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E30NA 27th June 2012 10:05 AM

Help needed- slight noise after mod.

I desided to convert my old Denon PMA925R into a power amp by bypassing the pre stage. The problem is that there now is a slight noise in both channels.

I have used the tape/dat input, and chosen to run the ground through the original path, and then bypas the pre stage with two wires directly from the connectors. The old signal path has been terminated.
The noise is constant, and does not get higher when the volume is turned up. It stops when i switch off the input source. The noise is not that high, but it is anoying.
What can cause this? Should i run the ground directly from the connectors also?

MarianB 27th June 2012 10:42 AM

Try to run bot ground and signal with shielded cable directly from the imput conector or path on the power amp, if the problem persists and the noise only apears when the audio signal source is turned on than maybe the foult is on it.

E30NA 27th June 2012 10:47 AM

Thanks, i will try that.
When i try the input source on an other amplifier, the noise is not there.

MarianB 27th June 2012 10:57 AM

Than it must be a problem with the signal path chosen for this amplifier, just for testing you could go in with the signal without using the conectors/plugs on the pannel, just connect a shielded cable on the input of the power amplifier and route it directly to the signal source, if this way all is good than you know for sure that the trouble is with the signal path, maybe with the ground, also make sure that you have a dc-coupling capacitor in series with the signal.

E30NA 27th June 2012 11:09 AM

Here is a picture of the signal path, and the red cirkles is where i have connected the signal. 1 is the signal in, and 2 is the ground.
I will try to connect both the signal and ground to these plugs.
How big should the dc coupling capacitor be? And should it be an electrolytic or a bipolar?

Sorry for the possible noob questions :)

MarianB 27th June 2012 11:15 AM

The dc-coupling cap on the input signal it usualy is something between 0,1uF and 10uF bipolar/MKT if possible, but as i see there already is this cap so no other one is needed.

E30NA 27th June 2012 12:27 PM

Ok, now i have tryed that, and there is no signal comming to the speaker now. It seems like the amplifier is going into mute mode.
Maybee it is missing a ground now, when it is only connected throug the connectors.

Nico Ras 27th June 2012 12:40 PM

Is the Tape/DAT connection not by-passing the pre-amp section anyway, why do you want to physically run wires?

E30NA 27th June 2012 12:53 PM

The dat/tape is not connected to the pre amp section anymore.
The plan was to use my DAC Magic Plus as a pre amp, and the Denon as a dedicatet power amp.

Nico Ras 28th June 2012 11:26 AM

From the PCB layout, it would appear as if the ground connections at these two input points you have chosen is not connected to ground. As you may notice the volume control uses the central ground pin, maybe this is the ground that is needed with your new signal leads.

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