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Old 31st December 2012, 10:21 AM   #1
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Default g'nded amp chassis making noise

hi,

i have this rockford amplifier that puts out high DC on the left channel.

i tried to track the problem and it comes from the left channel opamp.

Pin 1 puts out 390mvdc. while Pin 7 has only 1.6mvdc.

i tested it by powering it up and putting the headunit on pause and started checking out the output/input pins basing from the schematic.

the op-amp is a TL072. however i don't have this on stock so i went and replaced it with an opa2134 i had on the drawer. i went and changed 5 opamps in the signal path so they will all be of the same op-amp type. this reduced the DC output of the left channel down to -06.8mvdc. the right channel has 2.2mv.

i check another working rockford amp of the same series and the left channel does have -06.2mvdc and 3mvdc on the right channel.

i started to let it play and it does work and sounded nice.

the problem is when i start to put the amplifier back into the car on it's STEEL amprack. the left channel starts to whizz up and down. making strange noises when i touch the speaker cone. when i just put it on the mdf flooring, the noise goes away. is my amp oscillating when the chassis is grounded to the vehicle's chassis?

the speaker isn't grounded since it does the same thing on the workbench as soon as i ground the chassis to the negative terminal of the supply.

hoping for your assistance

thanks
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Old 31st December 2012, 10:31 AM   #2
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by the way guys,

i started to measure the DC output of the left channel when i ground the amp's chassis to the supply gnd.

the left channel's DC output increases to 54mv,the right channel increases to 15mv.

i tried it on the other rockford amp, grounded the amp's chassis to supply gnd and the DC output remained stable. no change at all.

i removed the bypass cap from the opa2134 since it was from supply pins to gnd, thinking it caused it. it didn't go away. soldered the 0.1uf bypass cap from v+ to v- and still no luck. i noticed that the pitch of the whizzing noise increases when i put my finger on any of the op-amps.

i was tempted to just put it on the mdf floor and call it quits but my install calls for it to be in the steel amp rack together with the other amps.
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Old 31st December 2012, 06:56 PM   #3
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There are 2 1N4003 diodes on the board, check those. Then also check to make sure you dont have a broken ground shield for the RCA jacks.

I cant tell you any locations since you didnt list the model of the amp or provide any pics.
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Old 1st January 2013, 12:40 AM   #4
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seankane,

thanks for the assistance.

here's the schematic for the main analog input section of the P500-2 amp.
Click the image to open in full size.

U3a was the defective TL072. which i changed to opa2134. i then changed the U4 opamp so they'll be the same.

here's the schematic for the xover section which is fed by the U4's output.
Click the image to open in full size.

i've been poking around last night while waiting for some assistance and i think i may have found the culprit..?? hopefully.

the xover section has a switch SW-5222 (S1-a) i have an external processor so i keep it in the 'all-pass' position. U103 has also been switched to opa2134 since they're in the same path. i kept u100 and u101 TL072 since i don't plan to use them.

however, if i switch to either highpass or lowpass. the amp operates like the stock ones even if i ground the chassis.

Pin 7 of U103B puts out a stable DC voltage even if i ground the amps chassis to supply gnd.

if i switch to all-pass, the input to U103B is fed by U103A which causes high DC voltage @ Pin 7. this also happens to the right channel but not as severe as the left.

U103a is set to unity gain.??

R51 and R54 of U103a are both 10kilo-ohm 1/4w
i have already checked diodes and they checked out good.

Last edited by bzhei; 1st January 2013 at 12:43 AM.
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Old 1st January 2013, 12:54 AM   #5
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here's the closer view of the highpass section
Click the image to open in full size.

all-pass. hi and lowpass switch prior to gain pot section.
Click the image to open in full size.
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Old 1st January 2013, 01:00 AM   #6
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Are you sure that you don't have a bad solder connection on any of the op-amps that you replaced?

It's possible that the crossover frequency selector pot is defective. Does changing the setting to either extreme make a difference?

Not all op-amps that are pin for pin compatible work in all circuits. Even though they're supposed to be unity gain stable, they may not be in this circuit.
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Old 1st January 2013, 02:59 AM   #7
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hi perry,

switching the selector switch from AP to either HP or LP makes a huge difference when the amps chassis is grounded to supply ground.

HP and LP is quiet and offers no huge DC output. and it does feed the opa2134 U103B without any problems in the U103B output.

AP is another story leading me to believe that the opamp U103A oscillates because of the unity gain setup.

BTW perry, i do notice that when at AP, the noise seems to appear then disappear when i move/rotate the 'gain' pot RV2. could the gain pot be defective? how can i rule it out? the pot is the PTD90 series from Bourns if i researched it correctly.

the frequency pot RV101 is ok since it doesn't offer any change in the opamp DC out.

i will also try to reflow solder to make sure the connections are good.

thanks
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Old 1st January 2013, 03:09 AM   #8
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If the gain pot was bad, I'd expect more significant problems.

The only time that it oscillates is when IC103A is in the circuit, correct?

If so and you want to leave the op-amps as they are, you could try reducing the value of the two resistors. Try 4.7k.
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Old 1st January 2013, 04:19 AM   #9
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hi perry,

i think i solved the problem. it was u103a that oscillates.

can you confirm if i did it right?

here's what i did.

i first plan to change the resistors as you instructed. but the rockford amp has SMD resistors which i didn't have.

i did however have a 330pf amtrans film and foil cap. i read the tangensoft article regarding working with cranky op-amps. i tried doing the bandwidth limiting cap. paralleled it with the feedback cap R54. the 330pf paralleled with the 10k feedback cap reduced my bandwidth to 48kHz.

when i did this, my DC out remained stable even if i ground the chassis of the amp to the supply ground.

what are your views on this?
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Old 1st January 2013, 04:33 AM   #10
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If you intend on using those op-amps, it's likely good enough if it doesn't affect the audio. I'd prefer to see the op-amps returned to the original part numbers with no modifications. At least you've found the problem.
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