Problems with passive level controls in effects loops

[Little Pigbacon]

I've built two passive level controls (voltage dividers) and tried them with combo amps -- a Fender Performer 650 and a Hot Rod Deluxe III. Both level controls exhibit the same behavior. That is, they will make the amplifiers hum in the bottom or top of their range, but not in the middle. The only exception is when the effects loop level switch on the Performer 650 is set to "Line/+4dBv/High", when there is no hum from that amp.

Anybody know why this sort of thing happens?

Certainly there are other questions that might be asked, like "Were you actually playing a guitar when you tested the level controls?" or "Why are you trying to do this?" or "Are you finally going to buy an appropriately sized amp now?" I'd like to set these and similar questions aside for the time being and focus on this electrical behavior that I've observed.

Or would I?

 

[mantis]

Re: Problems with passive level controls in effects loops

I get a similar result when using a Ernie Ball VP Jr 250k volume pedal in my effects loop. It's a Krank Revolution 1. I have it wired right from Send to IN on the pedal then OUT to my first effect in the loop which is a tc electronics Corona special edition Try Chorus. I don't notice it on my drive chanel just my clean channel. It's not loud or anything just a strange slight hum.
I gotta assume there is some resistance our preamps are not liking on some level. Your Voltage deviders, tell me how they are built.

 

[Little Pigbacon]

Re: Problems with passive level controls in effects loops

The first one was made with Radio Shack parts -- small plastic enclosure with an extra panel of light extruded aluminum for the face plate, a stereo 100k pot (reads more like 91k), and shielded audio wire from the pot to the two 1/4" stereo jacks in the back for I/O.

Despite the use of shielded wire, I thought there might still be enough opportunity for noise to get into the signal that I recently build a second voltage divider.

The second one was built in a light cast aluminum enclosure with a snug-fitting lid. I used a CTS 500k mono pot and two Switchcraft 1/4" jacks -- all from All-Parts. I connected it up with AudioQuest Alpha-Snake shielded audio interconnect wire.

In all cases, the incoming signal falls across the full value of the potentiometer. The ground from the input signal hits the other end of the pot, which is common with the output side. The output signal is taken from the wiper of the pot.

 

[devastone]

Re: Problems with passive level controls in effects loops

Try a 25K pot in the loop, 500K is too much, that's more for instrument level. Don't know if it will help the noise problem or not...

 

[Little Pigbacon]

Re: Problems with passive level controls in effects loops

25k pots are on order. Also the little brass sleeves that let you use set-screw knobs with the split shafts on the pots.

 

[Little Pigbacon]

Re: Problems with passive level controls in effects loops

Try a 25K pot in the loop, 500K is too much, that's more for instrument level. Don't know if it will help the noise problem or not...

It did. Thanks! :headbang:

 

[Mincer]

Re: Problems with passive level controls in effects loops

So the noise is gone?

 

[Little Pigbacon]

Re: Problems with passive level controls in effects loops

Not completely, but it's now below the threshold where it bothers me.

 

[devastone]

Re: Problems with passive level controls in effects loops

Glad that helped! 250K and bigger pots work better for instrument level signals, ~25K for line level, buffered signals. That's why most companies have high impedance and low impedance volume pedals. The high impedance versions work best right after a guitar with passive pickups, after a buffer or with active pickups go with the low impedance version. When you get into vintage fuzzes and stuff, it's kind of a crap shoot which one will work better after it.

 

[Little Pigbacon]

Re: Problems with passive level controls in effects loops

I was guessing with the 500k. I didn't think Fender would publish the output impedance of the preamp or the input impedance of the power section, but they're in the manual. Based in the specs -- 1.5k and 54k respectively -- the 25k pot makes sense. (I've really got to stop underestimating Fender.)