Regenerative feedback sustainer???

[mykey]

I've seen somewhere else a similar pickup/sustainer...think it was called the sustainiak or something.

I'm think something more along the lines of a foot-switchable effect. I'm very happy with the sound of my S-S-S guitar, and prefer not to mod it...I especially don't like the idea of mixing active electronics with my pickups.

Maybe it could be modded for setup like I suggested - more as mic placed in front of the cab, instead in the guitar?? I'll look it over some more.

The mic you are looking for is your guitar pickup. if you are not getting the sustain you seek, its an amp issue. start with a good tube amp such as a Marshall. switching the sustain on and off is accomplished with the guitar volume control, next zz top concert go watch and learn.

[mooreamps]

Well, but isn't that what sustain is ? Positive self oscillation. If you do this, I would put in a dial control to vary the amount of decay.
-g

[Satamax]

positive feedback IS used to create sustain in the first place. the sound coming from the speakers of the amp reaches your guitar and causes the strings to vibrate- this IS sustain, the root cause! the more gain your amp has, the more sustain (regenerative feedback) returns to the guitar strings.
in order to accomplish this the sound from the speaker must be IN PHASE with the motion of the guitar strings.
Also I have built amplifiers with positive feedback circuits that recirculate in phase signal from one stage backwards to the previous stages. this causes regeneration of signal (sustain) within the preamp circuit itself. so yes it is possible, and there are amps out there that have been built this way.
also very good question- someone is thinking.

Mykey, i think, if you can't get the signal in phase with the preceding stage since you can use the cathode as an input too I'm just thinking about controling the amount and decay of the local PFB, and a lightbulb came to mind. Lightbulbs increase in resistance with the heat they produce when passing a curent. That would make a nice limiter Which would start sustaining and when feed more than the desired level of PFB, would heat up and decay down the squeal By mixing this between dry signal going back to the preceding stage via a pot and through a lightbulb, it could be nicely controled. Thanks a lot to the first 741 oscilator i've made which was using something very similar

Sounds interesting. How did you go about controlling it, or keeping the amp from running away into oscillation?

Addendum: came accross this old ampage thread:
http://www.firebottle.com/fireforum/...7647161z7ka5jU

That's intresting, i might have read part of it at the time. What i thik of it now, is use a active pasive alike 2X12, feed the pasive speaker's signal back nto the amp. Well you would have to use a real speaker as pasive. May be the proper way to go about this would be to use some sort of impedance matching transformer between speaker's coil and input, as feeding back the signal of a speaker into an amp in an iso cab, made a constricted verry nasal and bassy sound.

[blodviol]

I'm going to revive this topic...I had the same crazy idea as Satamax too...came accross this thread after looking for precedents.

I've been wondering what would happen if you miked some of the output back into the beginning of the signal chain with a very small delay through a volume pedal. The idea is to get sustain and controlled howling feedback without the insane volume required to re-drive the strings on your guitar.

One idea I had to accomplish this would be to butt the headstock of second guitar up to your cab - so it's constantly resonating - and mix the signal in with your playing. Crazy? Worthless idea? Maybe a pickup could be modified to work like a mic near the speaker, only produce a resonant, rather than microphonic signal?

I've done something similar a couple of times, only i'll use two amps. one in a seperate room with a guitar in front of it, and that guitar hooked up to the amp in the playing room. This produces a nice reverby sound, not more gain, tho....

Tríste

[R.G.]

There is a DIY design for modding a pickup to do an in-the-guitar string redriver like the e-bow. I think that's what you want.

Using amplifier stages with positive feedback is much too tricky and rapidly gets unplayable.

[mykey]

I've done something similar a couple of times, only i'll use two amps. one in a seperate room with a guitar in front of it, and that guitar hooked up to the amp in the playing room. This produces a nice reverby sound, not more gain, tho....

Tríste

butt the guitar up against the amp, this is what Hendrix did. its not so much insane volume thats needed to produce sustain, its sensitivity of the preamplifier. this is why marshall amps (with very high sensitivity) are more preferred, and why people keep using footswitch preamps, its all an effort to increase sensitivity and thereby producing sustain. and absolutely sustain is the product of positive (in phase) feedback that reaches the strings from the amplifier, causing the strings to move. up to a certain point sustain will have decay, then afterwards the sustain will be infinite, then exceeding that level the sustain will reach a point where its out of control entirely. using the gain control on the amp and the volume on the guitar the threshold is set to where it can still be controlled, no other control is really required.

[Tiago]

At some time ago I builded a signal generator(well, I builded more, but those other were supposed to be amps). It uses a small lightbulb to mantain a 3:1 ratio in the positive feedback loop.
As this behavior of the lightbuld is slow it can be heard when it is trying to add gain or when trying to reduce it at startup(when powering on, or changing the frequency).
At that time I did some research and found that to mantain an oscillation it is needed a gain at least of 3. So even with positive feedback but with a gain of less than 3, it is possible to get that "sustain" without an uncontrolable oscillation.
So you can try doing experiments with lower gains.
This idea crosses my mind oftem...but I'm to lazy to try anything. I hope you are not as me...bring something new!

[rathmann]

I don't care how much gain an amp has; if it's not loud enough to re-excite the strings any oscillation you get will be the microphonic squealy variety.

Higher gain just lowers the threshold at which the re-excitment will be amplified and hence "fed back" into the system, but still the feedback loop has to incorporate the string vibration.

Dynamic modification of the gain is really just an expander, the opposite of compression. I suppose including a limiter might allow for crazy amounts of gain without letting the amp runaway, but I still kind of wonder how musical such an amp would be.

Some of the best musical feedback I've gotten was through a stock BF Super with a DOD Overdrive Preamp 250.

Perhaps another approach would be to use a small satellite speaker or floor monitor that could be aimed at the guitar and switched on to kick in the feedback/sustain.

And remember - controlled-feedback legend Santana has his amp techs put tape on the stage to mark the locations of all the feedback "hotspots".

[Tim Hall]

little experiment

The preamp in my amp is plenty sensitive...believe me, you hear every minute bump, scrape, and fumble I make when playing.

My problem is the amp only puts out 2W. That is, it's not a problem for keeping the wife and neighbors happy, but it is a problem when it comes to redriving the strings.

As an experiment I taped a small computer-monitor speaker to the back of my guitar's headstock...this might have some promising results. Not quite enough power to really swell into monster feedback, except for one note which I think might be the Q-point of my guitar. That one particular note sent the whole thing out of control in short order. Kinda cool giving it a sublte little finger tap, and it starts howling in seconds

I think I might try attaching a coneless speaker to the back of a fat CBS-style headstock...

[mykey]

Sounds interesting. How did you go about controlling it, or keeping the amp from running away into oscillation?

Addendum: came accross this old ampage thread:
http://www.firebottle.com/fireforum/...7647161z7ka5jU

there are several methods to controlling this and preventing runaway. first the positive feedback is routed through L/C network and attenuation circuit to control the amount and phase alignment of the signal that is returned to the previous stages. using a dual trace scope, the return signal can be aligned in phase with the previous stage, when the two signals are exactly in phase the tendency to produce cancellation is minimized.
second combining negative feedback with positive feedback:
connect 3-8 pf silver mica 1000V cap between plate and grid of V1A preamp. this is the most basic method of inducing negative feedback to prevent V1A from ringing or becoming "microphonic". then proceed through other preamp stages such as V1B, and add the appropriate amount of negative feedback to dampen the tendency of runaway oscillation. of course this "tuning" of the preamp stages is a matter of experimentation, testing with guitar over and over again.
another method is to add capacitance up to 250 pf between plate and cathode of preamp stages to produce damping.
another method is to add capacitance across plate resistor. for example, instead of using a 100K plate resistor, use two 50K in series, place your damping capacitor (such as 250 pf) across ONE of the 50K resistors. again experiment "tuning" the circuit.
this will induce some feedback from the plate to the power supply.
calling a preamp tube "microphonic" is usually a mistake. its the circuit that makes the tube microphonic, not the tube itself. keep this in mind when "tuning" your circuits to minimize ringing, runaway oscillation.
another factor (if you are not asleep already) is the quality of the components used. silver mica caps instead of ceramic disks, metal film resistors instead of carbon comp resistors, shielded wires instead of plain wires, all
contribute to lower noise, less ringing, less oscillation.
layout: keep the leads short, keep the input resistors away from the output, and keep the heater wires a far away from the audio path as possible (or use DC heaters).

[mykey]

I don't care how much gain an amp has; if it's not loud enough to re-excite the strings any oscillation you get will be the microphonic squealy variety

And remember - controlled-feedback legend Santana has his amp techs put tape on the stage to mark the locations of all the feedback "hotspots".

that's true, when inducing positive regeneration the acoustic power must be sufficient to re-drive the strings
but then again if the regeneration is in the amp circuitry, its less of a problem...
and the different positions marked on the stage are positions of standing waves
for example, stand here and your sustain will be a third harmonic
stand here and it will be a fifth harmonic
stand here and it will produce the octave.
this is because the lower the frequency, the longer the wave, and standing waves will be produced at full, half, quarter wavelengths, etc...
but you guys are definitely getting the root of the idea, and breaking down
the science of sustain into its component parts, hats off to you!
cutting edge thinkers.

[mykey]

of course iv'e been taking about doing this all with
tube amps, as i don't like the sound of transistors
and tube amps for me are easier to modify, build
play with.
but watch out as many know tube amps are high
voltage, and can shock the crap out of you.
so have fun with it and do it safely.

[Satamax]

http://www.firebottle.com/fireforum/ff.cgi?cmd=vt&fid=ga&tid=1087647161z7ka5jU
That's intresting, i might have read part of it at the time. What i thik of it now, is use a active pasive alike 2X12, feed the pasive speaker's signal back nto the amp. Well you would have to use a real speaker as pasive. May be the proper way to go about this would be to use some sort of impedance matching transformer between speaker's coil and input, as feeding back the signal of a speaker into an amp in an iso cab, made a constricted verry nasal and bassy sound.

Guys, i'm keen on hearing your opinion. I have a cab which could be candidate for this type of trick

Basicaly what i'm thinking, use one of the speakers as a normal speaker and the other one as a pickup, it would be connected to the input of the amp, instead of the output. The only problem i have, i've used a speaker as a pickup before, and the impedance is very badly matched. What kind of transformer could i use to match the speaker impedance with the input impedance? I was thinking about thoses pickup matching tranies.

Best regards.

Max.