Filter circuit question - unusual design?

Thread Starter

andy-h-h

Joined Nov 2, 2017
6
Bit of a beginner question - can someone please help me with the filter circuit in the attached schematic? It's a tone control / switch on a guitar pedal. I know what it sounds like - which is a tight band pass or high pass filter removing a lot of low frequencies.

Questions
  1. why are there resistors and capacitors in parallel? is the pre emphasis?
  2. why are there two of these in series? is this a second order filter?

Hope I have used the right terminology and thank you in advance for any assistance on offer

https://www.hobbielektronika.hu/forum/getfile.php?id=27379

Moderator's note: The document was replaced with a link to the original location.
 
Last edited by a moderator:

MrChips

Joined Oct 2, 2009
24,183
Remove the capacitors and what you have is called a T-pad attenuator.
Add the capacitors for high frequency compensation, i.e. add some emphasis to the high frequencies.
 

MrAl

Joined Jun 17, 2014
8,475
Bit of a beginner question - can someone please help me with the filter circuit in the attached schematic? It's a tone control / switch on a guitar pedal. I know what it sounds like - which is a tight band pass or high pass filter removing a lot of low frequencies.

Questions
  1. why are there resistors and capacitors in parallel? is the pre emphasis?
  2. why are there two of these in series? is this a second order filter?

Hope I have used the right terminology and thank you in advance for any assistance on offer
Hi,

That doesnt really look like a tone control circuit and i dont see any adjustment for tone control.
It is also not two circuits in series but two interacting circuits where part can be switched out.

That looks like and is even labeled a "Fuzz" circuit which is typically used for a guitar in rock and roll type music to obtain very long, drawn out notes with a lot of sustain and with a very distorted "clipped" output sound typical of some rock music. Some of the colorful notes played by the late Jimi Hendrix comes to mind :)

You can see the over driven clipper circuit right away which comes in the form of an amplifier and two clipping 'diodes' made of transistors Q5 and Q6. That provides the 'fuzz" sound as well as the sustain and the effect is switchable and adjustable for little or maximum effect.

If there are really two of these circuits in series (two complete circuits as shown in the pdf) then that would be to get even more sustain without redesigning the whole thing.
 

Thread Starter

andy-h-h

Joined Nov 2, 2017
6
Hi,

That doesnt really look like a tone control circuit and i dont see any adjustment for tone control.
It is also not two circuits in series but two interacting circuits where part can be switched out.

That looks like and is even labeled a "Fuzz" circuit which is typically used for a guitar in rock and roll type music to obtain very long, drawn out notes with a lot of sustain and with a very distorted "clipped" output sound typical of some rock music. Some of the colorful notes played by the late Jimi Hendrix comes to mind :)

You can see the over driven clipper circuit right away which comes in the form of an amplifier and two clipping 'diodes' made of transistors Q5 and Q6. That provides the 'fuzz" sound as well as the sustain and the effect is switchable and adjustable for little or maximum effect.

If there are really two of these circuits in series (two complete circuits as shown in the pdf) then that would be to get even more sustain without redesigning the whole thing.

Thank you - it is most definitely a fuzz pedal for guitar. I built it and it sounds great. There's one section which is a tone control (via a switch), and it is not the usual type you see in guitar pedals. Most guitar pedals have a combination of high and low pass filters with a pot to mix between the two. I'm just trying to understand a bit more about how these things work, and this particular filter is new to me.
 

MrAl

Joined Jun 17, 2014
8,475
Thank you - it is most definitely a fuzz pedal for guitar. I built it and it sounds great. There's one section which is a tone control (via a switch), and it is not the usual type you see in guitar pedals. Most guitar pedals have a combination of high and low pass filters with a pot to mix between the two. I'm just trying to understand a bit more about how these things work, and this particular filter is new to me.
Hi,

Oh i see, well that makes sense.

It looks like a basic high pass filter but why so complicated i am not sure yet as a regular high pass could be made by using less parts.

A quick look and it looks like it lowers the amplitude and slightly emphasizes the somewhat higher frequencies. On a guitar the highest frequency is roughly 1kHz though i think, so we are talking just a little higher frequency emphasis like maybe 1.5 to 1 where the higher frequencies have just slightly more gain.

The effect would be to make it sound more "raspie" but possibly with less overall effect too though. Maybe you can try it and let us know what effect you are hearing when you switch that filter in and out.
 

Thread Starter

andy-h-h

Joined Nov 2, 2017
6
Hi,

Oh i see, well that makes sense.

It looks like a basic high pass filter but why so complicated i am not sure yet as a regular high pass could be made by using less parts.

A quick look and it looks like it lowers the amplitude and slightly emphasizes the somewhat higher frequencies. On a guitar the highest frequency is roughly 1kHz though i think, so we are talking just a little higher frequency emphasis like maybe 1.5 to 1 where the higher frequencies have just slightly more gain.

The effect would be to make it sound more "raspie" but possibly with less overall effect too though. Maybe you can try it and let us know what effect you are hearing when you switch that filter in and out.

It's certainly raspie - drops a lot of lows, 1k is considered mid range on guitar due to the harmonics / overtones that make up the sound. Even though the fundamental frequencies are much lower
 

MrAl

Joined Jun 17, 2014
8,475
It's certainly raspie - drops a lot of lows, 1k is considered mid range on guitar due to the harmonics / overtones that make up the sound. Even though the fundamental frequencies are much lower
Hi,

Oh ok yes the high frequencies are being emphasized then.

Yes, i did not mean to imply that the highest harmonic was 1kHz, just the highest note frequency. That still says something about the sound, but i did not look up the highest harmonic. I would certainly believe that the 3rd and 5th should be easily heard, which would be 3kHz and 5kHz, and that's well within the range of human hearing, and even the 7th is only 7kHz still well within range. The gain ratio of those highs could be as large as 7 to 1 for that filter which would make it very raspie sounding.

For any filter the main idea is to find out how it affects all the frequencies of interest. It does not matter as much how it is made, as long as it works, and you can analyze it either with math or simulator. I did a quick math analysis which was approximate.
 

Thread Starter

andy-h-h

Joined Nov 2, 2017
6
Hi,

Oh ok yes the high frequencies are being emphasized then.

Yes, i did not mean to imply that the highest harmonic was 1kHz, just the highest note frequency. That still says something about the sound, but i did not look up the highest harmonic. I would certainly believe that the 3rd and 5th should be easily heard, which would be 3kHz and 5kHz, and that's well within the range of human hearing, and even the 7th is only 7kHz still well within range. The gain ratio of those highs could be as large as 7 to 1 for that filter which would make it very raspie sounding.

For any filter the main idea is to find out how it affects all the frequencies of interest. It does not matter as much how it is made, as long as it works, and you can analyze it either with math or simulator. I did a quick math analysis which was approximate.

thanks - very helpful
 

#12

Joined Nov 30, 2010
18,223
It seems to me that Q5 and Q6 are designed or drawn wrong. You don't need two transistors to handle 54 microamps of current but you need one of them to be a PNP to get symmetry in the wave shape. Take one out and I'll bet there is no difference in the sound.;)
 

MrAl

Joined Jun 17, 2014
8,475
It seems to me that Q5 and Q6 are designed or drawn wrong. You don't need two transistors to handle 54 microamps of current but you need one of them to be a PNP to get symmetry in the wave shape. Take one out and I'll bet there is no difference in the sound.;)
Hi,

I did not study the circuit in great detail but doesnt it look like those two are meant to be the clippers in the fuzz circuit? Granted, they may not be drawn exactly the way "transistors used as diodes" are drawn, but i would bet they function in a similar manner, and that is that one clips the positive peaks and the other clips the negative peaks and that's the heart of fuzz, and because the secondary function of those two 'diodes' is that of a limiter, the sustain effect is produced at the same time there. With just one transistor instead of two it would only clip one peak and not the other which may or may not produce a desirable effect. And i dont understand why you would want to use a PNP transistor because with the two existing NPN's i would think the base emitter diode drops are similar so that positive peaks get clipped as much a negative peaks and that's what we want for a standard fuzz circuit.

Better quality sustain circuits have a gain controlled amplifier in them too but it has a linear input/output characteristic so it does not have to fuzz as well. The one here must fuzz in order to sustain.
 

Audioguru

Joined Dec 20, 2007
11,249
Q5 and Q6 act like diodes and one has the signal on its anode and the other has the signal on its cathode so they clip and make awful odd harmonics distortion (fuzz) symmetrically. A "fuzz face" circuit also produces odd harmonics and adds even harmonics for more awful distortion.
I prefer a low distortion acoustic guitar sound.
 

MrAl

Joined Jun 17, 2014
8,475
Q5 and Q6 act like diodes and one has the signal on its anode and the other has the signal on its cathode so they clip and make awful odd harmonics distortion (fuzz) symmetrically. A "fuzz face" circuit also produces odd harmonics and adds even harmonics for more awful distortion.
I prefer a low distortion acoustic guitar sound.
Hi,

Ha ha, yeah. Some guitar players like to play that kind of rock so they use fuzz. I havernt used that since i was very young and in bands that played the local area schools and bars.
 

Audioguru

Joined Dec 20, 2007
11,249
I think that people who like to hear fuzz are deaf to the high frequencies produced.
I have normal high frequency hearing loss for my age (72) and fuzz is acceptable without my hearing aids. My hearing aids are programmed to make my hearing have the same frequency response as it was when I was young. I set my hearing aids to do "noise reduction" when I am in a crowded restaurant that does not have acoustical ceiling tiles. The noise reduction has no effect on fuzz.
I played a trombone in my high school band. I also played it for many years at midnight on New Year's eve.
 

Thread Starter

andy-h-h

Joined Nov 2, 2017
6
Well I guess there's no accounting for taste - I like the sound of fuzz. Not every minute of every day, but they are fun to play with, and build. You should see what this circuit does to a sine wave on an oscilloscope - destroyed...

Q5 and Q6 are acting as clipping diodes - which is unusual, as most guitar pedals just use actual diodes to do this. The rest of the circuit design is pretty normal for this kind of guitar effect. There's a lot of different brand names and types that are all basically variations on the same circuit in a different box. The funny thing is that some develop a hype that drives very high prices for vintage gear, where an almost identical circuit in a different box is nearly worthless by comparison.

Thank you all for the help though, much appreciated
 

MrAl

Joined Jun 17, 2014
8,475
Hi,

Yes as the saying goes Beauty is in the eye of the beholder, and here Beauty is in the ear of the beholder. Not all people like fuzz at all and some do.

For me in my earlier days of rock we had to use fuzz and sustained fuzz in order to be able to play some of the music of the times. Even if we did not like it we still had to play it, although most of us liked it because we were young and that was the sound of the times. As i progressed in music i moved on to Jazz which almost never uses fuzz and so i never used it im my later years in music.

I remember way back then i built my own fuzz/sustain/etc. box. I was a huge thing though because the circuit board was big and i wanted lots of switch options. It was about 18 inches wide, 12 inches deep,a and maybe 5 inches high. Part of the circuit came from either a Popular Electronics magazine or Radio Electronics mag, dont remember which now. The box itself was made out of heavy gauge sheet metal which made it kind of heavy to carry between gigs.

A little off topic but i see a lot of "Epiphone" guitars being sold that are Gibson Les Paul reproductions. I wonder how good they are.
 

recklessrog

Joined May 23, 2013
985
Replace Q5 and Q6 with two germaniun diodes (OA90, OA91 etc) wired in reverse parallel, Just try it, I think you will like the result :)
 
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