AMZ IC Buffer using LM386 N

[AM]

Hi,
I want to built an IC buffer following Jack Orman's schematics. Jack suggests to use a TL072. I have an LM386 N in my parts box. Could I use that instead?

[jefe]

I don't think so - LM386 is an audio amp, not an op amp. I think. Actually, don't take my word for it - hopefully someone more knowledgeable will come along and confirm...

[AM]

I think you are probably right. I know it as an audio amp too. I just want to make sure though. So I'll wait and see if some of the more knowledgeable guys chime in.

[Mike Burgundy]

Google is your friend. Very first hit:
http://www.datasheetcatalog.org/datasheet/nationalsemiconductor/DS006976.PDF

For this buffer you'd want cheap, high-impedance input quiet opamps - the TL07x series has just that.
There are a lot of pin-for-pin compatible dual opamps (such as 4558, 5532, the list vis as long as your arm) which will work to varying degrees and at different cost. The trick is to choose the one that maybe not necessarily does the *best* job in a certain circuit/application, but has the best balance between cost and certain merits of the opamp's design. For example: a TL072 has nice and high input impedance. A 5532 has somewhat low-ish input impedance, but is *really* quiet and clean. Etc.
The 386 can be a Smokey practice amp with a couple of caps, or a Ruby with a few parts more.
hih

[earthtonesaudio]

It's not a buffer, but you could use it similarly to a buffer.  But it's a tad extreme. 

First, it has a fair amount of gain pre-set by internal circuitry.  Okay, not so bad.  Gain can be a good thing.
Second, the input impedance is rather low for guitar.  So low that you'd likely want to put a buffer before it... which seems a bit redundant.
Then there's the output drive capability.  It can drive an 8 ohm load, which is pretty awesome.  However, you won't need it unless you have 1000 feet of cable between it and your amp.

Usually a single transistor buffer is more than adequate for guitar.

[Mark Hammer]

A buffer should have a high input impedance.  Generally speaking, audio amplifier chips do not meet this requirement.

[AM]

Thanks guys. I want to ask a couple of things more:

It can drive an 8 ohm load, which is pretty awesome.  However, you won't need it unless you have 1000 feet of cable between it and your amp.
Usually a single transistor buffer is more than adequate for guitar.

If I would use an MPF 102 I have here could that be enough to drive long cables? I 'm not talking a 1000 feet, but long. Is there an approximate limit in the length it could drive?

A buffer should have a high input impedance.  Generally speaking, audio amplifier chips do not meet this requirement.

Would that MPF 102 do a good job? Is it a Jfet or Mosfet? I've seen both description for it when I googled it.

Thanks for your help guys

[Mark Hammer]

Yes, MPF102s, although not the "platinum standard" for JFETs, have been widely used for just that purpose (buffers) for many years.

In fact, if you look here, you will see it listed. http://www.muzique.com/schem/eagle.htm

[AM]

Mark, any idea to whether this design would be adequate to drive about 60 feet of cable length?

[Mark Hammer]

You will have accomplished the high input impedance part with the MPF102.  Jack's site shows how to parallel some plain vanilla op-amps to produce a high-enough output current to pass safely through cables of 60ft and longer.  Just add the op-amp stage after the JFET stage.

[Ben N]

Someone, I think RG, posted a piggybacked jfet (e.g. MPF102) + bjt (e.g. 2N5088, 2N3904, MPSA18, etc.) buffer here. That would do a better job with that long cable than just the MPF102, which has a rather high output impedance for this purpose. OTOH, it is probably no better (and probably has higher distortion) than the opamp solution.

[earthtonesaudio]

Or how about this:

Add a series 1M resistor with the input of the 386.  Use the standard output network (resistor+cap to ground, and large output cap).

This takes care of the impedance issue and the gain issue simultaneously, and reduces the input current enough that you can likely direct couple without problems.  Voila.

[AM]

Jack's site shows how to parallel some plain vanilla op-amps to produce a high-enough output current to pass safely through cables of 60ft and longer.  Just add the op-amp stage after the JFET stage.

Is that the page you are refering to?
http://www.muzique.com/lab/superbuff.htm

According to what I read there I guess I could just do a single Opamp buffer without the preceding Jfet stage and get the same results, right?

Edit: I would just have to use one of the recommented opamps.

[Ben N]

Depends on where you use it and what opamps you use. Jfet input opamps like TL0xx and LF353 have very high input impedances, and buffering in front of them is redundant. OTOH, for maximum drive capability, you would parallel up something like a 5532, which has a rather low input impedance. If it is following a very low output impedance device, no problem; otherwise you might want to buffer the input with an fet. The general rule of thumb, IIRC, is that the input impedance of any device should be >=10x the output impedance of the device preceeding it (unless you want an impedance mismatch for tone-shaping).

[earthtonesaudio]

The basic idea, with a bunch of "extra" parts to make it behave better.

[AM]

Thanks. I think I'll try this and a few more possibilities and see what I like most. I also decided to build this too:

http://www.aronnelson.com/gallery/main.php/v/DRAGONFLY-LAYOUTS_0/album14/album143/DOD_201_JFET_PREAMP_VERO_001.gif.html

I don't know if I need to change any resistor values since I will use that MPF102 and not the J113. Especially the 220k one. Anybody has a clue?

Also, would it be OK to substitute the 0.047 uF with a 0,1 uF one? (or even a 1 uF)

[andrew_k]

Q: "Can I drill a hole with my hammer?"

Initial Answers: "No, your hammer is not a drill"

A short while later: "Here's a way you could use your hammer as a drill"

This is why I love this place. 

[earthtonesaudio]

Q: "Can I drill a hole with my hammer?"

Initial Answers: "No, your hammer is not a drill"

A short while later: "Here's a way you could use your hammer as a drill"

This is why I love this place. 

Me too!

[Ben N]

I don't understand how that 1M pot is supposed to work, with the wiper connected to both the cap and pin 3.

[petemoore]

  Any of the above methods design goal achievement ratings can be compared to using a short, HQ cable between the guitar and the amp.