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DIY Stompboxes => Building your own stompbox => Topic started by: Mark Hammer on April 21, 2016, 09:37:36 PM

Title: Understanding the EHX Y-Triggered Filter
Post by: Mark Hammer on April 21, 2016, 09:37:36 PM
Bit by bit, I'm repairing and bringing back to functionality a bunch of pedals that have been sitting unused for too long.  One of them is my old EHX Y-Triggered Filter.

It's an interesting beast, with a somewhat idiosyncratic sound.  I think it's a swept bandpass, but you know I'm not all that sure anymore.  It doesn't ever really seem to get thin, the way a bandpass would when swept high.  The circuit, even with the much more readable redraw from Scruffie (thanks, mate!) doesn't look like anything else out there, or at least isn't immediately recognizable as such.

The "Range" switch is essentially two different resonance settings: one high and one low.  Naturally, I just HAD to mod it, by installing a 3-way toggle to change the value of C13, for a higher and lower sweep range.  More amenable to bass now, when C13 = 6800pf.

It has a single bidirectional control that does direction and sensitivity in one knob.  Turn it clockwise and it sweeps down, counterclockwise sweeps up, and no real sweep in the middle.  But the correspondance between sweep and picking is not the 1:1 one finds on many envelope-controlled filters.  It just doesn't feel like a sensitivity control.  Moreover, as you can see below, the sidechain starts out with a clipping circuit, using a back-to-back diode pair that restricts dynamic range.  Now why would you want to do that if you were trying to extract an input signal envelope?  It also triggers weird, occasionally not accepting further plucks.

All of which leads me to realize that the name of the pedal does not lie.  It is NOT an envelope-controlled filter, it is an envelope-generator-controlled filter.  That is, somewhere in all of that (and I honestly can't figure out where), there is a preset envelope/transient generator that is initiated by the input signal reaching some threshold, which triggers a fixed sweep.  There may be something adaptive about it that might yield a changing decay time, but I can't see it.

Actually, I probably lied.  I gather it initiates two equal-but-opposite fixed sweeps, that cancel each other out when mixed 50/50.  Hence the other part of the name: Y-triggered.  Two parallel sweep generators converge on one point, and the knob adjusts their balance.

I cannot for the life of me figure out what sort of a filter topology this is.

So, if you have any ideas about how this beast works, I'd love to hear them.  A search for Youtube demos finds only two.

(http://i898.photobucket.com/albums/ac182/Scruffie_Crow/EHXYTFFin.gif)

Title: Re: Understanding the EHX Y-Triggered Filter
Post by: robthequiet on April 21, 2016, 10:48:39 PM
A couple of initial questions -- I take it R28 is your up/none/down sweep control? And unclear about the nomenclature on the switch, "on <-> up"... (Oh, "on" means "down")

Just from the description it vaguely reminds me of a multiplier circuit but is it really a kind of ring mod? What would happen if we feed a sine wave or square wave through it? Deep waters for my limited swimming skills but intriguing nonetheless.

Update -- went and looked at the you-tube vids and found the ehx ad from way back when. So it's an envelope filter,  and the upper part of the circuit is the "rms" and the OTA section is the drive, hence R28 adusts baseline current on OTA1 pin 5 to be modulated by the diff at pins 2 and 3. But the RC networks around U2 threw me for a loop.

My best guess without totally circuit-solving. Maybe the "Y-trigger" has to do with the signal taking a fork at the switch to either the cold or hot side of R27.

The claims that it would make your guitar into a synth were a bit exaggerated, imo.

http://www.ehx.com/blog/ehx-flashback-1976-y-triggered-filter
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Mark Hammer on April 22, 2016, 10:05:41 AM
I take it you mean R38, rather than R28

"Up" in the drawing refers to the position of the slide switch, when installed.  With the switch in that position, the resonance is higher.

In 1976, ANYTHING that resulted in a filter sweep with each strum or string-pluck was perceived as being synth-like.  Keep in mind that synthesis was not all that prevalent at the time, other than some very big and very expensive installations at university or commercial studios, or in the hands of wealthy musicians.  Guitar synthesis was just an emerging idea.  The very idea of a pedal that could provide filter sweeps for under a hundred bucks was pretty spectacular.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: duck_arse on April 22, 2016, 11:43:22 AM
that's funny, I've just been playing with up/blank/down. I'd hazard a guesss, under duress, that C5 and C9 might be timing sweeps, being tants and all.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Mark Hammer on April 22, 2016, 12:17:12 PM
Worth considering.  I'll try tacking on more capacitance and seeing if that makes a difference.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: robthequiet on April 22, 2016, 01:57:24 PM
So R38 acts as a mixer/pan control between a low sweep and a high sweep?
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: StephenGiles on April 22, 2016, 03:05:59 PM
It would be interesting to substitute the rectification circuits with the very clever envelope generator in the Space Drum which should match current levels required by the CA 3094.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: jatalahd on April 22, 2016, 03:20:54 PM
Mr. Duck might be on right tracks, to me it seems Q1 and Q2 are wired as simple switches. When off, the caps across them charge up relatively slowly  and when on, the caps discharge fast through the open switch. If this is the case, the waveform at the ends of R38 could be a mirrored sawtooth (ramp) wave, which, as a control signal to the OTA, could cause a sweep. But still, for me there are quite a lot of things in the circuit that I can't follow so well, especially around the OTA.

If we start to analyse block-by-block from the top left, there is a simple clipping amplifier U1B and a full-wave rectifier U1 (funny to do this for an assumed square wave input). Then I can't really say I could identify the two remaining op-amp configurations, although the one on the far right is again a basic non-inverting amp, with a few added caps around it.

Just sharing my thinking, but usually I take the wrong assumptions on the first go. It is an interesting circuit and I am also keen to know all details of it.

By the way, in the shown schematic the collector of Q2 should not be connected to the collector of Q4, it just does not make any sense.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: robthequiet on April 22, 2016, 10:32:55 PM
I'm having a hard time interpreting the signal path from the schematic, specifically the input jack.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Mark Hammer on April 23, 2016, 10:11:32 AM
So R38 acts as a mixer/pan control between a low sweep and a high sweep?
Well, between two equal-but opposite envelope voltages, yes. 

In something like a Mu-Tron, or any other similar pedal, changing sweep direction is a matter of either adding the rectified signal voltage to some basic standing level (usually zero), or subtracting it from some arbitrary maxmum value.  The rectified signal, however, could vary, depending on picking.   

If my suspicions are correct, and this design uses a fixed envelope voltage, then the middle position of R38 is able to cancel out the two opposite-but-equal envelopes....something a comparable control couldn't do in the usual envelope-controlled filter.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: duck_arse on April 23, 2016, 10:47:14 AM
+1 on Q2 shouldn't have CE shorted. and, does U1A appear to be ...
Quote
adding the rectified signal voltage to some basic standing level
... as mark so eloquently puts it?
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: robthequiet on April 23, 2016, 03:51:56 PM
Sorry if I'm getting noisy on this thread -- at any rate, I'm with Mark on the idea of the fixed envelope. I suspect that it is U2B that releases the stored charge when voltage at the inverting input goes over (or under) V/2. Looking at the size of R9 and all those diodes, it resembles a ramp wave generator. So the wave gets delivered to the base of Q1 and flows to the differential amp formed by Q3 and Q4, which are biased (correction: the current is directed) by R38 (pot, not bias resistor, on second guess) to favor either up or down output.

So the OTA gets the Iabc from R38 wiper and puts input-modulated current out on pin 8 to the R23 and R24 junction into what I presume is the filtering part. I would love to have a live circuit and a scope to help figure this out. Could C17 be the primary filter cap?

What is throwing me now is that the input seems to enter at the left side of C21 which also seems to be connected directly to the output jack. Can someone help to de-confuse me on this?
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: jatalahd on April 23, 2016, 04:08:16 PM
What is throwing me now is that the input seems to enter at the left side of C21 which also seems to be connected directly to the output jack. Can someone help to de-confuse me on this?

The Y-filter stompbox has one pot, one slide switch and the actual stomp-on, stomp-off  foot switch. This foot switch is directly connected to the output jack in the schematic. To me it seems that in the schematic the foot switch has been drawn to the state where the effect is in by-pass mode. Toggle it on and you can find the signal path when the effect is activated.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Mark Hammer on April 23, 2016, 04:37:16 PM

Sorry if I'm getting noisy on this thread -- at any rate, I'm with Mark on the idea of the fixed envelope. I suspect that it is U2B that releases the stored charge when voltage at the inverting input goes over (or under) V/2. Looking at the size of R9 and all those diodes, it resembles a ramp wave generator. So the wave gets delivered to the base of Q1 and flows to the differential amp formed by Q3 and Q4, which are biased by R38 to favor either up or down output.

So the OTA gets the Iabc from R38 wiper and puts input-modulated current out on pin 8 to the R23 and R24 junction into what I presume is the filtering part. I would love to have a live circuit and a scope to help figure this out. Could C17 be the primary filter cap?

What is throwing me now is that the input seems to enter at the left side of C21 which also seems to be connected directly to the output jack. Can someone help to de-confuse me on this?
Another vote for a fixed envelope, that should have dawned on me earlier: NO ENVELOPE RIPPLE.  Even the very best envelope-controlled filters, that use full-wave rectifiers, and optoisolators (both of which reduce ripple) have audible ripple.  If there is no ripple, that HAS to mean the envelope is not coming from an unstable source, like the guitar itself.

Which, if you think about it, makes the ad copy claims about sounding like a synth more legitimate.  It also make wanting to understand the circuit and know more about what is setting the envelope parameters.  Being able to vary the attack/decay in the unit would be VERY cool.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Scruffie on April 23, 2016, 09:17:07 PM
I fixed one recently and have voltages if that'll help you get yours back to life Mark :)

Also, that schematic, while I redrew it one to one to the schematics I had (and there were 2, they differed around the Q1 section if I recall) did have some mistakes compared to the actual unit (for a start the V.Ref is shorted out) so take it with a pinch of salt.

Also without any studying so this may possibly be a stupid statement, the clipping may just be an attempt at poor mans compression to reduce ripple.

The one I fixed had the same non-response to picking as you mention too for what it's worth.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: jatalahd on April 24, 2016, 12:30:46 AM
Mark, regarding attack and decay, did you try to swap the tantalum caps C5 and C9 as pointed out by Duck_? This is just a hunch, but I would say attack times for both sweep-up, sweep-down control signals are equal (about 15 milliseconds), determined by (R13+R19)*C5 and (R12+R14)*C9. For corresponding decay times my guess is R19*C10 and R11*C8.

Waiting for someone to challenge me on this :)
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: StephenGiles on April 24, 2016, 07:33:28 AM
Mark, regarding attack and decay, did you try to swap the tantalum caps C5 and C9 as pointed out by Duck_? This is just a hunch, but I would say attack times for both sweep-up, sweep-down control signals are equal (about 15 milliseconds), determined by (R13+R19)*C5 and (R12+R14)*C9. For corresponding decay times my guess is R19*C10 and R11*C8.

Waiting for someone to challenge me on this :)

............possibly!!!!!!

I currently have no chance of breadboading this because my wife hopefully returns home from 2 1/2 weeks in hospital with trench foot. No that's wrong - should be pseudomonas infection and cellulitis (look it up, interesting pictures!!) so nursing duties when at home!!

So I'm viewing this from the hospital via Teamviewer to my home PC on the recently upgraded "free" wifi!

Where does the trigger circuitry end, I can't get my head around this - is it pin 1 of U2, or is U2 and associated components just a conditioning filter for the trigger pulse which energises the offending "envelope generator", which, unless I am persuaded otherwise, should be replaced with something more useful with adjustable attack and decay times.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: jatalahd on April 24, 2016, 10:50:21 AM
Where does the trigger circuitry end, I can't get my head around this - is it pin 1 of U2, or is U2 and associated components just a conditioning filter for the trigger pulse which energises the offending "envelope generator", which, unless I am persuaded otherwise, should be replaced with something more useful with adjustable attack and decay times.

My current understanding is that, yes, all the 4 op amps in the top row are used for generating the trigger signal and pin 1 of U2 delivers short trigger pulses to the base of Q1.

Downward sweep: In steady state, the base of switching transistor Q1 is at VB, hence the switch is closed and the cap C5 is fully charged (it charges inversely, from VB towards 0 volts), collector of Q1 is close to 0 V. When the trigger pulse arrives (assuming it is a downward pulse), Q1 conducts and discharges C5, then collector of Q1 rises to VB. This would be an instant voltage rise, but it needs to charge C10 through R19, which slows things down. C10 pushes base of Q4 towards VB and the emitter follows the voltage rise of the base (VBE diode is free to move because of R20). After the trigger pulse has ended, Q1 is closed and C5 charges towards 0 V through R19 and R13. This reflects to Q4 as just explained and this causes the prominent sweep (VB towards 0 volts)

Upward sweep: at the same time when the trigger pulse affects base of Q1, it is seen inverted at the collector of Q1. In steady state, the base of Q2 is at ground and the switch is closed, C9 and C8 are fully charged to VB. The inverted trigger pulse is now upwards pulse and it triggers base of Q2 through C10. C9 discharges immediately through Q2 and this puts base of Q3 to ground, Q3 is open and lets C8 to discharge through R11. After trigger pulse ends, Q2 is closed, potential rises so that Q3 is closed and C9 charges from 0 to VB through R14 and R12.

So C5 charges from VB towards 0 V, and C9 from 0 V towards VB, simultaneously. The RC paths have the same RC time constants. These are the two opposite control signals, which cancel each other at the middle of R38 (the sum of voltages stays constant in the middle of R38).

Sounds good, but I am not sure :)   ... Actually just realized that C8 would charge up faster than C9, which makes a small hole to my theory :( Back to thinking mode.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: StephenGiles on April 24, 2016, 01:29:16 PM
Thank you, something to think about!
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: robthequiet on April 24, 2016, 02:52:53 PM
If I had one of these puppies I would test the difference between a series of long soft input signals vs. a short large signals, to test the charging/triggering rate. In other words, can the triggering be manipulated by the input or does it automatically draw a charge from the power supply, or both? The Youtube videos seem to indicated that it has a very smooth release, perhaps accounting for the overcomplicated (hat tip to StephenGiles) circuitry up top.

I have a question about the schematic -- the sleeve of the input jack goes to ground, but also seems to go through R30/470K to a ground point near the output jack switch. Kinda the way Q2 appears to have its collector and emitter tied to ground. Or am I misreading the schematic symbols?
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Mark Hammer on April 24, 2016, 03:06:02 PM
Nah.  Scruffie accidentally put a "ground" triangle symbol where a circle was intended.  The 470k is the terminating resistance, and the output switch selects between the 470k/0.1uf junction or the input.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: robthequiet on April 24, 2016, 03:42:33 PM
Ah. Got it. Still trying to parse the salad around the left side of OTA. Thx!
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Mark Hammer on April 24, 2016, 06:54:55 PM
that's funny, I've just been playing with up/blank/down. I'd hazard a guesss, under duress, that C5 and C9 might be timing sweeps, being tants and all.
Well, on mine C9 is the only tantalum on the board. C5 is a regular electro.

BUT, you win the prize, my clever friend.  Upping the value of either of them slows the sweep time.  As shown in the schematic, the down sweep time was noticeably faster than the up sweep time.  Which probably accounts for why there was a 5uf electro tacked on the copper side in parallel with the 10uf.  I removed it just to hear what happened, and the sweep time shortened, confirming both your guess, and the reason why the extra 5uf was there.

I think we are making progress!  :icon_biggrin:

So are these "one-way" envelopes? I.E., there is an attack but no decay?
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: duck_arse on April 25, 2016, 11:15:16 AM
the "original" scribbled copy of this that I have gleaned from the interwires shows no connection between the collectors of Q2 and Q4. I would have said that 470k looks like a de-pop for clinton bypass, but I really don't understand the drawing of the input jack, on the original or the redraw.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: robthequiet on April 25, 2016, 05:35:21 PM
@duck_arse, that was my question. It makes more sense if C21 and R2 form the envelope input path, and R27, R26 and R28 form the audio input --  but the output is a bit weird, though it seems to come out of pin 8 of the OTA with C15 as the output cap after R25, R24 being feedback (possibly) and R30 being the load/impedance setter... not too many spice models of CA3094 about, apparently. 
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: StephenGiles on April 26, 2016, 03:42:00 PM
@duck_arse, that was my question. It makes more sense if C21 and R2 form the envelope input path, and R27, R26 and R28 form the audio input --  but the output is a bit weird, though it seems to come out of pin 8 of the OTA with C15 as the output cap after R25, R24 being feedback (possibly) and R30 being the load/impedance setter... not too many spice models of CA3094 about, apparently. 

I used to have an EH design note on the CA 3094, but my paper copy has gone awol. This explained the various configurations required when using either pin 6 or pin 8 as its output. I'll keep looking.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: robthequiet on April 26, 2016, 04:28:31 PM
@StephenGiles -- Thank you -- I have located a datasheet or two and even found a story someone posted online having built one from discrete components. The EH notes would certainly be invaluable if they yield a clue. Of course, I am being lazy in not just hacking a model or two myself, although building a simulation out of the box may be more effort than it's worth if we could just build the actual, albeit having q's regarding the schematic.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: snap on April 30, 2016, 06:20:46 AM
this might be of help: depending on which side of the buffer (pin6 or 8 ) you take off the outputsignal, the inputs (pin2 and 3) each are inverting OR non-inverting:
http://www.mudpods.com/CA3094.gif (http://www.mudpods.com/CA3094.gif)
(http://www.mudpods.com/CA3094.gif)

http://www.synthdiy.com/files/2008/ca3094.pdf (http://www.synthdiy.com/files/2008/ca3094.pdf)
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: anotherjim on April 30, 2016, 07:01:22 AM
That's what I thought -  or rather it had to be an open collector buffer output - akin to the LM13700 with its open emitter buffer. Hence the output pin 8 is connected to supply, but via R23 as collector resistor. There is no emitter resistor fitted from pin 6, but there is ac negative feedback via C13 to pin 1.

Title: Re: Understanding the EHX Y-Triggered Filter
Post by: jatalahd on May 14, 2016, 09:10:37 AM
Let's get back to this one.

After thinking more about the sweep part of the circuit, I don't need to change much of my writings in earlier posts. The sweeps are not symmetrical as I first assumed, and the sweep time around Q3 is not that easy to determine because R12 makes Q3 a constant current source which loads up C9. Therefore, the sweep on the left side of R11 is a linear sawtooth sweep and the sweep on the right side is more exponential and it does not go down all the way to 0V because of the voltage divider R19 and R13.

Then the OTA filter. I made a simulation using the following simplified circuit:
(http://www.guitarscience.net/img/ota_schem.png)

For the XOTA I simply used a VCCS subcircuit model:

.SUBCKT OTAS        1   2   3   gm=0.009
RIN     1 2     10000k
Ggm   0 3 1 2 {gm}
.ENDS

and added the transistor buffer from discrete components with reference to the schematic snap posted. There is still some problems in the Y-triggered filter schematic, but I presume that the simplification of the input circuit is close enough. Don't mind the actual gain values I got, because R4 affects to those and I was not sure how to connect that properly due to the problems in the original schematic.

Here are the simulation results as a function of the OTA transconductance parameter:
(http://www.guitarscience.net/img/ota_freq.png)

It seems that it tries to mimic the original wah pedal frequency response with a combination of a high-pass filter (Chp) and a low-pass filter (Clp). The maximum transconductance value (gm=0.009) I calculated from the approximate formula gm = 20*Ibias, and started decreasing it from there. There are two options how it changes the "resonance" frequency. It is either using the fact that the output impedance of the OTA changes with Ibias and this forms a low-pass with the Clp capacitor or then Clp is used as a Miller capacitance in the feedback loop and its value changes with gain (which is controlled by Ibias). I am hoping it would use the former option, but the most obvious is to use the latter mechanism as the original wah pedal uses.

The transistor buffer with the ac feedback through Clp has a transfer function of a first order low-pass filter when it is connected to a voltage source with internal resistance Rs. The cut-off frequency is linearly related (but not directly) to the time constant of Rs and Clp. This would make it possible to alter the cut-off by modifying the source resistance, but I have not been able to verify how much the output resistance of the VCCS block changes with gm. This is why I would opt for the Miller theorem in this case.

Would be nice to know if anyone else has made progress in analysing the complete Y-triggered filter circuit.

Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Mark Hammer on May 14, 2016, 09:39:20 AM
Your diligence in this is deeply appreciated.  I am confident something useful will come out of this.

And thank you for confirming by your analysis of the circuit, something I've long held about the perceptual aspects of upward and downward sweep: they need to BE different in order to be PERCEIVED as similar but inverted.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on May 26, 2018, 01:43:28 PM
Am I the only one wondering why C16/C17/R34 etc are connected both to the ground and V/2?  :icon_question:

(https://s26.postimg.cc/gatpnpndl/Y_triggered_filter_EHX_questions.jpg)
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Scruffie on May 26, 2018, 02:18:47 PM
Because that's how it was drawn in the factory schematic, nobody ever said that factory schematic was right, I just copied it verbatim so it would be readable.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on May 26, 2018, 02:35:53 PM
Anyone having original pedal, so can check this?
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on November 14, 2019, 01:05:12 PM
I have finally had some time for this, so I tried to trace images of PCB that Mark Hammer posted here: https://www.diystompboxes.com/smfforum/index.php?topic=120148.msg1124242#msg1124242 (https://www.diystompboxes.com/smfforum/index.php?topic=120148.msg1124242#msg1124242). It seems that the schematic above is far from correct, and this is what I made. I'd be grateful to anyone who can confirm this schematic as a good one.

I don't have a CA3094, so I used it's schematic from the datasheet, hoping to re-create a discreet one.


(https://i.postimg.cc/xq00xBCP/y-corrected.jpg) (https://postimg.cc/xq00xBCP)
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on November 14, 2019, 01:09:53 PM
There were some attempts to recreate this schematic here https://www.diystompboxes.com/smfforum/index.php?topic=87545.msg736490#msg736490 (https://www.diystompboxes.com/smfforum/index.php?topic=87545.msg736490#msg736490), but the images are long gone.  :(
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Scruffie on November 14, 2019, 01:49:35 PM
First of all, there's no need to make a discrete OTA (and you'd need matched transistors etc.) just use a CA3080 or 1/2 a 13600/13700 with a discrete darlington output buffer.

Second... not to discourage you but having owned one (no I don't any more so can't take photos) I really wouldn't put that much effort in to copying it, it's not particularly wonderful. Different, yes, but the envelope triggering mechanism is a pain in practice.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Mark Hammer on November 14, 2019, 03:02:56 PM
Came across this YT demo from David Rolo,  Provides a nice compendium of the sounds possible, using both guitar and bass.  Gives you a good idea of the relative consistency of sweep.  It's clearly a very different sort of swept filter than most envelope-controlled units.  Would be worth understanding a little more; especially with an eye towards more control over the time constants of the envelope.  Glad the topic has been resurrected.

Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on November 14, 2019, 03:06:29 PM
First of all, there's no need to make a discrete OTA (and you'd need matched transistors etc.) just use a CA3080 or 1/2 a 13600/13700 with a discrete darlington output buffer.

Second... not to discourage you but having owned one (no I don't any more so can't take photos) I really wouldn't put that much effort in to copying it, it's not particularly wonderful. Different, yes, but the envelope triggering mechanism is a pain in practice.

Thanks! I though that it use CA3094 pins 1 and 8, which are not available on CA3080, so I wanted to try it with discrete OTA, since CA3094 is unobtainable.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Kipper4 on November 14, 2019, 03:21:08 PM
That's quite a nice filter there. Thanks for the vid Mark.
I noticed the envelope detector didn't trigger every time.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Scruffie on November 14, 2019, 03:53:56 PM
That's quite a nice filter there. Thanks for the vid Mark.
I noticed the envelope detector didn't trigger every time.
When you play it, it kinda keeps triggering and triggering and then just jams up, before starting again, you have to adapt your playing a lot and even then it's not particularly consistent.

Having said that, Mark isn't wrong, studying it more might lead to it being developed in to something more useful and yeah the filter itself is quite pleasant, it's the same type as found in the mini (not micro) synth.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Mark Hammer on November 14, 2019, 04:06:09 PM
Yep. Not your basic Doctor Q variant.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Kipper4 on November 14, 2019, 04:39:42 PM
I might have to reread that a few times, but yer 4046 pll right.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on November 14, 2019, 05:30:41 PM
This is ltspice model of a filter, hopefully w/o errors. Fixed so it can use LM13700 OTA, simulation works fine.

Code: [Select]
Version 4
SHEET 1 4004 1244
WIRE 1168 -752 816 -752
WIRE 272 -736 272 -768
WIRE 1472 -736 1232 -736
WIRE 1168 -720 1104 -720
WIRE -208 -640 -240 -640
WIRE -80 -640 -128 -640
WIRE 48 -624 -16 -624
WIRE 64 -624 48 -624
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TEXT -592 -320 Left 2 !;.tran .2
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Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on November 15, 2019, 03:28:10 PM
This is the final corrected schematic. There are still some parts (marked with ?) for which I'm not sure if they are really necessary, but this one works fine, at least in ltspice.


(https://i.postimg.cc/vgPxyKkS/y-triggered-final.jpg) (https://postimg.cc/vgPxyKkS)
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Mark Hammer on November 15, 2019, 04:10:28 PM
As much as I appreciate it, and the work involved, I am confused by your drawing.  Are you subbing a 13700/13600 for the original 3094/1048?
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Scruffie on November 15, 2019, 07:54:35 PM
As much as I appreciate it, and the work involved, I am confused by your drawing.  Are you subbing a 13700/13600 for the original 3094/1048?
He is, the current source/sink darlington from the 3094 is in the bottom right corner of the schematic with the "P1, P8" connections.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on November 16, 2019, 05:12:59 AM
As much as I appreciate it, and the work involved, I am confused by your drawing.  Are you subbing a 13700/13600 for the original 3094/1048?

Yes Mark, since I can't find CA3094, I decided to recreate this using LM13700. Also, I confirm that R21 and R37 on my schematic are not necessary - they're present in the "official" schematic, but not on the PCB photos you've shared. But I'm still not sure about C23 (10u), it's not present in original schematic, but I believe I saw it on the PCB. It definitely affects operation.

I must ask Mark or anyone else having an original pedal, to check out R2, R4, R5, R6 value. They seem as 1k to me (if photograph colors are right), not as 10k as shown on original schematic.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: StephenGiles on November 16, 2019, 08:16:01 AM
I echo Mark's appreciation, but I'll have to restate the 13700 rectangle with usual triangles to get my head around this!!
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on November 16, 2019, 01:40:59 PM
OK, like this:

(https://i.postimg.cc/vgPMxq4V/y-trigg.jpg) (https://postimg.cc/vgPMxq4V)

Just have in mind that this is the same as the previous schematic, just replaced LM13700 symbol. Also, previous schematic uses both halves of LM13700 in parallel.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: PRR on November 16, 2019, 02:14:31 PM
I have not digested plan in #49. But it appears if you use LM13700, then the two NPN between R42 and R43 can be done by the buffer inside the '13700, and IAC R42 should not be needed. Saves 3 parts.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Scruffie on November 16, 2019, 02:50:38 PM
I have not digested plan in #49. But it appears if you use LM13700, then the two NPN between R42 and R43 can be done by the buffer inside the '13700, and IAC R42 should not be needed. Saves 3 parts.
There's no access to the collector on the 13700's internal darlington.

You can do it with just one transistor if you want and then ignore R42.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on November 16, 2019, 02:56:53 PM
I have not digested plan in #49. But it appears if you use LM13700, then the two NPN between R42 and R43 can be done by the buffer inside the '13700, and IAC R42 should not be needed. Saves 3 parts.

I tried that in ltspice, but it doesn't work as expected.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: StephenGiles on November 16, 2019, 04:56:34 PM
OK, like this:

(https://i.postimg.cc/vgPMxq4V/y-trigg.jpg) (https://postimg.cc/vgPMxq4V)

Just have in mind that this is the same as the previous schematic, just replaced LM13700 symbol. Also, previous schematic uses both halves of LM13700 in parallel.

Many thanks!
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on November 16, 2019, 05:32:30 PM
I have not digested plan in #49. But it appears if you use LM13700, then the two NPN between R42 and R43 can be done by the buffer inside the '13700, and IAC R42 should not be needed. Saves 3 parts.
There's no access to the collector on the 13700's internal darlington.

You can do it with just one transistor if you want and then ignore R42.

Like this? This seems to work in ltspice.

(https://i.postimg.cc/D4b45FvK/y-trig-minimized.jpg) (https://postimg.cc/D4b45FvK)
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Scruffie on November 16, 2019, 07:45:40 PM
I have not digested plan in #49. But it appears if you use LM13700, then the two NPN between R42 and R43 can be done by the buffer inside the '13700, and IAC R42 should not be needed. Saves 3 parts.
There's no access to the collector on the 13700's internal darlington.

You can do it with just one transistor if you want and then ignore R42.

Like this? This seems to work in ltspice.

(https://i.postimg.cc/D4b45FvK/y-trig-minimized.jpg) (https://postimg.cc/D4b45FvK)
Yes, but I have had more consistent results with a discrete pair of transistors, but it will work like that.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on November 17, 2019, 05:13:37 AM
In that case, I will stay with a few extra parts :)

Did anyone with an original pedal had time to check values of resistors mentioned above?
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on November 17, 2019, 08:54:49 AM
I'm finally done with the PCB for this. I need some help - is this ground plane done well or not? I don't have ground loops, but I wander if it is OK regarding current flow. Please help!

(https://i.postimg.cc/XGXPRR5J/y-trigg-PCB.jpg) (https://postimg.cc/XGXPRR5J)

Single layer, with two SMD capacitors (for PS filtering right under ICs).
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: PRR on November 18, 2019, 10:35:01 PM
> There's no access to the collector on the 13700's internal darlington.

Duh. Sorry, brain fart.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on November 19, 2019, 08:22:52 AM
Both sides separated:

(https://i.postimg.cc/yDwxpQGN/y-trigg-PCB-sides.jpg) (https://postimg.cc/yDwxpQGN)
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: mookyj on November 21, 2019, 08:57:58 AM
I'm finally done with the PCB for this. I need some help - is this ground plane done well or not? I don't have ground loops, but I wander if it is OK regarding current flow. Please help!

(https://i.postimg.cc/XGXPRR5J/y-trigg-PCB.jpg) (https://postimg.cc/XGXPRR5J)

Single layer, with two SMD capacitors (for PS filtering right under ICs).

in reviewing what you have here:

You have it grouped by circuit function rather well. I would go back and review your placements within each grouping as they can be improved. That is always the case in 2nd and third looks, easy to say after it is done. I have marked up a few that you may find helpful. I would strongly consider moving the power input section the the upper left of center. Think about current flow as you you supply each circuit. Not a fan of single layer for reliability (non plated holes) and for lack of best impedance paths for signal flow. shortest return path is underneath forward path. Shortest distance is not always shortest impedance return, split planes is an example of that.  In the layout it does not have a short return path in bypass mode as return is forced up and above the switch and led circuit. For single layer you do provide much better return path that typically seen in some designs. If staying with single layer you can improve it some, that said with the observation you have applied plenty of effort to create more gnd pour in the layout.  Personally, I would default to a bottom layer ground plane would allow you to improve the V+ rails  and still allow a coplanar top ground pour also for much of your circuit.
(https://i.postimg.cc/z3yBBDcY/Capture.jpg) (https://postimg.cc/z3yBBDcY)
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on November 21, 2019, 02:54:14 PM
Thanks a lot, mookyj! I tried to keep power section close to input jack, because I use it's middle contact to disable power if pedal is unplugged. Is it really necessary to move power into top left corner? I use single side PCB because it is complicated to me to order dual layer from abroad, and I can get single layered locally. You're right, I really shouldn't be grouping input and output signal capacitors near each other. Is this better:


(https://i.postimg.cc/HJJK4ppS/y-trig-04-1.jpg) (https://postimg.cc/HJJK4ppS)
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: mookyj on November 21, 2019, 05:00:51 PM
Thanks a lot, mookyj! I tried to keep power section close to input jack, because I use it's middle contact to disable power if pedal is unplugged. Is it really necessary to move power into top left corner? I use single side PCB because it is complicated to me to order dual layer from abroad, and I can get single layered locally. You're right, I really shouldn't be grouping input and output signal capacitors near each other. Is this better:


It is incrementally better for sure.  As to disconnecting when unplugged, if not using a battery wouldn't matter to switch it.  I get why on a single layer, but I think I would try both and you can determine for yourself.   I would prefer not to return the power supply current back along the output. People do it, but I would be concerned with picking up noise, disturbance in any kind of gained pedal. With a battery or a very well filtered pedal board supply, I would say no worries. Anything coming from the wall wart could end up in your audio even with the filtering you have on board being so close to the output.  You have done a lot with a single layer.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on November 22, 2019, 03:18:41 AM
I would prefer not to return the power supply current back along the output.

Now I really need some help :)
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: mookyj on November 22, 2019, 07:46:51 AM
I would prefer not to return the power supply current back along the output.

Now I really need some help :)

it is easier than you might surmise, what would normally be switched by the jack would go to ground, move the series diode to the top as I mocked it up and that will straddle the trace going to the 1K Resistor, The lower circled section would need to move down some and you have plenty of space to do it. By rearranging the placements of the filter section after the power jack you will minimize the space needed. you could turn the jack upward it you want.

 
(https://i.postimg.cc/BXsJXZPh/Capture33.jpg) (https://postimg.cc/BXsJXZPh)
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on November 22, 2019, 09:16:11 AM
What about this solution - I left jack switching and moved jacks and 3PDT sw. upwards, so there's enough place for 9V battery now. Connected ground using one jumper, because it follows V+ current. OK?

(https://i.postimg.cc/JGpD7z3d/y-trig-04-3.jpg) (https://postimg.cc/JGpD7z3d)
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: mookyj on November 22, 2019, 03:06:25 PM
What about this solution - I left jack switching and moved jacks and 3PDT sw. upwards, so there's enough place for 9V battery now. Connected ground using one jumper, because it follows V+ current. OK?


My answer may frustrate you, but I would ditch the battery.  THe long switching routes essenstial distribut any noise from the wall wart along your anoalg sections. Kudos on clean work, I do like it.

Mike
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on November 22, 2019, 04:20:13 PM
Well, this could be the final version. What do you think, shall I remove the jumper and connect the ground plane here (red mark), or not?


(https://i.postimg.cc/fJ7fhrKT/y-trig-04-4.jpg) (https://postimg.cc/fJ7fhrKT)
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: mookyj on November 22, 2019, 04:45:01 PM
Well, this could be the final version. What do you think, shall I remove the jumper and connect the ground plane here (red mark), or not?



I would move the jumper closer together and connect the right side ground. If you find any issue with it, it is easily cut/scored and peeled.

Mike
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Mark Hammer on November 26, 2019, 06:00:31 PM
I'm about 2/3 of the way stuffing the board I etched the other day.
I dug my own unit out of the basement, and dickering with it just now, several things are clear to me:

1) It really needs a sensitivity control.  The unit is NOT an envelope-controlled filter that corresponds directly to picking strength, but rather employs a triggered envelope.  Triggering is not entirely independent of input level.  So something needs to be adjustable around U1.1 to allow it to retrigger reliably with both higher input levels, and lower ones.  You will note that R3/C1 roll off the bass sensitivity of that stage around 1500hz.  Changing the value of R3 to adjust the gain monkeys around with that rolloff.  So, in the absence of any strong rationale for using it, I'll suggest replacing R1 with something like a 250k variable resistor in series with 100k fixed resistor, to vary the gain of the stage between 22.3x and 75.5x (stock is 58.4x)

2) A filter range switch is useful.  On mine, I installed a 3-position toggle.  I replaced the stock 2200pf cap (equivalent to C22 in the redraw) with 2200pf and 6800pf in series.  The switch bridges the one or the other or maintains them in series, yielding three ranges.  Thinking it over I should have probably used 3300pf and 6800pf, since the highest range when using 2200 and 6800 is useless.  Putting 3300 and 6800 in series gets you 2200pf, which returns a stock sound.  The unit sounds really nice on lower ranges.

3) We know a transient is generated, and that two versions are produced: a rising and a falling version, equal in amplitude/sweep, and cancelling out when in 50/50 mix.  What we don't know - or at least I can't for the life of me figure out from just looking at the drawing - is where that transient is produced.  It would be nice to know a little more about how the time constants are determined.  Being able to vary the rise or fall time would be nice.  I'll haul out my scope later tonight and see if I can spot where appropriate intervention points might be, as well as learning a little more about the transient that is normally generated.

4) The Range switch is also a quirky one.  It sounds like it selects between two resonance settings.  It could really use a 3rd setting in between the two provided.  The stock arrangement provides a high-resonance setting, and another that is so bland it deters use.

I don't want to overcomplicate it, but this could be a much friendlier and more maleable filter pedal with just a couple of tweaks.  If it had variable sensitivity, the stock up/down control, a 3-position filter range switch, a better selection of resonance settings, and a 2 or 3-position toggle for fast/medium/slow sweep, it could be a really nice pedal.  Let's get this done.  Meanwhile, I'll continue stuffing my board and see if it fires up.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Mark Hammer on November 26, 2019, 09:58:34 PM
At the wiper of the Up/Down pot. the DC voltage  is high then momentarily swings low and back up again when sweeping up.  In the down-sweep setting, the DC voltage at the wiper is low, jumps high in response to triggering then returns low again.  NO movement when the control is centred.  NO great surprise there.

But the big surprise is that the schematic shows 4 transistors, and yet my board has only three.  And my personal big surprise is that the redraw actually includes a Speed switch.  Duh!  On mine, the default electro setting the time constant is 5uf.  Strapping in 10uf in parallel yields a modest, but still noticeable slowing of sweep that works well in both up and down directions.  I tried 33uf, and it worked fine in upward sweep but lousy in downward, for reasons I don't quite get.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on November 27, 2019, 04:13:29 AM
But the big surprise is that the schematic shows 4 transistors, and yet my board has only three.  And my personal big surprise is that the redraw actually includes a Speed switch.  Duh!  On mine, the default electro setting the time constant is 5uf.  Strapping in 10uf in parallel yields a modest, but still noticeable slowing of sweep that works well in both up and down directions.  I tried 33uf, and it worked fine in upward sweep but lousy in downward, for reasons I don't quite get.

Mark, I've traced this using your photos, and my schematic use only three transistors. There are two extra ones used to provide transistor output for LM13700 which is missing compared to CA3094. It could be also done with one transistor (using LM13700's buffer), but Scruffie said that he gets better results with two transistors, so I left the final schematic that way.

However, there are some photos available showing some kind of mod with the fourth transistor, but I didn't have time to trace that too.

Could you, please, confirm resistor values asked before (see schematic at https://www.diystompboxes.com/smfforum/index.php?topic=113930.msg1167164#msg1167164), resistors  R2, R4, R5, R6 value. They seem as 1k to me (if photograph colors are right), not as 10k as shown on original schematic.

(https://i.postimg.cc/PL3fk9HB/04-bottom.jpg) (https://postimg.cc/PL3fk9HB)

And this is the schematic with Mark's mods, if I understood him well:

(https://i.postimg.cc/9493xQbg/Y-trig-04.jpg) (https://postimg.cc/9493xQbg)

Both filter/speed switches shall be on-off-on type.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Mark Hammer on November 27, 2019, 08:58:35 AM
1) The photo of the copper side with the additional transistor is not from my particular pedal.  But there are several components soldered to the copper side on mine, so it is not unreasonable to think that EHX  modified the design and tacked on the additional transistor.  We need to remember that the pedal was made when there was NO layout software.  You would work it out on paper and then use rub-on tape to make the pattern on a transparency, photograph it or otherwise turn it into an image, and use that to make boards via a photosensitive method.  So there were "disincentives" to changing the layout, with add-on components being the preferred choice for smaller producers.  EHX was certainly "big" at the time, but nowhere as big as they are now.

2) The suggested mods to the circuit are generally faithfully reproduced in your drawing, but with a few changes needed:

I tried looking for the possible shape of the transient produced last night.  But my scope skills, and the capabilities of the scope itself seemed to be insufficient.  I could see when and where it went high or low, but not the entire transient from start to finish.  So I can't tell you much about it, unfortunately.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: MonkeyKult on November 27, 2019, 10:18:08 AM
What a coincidence, I just came across an old Y-Triggered Filter that was in pretty bad shape, and I just finished restoring it just yesterday. It was tricky, cause there were a lot of discrepancies between the circuit on my pedal, and what Ive read in this thread and elsewhere. I tried my best to piece together and here is what I found:

1. First, the similarities; I do have an additional 5 uF cap on the back side in parallel with the 10uF (not a tantalum, despite the schematic) to give 15 uF total across the resistor. This seems to work well.

2. Mine only has 3 transistors; 2 PNPs and 1NPN. This was a cause of a lot of confusion, as it was hard to reconcile these. Not sure which is correct.

3. Most notably, the circuit across the 4558 around R1 is totally different on my version. Instead, my R1 is 47K and there are no diodes in parallel. At first I thought this was a mistake, but I must have one of the first versions cause the resistor values are printed on the CB. So this is how mine is configured.

Lastly, mine needs to have a battery and a 3.5 mm DC plugged in to work properly.

Otherwise, I have to admit this sounds pretty good. I changed all electrolytic capacitors and the 1 tantalum. Im pretty pleased, though it was a challenging rebuild.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on November 27, 2019, 12:29:12 PM
If I can see well, those mods with a transistor from the image I posted before, should look like this, although I'm not so sure about resistor values (and transistor is 2N5087 actually):

(https://i.postimg.cc/7JxwQtxW/y-trig-04-4-mod.jpg) (https://postimg.cc/7JxwQtxW)

Tried this mod with ltspice, but doesn't seem to do much.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Mark Hammer on November 27, 2019, 12:30:37 PM
Congrats!  It's a fun pedal.  It tends to be confused with the DR. Q, that came out around the same time, but behaves quite differently.

Mine has the 3.5mm phone jack as well. I replaced it yesterday with a more standard 2.1mm barrel jack.  Wasn't easy machining the hole to make it bigger.  I think over the years I've replaced a few of the non-polarized capacitors as well.  I also replaced the original stompswitch with a DPDT, but now that I look at it, I think I'll stick a 3PDT in there, with a status LED.  If it was a rare beast, I'd be concerned about its "vintage" properties.  But I'm confident that it was produced in sufficient numbers that mine doesn't need to be preserved intact as a museum piece.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on November 27, 2019, 12:42:20 PM
Hmmm, shorting E&B of this additional transistor DOES make some change to the sound. I don't have this on a breadboard, but ltspice shows there's something going on. When they're not shorted, this mod seem to do nothing.


(https://i.postimg.cc/56hk4Yzx/y-trig.gif) (https://postimg.cc/56hk4Yzx)
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Mark Hammer on December 15, 2019, 08:37:07 PM
So, I have Rankot's board etched and stuffed, and modded (naturally) to have slow and fast time constant and three different filter ranges (two additional below stock).  I used 2700 and 5600pf in parallel with the stock 2200pf to get a medium and low range.  Sounds good.

Once I figured out that BC327 have the opposite pinout of a 2N5087, everything works as it should, except that the sound is very distorted and very dull.  I also need to turn my guitar volume down to about half or less to get reliable triggering.  It is possible that I may have mistaken values - the curse of using 1/8w metal film resistors. But I also wonder if the components around the LM13700 are appropriate.  The odds are that it is MY mistake, rather than anybody else's.  BUt I just thought I'd put it out there.  It would be nice to finally have this off the bench, so I can get to other things.  In the meantime, I'll go over the component values.  I'm usaing the layout posted here:

Both sides separated:

(https://i.postimg.cc/yDwxpQGN/y-trigg-PCB-sides.jpg) (https://postimg.cc/yDwxpQGN)
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on December 16, 2019, 08:02:12 AM
I'm still waiting for my PCB to arrive, our post is at strike for two weeks :(

However, you may check resistor values which I was not sure about, as I wrote few posts above:
Quote
Could you, please, confirm resistor values asked before (see schematic at https://www.diystompboxes.com/smfforum/index.php?topic=113930.msg1167164#msg1167164), resistors R2, R4, R5 and R6. They seem as 1k to me (if photograph colors are right), not as 10k as shown on original schematic.

I did use 1k on my layout, maybe they should be 10k?

Also, what you used is not the last version of my layout, there could be some mistakes there as well. I did finish a layout with some "bells and whistles", however, didn't want to publish it until I build it and make sure it's working. However, here it is, for reference, together with appropriate schematic:

(https://i.postimg.cc/mtsXzNKk/Y-trig-04-5.jpg) (https://postimg.cc/mtsXzNKk)

(https://i.postimg.cc/1V5KBJSr/Y-trig-04-5-schematic.jpg) (https://postimg.cc/1V5KBJSr)
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Scruffie on December 16, 2019, 08:15:05 AM
(https://i.imgur.com/muGQYaH.jpg)
10k.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on December 16, 2019, 08:24:14 AM
10k.

Thanks a lot! Then, what about R14/R15? 10k too?
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Scruffie on December 16, 2019, 08:56:26 AM
Pretty sure R14 is 10k, R15 is 1k.

Also, where did R105 & 106 come from?

And how come you've left out Q4 from the original schematic/found on the back of some of the boards.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on December 16, 2019, 10:13:19 AM
R105&106 are there to enable parallel usage of both OTAs in LM13700.

Q4 shown on those schematics was not very logical to me, so I've found a photo of a PCB with that transistor added, and it is actually shown on my schematic to the left of MOD switch, together with two resistors marked with * (R13 and R38). R37 is also seen on some of the schematics' but I'm almost sure it's unnecessary.

(https://i.postimg.cc/y30CtFn5/04-bottom.jpg) (https://postimg.cc/y30CtFn5)

If you carefully look at that old schematic (showing Q4), there's a wire shorting bot emitter and collector of Q2 to ground, which is not logical. I traced this photo and to me it seems that Q4 shall be connected like show on my last schematic. But please check, four eyes see much better than two :)
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Scruffie on December 16, 2019, 12:05:20 PM
You can still use the 13700 in parallel without them.

Sorry I was looking at an older version of your schematic.

The wire connecting Q2 to Q4 and thus Q2's collector to ground is actually a mistake on my original drawing, it's not on the vintage EHX drawing ('twas nearly 10 years ago I did it!).

So Q4 is working as a buffer before the up/down pot, EHX wouldn't have added it unless it was a worthwhile addition (I assume it improves the action on the pot). I think in the photo you just can't see a trace cut under that extra 10k resistor.

So R38 wouldn't be there, R13 would connect to B+, the emitter connects to R13 & the CW side of the pot, base connects to R19/C10 junction and collector goes to ground.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on December 16, 2019, 03:09:40 PM
(https://i.postimg.cc/NLvNXPxh/Y-trig-04-6-schematic.jpg) (https://postimg.cc/NLvNXPxh)

Like this?

Please notice LINK which I placed between Q4 base and Q2 collector, I couldn't find that trace on photos, but it is present in your old schematic. I presume it shouldn't be there, but I left it just in case.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Scruffie on December 16, 2019, 03:23:54 PM
Get rid of link and then yes.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Mark Hammer on December 16, 2019, 09:51:43 PM
I hate to be picky, but I'm quickly losing track of what is referring to whatIs it possible to identify the different transistors on the board by number?  I.E., which one is Q2, Q4, etc.  I'm hoping to be able to salvage this build without having to etch en entirely different board.  Changing R4 from 1k to 10k only succeeded in producing an "wiggly" oscillation, so I changed it back. 

Changing R14 to 10k improved things a bit, but it's still twitchy and dark.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on December 17, 2019, 02:24:31 AM
Sorry Mark, we were referencing the 'original' schematic, but I also tried to have the same part references on mine. Here's mine schematic with all the changes Scruffie noted, and with transistors numbered too.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on December 17, 2019, 02:48:36 AM
I hate to be picky, but I'm quickly losing track of what is referring to whatIs it possible to identify the different transistors on the board by number?  I.E., which one is Q2, Q4, etc.  I'm hoping to be able to salvage this build without having to etch en entirely different board.  Changing R4 from 1k to 10k only succeeded in producing an "wiggly" oscillation, so I changed it back. 

Changing R14 to 10k improved things a bit, but it's still twitchy and dark.

Try to remove C23 (10u) if you have installed it, I didn't see it on 'original' schematic, but found it while tracing. Maybe just my error? I've checked all the other values and they're the same as on 'original', except already mentioned resistors (10/1k), and R15 and R19. Here is the updated schematic, with all references the same as in 'original', and all values which looked different to me on the PCB photos I had are shown in ME/ORIGINAL form, for better tracking.


(https://i.postimg.cc/2bN50pzv/Y-trig-04-6.jpg) (https://postimg.cc/2bN50pzv)
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Scruffie on December 17, 2019, 07:16:25 AM
I hate to be picky, but I'm quickly losing track of what is referring to whatIs it possible to identify the different transistors on the board by number?  I.E., which one is Q2, Q4, etc.  I'm hoping to be able to salvage this build without having to etch en entirely different board.  Changing R4 from 1k to 10k only succeeded in producing an "wiggly" oscillation, so I changed it back. 

Changing R14 to 10k improved things a bit, but it's still twitchy and dark.
You're not the only one!  :D

That's not surprising if you just changed R4, you'd need to change R4,5 & 6.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: Mark Hammer on December 17, 2019, 12:44:17 PM
So, I removed C23, and the resulting sound is once again clear as a bell, and perfect volume balance to the bypassed signal...except there is now no sweep. Moving the up/down pot works as filter-tuning, but yields nothing that responds to picking.  Still a bit to go.  I'll need to comb through this thread for details I've overlooked.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on December 21, 2019, 01:16:32 PM
My PCB has finally arrived, so I will populate it right now!
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on December 21, 2019, 04:57:58 PM
First report: IT'S ALIVE!  8)

I've built it w/o C23 (10u), and I also didn't install Q4 which should drive Up/Down pot, I've just connected pot directly to joint of R19, C10 and R13.

R4/R5/R5 = 10k
R10 = 12k
R19 = 1k8
R105/R106 = 510Ω

R21/R37 not installed.

It is working, but I've noticed that there is some kind of 'reverberated' filter - it makes a quack on strum, but then there's another one (or two) quacks with less intensity after that. It doesn't happen always, I must check what's producing that - but it seems to be normal, and sounds almost the same as in this demo:



Also, Up/Down pot works OK, but not when turned totaly CW - then there's almost no sound - tone is heard only at the beginning of the note (during pluck), then nothing. So probably Q4 is necessary, but I will try that in the morning.

And one more thing - to my ears, that Range switch doesn't do much. I will also install Filter switch and Sensitivity pot (as proposed by Mark) in the morning, and report my findings.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on December 21, 2019, 05:29:36 PM
Not to mention that I'm extremely happy with this, because my first PCB for this filter didn't work at all!

 ;D ;D ;D ;D ;D
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on December 22, 2019, 04:15:17 AM
I've been ltspicing this this morning, and here are the conclusions:

- If R2/4/5/6 are 1k, then C23 should be there. If they're 10k, then no C23.
- R21 is making a difference in envelope shaping, so it can be switchable - I'll test it.
- The most difference is made with C12 value, and I will test it within 1u-10u range.
- R19 should be 1k8.
- R10 value is not so important (12/18k).
- R37 doesn't seem to be necessary.
- Q4 can make a lots of difference, so it can be also a nice idea to put it on a switch during testing (and maybe after).

So I will test this on a real pedal and report.

Cheers!
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on December 22, 2019, 07:05:02 AM
OK, here are the first findings:
- R21 doesn't make too much audible difference, regardless simulation, but it seem that makes things better with Up/Down pot at CW. I also tried to add C23 (10u) and it made things better a little bit, so I left it there too - it didn't make things worse, if nothing at all.
- I've found a point where U/D pot starts making trouble and measured it - it seems that it needs some 3k6 added resistance at CW side to work well. Maybe I should try R19 with 4k7? That's next step.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on December 22, 2019, 07:53:27 AM
Increasing R19 to 4k7 didn't make any improvement. So I've tried to add Q4 and R20 between C10/R19/R/13 junction and U/D CW. It seems to be a little bit better and filter works a little bit different, but there is still a dead zone when U/D is rotated to CW. I will add that resistance between pin 3 of CW and Q4 emitter. I will also try to make a switch so Q4 is switchable, to easily make comparisons.

Tested Sensitivity pot as proposed by Mark, but it didn't bring audible difference to me, so I've removed it.

Next is C12 value, hope it holds some surprises!
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on December 22, 2019, 11:51:34 AM
I tried to replace R10 from 12k to 18k, but it didn't bring any improvement to Up/Down action. After that, I tried a Filter switch, as proposed by Mark, and it brought some changes, although I like it the most when switched to 3n3. However, I'll leave it as on my schematic.

Switching that switch produces a thump, so any advice how to avoid that is welcome!

Next, I started experimenting with C12 value. Putting 1u there made sweeping shorter and also brought bigger "dead" zone at Up/Down pot CW side (and perhaps some at CCW), so it seems that this capacitor is the main problem. At 1u, dead zone is 22k big, compared to 3k6 with 10u.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on December 22, 2019, 01:29:34 PM
And finally - I decided to use 2u2 as a base value for C12, paralleled with another 2u2 and 10u mounted on On-Off-On switch. It gives a nice variation of sounds. There is also a 4k3 resistor between Up/Down pot's CW and rest of the circuit, to skip the dead zone. Seems to fit all above values for C12, but it can be up to 10k, try what suits you.

There is also a switch called Drive, to enable/disable Q4. I left it there for testing purposes, because I didn't have enough time to test impact of Q4.

As I have promised, here is the final version of a schematic, together with PCB files. Please note that there are two SMD (or you can try with small MLCC capacitors as I did) 100n capacitors for noise filtering above both ICs. Also, I used patterns for MLCC capacitors wherever I could (those not in a signal chain), but if you want to use polarized for some reason, check polarity according to the original schematic.

Merry Christmas and a Happy New Year!

https://docdro.id/N6ioW9N (https://docdro.id/N6ioW9N)

(https://i.postimg.cc/7bT2DBb4/YTF-final.jpg) (https://postimg.cc/7bT2DBb4)
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on December 24, 2019, 02:49:13 PM
I've been tweaking this in ltspice for a while, and it seems that switching Q4 could be interesting - if it is out of the circuit, width of pulse is shorter. Also, R19 seems to be in reverse relation to pulse amplitude, so I will try to put 1k resistor in series with 5k pot there, to is if there is any real sonic impact.
Title: Re: Understanding the EHX Y-Triggered Filter
Post by: rankot on January 06, 2020, 02:53:37 AM
OK, now really FINAL schematic and PCB files:
https://docdro.id/LgvaQ38 (https://docdro.id/LgvaQ38)

(https://i.postimg.cc/9z2j4Z9G/IMG-20200105-214741615.jpg) (https://postimg.cc/9z2j4Z9G)

Most of 1u, 2u2 and 10u capacitors are MLCC. If you want to use electrolytic, please check polarity according to schematic. Please note that there are two SMD 100n caps beneath both ICs.

I still have some glitches on lower settings for C12 (Speed switch), which can be omitted increasing R40, or you can use DPDT On-Off-On switch and add some resistances at the second pole to change R40 value according to C12, so to maintain maximum range for Up/Down pot.

Please note that I have added R42 at the Filter switch, to reduce pop. It minimally affects the sound, but pop is significantly reduced. Maybe this could be done another way, I don't know?

Also, there are a few final notes not shown in a schematic: