Wiring error! Sounds good! What’s going on?

[lion]

I've been working on a custom guitar EQ. After a long process of fine tuning and modding to my satisfaction, the circuit ended a bit messy – and I decided to tidy everything up a bit, get it prelim boxed, etc.

Powering the EQ up again I was very pleased with the circuit, but soon realized there was something (good) which hasn't been there before the tidy up.

Turned out that during tiddying the wiring to outboard parts I had got ONE wire soldered back on a wrong pot. See schem below - the wire from R11 should connect to the wiper of P2a, but got it wired to the bottom of P1a instead (which should have been left unconnected).


I'll keep the error (as a switchable option at least), but I'm curious as to what is really going on in the circuit.

As far as I can see there's at least 2 issues:
1) one of the 2 poles of the HP filter is left ineffective (changing the roll off slope to 6dB/oct instead of 12dB/oct).
2) the output of the HP (with the low end roll off set by the filter) is routed back to the input of the preceding LP filter – forming kind of a feedback loop.

Apart from the change in low end roll off steepness I can hear that there's some added midrange "sweetness" – I think due to the output of the HPF (with the low end roll cut set by the filter) reinjected into the LPF section. But - I can't figure out the details, and is probably missing something as well. Might be some unhealthy side effects also.

Can anyone explain? TIA.

Erik

[GibsonGM]

Feedback?  The lucky kind?

[lion]

Absolutely! Sounds good/better with the error - but why, ie but what's going on in the circuit?

[Keppy]

Warning: crude, amateur filter analysis below. Add salt to taste.

The wiper of P1a is low impedance at frequencies that pass through C5, so high frequencies aren't really fed back through the new path. The crossover frequency at which this happens is determined by R11 & C5, for a frequency of about 2.3k. Since IC2B also filters out lows, you have midrange feedback.

As the High Cut pot is turned for more cut, the mix between the output of IC1A and the output of IC2B changes to favor IC2B. Since they are mixed together through R6, R11, & P1A, any additional resistance on the outputs of those omitted EQ blocks could affect your result when using those blocks, and it might oscillate. Or not. I'm kind of surprised it doesn't oscillate already, since we're dealing with positive feedback here.

One way of looking at this is that the output of IC2B looks a lot like the output of IC2A (especially when not using much Low Cut), so it's a little like paralleling C5 with a 6.8k resistor.

[lion]

Thanks Keppy – much obliged, just what I was looking for.

I've found no oscillation, at any setting - and with/without the two para EQ blocks switched in and set for boost. Also no apparent levels jumps/changes etc from any switching.
Maybe the low gain in IC2A and IC2B (GAIN 1.3 in both), and the voltage divider between them, keep things steady.

[Kipper4]

What does it sound like any samples please?

[lion]

What does it sound like any samples please?

Hi Kipper
The EQ is a DIY build for my personal use, custom designed to give me (in comb with other effects) a certain sound spectre and character - think vintage instro music. The complete circuit comprises the lo and hi cut filters with the sweepable cut off points plus two individual SVF para EQ section - setup as a lo mid and hi mid boost with a preset Q. The concept is to create a (narrow) bandpass, with two possible and sweepable peaks inside the bandpass.

I'm not sure how relevant a sample would be (unless you are into the same music), and the resultant sound depends on the input as well as the filter settings with all the variables - but if you have any specific question I'd be happy to answer the best I can.

I notice you are in the UK - can I ask, are you the Kipper/Peter also a member on the CH&F forum?

[Kipper4]

CH&f forum?
What is that? My name is Richard so probably not.
I may not be into the same kind of music but it would give me an idea of what the filter is capable of and how your wiring mistake works.
I like to play reggae and Ska. Not everyone's cup of tea around here. Sure lots of people will listen to it and I'm sure wonder how the effects I like to use would fit into there set up too.
Thanks anyway
Rich

[Keppy]

I like to play reggae and Ska. Not everyone's cup of tea around here.

I love ska, at least the weirdo punk-influenced kind that went mainstream for awhile in the 90s. My day job is working at a music school, and I dream of starting a student band with a frontline of horns to play ska, funk, and classic R&B. My wife also works there, and she's currently teaching our top guitar class some chord voicings using "The Impression That I Get" by the Mighty Mighty Bosstones.

[lion]

Richard,
I can tell you the audiable effect of the wiring mistake is not a WOW thing, if that was the impression you got from my describtion. It's subtle, but noticable – and somehow fits what I want to achieve with the EQ.

I'll consider doing sound sample when I'm satified with the circuit, and have it properly box up and finished.

Here's the complete circuit as as I have it now, if anyone's interested - including the "error state" as a switchable option.



And here's a quick response plot of the circuit to show an example of the bandpass and 2 mid peaks dialed in. Lower plot shows the same setting just with the mid boosts switched out.



Erik