robot voice stomp pedal - Page 1
Forum section: electronics / beginners

author: electrolust
author_rank: Unknown
timestamp: June 19, 2020, 01:16:46 am
content: Trying to build a stomp pedal for a robot voice effect. Like a guitar stomp pedal but for a dynamic mic (shure SM58).My main ref ishttps://www.nutsvolts.com/magazine/article/build_a_voice_changer.Also, datasheets for THAT 1240 and 1646.I haven't found any ready-made circuit for this. The ones that are easy to find are all based on electret mic and built-in speaker, for a helmet toy. There are commercial voice mod boxes that include robot voice, but only as an additional feature to the main ones people actually want -- harmonies, auto-tune, etc -- so they are pricey. Anyway it will be a fun project.Please ref that URL and my attached circuit.I'm wondering do I tie the LED to virtual ground or to VEE (battery ground)? I'm only mostly sure that it either is actually fine, depending on LEDs I source.The balanced / unbalanced conversion sections, I understand that typically when you don't know where the output is going you use a coupling capacitor. Since this circuit is self-contained, I'm pretty sure it's fine without C6 and C7?U4,U5,U6 should all run fine ref to virtual ground -- 4.5V is within the input power range, and this maintains the correct virtual ground should C6 and C7 be removed.L1 and L2 are ferrite beads.I wonder if the switching on of the square wave source will cause a pop? If so, how would I soft-switch this? Should I prefer to use a MAX4066 vs some generic 4066 and would that resolve that potential issue?Per the 1240 datasheet, C3 should go to chassis ground. But surely they are considering a true +/- power supply. The input jack I want to use has pin 1 (ground) hard-wired to the shell, which I would connect to the chassis. So in that setup, chassis ground and signal ground are the same anyway. Does this present a danger, ie a net 4.5V available if you short the wrong things together? The battery won't be accessible, I intend to seal it in the box. So the user (a small child) can't really make such a contact. And if so, does the limited current capacity (20mA) of the TLE2426 help matters?I'm thinking battery life might be preserved better if I disconnect power from the U5/U6 pair when the effect is off. But when the effect is off, the output of U6 is open, so I'm guessing power consumption would be crazy minimal anyway?The last thing I can think of is that the signal level from a dynamic mic is < 20mV (-32dBu). The noise floor of the THAT 1240 is -104 dBu but I am still trying to get my head around if that translates to being able to process the very small mic signal without pre-amplification; and even if so, will the rest of the signal path deal with it as well. Comments about that aspect are especially welcome. This is a "middle box" effects pedal and the output is going into a preamp that already expects a dynamic mic input, as far as signal level and impedance. So the net output should be unity gain, so that I don't have to adjust mixer input gain if I take the box entirely out of the circuit. If I do need a gain stage, I'll need to either have the matching negative gain on output, or deal with level changes at the mixer, or dedicate a different input to the effect.Thanks for the look!

author: themadhippy
author_rank: Super Contributor
timestamp: June 19, 2020, 02:04:33 am
content: Quote80's style electronicshttps://worldradiohistory.com/UK/Everyday-Electronics/80s/Everyday-Electronics-1985-04-S-OCR.pdf

author: electrolust
author_rank: Unknown
timestamp: June 19, 2020, 03:14:34 am
content: brilliant!

author: alsetalokin4017
author_rank: Super Contributor
timestamp: June 19, 2020, 05:22:38 am
content: Yes, a great vintage ring modulator setup. But a RM doesn't have to be that complex.Here's a passive one I found that I'll be using soon. 2 inputs, one output, and you can use any of the jacks for out or in. Since it's passive it needs a fairly hot input; a guitar might need a preamp. Germanium diodes in the ring.If you are into Eurorack modular synths, I see that Behringer is getting ready to put out a relatively inexpensive module that includes a RM, along with a LFO and some other stuff.

author: electrolust
author_rank: Unknown
timestamp: June 19, 2020, 06:05:47 pm
content: I'm not into eurorack, but I did see a robot voice module, it was $99. To use it as-is I'd have to still make at least the foot box with a switch and consider how to power it. It'd be too much of a patch job so I think I'm better off full DIY.As to the RM, first off I've never even heard of that before starting this. So consider my comments appropriately.I've now looked at the Cybervox. It does not take a balanced input. (It's also doesn't supply power for a condenser mic, so not sure what the mic is expected to be.) It has a pre-preamp for low-in (G=100), chained to the preamp for high-in (G=2.5). So even for the "high in" signal, it's apparently not big enough to work with the RM. This is one of the things I wonder about the design I posted (I hesitate to call it "my" design) -- do I need input gain for it to work at all?I also realize now that just my LEDs will consume more than the 20mA that the TLE2426 can provide, so I should connect them to VEE not to virtual ground. The other components added together appear to be within the budget of 20mA.Referring to my original (N&V) URL, that author (and wikipedia) claim that the dalek voice is modulated by a sine wave. However, his circuit uses a square wave. It's easy to multiply by a square wave as we just switch the input signal on or off.The cybervox circuit uses the RM with a triangle wave carrier. This is a much more involved circuit.I'm not trying to precisely match the dalek voice, but I do wonder what the audible difference would be between the various carrier wave choices.Now that I re-read the N&V page, I think maybe the LM1496 is ideal. It has balanced input and output! -8V is not a problem. I could feed it either sine wave (wien bridge oscillator?) or square wave (555+4027) and even toggle between them. Looks like I have to buy from ebay, but that's ok.The rest of my questions from the original post still seem relevant, so any comments there are still appreciated.

author: electrolust
author_rank: Unknown
timestamp: June 19, 2020, 09:39:41 pm
content: I've learned that I can expectpeak1.6mV output from my mic (SM58). I don't know if that's p-p or rms. So, a very small signal.

author: electrolust
author_rank: Unknown
timestamp: June 22, 2020, 07:38:34 pm
content: I realized that for a "modulation" which is simple square wave on/off switching, I am not "processing" the signal and therefore do not need input and output drivers. I either pass the signal through or not. I only need to worry about phase error. So I can simplify the design and on the bench I can characterize how much phase error I can tolerate and the relationship to capacitance delta between the (-) and (+) signals.The articles I can quickly find are all about frequency-based phase error, of the kind induced by crossover networks, which is largely not audible. But this is a different kind of phase error, phase error between the 2 halves of the differential signal. After realizing this, I foundhttps://sound-au.com/articles/balanced-io.htmwhich helps me understand the problem but not the significance, since there seem to be 2 competing claims: "[correcting the phase] may not be worth the effort", followed quickly by a discussion how only 1% error is worth -41dB, which is very significant for an unamplified mic signal. I guess when he talks about it being worth the effort, it's with respect to a line level (1V) signal. Anyway, since I'm not processing the signal, this should be easy to avoid. It will be fun to measure.My other concern will be if I leave the signal circuits open or short during the "off" part of the cycle. I now see that mic mute switches short the pins, so this hints to me that this results in a softer on/off transition than leaving them open.Hopefully the square wave modulation produces the sound I'm looking for. Too bad I'm not already skilled in audio editing so I could see if I even like the effect. I got the parts this morning and it will take me under an hour to build and test hardware. vs days on the computer.

author: themadhippy
author_rank: Super Contributor
timestamp: June 22, 2020, 07:56:16 pm
content: Quoteuse a signal generator to produce the modulation signal,even a phone app should be able to produce something usable

author: electrolust
author_rank: Unknown
timestamp: June 23, 2020, 01:42:37 am
content: Tested it and it works perfectly except that I can't measure 1mV nor within 2° of phase, when zoomed in. Even with 2 probes hooked to the exact same signal at the "exact" same point I see as much as 2° phase shift when I zoom in between the two.However, if I zoom out to capture 1s of signal then I see 180° perfect phase input and 180° perfect phase output and 0° between them.So, I learned that the square wave modulation works just as I expected. In retrospect, duh of course it does!

author: electrolust
author_rank: Unknown
timestamp: July 01, 2020, 01:44:37 am
content: It actually didn't work, not for the actual mic signal. Well, it works, but the output isverylow compared to input. I did already know I can't adequately measure such a small signal but I expected unity gain anyway. I can't even determine how reduced it is because for the mic plugged directly I am fairly well cranked already on the preamp gain. With the mic going across the switch I can barely hear the output. So I'd guess at least 20dB.So why don't I see unity gain?I'm using TI CD4066B CMOS analog switch, 9V battery supply. On-state resistance for 9V is 180r; +/- 10 between any 2 switches. Input capacitance is 8pF. Vis is from -0.5 to Vdd+0.5; so it can't switch large negative going analog signals below Vss (ground), but my input signal is only 1mV, well within the spec. With the FG, I wasn't having a problem with 200mV +/- signals (400mVpp) with the +9V supply.So, my only guess here is that the input resistance (180r) is seen by the mic as the input impedance? And it just can't drive that. I'd have thought that the 180r is in series with the mic preamp connected to the output, which I assume must have very high input impedance, 50k at least but maybe 10M or more. And thus the 180r could only increase the input impedance the mic sees. Was that an incorrect assumption?Or is it that the 8pF input capacitance (to ground, I suppose) is seen as a filter by the mic and it's not the 180r per se, but the 180r with the 8pF that causes the mic to see the 180r as input impedance? Maybe for a 1V signal 8pF doesn't amount to much, but for a 1mV signal maybe that's causes a significant effect.Or, is it that the 180r in the switch is a voltage divider with the preamp's input impedance and so it's not that the input impedance (ref the mic) is less, it's that the preamp sees that much less signal due to the voltage divider? I don't think that's it -- at R1=180r and R2=50k (can't imagine smaller than this), Vout = Vin*.996.I'm going to try and do a better job measuring the negative gain across the switch @ 200mV and then @ 1mV using a uCurrent as scope input amplifier.

author: gcewing
author_rank: Regular Contributor
timestamp: July 01, 2020, 08:17:51 am
content: Can you show us a schematic of the circuit you're using?

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 01, 2020, 10:00:19 pm
content: Quote from: themadhippy on June 19, 2020, 02:04:33 amI love those circuits.

author: electrolust
author_rank: Unknown
timestamp: July 02, 2020, 04:26:29 pm
content: The circuit is the same as in the first post, minus the input and output drivers. Here is the simplified version I am using for the test. Just on/off, no modulation. The control line is connected to VCC. (9V)I didn't realize until now that the 4066 is a transistor switch. And apparently that means it isn't a "wire" but rather the pass-through signal is limited by transistor characteristics. Hence why the switched signal range is -0.5 to Vcc+0.5, I suppose.So then I realized that my mic preamp provides -48V phantom power, and as I've now learned, although the manual says it can be disabled, in fact it cannot. The mic itself doesn't need and ignores the phantom power, so it works correctly when directly connected. But the -48V is clearly well beyond the -0.5V capability of the 4066 switch! I haven't yet tested that this is indeed the problem, but it does seem so obvious it makes me do a facepalm.Is there another kind of switch, non-transistor I suppose, that is oblivious to the phantom power? I suppose that would be a relay or a transformer. Looking around on digikey I see only expensive and/or large parts, so blocking the DC seems a better way to go.

author: electrolust
author_rank: Unknown
timestamp: July 03, 2020, 01:02:44 am
content: Betweenhttps://www.digikey.com/en/articles/how-to-use-analog-switches-to-dynamically-switch-audio-and-video-signalsand AoE I've convinced myself that the input R and input C of the 4066 isn't the problem here and it is indeed the phantom power. Also that there are probably better choices than the 4066 for a mic audio signal.But (nothing is easy) adding a coupling cap to remove the phantom power will introduce a high pass filter so I'll have to pay attention to part values there. Or dissect the preamp and remove the phantom power via surgery.

author: electrolust
author_rank: Unknown
timestamp: July 20, 2020, 07:17:18 pm
content: it worked after blocking the phantom power with 100u caps. (just caps, not the load or discharge caps of the previous circuit I posted.) i can only get a relatively poor sense of it on the breadboard because breaking out the XLR in/out into long enough lengths to handle and patch comfortably (only a few inches!) adds quite a lot of noise to my sub-mV signal. I verified that it's noise from the breakout and not anything to do with the circuit, by just patching cable-to-cable direct. I could put some connectors on veroboard to minimize it but not really worth the effort.but, i can characterize it enough to learn that the effect "works" and does sound "robotic" as intended, but there is too much popping at the square wave transition. although the effect is supposed to be very high distortion anyway, the popping, detracts too much from the effect. even if i slow down the edges, this won't matter since the switch action itself is still on/off. It's not a short-circuit type of loud pop, it's just the way the sound comes on and off abruptly, there's a "pop" noise that goes with it.i'll have to build a real ring modulator after all.