Mini 9v Amp for DIY Talbox Pedal

[victorbignacio]

Hello everybody,
So I built a DIY Talkbox and I've been using it live with an A/B Box plugged in to my main amp (A) and an Orange Micro Terror (B) driving the talkbox speaker. It's working great, but I want to have a amp in a pedal format to carry way less stuff to gig.
I tried the RunOfGroove Ruby amp... And it's way quiet, not usable live. Can someone give me suggestions on a 9v pedal Amp (LOUD) to use with it? (Preferably with drive control)
Thanks!

[Sooner Boomer]

One thing to try would be to replace the gain pot, between pins 1 and 8, with a 10uF electrolytic.  This will allow the LM386 to run at a gain of 200.  Alternatively, you could use a 1.2K resistor in series with the cap for a gain of 50.  See the LM386 datasheet for other ways to adjust gain setting.

[GibsonGM]

If that doesn't work, you could explore amplifier IC's and maybe use a separate power supply for your amp-in-a-box that can deliver more current than what's powering your pedal chain.    Keep an eye on how much heat you're generating, though...

[Mark Hammer]

The Danelectro Free Speech pedal has a self-contained mini-amp (386 or functional equivalent) and speaker, running off a 9V battery.  I know I can go so far as to use mine like a tiny amp to plug into, although obviously the quality of sound from what seems to be a 3" speaker coming through a small hole meant to retrofit a vinyl tube to is not exactly a JTM45 through 4 twelves.  You can see more in this (Portugese) video: https://www.youtube.com/watch?v=_G652709Xac  But the nice thing is that it only takes one step to provide talk-box sounds, and another to pretend as if the entire assembly is not even there and you're going directly to the amp, or next pedal.

The old PAiA Pygmy battery-powered amp could also be used as the basis of a talk box, and they also sold a bezel to fit over the speaker and allow fitting of a tube to lead up to the vocal mic.  Unlike the Free Speech, however, one could not easily hit a stompswitch and send the guitar directly to the guitar amp.


The point is that a portable, self-contained unit IS feasible, and need not require running things back and forth between the amplifier and a speaker cab.  The one recommendation I would make is that it would be optimal to run the mini-amp off 8 AAA or AA batteries, or a reasonable 12V supply.  Powering any mini-amp intended to drive headphones, but now expected to push a speaker hard enough to have a reasonable output at the end of a vinyl tube, is not going to let a conventional, or even expensive, 9V battery live very long.  And many of the 8-pin mini-amp chips can deliver more oomph with 12V.

I guess the other aspect to consider is the mic-preamp you'd need to incorporate, to provide adequate sensitivity for whatever mic capcule you'd use.

[victorbignacio]

Thanks for the suggestions guys. I will try to set the gain to 200. So to do this I run a 10uf Electro between pins 1 and 8. But the Datasheet says that I need a bypass cap on pin 7 when seting the gain higher, but the ruby already has that cap on pin 7:



So the only thing I need to do is remove the gain pot and run a 10uf Electrolytic in place?
And since I'm losing the Gain control, will it already have distortion or I will have to run a pedal before?

[Mark Hammer]

Several years back, Nuts & Volts magazine had a piece on a box to plug in to the earphone jack on TVs for people with progressive high-end hearing loss.  The idea was to provide a nearby extension speaker that goosed the top end in order to make speech opn the screen sound less like mumbling to the viewer (particularly older ones). 

The circuit used 386 amp chips but configured the gain-setting loop between pins 1 and 8 in an interesting way.  Normally, one either uses a simple variable resistance, or inserts a medium-to-large cap in series with that resistance to bypass - selectively - the 1.35k fixed resistance internal to the chip itself.  The cap value assumes one is aiming for full bandwidth, or at least as wide a bandwidth as is sensible for the speaker involved (i.e., avoid "flubbing" of small speakers asked to reproduce frequencies too low for them).  But one can use smaller cap values to provide additional boost for the high end.

So, for instance, if one used 470R, in series with 100nf, between pins 1 and 8, if my calculations are correct, you'd provide boost beyond the default 20x for content above 3.3khz.  Much like the Proco Rat, multiple RC combinations can be used to provide different amounts of boost for different parts of the spectrum.  So, one could  get a bit more boost for the entire spectrum by sticking 1k in series with 10uf between pins 1 and 8, but even more boost by paralleling that with a smaller-value resistor and cap.

The overall objective is to emphasize those higher frequencies that a) tend to get lost in that vinyl tubing, and b) provide the most useful content for mouth-filtering.

[victorbignacio]

Thanks Mark. I will try the 470R in series with the 100nf between 1-8, forming a High-pass Filter right? Just so I can understand what I'm doing 

Edit: What is the difference on the sound volume (Chip Gain) and filtering between the 1k in series with the 10uF, and the 470R with 100nF?
I wish I had a protoboard

[Mark Hammer]

The simplest thing is to use a 1k-2k trimmer, in series with 10uf and adjust until you get a desirable balance between full-bandwidth boost and high-end boost.