This Week on the Breadboard: The Germanium Filter

I am curious about R3 and R4. I like to put filter pedals after dirt and sometimes modulation, delay. Buffers will change the sound so what happens if I change R3,4 to 10M like a typical Sho boost? I tried it on the breadboard and I dont think it affects the tone much. But my ears are fallible. What effect will the 10M resistors have?
 
Good question!
The input impedance is lower then you might think because the Miller Effect reduces R3's impedance by the gain of Q1. Q1's gain in the is circuit is around 36x, which reduces the input impedance to around 26K. Increasing R3 & R4 to 10M increases the input impedance to around 260K. The lower input impedance will dampen the pickup's resonance a bit and make the pedal more responsive to the guitar's Volume setting.

BTW, this is a filter and a dirt pedal. It's all about the knob settings.
 
Chuck D. Bones said:
Good question!
The input impedance is lower then you might think because the Miller Effect reduces R3's impedance by the gain of Q1. Q1's gain in the is circuit is around 36x, which reduces the input impedance to around 26K. Increasing R3 & R4 to 10M increases the input impedance to around 260K. The lower input impedance will dampen the pickup's resonance a bit and make the pedal more responsive to the guitar's Volume setting.

BTW, this is a filter and a dirt pedal. It's all about the knob settings.
Click to expand...
Thanks for the answer! Good to know. I googled Miller Effect so I have some reading to do. 👍
 
It affects the shape of the FILTER knob's freq response above 1KHz. There would be a lot less high freq content without C5. Q3 has a very low input impedance and it loads the FILTER network. This is not the typical BMP filter circuit.
The plot below shows the the FILTER circuit's freq response with the FILTER knob at noon. The blue trace is with C5 installed and the green trace is with C5 removed.

1781102854256.png
 
That was helpful. Made me realize I was barking up the wrong tree trying to adjust things to suit me a little more. I finally found the solution and it was C7. I think I will end up with a 3way switch to have 10n, 22n and 47n. It moves the notch around in a way I Iike very much.
 
Morbidpale said:
That was helpful. Made me realize I was barking up the wrong tree trying to adjust things to suit me a little more. I finally found the solution and it was C7. I think I will end up with a 3way switch to have 10n, 22n and 47n. It moves the notch around in a way I Iike very much.
Click to expand...

If you plan your vero layouts in advance, I'd be interested in comparing notes. I'm not where I can post my latest (maybe final) version at the moment, but I'll post it up soon. As I recall, I adapted the dirtboxlayouts version to match @Chuck D. Bones's schematic, which I positively loved on the breadboard. I may have moved a few components off board. I like smaller boards and am usually happy to do a little point-to-point to get there.

I also haven't tinkered with a pickup-simulating transformer yet, which I wanted to do before committing it to solder.
 
genXslacker said:
If you plan your vero layouts in advance, I'd be interested in comparing notes. I'm not where I can post my latest (maybe final) version at the moment, but I'll post it up soon. As I recall, I adapted the dirtboxlayouts version to match @Chuck D. Bones's schematic, which I positively loved on the breadboard. I may have moved a few components off board. I like smaller boards and am usually happy to do a little point-to-point to get there.

I also haven't tinkered with a pickup-simulating transformer yet, which I wanted to do before committing it to solder.
Click to expand...
I will definitely do a stripboard layout soon. I dont think Ill need to tinker with this circuit much more. It's pretty perfect. I just need to commit to some germaniums soon. There is a pile of them on the work bench to be tried and biased.

R3/4 are going to be 10M. It will let me put where I want in the signal chain. The Pickup simulator idea has not worked well for me. Its proving easier to create higher input impedance and adjust the circuit to taste.

There will also be trimmers for R9/15. Then the toggle for C7.

A fun idea I tried was this:
1710307277248.png
I put it in reverse beta with some 2n3904s. I don't think its right for this circuit because of the Mojo factor of it all but it sounded decent. For those who don't have or want to mess with Germs its worth a shot. Anyway, it was fun for curiosities sake. If a friend wants a build they can buy me some Germs or Ill just do this. 😉🤫
 
Morbidpale said:
I will definitely do a stripboard layout soon . . . .
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Germanium Filter VERO.png
I might be able to wedge that into a 1590B. If I get nervous about the fit, I think I can move R15 to leapfrog C6 and eliminate column 16. I should also be able to shift columns 2-4 one to the right, if I can stand the aesthetics of the result. That would definitely involve even more offensive irregularities in the layout than the leapfrog.

edit: Looks like I've got a residual cut and jumper on the last row not doing anything. For shame.

edit: I can also move the Level 1 connection to ground off board and C8 to row 16. That's tidier than moving rows 2-4, and R15 can stay put too.
 
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Morbidpale said:
A fun idea I tried was this:
View attachment 118770
😉🤫
Click to expand...
NO!!!

The topic here is the DBA Germanium Filter, not stupid transistor tricks.

Take this BS somewhere else please. 😁

Morbidpale said:
I put it in reverse beta with some 2n3904s. I don't think its right for this circuit because of the Mojo factor of it all but it sounded decent. For those who don't have or want to mess with Germs its worth a shot. Anyway, it was fun for curiosities sake. If a friend wants a build they can buy me some Germs or Ill just do this. 😉🤫
Click to expand...

Reverse beta is tricky business and there is no way to know what will happen with various transistors used that way in a fuzz circuit other than trying it. Not all Ge trannys work well in this circuit and the same thing can be said for Si.

Morbidpale said:
The Pickup simulator idea has not worked well for me. Its proving easier to create higher input impedance and adjust the circuit to taste.
Click to expand...

When it comes to pickup sims, I'm a skeptic. As soon as we back the guitar volume down, even a little bit, the output impedance of a guitar changes significantly. Pickup sims don't simulate that, except for maybe the Woman Tone.
 
Chuck D. Bones said:
NO!!!

The topic here is the DBA Germanium Filter, not stupid transistor tricks.

Take this BS somewhere else please. 😁
Click to expand...

I have taken my BS elsewhere!

I have some AC176's in that sound really good.
So do AC127 buuuuuut I only have 2 and am not sure i am committing them to this.
MN37 is the closest I have to what is stock in the DBA but they are a bit mild.
 
Chuck D. Bones said:
When it comes to pickup sims, I'm a skeptic. As soon as we back the guitar volume down, even a little bit, the output impedance of a guitar changes significantly.
Click to expand...

I'll save myself the trouble, gracias. I wasn't wild about making room in the enclosure anyway.

Chuck D. Bones said:
maybe the Woman Tone
Click to expand...

Even in Cream, I'd like to tweak those knobs significantly.
 
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