Wampler Faux Analog Echo

[Plexi]

Hi forum!
I finished this great delay a few weeks ago.


I replaced C19 for 100nf and Level pot to A100K.

Where I need to switch to have tails bypass?
Lift R12?
I get some "distorted" repetitions when level is almost 50% or more.. why is this happen?

[ElectricDruid]

Do you mean "Level"? I can see why you'd get distorted repetitions with the "Repeat" control up high, but if the signal is ok, altering the "Level" control should just make it louder. Unless it's starting to clip in the final mixer op-amp IC 4B, which is possible.
That said, there's a gain of two at the input, but that output mixer is designed to drop the level back down again (to help reduce noise) so if the signal at the top of the level pot is clean, changing the level pot shouldn't make it worse.

HTH,
Tom

[Plexi]

Thanks Tom
I forgot to mention that I replaced R12 (47k) for 100k (to oscillate easyly).
As you mentioned, and I understand right, it affects "repeats".

I tried 100k res across a100k pot to see it they reduce distortion, and it's the same.
I'll try the same with R12, to get back to 50k (closer to 47k).

[ElectricDruid]

Changing R12 from 47K to 100K probably won't help it oscillate. If I'm reading it right, R10, R11, R12 and C11, C14 are a multiple-feedback filter. Sticking the values into this tool tells us what it's up to:

http://sim.okawa-denshi.jp/en/OPtazyuLowkeisan.htm

With R12=47K, this gives me 2.5KHz, Q=0.85, a reasonable cutoff and a bit of a hi-mids peak.
With R12=100K, the cutoff comes down to 1.76KHz, a bit darker, and the Q drops to 0.63, a much softer roll-off.

Since it'll reduce the overall level more than 47K does, it won't make it oscillate more easily.

There's another RC LPF (R9/C9) stuck on the front that I've ignored, but that hasn't changed.

HTH,
Tom

[Plexi]

Tom, thanks a LOT for your explanation.

Update, I tried in R12:
- 50k (100+100 parallel)
- ~79k (100+390 parallel)
- 100k

The only way it oscillates easy is with 100k 
But, with less than 80k, it doesn't get that "dirt" I mentioned.


I added a momentary DPDT with a small res to the ground at the time pot tab 3, plus the regeneration pot attached all the way up.
That way it starts to oscillates getting a "skating tape" sound.

[ElectricDruid]

If you're having trouble getting it to oscillate, rather than messing with the input filter (which is what R12 is part of) which has all sorts of other side-effects, I'd have a play with R22. Reducing the value of that (18K? 15K? or a 22K trim) should boost the level of the repeats signal going back into the input of the delay.

That'll let you put the 47K back for R12 and avoid the dirt.

HTH,
Tom

[Plexi]

If you're having trouble getting it to oscillate, rather than messing with the input filter (which is what R12 is part of) which has all sorts of other side-effects, I'd have a play with R22. Reducing the value of that (18K? 15K? or a 22K trim) should boost the level of the repeats signal going back into the input of the delay.

That'll let you put the 47K back for R12 and avoid the dirt.

HTH,
Tom

You got it, Tom!

Thanks 

[anotherjim]

I'd go for reduced R22 too.
You may find when it goes into runaway repeats it gets dominated by the bass end, in which case also try reducing C10.
R22/C10 values will interact since reducing the bass with C10 will reduce the overall tendency to feedback.

[Groovenut]

For tails switching you'll want to break the line between IC4A pin 1 and C4. A FET switch will work very well here. For inspiration in implementation, have a look at Merlin's Small Time delay.

[Plexi]

Thanks anotherjim and Groovenut.
Tomorrow I'll put hands on the solder and back with updates

[Plexi]

Updates!
Back R12 to 47k
Reduced R22 to 15k and then to 10k; it oscillates a bit more, but only with tone control almost all way up (brighter).
I guess it happen in conjunction with C10, as anotherjim says.

Tail switch works fine!
Really great buffer section in this delay.
But, it start to oscillates as crazy when is in bypass, and oscillation reduce when is engaged.
Why this could happen?

[ElectricDruid]

But, it start to oscillates as crazy when is in bypass, and oscillation reduce when is engaged.
Why this could happen?

I don't know, I'm afraid. At this point, that's beyond me. There must be some interaction between the signal coming in and the signal recirculating, but I don't understand why it'd reduce the oscillation. My first thought was "something's out of phase!", but it's not - there's an invert MFB filter on the input of the delay, and another on the output, so the signal after C16 at the top of the repeats pot is in phase with the input signal.

Sorry!

Tom

[Plexi]

No problem, Tom, there's nothing to apologize about.
I'll be tweaking and reading about pt2399's circuits.

And if I want to get some "dirt",à la tape delay?
I'm planning to put an jfet preamp before.

[anotherjim]

Increased feedback when the input is disconnected is due to the removal of the backload seen by the repeats pot provided by the low impedance of IC4A pin1. I guess you might find a way for the switch to put a low resistance from the input side of C4 to Vb when the input is meant to be disconnected?

[Groovenut]

You might try adding a 100k-1M resistor from the connecting point of the Jfet switch and the C4 cap to the bias rail. I am wondering if the high impedance on the signal line when the jfet is open is causing the signal line to oscillate.

Although you might be able to fix it with layout lead dress. I thing this may be the culprit as I have never had any issues with Merlin's Small Time oscillating when switched.

edit: Jim beat me to the punch 

[Plexi]

Thanks both!
I'll see and compare with others delays feedback section and test some tweaks to see what happen.

[anotherjim]

Have you seen my "Soft Delay"?
http://www.diystompboxes.com/smfforum/index.php?topic=114973.0
Has the same (cough) "feature" that tails bypass can lead to runaway feedback. Although this uses a MOSFET switch to disconnect the incoming clean from the delay, there is the same problem with the impedance load on the regen signal rising when bypassed leading to a stronger regen. A changeover switch would be needed to cure it.
Note that many designs have simply set the maximum feedback level so it won't go into runaway feedback even if you wanted it to. Soft Delay has 2 alternate feedback paths from the repeats (Regen) pot, one will runaway, the other won't.

The rest of the MOSFET's in that 4007 chip give it a class A and class AB type signal path into the PT2399 for a very analogue sounding delay.

[Plexi]

Sounds really interesting, but I can't see the schematic/pic

[Groovenut]

Soft Delay schematic

http://i.imgur.com/tdKW9tf.png

[Groovenut]

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