Hey. This amp powers and produces sound output (+-125 rail voltage); however theres a high frequency pitched wheezing sound coming off the speaker terminals. I'm really not sure where to look so I looked at a lot of things to start. This board is half double sided on some of the bigger components however the solder job wasnt the best. I went ahead a re-flowed al the large transistors, caps, and coils the best I could but it didnt help. Grounding the RCA shields changes the noise just slightly but does not make it go away. None of the crossover controls help or change the noise too much.
One thing I noticed is that the opAmps are only getting -7.5vDC and 10.5vDC. Is that enough voltage?
One thing I noticed is that the opAmps are only getting -7.5vDC and 10.5vDC. Is that enough voltage?
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Heres what the amp is:
Here is what comes out of the speaker terminals at all times: .1v/Div
Next thing I noticed is that at higher volume the signal starts to distort badly like theres a bad driver.
Also, this unmarked 24 pin IC gets pretty hot after about 30 seconds. I measured 150F.
Here is what comes out of the speaker terminals at all times: .1v/Div
Next thing I noticed is that at higher volume the signal starts to distort badly like theres a bad driver.
Also, this unmarked 24 pin IC gets pretty hot after about 30 seconds. I measured 150F.
With no load, and your scope set to 10v/div, drive a sine wave into it that will make the trace deflect ±3 major divisions. Set the timebase so that 2-4 full cycles are displayed.
Heres the output with no load, scope set to 10v/div with a sine wave driving up 3 divisions.
Same:
This probably tells you there is nothing wrong thus far. With a load the output sounds pretty terrible and its hard to see with a still image on the scope, so heres a quick 20 second video. This is with approximately 1-2v/Div, and I adjust things a little throughout. The frequency source is ~40-60hz, and throughout the whole time the test speaker crackles. Theres very little audible sound in this video.
jl audio :: VID_20110110_084612.mp4 video by unclemeat2010 - Photobucket
Same:
This probably tells you there is nothing wrong thus far. With a load the output sounds pretty terrible and its hard to see with a still image on the scope, so heres a quick 20 second video. This is with approximately 1-2v/Div, and I adjust things a little throughout. The frequency source is ~40-60hz, and throughout the whole time the test speaker crackles. Theres very little audible sound in this video.
jl audio :: VID_20110110_084612.mp4 video by unclemeat2010 - Photobucket
Also wanted to mention, this amp blew a couple of its owner's good speakers.
I'm really not too sure though. Its kind of like chasing a rabbit's tail
I'm really not too sure though. Its kind of like chasing a rabbit's tail
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Are you sure that the speaker isn't damaged? I don't see anything there that would make a crackling sound.
I'm assuming the video was with a speaker load.
I'm assuming the video was with a speaker load.
Like I said its like chasing a rabbit's tail. I changed out the test speaker and results are similar. The speaker is playing a high frequency hissing/whining and the scope shows what is in the second post. The hissing reminds me of an HVAC system inside of an airplane while sitting at the terminal.
I'm thinking one of the opamps might be dirty, then again they are only getting -7.5vDC and 10.5vDC
I'm thinking one of the opamps might be dirty, then again they are only getting -7.5vDC and 10.5vDC
You didn't state the timebase for the photo showing the noise but I'm assuming it's about 10uS. If so, that's normal for a lot of class D amps.
I think you first have to identify the static/noise. Then you can track it down.
I think you first have to identify the static/noise. Then you can track it down.
Yes that was .1v/div at about 10uS on the scope.
This is something a little more promenent. Referencing amp ground, I get the following at both the positive and negative speaker output terminals with no source or load attached. .5v/div and 10uS
This is something a little more promenent. Referencing amp ground, I get the following at both the positive and negative speaker output terminals with no source or load attached. .5v/div and 10uS
When looking for noise in the audio section of the amp, you need to use the secondary ground. The primary ground isn't reliable.
Do you have two 22k ohm 3 watt resistors near the rail caps?
Do you have two 22k ohm 3 watt resistors near the rail caps?
Perry; you are so slick! I dont want to bounce all over the room just yet, but these two resistors R30 and R31 are measuring 4.5M on the fluke... But I have to ask the question; why?
EDIT: False alarm. Off the PCB these measure 22K.
EDIT: False alarm. Off the PCB these measure 22K.
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They were likely measuring different in the board due to residual voltage across them.
Those are drain resistors for the rail caps. Find the ends of those resistors that have no significant voltage when the amp is powered up. That should be the secondary center tap. Use that as the reference when looking for noise.
Those are drain resistors for the rail caps. Find the ends of those resistors that have no significant voltage when the amp is powered up. That should be the secondary center tap. Use that as the reference when looking for noise.
I'm no expert at troubleshooting noise on the source side of things, but the majority of the opamps portrey a signal like this.
I'm on the trail using the RS Miniamp. The same noise I find through the test speaker can be observed through the mini amp on most of the opAmp inverted inputs and outputs.
I'm on the trail using the RS Miniamp. The same noise I find through the test speaker can be observed through the mini amp on most of the opAmp inverted inputs and outputs.
Yes I did; and the RS mini amp as well. BTW its a nice $15 tool when you need it; its outputing exactly the same noise of the speaker terminals.
Theres is very little noise between the RCA center first opAmp's inputs. A few SMD caps and resistors. The resistors all measure out OK, and the caps as far as I can tell while on the PCB.
In this photo for example, there is no noise on the RCA center lead through the first cap then leading to R101. From R101 goes under the board and then into the TL074C's 3rd pin. The inverted input pin 2 has a lot of noise, as does the output pin 1. This is similar accross 75% of the opAmps inputs/outputs.
Theres is very little noise between the RCA center first opAmp's inputs. A few SMD caps and resistors. The resistors all measure out OK, and the caps as far as I can tell while on the PCB.
In this photo for example, there is no noise on the RCA center lead through the first cap then leading to R101. From R101 goes under the board and then into the TL074C's 3rd pin. The inverted input pin 2 has a lot of noise, as does the output pin 1. This is similar accross 75% of the opAmps inputs/outputs.
Yes without an input source the noise exists. If I touch or ground the RCA shields the noise changes pitch slightly but the amplitude does not change.
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