401k Compressor (Keeley 4-Knob)

[psb962]

Next up on my list of winter pedal projects was the 401K, which I just completed. This build specified some resistor values that I didn't have (27k->22k, 56k->47k or 68k, 390k->360k, 4M7->2M), so I had to make some decision about what matters and what doesn't when subbing in other values. Also, the pcb is built for a bunch of 1u electrolytics but I managed to use 1u film by bending one leg on each. It all seems to have worked out OK as I'm very pleased with the way the pedal sounds and it seems to perform just like the Keeley 4 knob does in the many reviews online. I don't really care for the Clip function but the other controls all do useful stuff.

The offboard wiring on this one went very quickly as I'm now getting pretty good at using Blu Tack to hold stuff where it needs to be for soldering - highly recommended.

This build worked at second attempt; at first try when plugged into the Protoboard it was silent when on but ok on pass through, so a reinspection of the soldering showed the leads of two adjacent transistors connected by a solder blob when they shouldn't be, so one snip and all is well.

I put this one in a silver box - of course - but imagination failed me when naming the pedal so 401k will have to do.

 

[cdwillis]

Out of curiosity are you using a guitar with single coils or humbuckers with it? I've tried a few dyna/ross comps, but they pretty much always sound like ass because I'm using humbuckers. I think the clip control is supposed to help with that. It's something I've been curious about.

 

[psb962]

Out of curiosity are you using a guitar with single coils or humbuckers with it? I've tried a few dyna/ross comps, but they pretty much always sound like ass because I'm using humbuckers. I think the clip control is supposed to help with that. It's something I've been curious about.

I was using two guitars, both with humbuckers, but one was.tapped. From the circuit diagram I think the clip function is a simple hard clipper, so it limits the signal and adds a little grit.

 

[psb962]

From the circuit diagram I think the clip function is a simple hard clipper, so it limits the signal and adds a little grit.

On second look it does seem the Clip adjusts level into the opamp by changing the resistance of the coupling resistors. The clipping diodes appear much later in the signal chain.

 

[cdwillis]

The pot is in parallel with the 390k resistor, so as you turn it the clipping pot up it goes from 390k resistance to about 152k. Plus that 2.2nf cap is in parallel so it's filtering some signal out depending on the resistance.

 

[psb962]

The pot is in parallel with the 390k resistor, so as you turn it the clipping pot up it goes from 390k resistance to about 152k. Plus that 2.2nf cap is in parallel so it's filtering some signal out depending on the resistance.

Help me out here - I last did an EE class in 1982. What I see on the schematic is that with the pot at 0%, the pot is open, so there is 0 resistance and the cap is bypassed, so maximum signal flows. With pot at 100%, you have the pot at 250k in parallel with 390k and 2n2, which is 152k || 2n2. Is that correct? I certainly seem to get a lot more signal with pot at 0 than at 100.

 

[Giorfida]

View attachment 66956
Help me out here - I last did an EE class in 1982. What I see on the schematic is that with the pot at 0%, the pot is open, so there is 0 resistance and the cap is bypassed, so maximum signal flows. With pot at 100%, you have the pot at 250k in parallel with 390k and 2n2, which is 152k || 2n2. Is that correct? I certainly seem to get a lot more signal with pot at 0 than at 100.

That makes sense. With the pot at 0, 100% of the signal passes, with the pot all the way up, 100% of the upper mids/treble pass, but only a portion of the lows and lower mids… so the overall volume would be less as a portion of the signal is being blocked. It is how the internal trimpot is configured on the Keeley