Is the hiss level relevant to being used with a LM386 amp? I mean, those things aren't exactly hi fidelity. And neither is my hearing these days.

Yes, an LM386 amplifier also has some hiss but a Jfet preamp has low hiss that will not be amplified by the LM386.
The Cmos rail-to-rail preamp has a lot of hiss (about as much as a lousy old 741 opamp).

My hearing is pretty good because I took care of it (hardly any guns and no acid rock concerts). I hear about as well as a 12 years old kid before his first ipod damages his hearing. I am 67.

 

My hearing is pretty good because I took care of it

The hearing screeners tell me I'm well above average, but the typical hearing test doesn't go into high frequencies. I have frequency generator apps on my iPod, and my kids shriek when I turn on 10kHz. I can sense it but just barely, above their screams. I'm 54

 

Hearing test are for only "telephone quality". They don't care if you can't hear the very high frequency silibants spoken by young people nor the sizzling of music.

My local TV station cuts all high audio frequencies from the news anchors who read the news. But outside reporters and commercials sound fine with lots of high audio frequencies. I think their sound man is deaf.

Local AM radio stations boost the mid-range (because they do not broadcast high audio frequencies). Then it sounds tinny but maybe a little clearer.

 

Is the hiss level relevant to being used with a LM386 amp? I mean, those things aren't exactly hi fidelity. And neither is my hearing these days.

And neither is the signal from an electric guitar pickup, which also has ton's of noise and hiss.

There have been lots of commercial portable guitar amps over the years using LM386, and you don't need any JFET or opamp input stage. A magnetic guitar pickup is lowish impedance and puts out significant voltage typically 2v p/p when playing quietly, and if played by an aggressive teenager with "hot" pickups you can get over 4v p/p!

How much gain do you need on a 9v amp when the input signal is 2v p/p?

But personally if the OP wants a fun project for a portable guitar amp I would look at some of the new class-D SMPS amp chips, they are simple enough to use, have enough gain to not need an input stage, and will be VERY LOUD compared to a LM386 and still be very battery efficient.

 

How much gain do you need on a 9v amp when the input signal is 2v p/p?

I played with a direct guitar-to-LM386 amp on breadboard and this explains why I didn't like it. It was clipping a LOT.

...I would look at some of the new class-D SMPS amp chips, they are simple enough to use, have enough gain to not need an input stage, and will be VERY LOUD compared to a LM386 and still be very battery efficient.

Very interesting. Maybe my next project.

 

But personally if the OP wants a fun project for a portable guitar amp I would look at some of the new class-D SMPS amp chips, they are simple enough to use, have enough gain to not need an input stage, and will be VERY LOUD compared to a LM386 and still be very battery efficient.

Please advise a part number for one you like. I would give it a try. Thanks.

P. S. A schematic would be welcome as well.

A magnetic guitar pickup is lowish impedance and puts out significant voltage typically 2v p/p when playing quietly, and if played by an aggressive teenager with "hot" pickups you can get over 4v p/p!

Do you happen to know what the output is from a typical piezo pickup? Thanks.


@blah2222 I hope we aren't taking your thread too far afield from what you wanted. If so, just say the word and I'll ask a mod to split it off.

 

Do you happen to know what the output is from a typical piezo pickup?

Oops! Are you using a pickup on an acoustic guitar?
Sorry, I only have a Fender Strat to test this with.

 

Oops! Are you using a pickup on an acoustic guitar?
Sorry, I only have a Fender Strat to test this with.

Yes, but I am still very interested in your project and results. And I have started prototyping my circuit on perfboard before ordering PCB's.

 

Please advise a part number for one you like. I would give it a try. Thanks.

P. S. A schematic would be welcome as well.



Do you happen to know what the output is from a typical piezo pickup? Thanks.


@blah2222 I hope we aren't taking your thread too far afield from what you wanted. If so, just say the word and I'll ask a mod to split it off.

No no, I am glad so much interest was shown. I am picking out bits here and there and am realizing that my simple BJT design is not robust enough.

Also, is the main use of the JFET to buffer the input to lower the impedance seen by the 386?

Also, how are y'all setting up your guitar output jacks up? Right now I've got my SG patched to a 1/4 audio jack crudely soldered (no shielding) to plug into the breadboard. I'm thinking that I might be losing too much signal just in the cable->jack.

Thoughts?

 

No no, I am glad so much interest was shown. I am picking out bits here and there and am realizing that my simple BJT design is not robust enough.

Also, is the main use of the JFET to buffer the input to lower the impedance seen by the 386?

Also, how are y'all setting up your guitar output jacks up? Right now I've got my SG patched to a 1/4 audio jack crudely soldered (no shielding) to plug into the breadboard. I'm thinking that I might be losing too much signal just in the cable->jack.

Thoughts?

My in and out jacks are going to be on an enclosure wall, and will be connected with short lengths of wire to the PCB. I hadn't really thought about using shielded cable for those short connections, but it might be a good idea.

 

There have been lots of commercial portable guitar amps over the years using LM386, and you don't need any JFET or opamp input stage. A magnetic guitar pickup is lowish impedance.

No.
The electric guitar preamp circuit I posted says the minimum input impedance of a preamp is 1M ohms. The LM386 without a Jfet preamp is 50k ohms.

Almost everybody uses a high input impedance vacuum tube or Jfet as the preamp for a pickup. The high impedance causes a peak at 4kHz which might cancel some of the high frequency loss from the capacitance of a connecting cable.

Attached is a graph showing the amount of peaking with different load resistances. The 50k input of an LM386 amplifier will cause a cutoff at only 1800Hz, plus more high frequency drop if the amplifier is at a distance from the pickup.

Here are links to some recordings of the Little Gem (LM386 by itself) and to the Little Gem MkII (added Jfet preamp)
http://www.runoffgroove.com/salvo.html#littlegemMK2

Articles say, "The input buffer helps to retain treble detail going into the 386 chip.

Depending on your personal tastes, there may be a potential down side to the 386 designs. The input impedance is listed at 50k, according to the datasheet. This is quite low and causes some loading of the guitar signal. There is some degree of high end signal loss.

The result of the input buffer is easily noticeable. There is increased sparkle in the signal."

 

I played with a direct guitar-to-LM386 amp on breadboard and this explains why I didn't like it. It was clipping a LOT.

Of course it clips a lot. Its maximum output is only 0.3W into 4 ohms or 0.45W into 8 ohms at low distortion before severe clipping. About like a cheap clock radio. If you turn it up to be louder then it clips like crazy.
If you want more power then get an amplifier with more power.

Clipping is FUZZ that everybody plays.
FUZZ is severe distortion.
FUZZ is also called "overdrive".
FUZZ is also called "nice breakup".

 

Do you happen to know what the output is from a typical piezo pickup?

We were talking about an electric guitar that uses a magnetic pickup that has coils inside. The coils pickup magnetic vibrations directly from the strings. So the body of the guitar does not affect the sound much.

Isn't a piezo pickup used on some acoustic guitars? It is mounted on the body of a guitar to pickup resonances of the body. I have not seen an article about it.

 

Isn't a piezo pickup used on some acoustic guitars? It is mounted on the body of a guitar to pickup resonances of the body.

That is correct. There is a school of thought that a piezo is grossly inferior to a magnetic pickup (sounds "muddy,") but that is what I plan to use at least in the beginning. I wish I knew more about piezo transducers used as sensors, but I haven't found any specifications.

 

So the body of the guitar does not affect the sound much.

Les Paul said in an interview that his first electric guitar was a wire between two pegs anchored into a length of torn up railroad rail. He used a telephone mic/speaker as a pickup and sent the signal into his mom's radio. He said he loved the sound and was so excited when he ran to his mom and announced he had invented the electric guitar. She told him he'd never get a cowboy to carry a length of rail on his horse, so maybe it needed a bit more work.

 

I tried the Fender with electric guitar pickups directly into the LM386, basically the same circuit posted by Audioguru and also shown in the LM386 data sheet. The sound was actually quite good but wouldn't blow the roof.

You can increase the gain by adding a 2kΩ variable resistor between pins 1 and 8 as described in the data sheet.

 

I tried the Fender with electric guitar pickups directly into the LM386, basically the same circuit posted by Audioguru and also shown in the LM386 data sheet. The sound was actually quite good but wouldn't blow the roof.

The LM386 has a fairly low input impedance that cuts high audio frequencies from a magnetic guitar pickup.
The maximum output is only 0.45W at low distortion which is as loud as a cheap clock radio.

 

Actually, I'm so impressed I am going to build me another portable amp with an 8" speaker but not with the LM386, something with more power running off a 12V SLAB.

 

The LM386 has a fairly low input impedance that cuts high audio frequencies from a magnetic guitar pickup.
The maximum output is only 0.45W at low distortion which is as loud as a cheap clock radio.

Most of the time I play blues and jazz and I prefer a deep bottom. Besides I can't hear anything over 3kHz anyway.

 

Here's my perfboard assembly; it works very well. After I clean up the external wiring, I'll try to make some measurements with the scope.