Guitar Echo design questions and Ideas  - Page 1
Forum section: electronics / beginners

author: Dellyjoe
author_rank: Regular Contributor
timestamp: December 08, 2020, 12:15:55 am
content: Hello everyone,I have a couple of questions, When I think of a echo circuit it really just means creating a delay within the circuit and over lapping the delayed signal with the original signal.I would like to have the original signal, think of a by pass signal go right into the output, but then have a feedback loop create an op-amp attention circuit, so each time the signal goes through the circuit it gets small and small until you can't hear it anymore.My question is how do you make an op-amp attention should I just use an inverter op-amp with R1 bigger then R2? and how do I over lap the signals? do I use a summing op-amp ?Sorry If this isn't explained well I'm having a hard time with the idea.Thank you for reading, I look forward to any suggestion you might have.Joe

author: fourfathom
author_rank: Super Contributor
timestamp: December 08, 2020, 12:50:14 am
content: Yes, a guitar echo effect does take an attenuated and delayed version of the output, and feeds this back into the input. But you can't do this with just opamps -- these will typically have nanoseconds of delay and you need hundreds of milliseconds (or more) worth of delay. Here's how you can get delay, from oldest to newest technology:* A mechanical spring-based reverb unit. This has magnetic "record" and "playback" heads at either end of a spring. This provides delays of tens of milliseconds, and is used to create a "reverb" style of echo.* A magnetic tape delay, using record and playback heads, with the passing tape providing the delay.* An analog "bucket brigade" or "charge coupled" delay line as the delay element. I don't know if these are even made anymore.* An Analog/digital converter, writing into RAM memory, and a digital/analog converter reading it back out. This is how it's done these days.

author: Dellyjoe
author_rank: Regular Contributor
timestamp: December 08, 2020, 01:39:23 am
content: Quote from: fourfathom on December 08, 2020, 12:50:14 amhmm ok, " I will have to look into the Analog/digital converter, writing into RAM memory, and a digital/analog converter reading it back out. This is how it's done these days." seems like this will be more complicated then I thought.Thanks again, I will look into this.

author: Dellyjoe
author_rank: Regular Contributor
timestamp: December 08, 2020, 02:13:49 am
content: Quote from: fourfathom on December 08, 2020, 12:50:14 amTo be clear fourfathom I can use a AD7910 to do the ADC and then into a AD5626 to go from DAC and this will be the feedback loop in my circuit. I will figure out how to adjust the delay.And do you know if these components have a LT Spice model ? or a model from another application ?Also do you think this is the right way to go about this?

author: msat
author_rank: Regular Contributor
timestamp: December 08, 2020, 02:40:16 am
content: Bucket brigade chips can certainly be found, especially if you're not looking for a huge quantity of them. A while back, a popular chip which had been discontinued was recommissioned for production. There are plenty of boutique pedal manufacturers out there, and many of them want an analog solution rather than resorting to ADCs, DACs and DSPs.

author: fourfathom
author_rank: Super Contributor
timestamp: December 08, 2020, 03:34:32 am
content: Quote from: Dellyjoe on December 08, 2020, 02:13:49 amIf you can find the bucket-brigade chips (as msat mentions), that is probably the easiest way for you to go.The analog/digital/analog method probably needs a higher-resolution ADC/DAC that that AD7910 -- that is 10-12 bits and you probably want at least 16-24 bits for acceptable distortion. You will then need a controller or FPGA to trigger and read data from the ADC at a fast rate (96 HKz?) , then write it into a memory buffer. The controller then needs to read data from the buffer -- probably from several locations and then scale and combine the data, writing the result out to the DAC. You will want to have varying amounts, durations, and types of echo, and this will be done under firmware control. You will need some type of user interface to let you actually use this thing.This is complicated stuff -- well-worth it if you are developing a high-volume commercial product, but probably more than you want to take on as a hobbyist.So try to find an analog bucket-brigade design you can look at.

author: floobydust
author_rank: Super Contributor
timestamp: December 08, 2020, 04:59:15 am
content: Lowest cost to experiment with arePrinceton PT2399,on $4 boards from eBay or Ali, for delay or reverb.The IC has all the modules necessary, and there are guitar projects using it on the web.

author: mag_therm
author_rank: Frequent Contributor
timestamp: December 08, 2020, 02:46:05 pm
content: I see there are spring line reverberation tanks still available as components.We have one here in an old Electar 30 guitar amplifier.I think it adds artifacts of its own, giving the classic old sound.

author: themadhippy
author_rank: Super Contributor
timestamp: December 08, 2020, 03:32:31 pm
content: Quoteespecially if you thump/kick the ampIf you can find one an HT8955 does most of the work for you.

author: Yansi
author_rank: Super Contributor
timestamp: December 08, 2020, 03:49:04 pm
content: Get yourself some inspiration here:https://www.eevblog.com/forum/microcontrollers/reverb-effects-box-code/

author: Jan Audio
author_rank: Frequent Contributor
timestamp: December 08, 2020, 05:16:09 pm
content: You need a summing opamp for the mixing, the feedback you can mix in the bbd input without opamp,the delay-output does need a opamp for filtering and buffering, from there you take the feedback.The delay-input also needs a opamp for the same filtering as the delay-output, this is not used for the mixing, for mixing you use the original signal.Take note with inverting opamps, look in your oscilloscope so you dont have a delay that is constant changing polarity.

author: Benta
author_rank: Super Contributor
timestamp: December 08, 2020, 06:02:02 pm
content: You need to be a bit more specific. "Reverb" and "Echo" are not the same and they sound differently.A main difference is the delay time, but mixing and remixing are different, where "echo" has multiple adding repeats.

author: Dellyjoe
author_rank: Regular Contributor
timestamp: December 09, 2020, 12:34:14 am
content: Hello everyone,Thank you for replaying I have learned alot from reading everything, I know I can use the PT2339 how ever this seems like a little bit of a cheat.I should be able to do it without using the PT2339 So this is the block diagram I came up with.See attachment (1):Does anyone think this will not work?Also Why do I need 16-24bit converters ? Will the sound just be distorted if I don't ?Thank you in advanced,Joe

author: langwadt
author_rank: Super Contributor
timestamp: December 09, 2020, 12:57:31 am
content: Quote from: Dellyjoe on December 09, 2020, 12:34:14 amyou need buffer memory between the AD and DA, else the delay will only be one sample period with is either fast to short or far too slow a sample rate

author: Dellyjoe
author_rank: Regular Contributor
timestamp: December 09, 2020, 01:03:20 am
content: Quote from: langwadt on December 09, 2020, 12:57:31 amHi Langwadt,Is this a chip that I can buy? or do I have to use a micro controller ?Joe

author: jonovid
author_rank: Super Contributor
timestamp: December 09, 2020, 01:09:15 am
content: been down this rabbit hole. IMO buy off the shelf hardware & reverse engineer to your individual requirements.in the end I setout make anautoguitar, it is a type of spring line reverberation useing guitar strings.but also musical instrument in its own right. able to play hurdy gurdy, vilin and steel guitar sounds from an arduino or any micro controller.a work in progress because of parts delayed in postage covid.

author: RJSV
author_rank: Super Contributor
timestamp: December 09, 2020, 03:37:02 am
content: I agree with jonovid, get an existing project board, as that will be documented. The reference to the 4 dollar kit board, with PT2999 (yeah, I got that wrong too), looks like 'learning opportunity' to me.I had bought a cheap guitar reverb pedal, that had used the IC LM1458, and try various guitar boards.I mean, here is a test: Can you write a diagram for a basic, 2X ac amplifier, and discuss how it works ?That would be your input buffer.Then: Can you diagram an audio mixing stage, taking your buffered guitar input, mixing with feedback line, from your output buffer (that drives the actual output jack)?Without those two simple methods, using the LM1458 for each circuit, you are lost. Just a short study, and you could rapidly understand the OP AMP use, in this context.It's the mixed signals that goes to the delay IC, with the fed back (delayed sound) signal going thru a potentiometer, for dialling in how much effect / clean.Actually, 'cheating' is asking questions like "does anyone have a spice model so you can just, blindly hook up a bunch of components when you have no clue. So, best place to re-start, maybe try some of the blogs around that 4 dollar kit, and keep checking in here with more specific questions.Oh, and if you do go with the bucket brigade IC route, that IC needs a CLOCK pulse supplied (I think that's around 30 khz)There's plenty to be learned there, nice thread, please continue this, and any questions as you go.P.S. I bought my pedal at local pawn shop! Cheap!Thanks

author: RJSV
author_rank: Super Contributor
timestamp: December 09, 2020, 04:05:08 am
content: (postscript in hindsight)Joe:. Nice question, if you can give me a day or so, I will follow up my comments, with a schematic and more comments!(I was born 2 weeks late, so...)Schematic will be hand - drawn, but I don't think a bucket brigade type (project) will be that hard to follow, assuming I can write it coherently.

author: RJSV
author_rank: Super Contributor
timestamp: December 21, 2020, 06:50:13 pm
content: I owned a DOD brand Reverb Pedal, circa 1985. See photo diagram of controls. It had a RIDICULOUSly low clock, at 6 khz ! Or even lower, maybe, as the delay put through the IC (2 k Bucket Brigade chip) turns out to delay any audio by about 333 mili-seconds.On the other end, of speed, a fastest clock setting of 48 khz, for example, will cause a 40 milli-second audio delay. So the range of that first knob is about 1 to 8.For sound effect, only the first two knobs are used (having a 'Program Regen.' knob, for echo repeats (persistence).So for a 1950 s slap-back sound, you would turn down the REGEN, and turn up MIX. That gives a 40 millisecond retort, barely audiably delayed, but only one (one loud echo).The pedal came with schematics: most OP AMP circuit stages are simple, with AC gain of X5 etc.Many musicians like effects, a bit dialed back.The circuits helped, as an output IC would drive a cable, things like that.I added a second channel, so reverb - echo could be fed to stereo recorder.

author: RJSV
author_rank: Super Contributor
timestamp: December 26, 2020, 11:07:41 pm
content: Dellyjoe, your block diagram of echo pedal is great, so here is some helpfully info, on putting your OP-AMPs.I like to think as it's a servo, see diagram-photo. So the op-amp IC is connected to constantly keep (the so-called virtual ground).If you've ever seen a juggler, with a spinning plate on a tall stick, they 'wiggle around' in comical ways, to keep a plate, up on the stick. So the op-amp does not know about 'waveforms', rather, it will quickly stabilize, in a few tens of nano-seconds, and do that 'hover' act, to keep things steady.It is hard to wrap your head around, the concept of the op-amp IC contrtolling it's own input (that's classic negative feedback). My thoughts were, Why 'control' there, at the Negative input node ?This is what is meant by ' virtual ground', it's a real voltage, just can't be loaded down, much.