Advice for a custom slapback echo build please?

Started by Badpaw, March 12, 2019, 02:10:38 PM

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Badpaw

I play blues harmonica and really want a slapback echo with some decent adjustment to tweak my sound...not just the lower 5-7% of the dials on a standard delay...

I have found the following and it sounds sweet and offers a liberal fine tuning range on the pots...

https://m.youtube.com/watch?v=6J3Yhm1Ew7w&time_continue=114

...I don't know how to build from schematics or electronic design principles but can build off Vero designs very well when presented with an accurate BOM.

Can anyone help?

Mark Hammer

The general principle of most digital, analog, or hybrid delays is that longer delays are achieved when the clock  is set by a larger resistance.  If only the first little bit of the delay-time range is of use to you, then the most straightforward thing to do is either replace the delay-time pot with a lesser value one, or tack on a parallel resistor to lower its effective value.

For instance, here's a schematic for yet another PT2399-based delay (from a reliable source).  (I know some folks will accept nothing less than analog delay, but the PT2399 is ubiquitous and used in many well and widely-received delay pedals.  And it's cheap.) The resistance between pins 4 and 6 set delay time, and seems to be selected for somewhere close to the maximum range of possible usable delays from the chip.  If you tacked an 8k2 fixed resistor to the outside lugs of the 50k pot, you'd have a smaller range of short delay times to dial in, allowing you to nail the one/feel you want.  Personally, I like to shave off a bit of the top end in the repeats path, but that's me.


Here's a stripboard layout of the circuit.  http://www.bouron.org.uk/marc/smalltime.gif

Badpaw

Thanks for that! I almost kinda get that.

You totally lost me with...

"Personally, I like to shave off a bit of the top end in the repeats path, but that's me."

..I get "Personally... " but ..."shave off a bit of the top end in the repeats path"....Ummmmmm

:icon_eek: :icon_eek: :icon_eek:


Badpaw

Also, in classic Slapback there could well be only a single repeat. Is there a similar pot mod that can be done to reduce the number repeats?

bluebunny

Quote from: Badpaw on March 12, 2019, 03:37:22 PM
Is there a similar pot mod that can be done to reduce the number repeats?

Turn it all the way counter-clockwise?

Quote from: Mark Hammer on March 12, 2019, 02:32:12 PM
Here's a stripboard layout of the circuit.  http://www.bouron.org.uk/marc/smalltime.gif

Not my finest moment (it was a long time ago, in a galaxy far, far away), but at least all the track cuts are in neat columns!   :icon_biggrin:   (And it is known to work.)
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Ohm's Law - much like Coles Law, but with less cabbage...

anotherjim

Repeats path is from the wiper of the Repeats control pot back toward pin 16 of the PT2399.
Naturally, echoes lose some treble because those sound waves get absorbed and defused more by the surface it's bouncing off than the mid frequencies. Also, a different tone to the echo helps us tell it apart from the original sound.

However, for slapback, you don't need repeats, so that control can be left out. If I wanted the echo to sound duller, I'd try a larger cap over that 2.2nF between pins 13 & 14 of the PT2399.



Mark Hammer

Jim summarizes it well.  I'll simply add to it by noting that personal definitions of "slapback" vary.  Some folks really mean something like the short-delay reverb in a tiled shower or bathroom, in which case there is a desire for having just a little more than one single repeat, but with a quick perceived decay.  Having every repeat after the first one lose more treble with each subsequent repeat, quickly pushes the repeats to the background, maintaining the perceived impact of that first repeat, but keeping the decay without it cluttering up the sound.

The way I do it is by taking the resistor normally found between the Repeats control and the repeat+new mixing node, and dividing it in approximately half.  I then run a cap from the junction of those two resistors to ground, adding a single pole of lowpass filtering.  This has no impact on the first repeat, only on subsequent repeats.

In the case of the Small Time I showed below, the resistor in question is the 10k after the Repeats pot and 47nf cap to ground.  Were I to be doing the mod to this circuit, I would translate 10k into a 4k7 and 5k1 resistor in series.  If I ran a 39nf (.039uf) cap to ground from the junction of those two resistor, 4k7 and 39nf gives me a 6db/oct rolloff beginning around 870hz, on top of whatever treble cut the existing 47nf cap to ground provides.  You will note that the 1k/47nf pair between the Repeats and Mix controls also rolls off top end, but that 6db/oct rolloff starts around roughly 3.4khz

Conversely, if one wanted repeats that got more shrill (like what we often hear in dub music), you'd reduce the value of the 100nf cap between the Repeats control and the existing 10k resistor.

Hopefully all of that makes sense to you.

Badpaw

Thanks guys...unfortunately, as I said, I'm not electronics engineering literate...some of the ideas kinda land but differentiating between "input, output, tone, and filtering caps" and the like is lost on me...let alone what they do. "Halving" values and increasing and decreasing values...and how that relates to numbers of nf's "k's" etc etc also gets me in a muddle...all over my head I'm afraid. Show be a vero with components and I'm your man, it'll get built fast and neat but ask me to point to the "low pass filtering goober triode" and you may as well be discussing Nietzsche with an aardvark!

MaxPower

Great info. Try a Google search for "slapback echo vero board". Then ask here about any that seem interesting. Or do a search here as there's probably already been a topic about them.

Going off topic: Has anyone here messed with the korg monotron delay? The delay is based on the pt2399 and the schematic can be downloaded at korg's website. Looks simple enough.
What lies behind us and what lies before us are tiny matters, compared to what lies within us - Emerson

Mark Hammer

I have one, and have "messed" with it.  Just be forwarned that it is surface mount, and not especially friendly to mods.   

Badpaw

Is there, perhaps, a variation of a Rebote 2.5 build that I could make that would result in  a decent Slapback unit with a bit of "elbow room" so that I'm not just using only the bottom 5% of every pot? That gives me no real "tweak room". It feels like trying to balance a ball bearing on the edge of a razor blade. Something that shares that lower 5-20% of the pots control over the whole sweep of a given pot? I just don't need endless delays and repeats. I would guess 50-250ms max delay and 1 or two repeats max, a bit of mix control as well would be great. If there is such a possibility is there a plan I could follow with a BOM for the suggested component replacements for given functions? Im a bit simple you  see... :-[

blackieNYC

The pt2399 circuits are generally 50-300 ms already.  So you're almost there already. 
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Badpaw

Ok...I THINK I'm starting to grasp the basic concept....

1) Repeats = the literal number of copies of the input "played back"

2) Delay = the time between the original input and the first (and subsequent copies), the rhythm as it were.

3) Feedback/mix/level = the style, decay and tonality of the "copies" of the input

...am I in the ballpark?

So if I could TOTALLY lock off the repeats to a single one (maybe a switch for two repeats) I would have my starting point?

If I could have a pot for delay that (through it's full sweep) covers 50ms -250ms that would cover the life of the echo

The mix/feedback/level could go from clean to as "coloured" as is wanted.

Do you think this is doable with a Rebote 2.5 with some component variations? If so could someone guide me through the changes/mods?




Mark Hammer

Getting closer...

It is generally difficult to restrict a delay to a defined number of repeats beyond one.  One repeat is zero feedback.  Introducing some amount of feedback can result in multiple repeats.  The degree to which the repeats will seem to disappear after X number will depend on how much feedback is provided.  If the feedback/repeats pot cuts the volume of each repeat by half, for example, then repeat #2 is half the volume, repeat #3 half of that volume, and repeat #4 half of that, etc., such that the 4th repeat is 1/8 the volume of the first one.

But let's distinguish amplitude per se, from perceived amplitude.  If subsequent repeats get progressively duller, then the drop in perceived volume of repeats is quicker.  Earlier, I suggested, using the Small Time as an example, a 6db/oct rolloff at 870hz.  6db/oct is a pretty shallow  rolloff, but if the delay passes through that same filter a second time, it now becomes a 12db/oct rolloff, functionally, and 18db/oct the third go-around.  With the delay signal reduced by half, via the Repeat control, the third repeat is 1/4 the volume of the first repeat.  But because of the repeated filtering via the feedback path, that third repeat will be much less apparent than 1/4 volume would suggest.  The first repeat is nice and bright, but you get just a hint of a "trail" that moves to the perceptual background quickly.

PRR

> It is generally difficult to restrict a delay to a defined number of repeats beyond one.

For discussion (you know this):

Feedback will always give "infinite" repeats if fed-back exactly unity (which is not practical); infinite-fading-to-nothing if feed back is less than unity. (And as you say: the treble usually falls off faster.)

To get exactly X repeats and then stop, you need X delay lines.

Special case: you want exactly, say, three repeats and stop altogether. Feed-back as near unity as possible and cut-off after the third repeat. I have rigged a minute of tape across a stage. Performer played one 1-min round then that played back as he played over it. At the end of the 4th minute I killed the audio as the splice came over the head.
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Mark Hammer

Good point.  I was thinking in terms of only one delay chip.  But you are correct.  If a person wanted a specific number of repeats, they could do what the Binson Echorec did with tape heads, only with PT2399s.  That is, signal goes to chip 1, whose output is tapped, then to chip 2, whose output is tapped, and then chip 3, whose output is tapped.  The three tapped outputs are then mixed down to mono, so that, in the complete absence of any feedback, a person gets exactly 3 repeats

BluffChill

My Shagpile double tracking is a relatively simple double-tracker, but it has an envelope follower to modulate delay time, making the repeats a bit 'wobbly'. I'd be interested in sharing some design advice with you if you like. Using a PT2399 to create a single repeat at a short time is quite simple, and adding feedback (for repeats) only slightly less so.

Kits & Pedals! EctoVerb - HyperLight - Shagpile - http://bluffchilldevices.bigcartel.com/

Badpaw

Hello folks. Well I figured I'll either get started with making basic pedals and trying mods or I could grab my "Deep Blue" clone and jab around with a soldering iron...I did that second one.

I replaced the 50k delay pot with a 25k one and now the delay goes from its minimum to perhaps 250ms...perfect...didn't want anymore than that.

I then settled on fully removing the feedback pot altogether.

I now have a (I know, I know) basic slapback pedal that goes from barely noticeable to kinda weird with lots of iterations inbetween available on two knobs that I can move more than .4269 of a degree of rotation!

Thanks for all you "feedback" (FIRST time that joke was EVER made on this forum I'm SURE!)

That'll get me going until I get obsessive about the nuances and then open bigger cans of worms!

Mark Hammer

Great to hear.  Congrats!  Now all you have to do is steal my tweed Princeton (one of the greatest harmonica amps ever).

I've been on this forum for over 20 years, and I do think that IS the first time I've heard that joke.  Mind you, there were an awful lot of threads I never looked at, so it is important to distinguish between "ever used" and "ever heard".  :icon_lol:

Badpaw

Mark, bring that amp to the Highlands of Scotland for a Blues and BBQ session and you might just leave without it!! You'll have a stomach full of smoked and slow cooked ribs, burgers, mac and cheese and whiskey but 1 amplifier lighter!