Infinity 7541a Distorted high frequency / noise in all channel - diyAudio
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Old 1st April 2012, 08:01 AM   #1
hyoo is offline hyoo  United Kingdom
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Default Infinity 7541a Distorted high frequency / noise in all channel

Dear All, Please I am new in the forum and need some advice to find out the problem.

I have a problem with a Infinity 7541a amplifier, it is a used one and I have connected to the speakers and headunit without any problem. The amplifier could start and all 4 channel work without any problem of going into protect or anything. The HU is a Pioneer P4900IB with 3 set of 2V RCA pre-out Front, Rear and Sub ; and I have connect the Front and Rear to the Infinity amplifier for speakers in the cabin then a powered sub was connected to the HU sub out RCA(s); I have adjust the front 2 channel crossover as 'Flat' and High pass for the rear; then the gain of the infinity are setted less then a half.

Problem is, I discover that all 4 of the channel produce some high frequency distortion or noise may be, when music playing in low volume and in particular frequency. for example, when playing sound of piano, it will have distorted sound or some source of noise, but sound ok will higher note or bass!

Before I asking expert for help, I have tried to see if there are anything I could do 'Externally' for the amp, I have tried to check if there are anything went wrong for the installation, and search though the forum and internet so as to diagnostic the problem, so far I have tried, and the the symptom of it are:
  1. The amp will produce the noise When the both the Engine either on or off, so it is not the alternator whine, so I assume that is not the interference from the power.
  2. When there are no music / signal to the Amp, there are no hum or noise only hissing
  3. Tried to disconnected the HU and leave the RCA connected to the amp, there is no noise and sound normal
  4. Also tried to connected the Pioneer RCA out to a headphone amplifier (PA use) and listen to the sound produce, it did have a tiny bit of noise, but not as much.
  5. I tried to use a Disc man to connect to the RCA input of the amplifier, it also have the noise in all 4 channel, BUT not as much as the Pioneer produce!
  6. Tried another pair of RCA and use the Dicsman as a signal source, again, it doing the same! so I assume is nothing to do with ground loop?!! (am I right?!)
  7. Check the Peco fuse of the Pioneer (as I found on the net that it is a comment issue), by using multimeter to check the grounding of the RCA(s) of the HU, they all connected to the HU chassis ground, so I assume that it is normal.
  8. Then tried to connect the chassis ground of the Pioneer HU to the power ground ( I know that is wrong!) even more noise!
  9. Tried to ground the RCA in the amplifier side to the power ground, again Wrong! even more noise!
  10. Use a multi meter to check the speaker cable, and they all look fine for me, as they are NOT touching / shorting with the car chassis.
  11. Use a multi meter to measure the resistance between the RCA to the Amplifier chassis ground and found that is not connected, so I tried to connected all RCA together then connect to the amplifier chassis ground, again, doesn't help
  12. After that, tried the amplifier side, I have tried to connect all RCA ground together, but not connected to the chassis ground, no hope
  13. Swap the front and rear channel RCA and see if all channel doing the same, and found that all 4 channel having the same problem!!!

SO, after all the test above, I assume it is the problem of the Amplifier itself, but I believe it is not related to the signal input stage or amplifier output stage, as if any channel got problem, they will behave differently, in my case, they all the same! I am not sure if it is the problem of the grounding inside the amplifier or the internal power supply?! or may be a doggy Cap inside??!!

If we going back to the point 11 above, should the RCA(s) ground of the amp input connected to the amp chassis ground internally??!!

Any other suggestions I could try Externally for the amp???!

Will that be a problem about the matching between the Pioneer and the Infinity??

Thanks for any help in advance!
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Old 2nd April 2012, 06:39 PM   #2
sdoom is offline sdoom  Germany
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Hi

regarding point 11: not all amplifiers connect the RCA GND signal to Chassis GND, some (most of the times higher value amps) use some isolator-amplifiers to surpress engine noise entering the pre-amplifiers ( I know some Alpine, Philips and Kenwood amps do)

It sounds like the amplifier itself needs to be checked thoroghly , I dont know the model myself and I dont have a schematic for this particular amp but I know from the past that solder joints are pretty bad thing throughout all price-ranges of amplifiers. So I usually re-sodler all circuit borads before I start checking for defective parts. So if this was an oldr amp , maybe you wanna open your unit and check the board(s) thoroughly (use a magnifying glass if you have one) for bad solder joints.

Another thing that sometimes happens is that the GND line of the cheaper-type RCA inputs just brakes or has very bad contact.
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Old 2nd April 2012, 08:04 PM   #3
hyoo is offline hyoo  United Kingdom
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Hi, Firstly, Thanks for your reply, I have the similar idea! I have found myself the schematic diagram as well, so hopefully will be able to have it open and diagnostic the problem.

the weird thing is, it happen on ALL 4 channels so make me think that it is not the problem of the pre-amp stage or power amp stage, sound more like teh power supply or the floating ground issue (maybe)?!
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Old 2nd April 2012, 09:03 PM   #4
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If you decide to open the amp, first look for obvious problems. If it looks OK, confirm that you have the correct voltage 10% on the output transistors and on the power supply pins of the audio op-amps.
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Old 3rd April 2012, 05:57 AM   #5
hyoo is offline hyoo  United Kingdom
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Hi,

Thanks for your information, and I will try to have it open today and start the visual inception first, then probably will check the power supply stage first, will update if I could possibly find anything!
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Old 4th April 2012, 05:52 AM   #6
hyoo is offline hyoo  United Kingdom
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I have manage to get the amp out of the car and have it open and test, apparently can't see any thing went wrong, the nothing burns inside, and I have check the capacitors and resistor which connected to the floating ground, and also have check the RCA grounding from the input, they all fine....

Tried to poke the signal into the power amp stage, (the resistor R120 on the board) which bypass the preamp and crossover, but still, can't see what went wrong....

Finally have check the voltage which feed into the crossover board, and look look to me, it measure around 14.5V, which suppose to be 15v, then remove the crossover board, and have the connect clean, as they look a bit tarnish, then put it back on and re- test with a signal source (a Discman) and a speaker (a 4 ohm and a 8 ohm) in the branch, sound like it improve a bit, and I will put it back to the car and see what happen first, and see what happen next.

Please are there any suggestion for what I should check next if it still here???
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Old 5th April 2012, 11:15 AM   #8
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Did you have both positive and negative supply voltage on the op-amps?

12-18 is close enough to 15v.

Did you have both positive and negative rail voltage on the output transistors?
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Old 5th April 2012, 04:30 PM   #9
hyoo is offline hyoo  United Kingdom
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Yes, both positive and negative have nearly 15 v which read approximately 14.6v.

But I can't get the output transistor rail voltage, which particular point should I look at?
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Old 5th April 2012, 05:20 PM   #10
hyoo is offline hyoo  United Kingdom
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Hi,

I have got the Voltage measurement:

In my test bed and when the battery is 13.6volt the output to preamp is +14.3v, and -14.2v

The Transistor output rail, (The output from the power supply section is +36.8v and -36.8v

They both look ok to me, as the service manual mention that it should be +/-15v and +/- 38v when the battery is 14.4v

BUT one think start worrying me is, my Fluke display tell me that the high voltage rail (+/-36.8v rails) have AC voltage! which measure as +8mV and -10mV, is that mean the diode D41 and D42 (YG225D2) is dying?!
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