Requesting Thoughts On This Small Guitar Amp Circuit

[phaeton]

Hello again,

I've recently built a small guitar practice amp circuit found here:

http://www.generalguitargadgets.com/richardo/practiceamp/lm386.gif

I've seen generalguitargadgets.com mentioned in ths ng before, and
there are a few players here, i see. I'm betting that someone has come
across and possibly built this circuit themselves, or that it's simple
enough (and probably generic enough) that most of you can look at it
and understand it (i'm still working on understanding *how* things
work as a whole, myself).

Anyways, it soldered it up and it works. I've put it together exactly
how it is in the diagram, except that for the output i've got it
driving a 40W 6"x9" 4-ohm speaker instead of headphones. It's
surprisingly loud for being 1/2W- not earth shaking, but you have to
raise your voice to talk over it. Not expecting stellar sound quality,
yet the distorted guitar tones are pretty good for what it is- probably
better than most really small, solid state practice amps.

However, speaking of harmonic distortion, there is a LOT of it. It's
great for rocking out (Toni Iommi In A Box!) but i can't clean it up at
all. There is no volume or gain control for the circuit, so it relies
on the guitar's volume knob for net output. Turning down from the
guitar will reduce the distortion just a little bit, but it's still
very present all the way down to "0". There is a noticeable change in
overall volume by toggling the switch (between pins 8 and 1) but the
relative level of distortion doesn't change. The guitar i'm using is a
stratocaster with stock, passive singlecoils, and i'm not using any
sort of effect pedal or preamp with it. Just guitar-->cord-->amp.

Distortion is great, and desireable for some guitar applications, but
seeing how this OpAmp was possibly created with Walkmen, Discmen and
other small audio circuits (Cheap T.V.s?) in mind (or not), is this
level of distortion sound excessive? Or does it sound as if i may
have overheated something whilst soldering? (I'm getting better, but
i'm not perfect yet). Or have i overlooked something re: speaker type
and compatibility? (I won't lie, it was less than $10 and it's targeted
at the piss-poor car audio market).

Any thoughts or direction are appreciated. I apologize that this is so
wordy and long-winded. I pared it down as much as i could, but
articulation is not one of my strengths either :-/

-phaeton

 

[dB]

"phaeton" <blahbleh666@hotmail.com> wrote in message news:<1113367707.479165.80440@o13g2000cwo.googlegroups.com>...

Hello again,

I've recently built a small guitar practice amp circuit found here:

http://www.generalguitargadgets.com/richardo/practiceamp/lm386.gif

The TBA820M, also in an 8 pin dil package, will develop a watt in 8 ohms.

articulation is not one of my strengths either

-phaeton

There's nothing wrong with your articulation.

 

[phaeton]

Lord Garth sez:

Okay, take a look at the datasheet:
http://cache.national.com/ds/LM/LM386.pdf

Well, there's a novel idea! ;-) These are actually the -3's, now that
i pull them out of the tray and look at them. I had read in another
writeup (not this pdf, but another document on National's website) and
saw the "0.2%THD" bit, but i suppose i didn't see the "at 1/8W" part,
nor the "10% THD" part, or the "32-ohm" load part. Silly me...

At what point does THD become noticeable?

designs that might serve you better though most of what I find is
toob
based.

I've got some toob amp schematics too, but i'll mess around with the
solid state stuff for awhile first

Roger Johansson says:

What kind of distortion are you hearing, describe the sound please.

Soundclips are worth 1000 words:

http://home.mia.net/~phaeton/Anklebiter/200-100.mp3 (707KB)

The amp up all the way, the guitar's volume up all the way. Apologies
up all the way to the Yardbirds. The switch between pins 1 and 8 is
closed (gain == 200). A distorted sound here is expected, no biggie...

http://home.mia.net/~phaeton/Anklebiter/200-30.mp3 (557KB)
Same as above, but with the guitar volume turned down to 30%.
Interestingly it didn't sound as distorted when i played it back, but
the distortion is still there. Apologies turned down to 30% as well.

http://home.mia.net/~phaeton/Anklebiter/20-100.mp3 (670KB)
The switch between 1 and 8 opened (gain == 20) and the guitar volume up
all the way. You would expect distortion here, and note that there's a
little less (for obvious reasons).

http://home.mia.net/~phaeton/Anklebiter/20-65.mp3 (637KB)
Same as above, but with the guitar volume turned down to 65%. As you
can hear, the resonance of the guitar itself (it's a solidbody guitar)
is about as loud as what's coming out of the amp. Still some
distortion there. If i turn it down any lower it seems to fall under
some threshold and nothing comes out at all (which is ok, it's a
simple, cheap toy )

Then i tried it using a stompbox (Ibanez Tubescreamer) that wasn't
switched on. The TS does not have true bypass switching, so AFAIK in
"bypass" mode it still acts like a small preamp:

http://home.mia.net/~phaeton/Anklebiter/Rusted_Nosering.mp3 (972KB)

There is no distortion coming from the pedal, it's all amp. This is
with gain at 200, volume all the way up, etc. I would expect
distortion here too, and this is fine, but it remains mostly the same
as i turn the guitar down.

I'll try the resistor, like you suggested.

dB says:

The TBA820M, also in an 8 pin dil package, will develop a watt in 8
ohms.

I was looking at an 8W TDA2002v circuit before this one, but decided to
go with the smaller of the two

There's nothing wrong with your articulation.

You are so kind!

In fact, all of you are so kind.... it's starting to look like i've
asked some really naive questions, esp regarding the docs.... oops....

-phaeton

 

[phaeton]

I don't think it is possible to hear the resonance of the body
through
all that distorsion, but I can be wrong.

Actually I was referring to the acoustic sound of the guitar being
picked up by the mic directly. While recording, the amp was at my
feet and i was sitting down, and if you listen closely you can hear the
pick and strings snapping.

Strange actually. Maybe there is some non-intended noise gate
function
in the circuit.

yeah, that's exactly what it acts like- an overzealous noise-gate with
a high threshold. I would have just said "noise gate" but i didn't
know if you were a player and/or would know what i meant

There are many versions of the TS

In this case it is a TS-15....a mid-90's era "Soundtank"- style. Not
the best example of a TS, i know. I've never looked at the schem for
this one, but i'll assume you are right- it's got a "bounceless" type
of switch, which could very well be a momentary switch and some FET,
and is definately not a true bypass type.

(I've also got a TS-10 with the classic "Jack separating from the PCB"
syndrome, and i intend on cloning a TS-9 and/or TS-808 later this
summer. I promise not to bug you guys about those )

I think the sound is very different when the TS is connected, so
there
is distorsion from that too, but a good kind of distorsion I would
say.

I like the distorted tones overall. I still have yet to look at your
links, and try (all) your suggestions, but I will (long story short,
i have limited time at home to actually play with things)

Perhaps i should have done this first anyways- built one up (even if a
duplicate) on breadboard and just experimented on it. I'd like to do
this anyways.

Thanks to y'all for all the suggestions and tips!

-phaeton