Drive Pedal - Impedance issue?

[Barracuda]

So a quick introduction on what I'm working on, you'll see some similarities in the design to a rangemaster. Although this isn't built to be a boost pedal at the start of a chain, this is to offer a drive at the end of my pedal chain, as an alternative to an amp overdrive. I felt I could customize a lot more how my signal is being driven this way and just have a nice touch of distortion over everything to add colour, like a cranked valve amp, but with a couple adjustments to frequency response.

So what I have is sort of a slightly cocked wah/bandpass fuzzy driven overdrive, I suppose. Here's the simply drawn schematic -



This works great when my guitar goes straight into it. However since this is going at the end of my pedal chain, I thought I'd test it with another effects pedal that I use - "Mooer Elec Lady" flanger. Like I said the drive is last in the chain so the signal goes to the flanger and THEN the drive. So when I turn the flanger on, I get a very nasty high end distortion ruining the tone of the drive completely. If you notice the drive design it has a 10nf cap grounded to make a low pass filter before anything else can be affected, so I'm a bit confused. I thought it may be an impedance issue, so I added a buffer, the TS9 input buffer to be exact..





The issue I had with this, is after the buffer, I add in the 10nf cap to ground and it really doesn't change the frequency response much at all, even after trying a 10uf cap to ground just to test it wasn't my breadboard, it just reduces the volume more than anything really, not much change in the frequencies. I'm looking for a way to solve/understand whats happening.

I tried to add the drive circuit in anyway, I needed to add a resistor in the signal chain of about 20k to feed the correct amount of signal into the drive transistor, and it does help some what, but I can no longer make a low pass/high pass filter like I did in the original design, or at least, don't know how to rectify the signal to be able to do so.

I'm somewhat uneducated on the actual theory of electronics so I thought I'd ask to see what the issue might be. I can upload some sound samples if necessary.

I'm using BC109 transistors by the way!

Thanks for your time!

[Aleph Null]

I'm not sure exactly what the issue is but...

I capacitor will pass frequencies above it's cutoff and block (attenuate, actually, at 3db/8ve) those below. The higher the capacitor value, the lower the cutoff frequency. This means a cap to ground will actually act as a low-pass filter; the frequencies above the cutoff will be passed through the cap and shunted to ground.

[Barracuda]

My intention was to be able to severely smooth over the harsh frequencies in the guitar signal, and cut the bottom end (C1/C2). With the flanger pedal active in front, it doesn't at all, and ruins the tone of the drive completely. Almost like it was ignoring the low pass (C1).

[Frank_NH]

Put a resistor before your 10n cap to ground so that you have a proper LPF.  For example, a 2.2K resistor with the 10n cap to ground would filter out frequencies above about 7.2 kHz.

[midwayfair]

A slightly more thorough answer: An RC filter is formed with resistance and capacitance.

Your cutoff frequency is calculated by 2pi*R*C (where R is your resistor value in Ohms and C is your capacitor value in Farads).

You have a 10nF cap at the input of the effect, so your capacitance is really obvious.

But what's your R?

Well, we don't necessarily know. It varies depending on what's in front of it. Your pickups are a high impedance source, while your flanger is a low imepdance source. Impedance is just the alternating current way of expressing resistance -- that is, your audio signal is AC after it's been decoupled by a capacitor, or in its original form from your pickups. (Resistance is for direct current if you're curious.)

This is of course what the buffers for. You can make the input impedance high enough that it'll make the guitar pickups happy and other effects generally won't care if the input impedance is 510K or 1M or even 22K most of the time. You've gone that far. What you didn't do was take into account that now your impedance preceding that cap is perhaps as low as dozens of Ohms.

So as Frank said, you need to add a resistor. I suggest 10K since that should sound a little like that it did with your guitar pickups, though it's going to be best to socket it or experiment with a pot or on a breadboard before you make your final decision.

[Barracuda]

Hi guys, thanks for the replies. Just a little note, I am breadboarding this with the buffer preceding the drive/voltage divider biased transistor.

I did add a 10k resistor with the 10nf cap making the corner freq roughly 1.6kHz, this works fine without the flanger engaged, and it did before. But the issue is when the flanger is turned on, it seems to completely ignore the filter, this is where I thought the buffer would help. If I try this 10k/10nf combination after the buffer, the cutoff is basically nullified, or at least changed drastically. So I'm wondering the correct place to filter out the harsh frequencies of the flanger, whilst also filtering out the same frequencies on the guitar without flanger.

To put it simply, the drive sounds harsh with the flanger engaged, I'm going to try and upload a sound sample which should make this a lot clearer.



EDIT:

So here is the drive with no buffer

https://www.dropbox.com/s/3g1vgzpk1d6hxs3/Flanger%20Impedance.mp3?dl=0

Here is the drive with the buffer,

https://www.dropbox.com/s/apzgox0rduoqlio/Buffer%20Drive.mp3?dl=0

-After the buffer stage is a 4.7uf cap (purely for dc blocking purposes) followed by..
-Low pass : a 22k Resistor and then the 10nf cap to ground. According to AMZ R-C Filter calculator this rolls off at 723hz Followed by..
-High pass : 10nf cap into the drive stage
I'm using a strat, and I believe single coils should have an output impedance of about 6-7k, so a resistor of the same value should have done nicely? but I tried going more extreme to demonstrate the problem.

The buffer has definitely let the flanger signal in nicely, but I don't think the buffered drive is meant to be so sharp/harsh, this is what I'm having problems with. It seems to ignore the low pass filter, even though the high pass works fine. I'm very happy with the sound of the unbuffered drive, but I'd like it to work with my flanger, hence the buffer issue.

[midwayfair]

I'm not totally clear -- you're saying that your chain is:
flanger>
buffer>
10K/10nF low pass filter
booster

And it suddenly gets brighter when you add the flanger?

The flanger might have a good deal more high frequency content or it might be introducing some distortion. Have you tried other pedals instead of the flanger?

[Barracuda]

Yes, that signal chain is right. I did not try other pedals at the time I had that breadboarded, but I will try it. I've tried this flanger with a big muff before and it seemed to handle it a lot better than this. Most likely input impedance is higher on a big muff, which is what makes me think that's the issue.

My problem is that now I have added a buffer, I can't seem to create a low pass, which was to roll off some of the high freq content of the flanger. Either that, or there is something seriously wrong with the impedance relative from the output of the buffer to the input of the boost. I guess my question is to ask if that is the case? would impedance cause that "harshness" issue? how can I alter it?

Not expecting any big paragraphs - Links are always helpful!

[midwayfair]

Big muff's input impedance is ~47K + 39K blocking resistor. It's not loading your flanger but it sure as heck is loading your guitar.

I am not sure if something is wrong with your low-pass filter, like a short or something. You should absolutely hear a difference if you lift either the resistor or the 10nF. The cutoff's 1.5K and unless you're using a bunch of distortion that's pretty much right in the most prominent guitar frequencies.

Again, though assuming you aren't using a long cable, your guitar and the pedal should care equally about the input impedance of that buffer. 510K is not even in the audio band for anything shorter than a 25-foot cable with typical guitar pickups.

I mean, if you think that the impedance is causing an issue, you can test this and listen for yourself. Make the 510K smaller and you'll drop the input impedance. Make it bigger and you'll increase the input impedance. Try 1M. Try 10k. Try playing with the low pass filter values. Here's a calculator if like most people you can't multiply by 2pi in your head: http://sim.okawa-denshi.jp/en/CRtool.php