old 5th July 2012, 07:47 PM   #12
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The problem is the ic...
After i change the ic, ive got 1mv on both channel

But something weird happened....
After im wiring a preamp into the lm1875, ive got output 17v ( again ) and 10mv on the other one.... I dont know what happened..... :'(
When i put off the preamp, i still got the same dc offset... 17V

*havent wiring any grounding yet, and dont put anything on input and output*
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Old 5th July 2012, 11:01 PM   #13
gootee is offline gootee  United States
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Quote:
Originally Posted by Overdream View Post
@gootee : no not yet.... i was wiring v+ g- g+ v- from power supply to amp board.... haven't do the other wiring... is this the problem ??

<snipped>
What is the ground of your amplifier output connected to?

How do you have G- and G+ wired to the amplifier circuit??

There can be only ONE ground.
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Old 8th July 2012, 03:47 AM   #14
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Guys, please help me to do the grounding

as attach is dual mono lm1875 integrated with stereo CMoy as a preamp

i have 4 different output through 1 rotary switch and 2 different output through 1 rotary switch

nb. please ignore the 10ohm resistor at the pic
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Old 8th July 2012, 06:24 AM   #15
gootee is offline gootee  United States
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Quote:
Originally Posted by Overdream View Post
Guys, please help me to do the grounding

as attach is dual mono lm1875 integrated with stereo CMoy as a preamp

i have 4 different output through 1 rotary switch and 2 different output through 1 rotary switch

nb. please ignore the 10ohm resistor at the pic
See the attached jpg image.

I couldn't fit everything on the image that I wanted to include. But the important ones are:

1. Flood-Fill input area with copper, CLOSE to all traces and pads (to eliminate enclosed loop area between signal and signal ground.

1a. DISCONNECT input ground from all other grounds.

1b. Run a SEPARATE WIRE from the input ground area to the star ground.

2. Change Zobel to connect very close to output pin and directly to ground between caps.

3. Move 0.1 uF caps to bottom of board, connecting right at power pins.

4. Move large caps closer to power pins.

5 Widen the ground, power rail, and output traces.

6. Make the star ground point be at the output of the power supply, in the smoothing caps' ground but AFTER the last smoothing cap.

7. Run a separate wire to the star ground, for each amp's main ground.

I'm not sure if the speaker ground should go directly to the star ground, separately, or should come back to the amp board first.

Cheers,

Tom
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Last edited by gootee; 8th July 2012 at 06:27 AM.
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Old 9th July 2012, 06:14 AM   #16
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Hi Tom
thank you for your advice

yesterday i was wiring the grounding
i tied together input ground and output ground ( from binding post and rca )
then i mount to chassis

what i've got ???
no sound at my cpeaker.. just clap... clap... clap... clap...clap....

@tom : maybe you can give me an advice without changging the pcb ???
thank you very much
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Old 10th July 2012, 06:58 AM   #17
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Connect the speaker ground connection(s) directly to the filter capacitor.
The ground connection to the RCA input should be physically isolated from the chassis.

More pictures giving clear view of the details will help.

Gajanan Phadte

Last edited by gmphadte; 10th July 2012 at 07:00 AM.
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Old 11th July 2012, 02:38 AM   #18
gootee is offline gootee  United States
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Overdream,

It looks like the power supply has named outputs that match the amplifier boards' named connections.

You will need to make the wires mentioned below, if you don't already have them installed. If you DO already have some or all of the wires installed, make sure that they are correct.

Make two sets of two pairs of wires each, for two sets of a) V+, G+ pair and b) V-, G- pair, that will go from the power supply to the two amp boards. Twist each pair before cutting off and stripping the ends of the wires, with about four twists per inch, so you will get the lengths right, i.e. including the twisting.

Connect the V-, G+, G-, and V+ from the power supply to each of the amp boards, using the wire pairs you just made. Those should fill positions 3, 4, 5, and 6, of both amplifiers' terminal blocks.

Twist/measure/cut two more pairs, for the left and right IN and GND, that will go from the signal input jacks to the preamp board IN and GND connections. NOTE that the signal input jacks' grounds should NOT be connected to the chassis, near the jacks. Check them with your multimeter, before the other ends of the wires are connected (Ohms not small; should be "off-scale high").

Twist/measure/cut two more pairs, for the left and right IN and GND, that will go from the preamp OUT and GND connections to the amp boards' IN and GND, which will be positions 1 and 2 on the amps' terminal blocks.

Install a bolt in the chassis, which will be the star ground. The threaded part of the bolt will be inside the chassis, and needs to be positioned where you can access it.

You will need round lugs that fit onto the bolt, that also can have a wire crimped and/or soldered to them.

Make two wires, to go from each amp's terminal block's Position 7 to the star ground bolt.

Make two wires, to go from each speaker jack's GND or "-" to the star ground bolt.

Make two wires, to go from the amp boards' terminal blocks' Position 8 to the speaker jacks' "+" or OUT connections.

I'm not sure if a ground connection from the power supply to the star ground is still needed or not. (You can try it as decribed, so far, and maybe we will find out.)

I think that you will either need a wire from the power supply output ground to the star ground, OR, connect the speaker jacks' GND wires directly to the power supply's G+ nd G- connections on the power supply PCB.

You should use a bulb tester between the AC Mains and your power supply, until everything has been checked out.

Before connecting any speakers, you should short each IN to GND at each signal input jack and measure the DC voltage across each speaker OUT/GND connector and make sure that the DC voltage across both speaker connectors is near zero, for every volume setting.

Once all of those wires are connected and the DC offsets have been tested, it might give you some sound. If not, maybe I forgot something (or maybe something else is wrong). If there is sound, there might also be some hum. If there's hum, we can go from there, and try to eliminate it, after we can get some output sound.

Cheers,

Tom

Last edited by gootee; 11th July 2012 at 02:48 AM.
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Old 22nd July 2012, 04:40 PM   #19
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Hi guys
Problem solved !!!
Thank you all

This is the solution...

I put the input ( rca ) ground and the potentio ground to preamp board ground....
I put the output ground to the amp board

Then i put all ground channel ( pre amp, and amp board ) together as a star ground then connect it to chassis through 10ohm resistor...

I've got no humming, small dc offset, a lil bit hissing on my headphone out, but its ok...
Thank you all
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Old 4th October 2012, 12:48 PM   #20
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Quote:
Originally Posted by Overdream View Post
Hi guys
Problem solved !!!
Thank you all

This is the solution...

I put the input ( rca ) ground and the potentio ground to preamp board ground....
I put the output ground to the amp board

Then i put all ground channel ( pre amp, and amp board ) together as a star ground then connect it to chassis through 10ohm resistor...

I've got no humming, small dc offset, a lil bit hissing on my headphone out, but its ok...
Thank you all
somebody should alert him...
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