@mooly
So I hooked it up again and confirmed I get the buzz when touching the appropriate end of R1L (47k) and finally on the hot pin of that channel on the output socket.
As you might be able to tell from the image, I'm not really using the appropriate socket type for panel mounting (but all I had on me). The sockets have a pin for ground shield on each channel, and then 2 'hot' pins on each channel. The buzz I referred to is what I get when I touch the 'center' hot pin of that channel. If I touch the outer hot pin of that channel, I get a very loud buzz (much louder than on the center hot pin). Not sure if this means anything.
The buzz doesn't seem to be specific to the one channel.. but maybe I'm just going crazy at this point:O
So I hooked it up again and confirmed I get the buzz when touching the appropriate end of R1L (47k) and finally on the hot pin of that channel on the output socket.
As you might be able to tell from the image, I'm not really using the appropriate socket type for panel mounting (but all I had on me). The sockets have a pin for ground shield on each channel, and then 2 'hot' pins on each channel. The buzz I referred to is what I get when I touch the 'center' hot pin of that channel. If I touch the outer hot pin of that channel, I get a very loud buzz (much louder than on the center hot pin). Not sure if this means anything.
The buzz doesn't seem to be specific to the one channel.. but maybe I'm just going crazy at this point:O
I forgot to mention.. I did notice that maybe the connectors I'm using on the INPUT pins are no good. Seems strange that I can't get a signal even though the output is obviously doing something (hence the buzzing).
I'm going to try taking off the connectors and soldering the input wires directly to the input pins.
I'm going to try taking off the connectors and soldering the input wires directly to the input pins.
Its difficult making things out in pictures. The board accepts a standard unbalanced connection so you shouldn't have two 'hot pins' as you call them per channel. You have a single 'hot' input feeding to the top of R1, and a single ground per channel that connect to the ground part of the socket.
Having two pins that seem to buzz at different levels (per input ? that's how it reads) suggests you have the sockets wired incorrectly somehow.
You should really be using single RCA sockets per channel input and per channel output.
Having two pins that seem to buzz at different levels (per input ? that's how it reads) suggests you have the sockets wired incorrectly somehow.
You should really be using single RCA sockets per channel input and per channel output.
@mooly
I probably worded that incorrectly. The sockets don't really have 2 'separate' hot pins. It just has two tabs for each channel which are both the same hot pin.
Anyway I realized maybe I messed something up royally here - do I have the INPUT and OUTPUT switched based on the diagram?
You mentioned the input feeding R1, but I believe I have R1 near the output.
I probably worded that incorrectly. The sockets don't really have 2 'separate' hot pins. It just has two tabs for each channel which are both the same hot pin.
Anyway I realized maybe I messed something up royally here - do I have the INPUT and OUTPUT switched based on the diagram?
You mentioned the input feeding R1, but I believe I have R1 near the output.
Well I'd look at the circuit diagram as a starting point. The board layout you posted has the 47k marked at the left along with U1. U1 on Rods diagram is the first opamp. So yes, input at the left, that's conclusive enough.
(You would normally have the sensitive inputs kept well away from noisy power inputs and the power supply area of the board)
(You would normally have the sensitive inputs kept well away from noisy power inputs and the power supply area of the board)
It's working!
Well, almost. It goes to show you can't assume anything.
I have my turntable feeding into my interface and then to my computer. I noticed it's only picking up the left channel. I though maybe its the channel with the bent capacitor, but that's on the left channel which is working.
How can I go about troubleshooting this from here? I guess I would have to see where the right channel is failing - but how can I do that with the multimeter?
It sounds really good (and quiet) compared to my other preamp though, even with one channel working
Well, almost. It goes to show you can't assume anything.
I have my turntable feeding into my interface and then to my computer. I noticed it's only picking up the left channel. I though maybe its the channel with the bent capacitor, but that's on the left channel which is working.
How can I go about troubleshooting this from here? I guess I would have to see where the right channel is failing - but how can I do that with the multimeter?
It sounds really good (and quiet) compared to my other preamp though, even with one channel working
So its getting there.
Try this. (And have the circuit in front of you so it makes sense)
1: First be 100% sure the problem isn't in the connecting leads. Swap the output leads around and make sure the fault transfers to the other channel.
2: Make sure that the DC voltage on pin's 1 and pins 7 of both opamps is zero.
3: Buzz the input of U1 with a metal screwdriver. That means touching pin 3 and pin 5 in turn. The good channel will buzz extremely loudly so have the volume turned right down.
If both buzz at the same level then the problem is with the input wiring.
4: Buzz pin 2 and pin 6 of U2 in the same way. The buzz will be much lower than buzzing U1 but should be equal on both channels.
If that is OK then the problem is around U1.
Try this. (And have the circuit in front of you so it makes sense)
1: First be 100% sure the problem isn't in the connecting leads. Swap the output leads around and make sure the fault transfers to the other channel.
2: Make sure that the DC voltage on pin's 1 and pins 7 of both opamps is zero.
3: Buzz the input of U1 with a metal screwdriver. That means touching pin 3 and pin 5 in turn. The good channel will buzz extremely loudly so have the volume turned right down.
If both buzz at the same level then the problem is with the input wiring.
4: Buzz pin 2 and pin 6 of U2 in the same way. The buzz will be much lower than buzzing U1 but should be equal on both channels.
If that is OK then the problem is around U1.
@Mooly
1. Do you mean just switch the plugs on the output? Or actually resolder the leads so L is R and R is L. If I just swap the plugs then yes the music comes through just the right channel (instead of just the left).
2. Confirmed DC voltage 0 on pins 1 and 7 on both opamps.
3. U1 pin 3 buzzes very loud when I touch it. It buzzes in the left channel (but because I swapped the outputs) coming out of right speaker.
U1 pin 5 buzzes very softly when I touch. Its soft so I can't quite tell but seem to be in the right channel (coming out of left speaker with swapped outputs).
4. U2 pin 2 buzzes (softer than U1 pin 3 but louder than U1 pin 5) in the left channel.
U2 pin 6 doesn't seem to buzz at all.
Maybe some other useful information:
U1 pin 2 buzzes and distorts VERY loud on left channel when I touch it. U1 pin 6 starts picking up a radio station on right channel, almost crystal clear when I touch it
1. Do you mean just switch the plugs on the output? Or actually resolder the leads so L is R and R is L. If I just swap the plugs then yes the music comes through just the right channel (instead of just the left).
2. Confirmed DC voltage 0 on pins 1 and 7 on both opamps.
3. U1 pin 3 buzzes very loud when I touch it. It buzzes in the left channel (but because I swapped the outputs) coming out of right speaker.
U1 pin 5 buzzes very softly when I touch. Its soft so I can't quite tell but seem to be in the right channel (coming out of left speaker with swapped outputs).
4. U2 pin 2 buzzes (softer than U1 pin 3 but louder than U1 pin 5) in the left channel.
U2 pin 6 doesn't seem to buzz at all.
Maybe some other useful information:
U1 pin 2 buzzes and distorts VERY loud on left channel when I touch it. U1 pin 6 starts picking up a radio station on right channel, almost crystal clear when I touch it
@Mooly are you still there?
Kind of stuck at this point and not sure what the next step is. Rod Elliot did get back to me and mentioned:
"Pins 2 and 6 are the inverting input pins, and are fairly low impedance. Each channel should have similar noise output when they are touched. This is a risky way to test opamps though, as the static charge on your body can damage input stages."
I'm not sure where the right channel is going bad
Kind of stuck at this point and not sure what the next step is. Rod Elliot did get back to me and mentioned:
"Pins 2 and 6 are the inverting input pins, and are fairly low impedance. Each channel should have similar noise output when they are touched. This is a risky way to test opamps though, as the static charge on your body can damage input stages."
I'm not sure where the right channel is going bad
Odd. I remember replying to this but also remember one morning my internet connection dropped out for a few moments. I wonder if it got lost in space.
So, DC offset voltages are fine. I asked you to swap the leads just to be 100% sure we weren't chasing a problem that wasn't the preamp.
Buzzing pin 1 of U1 is obviously the good channel. Pin 5 of U1 should behave the same... and it doesn't. So we need to look first at U2 . Buzzing pins 2 and 6 of U2 should give identical results in their respective channels.
So it sounds as though there is a problem around U2 on the pin 5/6/7 side.
You need to check that pin 7 (the output pin) really does connect to that 820 ohm, and that the other end of the 820 ohm goes to the 1uF cap and from there to the output.
Also check that R6 really is 100k (you can't measure in circuit). If you need to confirm it then pull the opamp from the socket first. Make sure R7 is 2k7 and that it connects as shown through a 22uf cap to ground.
Also make sure you haven't blobbed solder across C4. Measure across the cap and make sure it does not read short circuit.
So, DC offset voltages are fine. I asked you to swap the leads just to be 100% sure we weren't chasing a problem that wasn't the preamp.
Buzzing pin 1 of U1 is obviously the good channel. Pin 5 of U1 should behave the same... and it doesn't. So we need to look first at U2 . Buzzing pins 2 and 6 of U2 should give identical results in their respective channels.
So it sounds as though there is a problem around U2 on the pin 5/6/7 side.
You need to check that pin 7 (the output pin) really does connect to that 820 ohm, and that the other end of the 820 ohm goes to the 1uF cap and from there to the output.
Also check that R6 really is 100k (you can't measure in circuit). If you need to confirm it then pull the opamp from the socket first. Make sure R7 is 2k7 and that it connects as shown through a 22uf cap to ground.
Also make sure you haven't blobbed solder across C4. Measure across the cap and make sure it does not read short circuit.
Thanks Mooly, which 820 ohm resistor are you referring to? I can't seem to find that anywhere on the circuit.
Ok the 820ohm you referring to is actually 750 ohm on my circuit (maybe the difference was an older version of the circuit you were referencing).
I used a continuity check on the multimeter and confirmed the following:
- U2 pin 7 (right channel output) is connected to one leg of the 750 ohm resistor.
- other leg of the 750 ohm resistor is connected to 1 leg of the 1uF cap.
- other leg of the 1uF cap is connected to output.
I haven't checked the resistors values yet but you mentioned I will have to pull U2 out to do so - why is that?
It looks like the output chain from U2 pin 7 is ok - I'm thinking maybe the signal is never getting to there and is failing before that point?
I used a continuity check on the multimeter and confirmed the following:
- U2 pin 7 (right channel output) is connected to one leg of the 750 ohm resistor.
- other leg of the 750 ohm resistor is connected to 1 leg of the 1uF cap.
- other leg of the 1uF cap is connected to output.
I haven't checked the resistors values yet but you mentioned I will have to pull U2 out to do so - why is that?
It looks like the output chain from U2 pin 7 is ok - I'm thinking maybe the signal is never getting to there and is failing before that point?
The 820 ohm is the one connecting the final opamp output to the output coupling cap.
You can not read resistors in circuit because of interaction with other components (such as the opamps), the readings can be meaningless.
What you can also try as a test. This will prove if U1 stage is OK.
1/ Remove U2
2/ Link pin 3 to pin 1 of U2 socket.
3/ Link pin 5 to pin 7 of U2 socket.
That will bypass U2 stage completely. You should have audio in both channels but at low volume.
You can not read resistors in circuit because of interaction with other components (such as the opamps), the readings can be meaningless.
What you can also try as a test. This will prove if U1 stage is OK.
1/ Remove U2
2/ Link pin 3 to pin 1 of U2 socket.
3/ Link pin 5 to pin 7 of U2 socket.
That will bypass U2 stage completely. You should have audio in both channels but at low volume.
Ok I did this. I removed U2 and used some spare pcb pins to link pin 3 and 1, and then pin 5 and 7 on the socket.
I could not hear anything. I turned it way up on my speakers and still could not hear anything from the turntable. I did however hear a loud hum on the right channel.
Btw those resistors I did check and seemed to be the correct values. I was able to check them without anything out.
I could not hear anything. I turned it way up on my speakers and still could not hear anything from the turntable. I did however hear a loud hum on the right channel.
Btw those resistors I did check and seemed to be the correct values. I was able to check them without anything out.
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