With the gain all the way down it has no audio output at all. I’m using a Pioneer head unit for my signal source, even with the volume all the way up on the head unit the amp produces no audio at all with the gain at minimum.
When I turn the gain to where the audio starts to come in, the amplitude does not seem real steady, seems to go up & down a little even without touching the gain control. With the gain all the way up it is pretty consistent.
Again, with the gain up the audio sounds ok & the sine wave looks pretty good on the scope, but looking closely with a lower level signal I do see a little high frequency noise with it.
When I turn the gain to where the audio starts to come in, the amplitude does not seem real steady, seems to go up & down a little even without touching the gain control. With the gain all the way up it is pretty consistent.
Again, with the gain up the audio sounds ok & the sine wave looks pretty good on the scope, but looking closely with a lower level signal I do see a little high frequency noise with it.
I was still under the impression that the outputs were intermittently oscillating. I guess I missed something.
If the speaker output essentially looks clean with the scope set to 10v/div and the signal swinging ±3 divisions, I think the output section is OK. There may be an issue with the volume control if it's sensitive to touch.
This gain pot isn't like a normal potentiometer so if you replace it, you'll have to get one from an MTX amp. Salvaging one isn't easy, generally because the heat required to remove it relieves the tension between the legs and the resistive element and makes the pot act much like the one you have now (assuming that it's sensitive to touch).
Is there any significant DC on the output pins any of the audio op-amps with the gain at the maximum (no audio)?
If the speaker output essentially looks clean with the scope set to 10v/div and the signal swinging ±3 divisions, I think the output section is OK. There may be an issue with the volume control if it's sensitive to touch.
This gain pot isn't like a normal potentiometer so if you replace it, you'll have to get one from an MTX amp. Salvaging one isn't easy, generally because the heat required to remove it relieves the tension between the legs and the resistive element and makes the pot act much like the one you have now (assuming that it's sensitive to touch).
Is there any significant DC on the output pins any of the audio op-amps with the gain at the maximum (no audio)?
Well, I too was under the impression the oscillation was still intermittent myself, so I may not have been clear there, sorry about that, but I realized it has not been going off like it was, & changing Q110 may have helped there?
Interesting about the gain pot, hopefully I won’t need to replace it.
One of the op-amps has -15.99vDC on both of its outputs, seems a little high for an op-amp output? Or is that normal for the circuit its being used in?
Since I do not see any board designation numbers for the op-amps, I will label each one with a letter & include a picture so you can see which ones which.
Below is the DC voltage on the output pins (pin-1 & pin-7) of the op-amps, using the ground terminal of the amp as my ground reference (gain at maximum):
Op-amp (A):
Pin 1: 0.413v
Pin 7: 0.506v
Op-amp (B):
Pin 1: 0.335v
Pin 7: 0.405v
Op-amp (C):
Pin 1: 0.335v
Pin 7: 0.336v
Op-amp (D):
Pin 1: 0.332v
Pin 7: 0.332v
Op-amp (E):
Pin 1: -15.99v
Pin 7: -15.99v
Op-amp (F):
Pin 1: 0.010v
Pin 7: 0.002v
Interesting about the gain pot, hopefully I won’t need to replace it.
One of the op-amps has -15.99vDC on both of its outputs, seems a little high for an op-amp output? Or is that normal for the circuit its being used in?
Since I do not see any board designation numbers for the op-amps, I will label each one with a letter & include a picture so you can see which ones which.
Below is the DC voltage on the output pins (pin-1 & pin-7) of the op-amps, using the ground terminal of the amp as my ground reference (gain at maximum):
Op-amp (A):
Pin 1: 0.413v
Pin 7: 0.506v
Op-amp (B):
Pin 1: 0.335v
Pin 7: 0.405v
Op-amp (C):
Pin 1: 0.335v
Pin 7: 0.336v
Op-amp (D):
Pin 1: 0.332v
Pin 7: 0.332v
Op-amp (E):
Pin 1: -15.99v
Pin 7: -15.99v
Op-amp (F):
Pin 1: 0.010v
Pin 7: 0.002v
Last edited:
Ok, this one has the THAT 2155 in it.
One question on that.
Connect a 1K resistor from pad? to pad 8
Did you mean pad 1 to pad 8?
According to the data sheet that would be the input / output pins. I seen in another thread where you mentioned to someone else about bypassing the THAT 2155 with a 1K resistor from its input / output, so I will assume that is what you meant.
Thank you for all the help Perry, I can’t tell you how much I appreciate it!
One question on that.
Connect a 1K resistor from pad? to pad 8
Did you mean pad 1 to pad 8?
According to the data sheet that would be the input / output pins. I seen in another thread where you mentioned to someone else about bypassing the THAT 2155 with a 1K resistor from its input / output, so I will assume that is what you meant.
Thank you for all the help Perry, I can’t tell you how much I appreciate it!
Alright, I removed the THAT 2155 and then put a 1.2K ohm resistor from pad 1 to pad 8 bypassing it.
I used a 1.2K as I have a bunch & could not find a 1K at the moment.
So far everything seems about the same as it was, I can’t tell a difference from when it was in there to now with the resistor in its place. Do you think the 1.2K (1200 ohm) resistor is ok there instead of the 1K.
I went over the output voltages on the op-amps & they appear to be about the same, or very close to where they were.
I noticed as well that if I adjust the FREQ pot it makes the speaker flutter as well, it did this before I changed out the THAT 2155 though as I was adjusting the controls before I changed it.
I used a 1.2K as I have a bunch & could not find a 1K at the moment.
So far everything seems about the same as it was, I can’t tell a difference from when it was in there to now with the resistor in its place. Do you think the 1.2K (1200 ohm) resistor is ok there instead of the 1K.
I went over the output voltages on the op-amps & they appear to be about the same, or very close to where they were.
I noticed as well that if I adjust the FREQ pot it makes the speaker flutter as well, it did this before I changed out the THAT 2155 though as I was adjusting the controls before I changed it.
The output is not unexpected from the input voltages but I'd only expect this much DC on the outputs if it's essentially operating open loop which isn't likely unless it's connected to a defective potentiometer. It's also possible that the op-amp has failed and is leaking internally and causing the offset.
I think replacing the op-amp would be the simplest/quickest way to determine where the problem is. If you decide to do this, measure the voltage on the pads with the op-amp out of the circuit. You don't need to post them unless replacing the op-amp doesn't help.
I think replacing the op-amp would be the simplest/quickest way to determine where the problem is. If you decide to do this, measure the voltage on the pads with the op-amp out of the circuit. You don't need to post them unless replacing the op-amp doesn't help.
Ok, well I may have to wait until I can place an order before troubleshooting much more.
I don’t think I have any SMD NE5532 op-amps (I’ll look), but I do have a new NE5532 through hole / DIP package, would it be a bad idea to try to put that in? I think I could get it in there with some thin short wire I have without a problem. Probably not a good idea, maybe I should just wait until I can order parts.
Thanks again for all your help!
I don’t think I have any SMD NE5532 op-amps (I’ll look), but I do have a new NE5532 through hole / DIP package, would it be a bad idea to try to put that in? I think I could get it in there with some thin short wire I have without a problem. Probably not a good idea, maybe I should just wait until I can order parts.
Thanks again for all your help!
Alright one more dumb question here before I wait like I am suppose to. 
I have a couple 4558 SMD op-amps that I am sure are good, there on an old stereo board out of a stereo system that was parted out.
I know changing op-amps can cause problems such oscillation & stuff, but do you think the 4558 would work in here for testing?
I’m sure its better to wait for the proper parts, was just curious is all.
I have a couple 4558 SMD op-amps that I am sure are good, there on an old stereo board out of a stereo system that was parted out.
I know changing op-amps can cause problems such oscillation & stuff, but do you think the 4558 would work in here for testing?
I’m sure its better to wait for the proper parts, was just curious is all.
I removed the op-amp & replaced it with the 4558; the amp seems about the same as it was. The output pads still have the DC offset, the solder pads had DC on them even with the 5532 removed, pad 3 & pad 6 now have +/-5v on them as shown below, & they continued to measure about the same when I put the 4558 in the circuit, the DC voltage readings with the op amp “out of circuit” as well as with the 4558 in circuit are below.
Voltage on all 8 pads with op-amp out of circuit:
Pin 1: -16.61v
Pin 2: 0.386v
Pin 3: -5.08v
Pin 4: -16.95v
Pin 5: 0.384v
Pin 6: 5.95v
Pin 7: -16.61v
Pin 8: 18.53v
Voltage on all 8 pins with the 4558 in the circuit:
Voltage on all 8 pins:
Pin 1: -15.91v
Pin 2: 0.383v
Pin 3: -5.08v
Pin 4: -16.94v
Pin 5: 0.382v
Pin 6: 5.94v
Pin 7: -15.91v
Pin 8: 18.52v
Voltage on all 8 pads with op-amp out of circuit:
Pin 1: -16.61v
Pin 2: 0.386v
Pin 3: -5.08v
Pin 4: -16.95v
Pin 5: 0.384v
Pin 6: 5.95v
Pin 7: -16.61v
Pin 8: 18.53v
Voltage on all 8 pins with the 4558 in the circuit:
Voltage on all 8 pins:
Pin 1: -15.91v
Pin 2: 0.383v
Pin 3: -5.08v
Pin 4: -16.94v
Pin 5: 0.382v
Pin 6: 5.94v
Pin 7: -15.91v
Pin 8: 18.52v
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