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Ibanez Auto Filter AF201 - Question

3/22/2000 10:07 AM
Jukka	Ibanez Auto Filter AF201 - Question Hi I've just bought this rare Ibanez pedal. After checking Harmony Central and Tonefrenzy's reviews and twisted it a little bit around myself I realized that it is difficult pedal to get it show its "sweet spots". Anyhow I was able to get it sound "right" or should I say "funky", only after turning the range trim pot almost to its limits at the right end (default was about 15:00). Now the sound is almost as good as with the good wah pedal, but it is sligthly distorted (not the straight signal, just the wet), low frequencies are almost rumbling, not bad at all, but it isn't as clean as can be heard on Tonefrenzy's clips. Could this be, because of bias shift, somehow, or what ?. Does anyone know the circuit ? If I set the trim back in direction of the default, the effect is weak Thanks Jukka PS: The setup is 79' Strat with Fralin's, to a TS-808 (just to beef up the signal, which this pedal needs, so it can trigger the filter), to a compressor For simplifying the things to trace the problem, all slides and switches are in up position.

3/22/2000 10:07 AM

Jukka

Ibanez Auto Filter AF201 - Question

Hi

I've just bought this rare Ibanez pedal.
After checking Harmony Central and Tonefrenzy's reviews and twisted it a little bit around myself I realized that it is difficult pedal to get it show its "sweet spots".
Anyhow I was able to get it sound "right" or should I say "funky", only after turning the range trim pot almost to its limits at the right end (default was about 15:00). Now the sound is almost as good as with the good wah pedal, but it is sligthly distorted (not the straight signal, just the wet), low frequencies are almost rumbling, not bad at all, but it isn't as clean as can be heard on Tonefrenzy's clips.
Could this be, because of bias shift, somehow, or what ?. Does anyone know the circuit ?
If I set the trim back in direction of the default, the effect is weak

Thanks
Jukka

PS: The setup is 79' Strat with Fralin's, to a TS-808 (just to beef up the signal, which this pedal needs, so it can trigger the filter), to a compressor
For simplifying the things to trace the problem, all slides and switches are in up position.

3/22/2000 4:09 PM
Jay Doyle	I don't know if this will help but, you may want to take out the 808 and especially the compressor from in front of the auto filter. The auto filter is designed to track the levels of you signal, so when you pick harder the envelope moves a greater distance than when you pick softer. Both the 808 and the compressor will mask any of the dynamics of your pick attack. That is what a comprssor is designed to do, and the 808's diodes limit the level of the input and as the output is log to the input a huge change in input to the diodes results in a minimal change in the output. As for the distortion, there may be a trim pot inside the pedal to tune the VCF. Mark where it is now and then adjust to see if the sound improves. Hope this helps, Jay Doyle

3/22/2000 4:09 PM

Jay Doyle

I don't know if this will help but, you may want to take out the 808 and especially the compressor from in front of the auto filter.

The auto filter is designed to track the levels of you signal, so when you pick harder the envelope moves a greater distance than when you pick softer.

Both the 808 and the compressor will mask any of the dynamics of your pick attack. That is what a comprssor is designed to do, and the 808's diodes limit the level of the input and as the output is log to the input a huge change in input to the diodes results in a minimal change in the output.

As for the distortion, there may be a trim pot inside the pedal to tune the VCF. Mark where it is now and then adjust to see if the sound improves.

Hope this helps,

Jay Doyle

3/23/2000 10:46 AM
Jukka	All you are saying is true, but.. Maybe I wasn't clear enough. The compressor is connected after the auto filter, of course. The filter has so high peak output level, so that it is a must there. The "Peak" slide pot does not seem to effect to the peak level enough, which is according to the manual (got it with the pedal) supposed to control that. Ok ! 808 is not the best choice if we are talking about the headroom. The same result with the other booster devices, anyway, but that it is not the point. Yes, there are 3 trimmers, exactly , "Range" (which my question was about), and "Up" and "Down" , which are only changing the envelope behavior. I left these two in their original positions after tested their effect. Tone is great , but the distortion in the wet (processed) signal bothers me a bit Thanks Jukka

3/23/2000 10:46 AM

Jukka

All you are saying is true, but..
Maybe I wasn't clear enough. The compressor is connected after the auto filter, of course. The filter has so high peak output level, so that it is a must there. The "Peak" slide pot does not seem to effect to the peak level enough, which is according to the manual (got it with the pedal) supposed to control that.

Ok ! 808 is not the best choice if we are talking about the headroom. The same result with the other booster devices, anyway, but that it is not the point.

Yes, there are 3 trimmers, exactly , "Range" (which my question was about), and "Up" and "Down" , which are only changing the envelope behavior. I left these two in their original positions after tested their effect.

Tone is great , but the distortion in the wet (processed) signal bothers me a bit

Thanks

Jukka

3/23/2000 5:06 PM
Jay Doyle	Sorry, I guess I misunderstood you. Without the schematic I can't be of much help but I have a feeling that it is the attack or decay response of the filter itself which is adding the unwanted distortion. If the "up" and "down" trimmers change these parameters than you may want to tweak those out of the original positions to try and improve the sound. Are these trim pots inside the pedal or the sliders on the outside of the pedal itself?

3/23/2000 5:06 PM

Jay Doyle

Sorry, I guess I misunderstood you. Without the schematic I can't be of much help but I have a feeling that it is the attack or decay response of the filter itself which is adding the unwanted distortion. If the "up" and "down" trimmers change these parameters than you may want to tweak those out of the original positions to try and improve the sound. Are these trim pots inside the pedal or the sliders on the outside of the pedal itself?

3/23/2000 6:16 PM
Mark Hammer	Not all distortion is the same, but I gave a listen to the sample at Tone Frenzy, and some of the distortion you are hearing may well be the shortcomings of the envelope detector circuitry. Look at almost any envelope filter (the Dr. Q, Bassballs, and Mutron are good examples), and you will likely see an RC time-constant combination in the envelope section, generally after a diode. This is a small value resistor connected to the + end of a cap, and the cap goes to ground. The resistor sets the time it takes to charge up the cap (envelope attack time), and the cap value determines how long it takes for that charge to drain (envelope decay time). Sometimes (as in the case of the Bassballs) there is a resistor in parallel with the cap to speed up discharge. Many designs will use a small value cap in that circuit to achieve a fast decay, fast attack/decay being "funkier" and more lead-oriented. The trouble is that the small cap value not only provides faster decay, but also provides less filtering for the envelope signal, and lets a lot of envelope ripple through. This is most especially true with half-wave rectified envelope detectors (like the Dr. Q). Listen to the sample at Tone Frenzy, and towards the end of the sample you will hear a burbling sound underneath the notes. It SOUNDS like distortion, but is kind of a "micro-sweep" at an audio frequency. Think of it like a wah that sweeps back and forth about 5 degrees, at a random speed ranging between about 5 and 15 sweeps per second. Not quite pronounced enough to be heard as a wah, but enough to provide an audible "grumbling" under the note. Although a complete redesign of the envelope detector circuitry from half wave to full-wave rectifier would be in order, you obviously can't do that on the board. The next best thing is to identify the RC combination in question, and increase the value of the decay-setting cap so that the decay is a bit slower. For example, if it is 4.7uf, increase it to 10uf. On the two Dr. Q's I built recently, I installed switches to select between the stock cap value, and another one twice as high. The higher value not only gives a longer decay (better for rhythm), but it virtually eliminates the ripple I described (which is very evident with the stock value). I should point out that this doesn't solve all the problems you describe, merely the distortion alluded to.

3/23/2000 6:16 PM

Mark Hammer

Not all distortion is the same, but I gave a listen to the sample at Tone Frenzy, and some of the distortion you are hearing may well be the shortcomings of the envelope detector circuitry.

Look at almost any envelope filter (the Dr. Q, Bassballs, and Mutron are good examples), and you will likely see an RC time-constant combination in the envelope section, generally after a diode. This is a small value resistor connected to the + end of a cap, and the cap goes to ground. The resistor sets the time it takes to charge up the cap (envelope attack time), and the cap value determines how long it takes for that charge to drain (envelope decay time). Sometimes (as in the case of the Bassballs) there is a resistor in parallel with the cap to speed up discharge.

Many designs will use a small value cap in that circuit to achieve a fast decay, fast attack/decay being "funkier" and more lead-oriented. The trouble is that the small cap value not only provides faster decay, but also provides less filtering for the envelope signal, and lets a lot of envelope ripple through. This is most especially true with half-wave rectified envelope detectors (like the Dr. Q). Listen to the sample at Tone Frenzy, and towards the end of the sample you will hear a burbling sound underneath the notes. It SOUNDS like distortion, but is kind of a "micro-sweep" at an audio frequency. Think of it like a wah that sweeps back and forth about 5 degrees, at a random speed ranging between about 5 and 15 sweeps per second. Not quite pronounced enough to be heard as a wah, but enough to provide an audible "grumbling" under the note.

Although a complete redesign of the envelope detector circuitry from half wave to full-wave rectifier would be in order, you obviously can't do that on the board. The next best thing is to identify the RC combination in question, and increase the value of the decay-setting cap so that the decay is a bit slower. For example, if it is 4.7uf, increase it to 10uf. On the two Dr. Q's I built recently, I installed switches to select between the stock cap value, and another one twice as high. The higher value not only gives a longer decay (better for rhythm), but it virtually eliminates the ripple I described (which is very evident with the stock value).

I should point out that this doesn't solve all the problems you describe, merely the distortion alluded to.

3/24/2000 3:27 PM
Jukka	Thanks Mark As I listened again Tone Frenzy's sample, it was that "burbling" sound, what I tried to describe. I guess it comes with the package Hmmm .. Time-constant, maybe those "Up" and "Down" trimmers are in RC network to set the decay time ? I'll have to check those again. Jukka

3/24/2000 5:25 PM
Mark Hammer	"...maybe those "Up" and "Down" trimmers are in RC network to set the decay time..." No. Take a look at the schematic for the Mutron III on CJ Landry's page (http://www.compassnet.com/~cjlandry/schems/mutron3.jpg). On the bottom, you will see one op-amp with two diodes connected to it. That is the envelope detector stage. Immediately after it, you will see a 330R resistor and a 4.7uf cap. That sets the attack and decay time (this should have less "burbling" because it is a full-wave, rather than half wave rectifier, and a smaller cap is sufficient there). Immediately after it, you will see another op-amp connected to the up/down switch. In combination with the previous op-amp, the up/down switch either subtracts the envelope signal from some preset voltage, or adds it. If it subtracts the envelope signal, then what is driving the photocells starts out high, and then swings low, which we hear as a downward sweep (an "ow" tone). When added, it sweeps in an upward manner. One thing you will have to clear up for me is your use of the word "trimmer". The AF201 has controls for up/down, range, filter type, etc. But does it also have trimpots on the board related to these things? I am not sure if your questions have been about setting the controls of the unit correctly, or setting up the fine adjustments on the PC board so that the controls COULD work correctly.

3/24/2000 5:25 PM

Mark Hammer

"...maybe those "Up" and "Down" trimmers are in RC network to set the decay time..."

No. Take a look at the schematic for the Mutron III on CJ Landry's page (http://www.compassnet.com/~cjlandry/schems/mutron3.jpg). On the bottom, you will see one op-amp with two diodes connected to it. That is the envelope detector stage. Immediately after it, you will see a 330R resistor and a 4.7uf cap. That sets the attack and decay time (this should have less "burbling" because it is a full-wave, rather than half wave rectifier, and a smaller cap is sufficient there). Immediately after it, you will see another op-amp connected to the up/down switch. In combination with the previous op-amp, the up/down switch either *subtracts* the envelope signal from some preset voltage, or *adds* it. If it *subtracts* the envelope signal, then what is driving the photocells starts out high, and then swings low, which we hear as a downward sweep (an "ow" tone). When added, it sweeps in an upward manner.

One thing you will have to clear up for me is your use of the word "trimmer". The AF201 has *controls* for up/down, range, filter type, etc. But does it also have trimpots on the board related to these things? I am not sure if your questions have been about setting the controls of the unit correctly, or setting up the fine adjustments on the PC board so that the controls COULD work correctly.

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