Input pot popping?

[scott2000]

Trying to figure out why when the channel 2 input pot (R82)is creating popping noise on the output when rotating...(not the pot popping).. I'm guessing it's DC... The coupling caps which are bipolar  have been changed...... anywhere else I should be looking? I've tried a new op amp and it doesn't help....

It's not a scratchy pot. Just random spikes when I rotate it.... Could a bad pot act like this??

Thanks!

 

[Bo Deadly]

There is DC there because of the offset current but TL072 being JFET it should be very small. So I'm not sure why you would be getting popping noises. Maybe the pot is just bad. A conductive plastic pot and JFET op amp should not make popping noises. If the pot is an old cheap cabon type, maybe that is the problem. What does the pot look like? Post a pic or if you know the exact part, post a part number. The cleapo Alpha RD901 pots come in dual B100K and they're conductive plastic. You might even find one with the center detent.

 

[scott2000]

Here's the pot.....alpha

https://s20.postimg.cc/65zh8hlq5/b1ook.jpg

It does have center detent..... I guess I could swap one around from the compressor section to see if it's that???

 

[pucho812]

popping is definately dc.  It may even be arcing in the pot.  I had that happen to me before.

 

[JohnRoberts]

IMO a design flaw... wiper bounce leaves the op amp - input open, so with no negative feedback connection the op amp swings to one rail or the other.

At a minimum the NF cap should have been connected to the - input, not the pot, but that might require a smaller value cap to avoid HF rolloff at high gain (i hate redesigning other peoples products).

Short answer replace the pot, long answer replace the product.  :

JR

 

[ruffrecords]

Sounds like an opportunity for modding.

Cheers

Ian

 

[JohnRoberts]

Sounds like an opportunity for modding.

Cheers

Ian

Moving the cap directly across the op amp will reduce a lot of the wiper noise.

Potentiometer design pays attention to wiper mechanics. The better pots use 3 fingers on the wiper contact so even if two fingers bounce off there is still one finger making electrical contact.

Dirty/damaged pots can exacerbate this but I repeat IMO a design flaw.

JR

 

[scott2000]

Thanks!


I can't just move the cap leg to the wiper can I????

 

[JohnRoberts]

Thanks!


I can't just move the cap leg to the wiper can I????

At max gain the feedback R total  is 120K with 47pF that makes a -3dB LPF at 28kHz ... marginal IMO but probably OK. 

I would be happier using maybe a 22pF but you don't need to make me happy.  8)

JR

PS: I used a gain stage like that (back in the 80s)  in the front end of a parametric EQ (LOFT 401)  so it could double as bass preamp with 20dB of possible gain.

 

[scott2000]

Thanks! I removed the 47pf and tried a 27pf across 1 and 2 and it still does it....Funny I never noticed but both channels do it so I guess it is a design thing???? I can tell it's not like a scratchy pot .... More like  a start and stop of a pot rotation makes it appear...... I had a guy wanting it for PA use but, this seems like it would create some issues on a big system....he said he already uses a few of them so maybe not....I'll need to reach out to him...... I guess it is a cool gain stage.... I actually never thought about that but I think I do remember reading it as a bullet point in the manual..... ........

 

[ruffrecords]

Try adding a couple of 1Meg resistors. One from the wiper to the top of the pot and the other from the wiper to the bottom of the pot.

Cheers

Ian

 

[scott2000]

Alright...I hope I have something...I know I have some giant AB carbons...lol

 

[ruffrecords]

Alright...I hope I have something...I know I have some giant AB carbons...lol

The exact value is not important as long as it is several times the value of the pot. 470K would probably be OK.

Cheers

Ian

 

[scott2000]

I had some 560k I tried and the problem is still there.....

I'll keep digging.... kinda suspect of the pots.....the right channel is worse and in one particular part of the rotation more or less....the left is pretty much only at one end of the rotation when it acts up.......???

Thanks!

 

[benb]

I had some 560k I tried and the problem is still there.....

I'll keep digging.... kinda suspect of the pots.....the right channel is worse and in one particular part of the rotation more or less....the left is pretty much only at one end of the rotation when it acts up.......???

Thanks!

In addition to those resistors (I'm thinking one from either end of the pot to the wiper should be enough),  there's C6 that says 22/25 - 22 uF at  25V. The previous op-amp's output is theoretically at zero volts, and this input is theoretically at zero volts, but they're probably not, and 22uF is surely and electrolytic which is probably leaky enough to cause this problem. Looking at the resistances in this circuit, 22uF is overkill.  Use a 1uF (any voltage, but a lower voltage is physically smaller)  something-or-other cap, NOT an electrolytic (nor a tantalum).  1uF should still give full low-end response even at full gain, where the load looks like the 20k resistor.

 

[JohnRoberts]

Or replace the pot...

A single large value resistor*** from op amp output to - input could help (that's what the cap is supposed to do), but problem seems like bad wiper bounce or dirty pot element.

JR

*** this added resistor will reduce the amount of boost available at max gain.

 

[scott2000]

Yeah....just tried the last suggestion...Had some 1uf poly....no luck....I'll swap a ratio  pot to see if it helps...They're all the same aside from the indent......

Thanks!

 

[scott2000]

I guess it was the pot.....??? This one did the same thing kinda in a different section of the pot but after turning it a few times it's all good it seems..... but it still does it randomly in that spot....maybe this pot is bad too...The design must have something to do with it being so sensitive in this ???? I noticed other Rane stuff has weird things like big fluctuations when you first turn the pots and I've mentioned this before.....  way down in the 10hz range......


  pulled a couple pads too .... these are silly.....

... The face plate on this thing is heavy duty though..... little 34vct 170ma transformer..... :-\

Anyhow thanks so much!!!......

 

[JohnRoberts]

I guess it was the pot.....???

QED

This one did the same thing kinda in a different section of the pot but after turning it a few times it's all good it seems..... but it still does it randomly in that spot....maybe this pot is bad too...The design must have something to do with it being so sensitive in this ???? I noticed other Rane stuff has weird things like big fluctuations when you first turn the pots and I've mentioned this before.....  way down in the 10hz range......

pots are mass produced, could be bad batch, exposed to bad environment (if not sealed), worn, etc....

  pulled a couple pads too .... these are silly.....

... The face plate on this thing is heavy duty though..... little 34vct 170ma transformer..... :-\

Anyhow thanks so much!!!......

Glad it worked

JR

 

[JohnRoberts]

Swapping pots may only be a temporary solution, while moving the cap should help some.

If you are willing to forgo some of the max gain available, something like Ian suggested where the op amp is never left completely without negative feedback, would be a more robust solution. 

or sell it while it is quiet... 

JR