I was told this design could cause scratchiness in the pot because of loading and after about 4 months of use it seems that may be the case.
I'm seeking advice on two changes I was considering.
Replace the conductive plastic Alps RK27 (from Ebay so probably a knockoff) with this Vishay cermet, and swapping out the LM4562 with a FET input OPA2134 to reduce the pot load. I know an OPA1642 would be a better choice but it's not available in DIP8.
Output stage shown.
VR4 is 50K so R112/212 is 6k8 and R113/213 is 15k, per Rod's instructions.
I'm seeking advice on two changes I was considering.
Replace the conductive plastic Alps RK27 (from Ebay so probably a knockoff) with this Vishay cermet, and swapping out the LM4562 with a FET input OPA2134 to reduce the pot load. I know an OPA1642 would be a better choice but it's not available in DIP8.
Output stage shown.
VR4 is 50K so R112/212 is 6k8 and R113/213 is 15k, per Rod's instructions.
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There doesn't seem to be many options if one wants tone controls. The Chinese boards I tried were too noisy.
There's no way you could bodge in a capacitor in series with R112/212?
BTW, how much voltage drop are you getting across these? The chip has bias current cancellation (which is also the likely source for some of its quirks), so while not FET grade, it should be a good bit less concerning than a lot of other traditional bipolars.
BTW, you may want to consider using little SO-8 to DIP adapter boards, which should be relatively common.
BTW, how much voltage drop are you getting across these? The chip has bias current cancellation (which is also the likely source for some of its quirks), so while not FET grade, it should be a good bit less concerning than a lot of other traditional bipolars.
BTW, you may want to consider using little SO-8 to DIP adapter boards, which should be relatively common.
Thanks. It looks like I'm not going to have an opportunity to work on it for awhile.
The pot is divorced from the board by a short distance of wire so I could certainly attach caps to the wiper leads. What type and value would you suggest?
I thought about using an SO-8 to DIP adapter but I don't have a heat gun. I've soldered allot but I've never tried SMD, perhaps it's time I give it a go. Is a 2.9nV difference in noise worth the trouble though?
The pot is divorced from the board by a short distance of wire so I could certainly attach caps to the wiper leads. What type and value would you suggest?
I thought about using an SO-8 to DIP adapter but I don't have a heat gun. I've soldered allot but I've never tried SMD, perhaps it's time I give it a go. Is a 2.9nV difference in noise worth the trouble though?
With R112=R212 = 6.8 kΩ you'll want C ≥ 1/(2*pi*6800*2) -> C ≥ 11.7 µF for a lower cutoff frequency below 2 Hz. I'd probably go with 22 µF as that's a standard value. That pushes you towards the electrolytic types. The Nichicon Muse UES caps are good.
You don't need a heat gun. If you use any hot air, get an actual hot air soldering tool. But that's not needed either. A fine tip on your soldering iron will work just fine. I like the bent conical tips, but a 0.8 mm chisel tip would work also. Just add solder to one of the corner pads. Place the IC on top and heat up the solder while gently pressing the IC down with tweezers. Make sure it's centred on the footprint and remove the heat. Once the solder has cooled it's easy to solder the remaining pins.
Given the high impedances already in the circuit, probably not. The dominant noise source is likely the thermal noise from R112, R113, and the volume pot (and the corresponding components in the other channel).
Tom
Thanks Tom,
It's looking like this may have been a defective pot afterall.
The shaft had some in and out play that I didn't really notice until I started to remove it. There's a push nut holding the wiper disc in place and it was almost off the end of the shaft. I pushed it farther on with a small socket and now the play is gone.
Rather than put this back in though, I'm just going to install the cermet I already bought and see what I get.
Then I will try the other mod's if necessary.
It's looking like this may have been a defective pot afterall.
The shaft had some in and out play that I didn't really notice until I started to remove it. There's a push nut holding the wiper disc in place and it was almost off the end of the shaft. I pushed it farther on with a small socket and now the play is gone.
Rather than put this back in though, I'm just going to install the cermet I already bought and see what I get.
Then I will try the other mod's if necessary.