"P-Dub" Passive Bass Dub circuit

[Earthscum]

Hey all. So, I have been playing around with inductors lately (ironically, right after I started playing, a new thread appeared: Where are all the inductors?)

So, I decided to go ahead and try out a passive Butterworth low pass filter design for the hell of it, and it actually worked quite well! It has relatively low volume loss... almost negligible.

And SUPER SIMPLE!

Here's the schem:


By earthscum at 2010-10-29

I found .22uF to be the best balance. .1u still let a bit of jangle through. .47 started to cut the volume.

15k gave me the best performance. 12k and 10k started to realize volume drop. 18k wasn't too bad, and 22k started to let the jangle through.

I'm guessing a 42TM013 primary should give similar results, but I'm not sure.

[ashcat_lt]

What's the actual inductance there? 

Anybody ever try sticking an old pickup into a box for this type of filter?

[Earthscum]

The inductance for the RS transformers (primary) is a bit over 500mH. You're looking at about a 250mH/250mH split.

[Taylor]

That's pretty neat. So this is a 3-pole filter?

Better watch out, I wouldn't be surprised if that pedal company that only makes passive pedals comes out with a dub filter in a month... 

[Earthscum]

Yep... Third Order Butterworth Low Pass Filter

http://en.wikipedia.org/wiki/Butterworth_filter

I used some online calcs to get in the ballpark... not even close. They were calling for about a 1k to ground, so I started with what I knew worked already... a .1uF cap in a t-filter, then used a pot to get the resistance where I was just above volume drop. I popped in a .22 and got total dub without even touching the tone on my bass.

I tried a 1n5817 as overdrive... didn't do a damned thing, sonically. The cut wipes out any overtones caused by clipping. I'm actually thinking about putting a rotary switch in my bass with this as one of the selections... a schottky passive overdrive in another, and maybe some kind of resonant circuit (working on that now... but I think I'm gonna have to wait for the 42TM019's to come in the mail).

Lol, yeah... wouldn't be surprised if we see a dub passive. Another one I'm working on (which is how this circuit came about) is a passive wah pedal, which I have seen... I think Fender even makes one (just a passive tone control foot pedal).

[PRR]

> a 250mH/250mH split.

The center-tap is not half impedance or inductance.

Also this is NOT a 2-coil 1-cap filter because the two coils are 99% coupled.

What you really have is some form of T-coil. This is an insanely tricky thing used to extend oscilloscope response. The formulas are not simple, and at some point even the best designers had to "stick it and see".

[PRR]

WAIT.... this is driven by a pickup? The pup self-inductance is much greater than the 1K transformer inductance. It reduces to 0.22u||15K load on the pickup. 2-pole (12dB/8ve) 150Hz Q=3 low-pass, more or less depending on pickup. Yes, 10K will load the pup and 22K is awful ringy.

[Earthscum]

ok... throw a cap in parallel with the resistor for that extra drop. 

Paul, can you explain more? I understand that the 2 halves are magnetically coupled, and that affects things. I was getting the same results, though, with a pair of 1H inductors (3 Ohms DC), with different cap and resistor values, of course.

[PRR]

What do you get with just the cap and resistor, no transformer?