Hello I need some advice I own a 04 mkii
I did mods to a few of them already a good amp for mods
Typically I double the size of psu caps and use some bypass caps
Then adjust dc offset and bias to specs. I got info for some very helpful people here
This amp does some odd things
The procedure for adjusting is measure an imiter res and adjust
Then short inputs and adjust. This goes fine but when I remove input shunts the dc at outputs is high like over 90 mv. The amp also is dull little jump
I have measured the dc at psu on all 4 dc buss all is ok 68 volts
I need Advice please.
Al dattolo
I did mods to a few of them already a good amp for mods
Typically I double the size of psu caps and use some bypass caps
Then adjust dc offset and bias to specs. I got info for some very helpful people here
This amp does some odd things
The procedure for adjusting is measure an imiter res and adjust
Then short inputs and adjust. This goes fine but when I remove input shunts the dc at outputs is high like over 90 mv. The amp also is dull little jump
I have measured the dc at psu on all 4 dc buss all is ok 68 volts
I need Advice please.
Al dattolo
I do not understand your comment "The amp also is dull little jump".
One thing that you could check is there is a resistor R25 which is 20k on the input to ground. I don't know why it would've opened up but you could check that you're measuring about 21k ohms from the RCA center pin to RCA ground. Aside from that it doesn't sound like the amp is acting in an unusual manner. The reason Mondial tells you to short the inputs in order to measure/adjust DC offset is to ensure that any noise which may be present on a floating input is mitigated. Once you connect a preamplifier to the input the situation will likely change again.
I realize that you may have not seen this offset behavior on your prior amps that you have worked on, but the fact that Mondial specified that the inputs be shorted tells me that they knew it could be a problem.
In the end the only way to truly know if the amplifier has a problem would be to connect it to a dummy load and audio analyzer and run some tests on it to see if both channels meet distortion and power output specs.
One thing that you could check is there is a resistor R25 which is 20k on the input to ground. I don't know why it would've opened up but you could check that you're measuring about 21k ohms from the RCA center pin to RCA ground. Aside from that it doesn't sound like the amp is acting in an unusual manner. The reason Mondial tells you to short the inputs in order to measure/adjust DC offset is to ensure that any noise which may be present on a floating input is mitigated. Once you connect a preamplifier to the input the situation will likely change again.
I realize that you may have not seen this offset behavior on your prior amps that you have worked on, but the fact that Mondial specified that the inputs be shorted tells me that they knew it could be a problem.
In the end the only way to truly know if the amplifier has a problem would be to connect it to a dummy load and audio analyzer and run some tests on it to see if both channels meet distortion and power output specs.
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My sound comment is based on my other amps from them
Dull meaning a poor class a sound little dynamics
No jump as amp moves from class a to ab on dynamic hits like drum or cymbal crash
I'll measure the resistor as asked and yes of the preamp is Connected the dc st speakers is low and to spec.
It's odd to this amp is just not like the other two were and is the same parts inside
Thanks for your reply
Dull meaning a poor class a sound little dynamics
No jump as amp moves from class a to ab on dynamic hits like drum or cymbal crash
I'll measure the resistor as asked and yes of the preamp is Connected the dc st speakers is low and to spec.
It's odd to this amp is just not like the other two were and is the same parts inside
Thanks for your reply
Yes odd in deed. I'll check for input resistors tonight. What I also see is this
Unshorted input yields about 100 to 200 mv it fluctuates
I put my preamp to the inputs this makes the dc offset even higher average 250 and it inverts the dc polarity. Now if I put a dummy load of 20 ohms the same dc mv is there but the polarity is normal. This is with preamp connected. I know this makes me sound dumb lol. I'm not man this amp is odd in deed.
I'll post tomorrow for the dc res. All voltages on psu are 70 vdc and don't vary.
Unshorted input yields about 100 to 200 mv it fluctuates
I put my preamp to the inputs this makes the dc offset even higher average 250 and it inverts the dc polarity. Now if I put a dummy load of 20 ohms the same dc mv is there but the polarity is normal. This is with preamp connected. I know this makes me sound dumb lol. I'm not man this amp is odd in deed.
I'll post tomorrow for the dc res. All voltages on psu are 70 vdc and don't vary.
Maybe the input coupling caps have become leaky ?
Especially if they are electrolytics.
Is the amp DC coupled at the input ?
Especially if they are electrolytics.
Is the amp DC coupled at the input ?
Not sure will look at schematic what's killing me is both channels On dual mono amp.
Going to go through amp one channel at a time Tonight.
Going to go through amp one channel at a time Tonight.
Are you performing these offset tests with the covers on or off the amp? The reason I ask is last year I was performing DC offset checks on an Acurus A250 and the offset was drifting around badly and I don't mean over a 10min period. Like 10mv then 15secs later it would be 50mv. I was stumped by it for a bit and I finally put the top cover back on the amp and the offset settled to a solid single digit number and didn't move. Its possible the environment you're measuring in is noisey.
I thought of this too. Cover is off
This is just wild I'm going to try a set of cheap speakers to see how it sounds
But this amp was working ok.
I had in my mod used larger caps from 18k stock to 36 k uf
Mundorf it was ok but caps stink slow and dark. One even had an intermittent short too.
I had to use a light bulb for a fuse to Find it.
It now has TDK much better. It also has three by pass caps
.1uf / 36uf and 360 uf caps. Making the amp sparkle and a much lower noise floor too.
I'll take it apart in the am and I thank all for your help here.
This is just wild I'm going to try a set of cheap speakers to see how it sounds
But this amp was working ok.
I had in my mod used larger caps from 18k stock to 36 k uf
Mundorf it was ok but caps stink slow and dark. One even had an intermittent short too.
I had to use a light bulb for a fuse to Find it.
It now has TDK much better. It also has three by pass caps
.1uf / 36uf and 360 uf caps. Making the amp sparkle and a much lower noise floor too.
I'll take it apart in the am and I thank all for your help here.
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