CMOY H/Phone Amp - I know you've had this a million times but....

[VamP1R4T3]

I know you guys have probably had this question a million times and I know being an electrotech student I should be able to figure all this out myself but I can't. So anyways here's the deal;
I have a Bass guitar which I'm learning to play and I'm rubbish at ATM and I want to hook it up to headphones without having to plug it into a full size amp etc because that'll just be something else I have to carry when i travel. I was going to use my computer to try and do it but my laptop refused to play the game so I decided building something was the way to go. Anyways, back story out of the way here's what I had in mind....



Sorry the pictures a bit big but anyways, I drew it up in oregano so hopefully its neat enough for you guys to figure out what's going on.
SW1 and SW2 will be a dipole switch, I just couldn't find the right symbol in the program I used. And Signal In will actually be a 6.5mm audio connector not a BNC connection which is what the diagram makes it look like.
Credit where its due I did read This topic. And borrowed a lot of the design etc from there but now I have a few questions.

The first of which being will it work?

And do I need the JFET Buffer if I'm only having the single input? I think, please correct me if I'm wrong, that that's just in the schematic in that topic because jasonsmusicgear wanted the second line in.

And thirdly, this will probably sound stupid to everybody but do I need to protect the guitar in any way?

Also do you guys think the values of my components look okay? And does splitting the input to go into both opamps of the chip seem like a good idea?

Sorry if I'm wasting everyone's time, thanks in advance.

[jasonsmusicgear]

Mine works great and sounds even better with a condor cabsim in front of it.  I put the buffer on the guitar input only.  The monitor input did not have a buffer.  You may want to include the buffer because you are plugging in a guitar.
Jason

[VamP1R4T3]

I did find another problem, I can't find a place to buy the 2N5089 JFET from, does anybody know what I could substitute it with?

[PRR]

> being an electrotech student I should be able to ...

Should learn how to scale your drawings to fit the thesis or book.

A simple thing, but not knowing it annoys professors and publishers.

At first I thought you were going to drive phones with a little FET.

No, you do not need the FET.

And you do not need R8 R10. (And if you did, that's the wrong value... SB R6||R7 or 9.17K.)

I dunno why 110K, 100K is common-value, unless you have a shoebox of 110K? (I done that.)

But you DO need another R6. Each op-amp must find its own bias. As you have it, the right end of R6 will tend to be the average offset of the two amps, and each amp will amplify the difference between that and its own offset.

There's not enough gain here unless the bass is very hot and the headphones very sensitive.

Why inverting mode? Non-inverting will allow the opamps to have a 100K input impedance with a low-Z feedback network, less loading on the 100K volume pot. Then you can bring the gain to 100 or more without the very low input impedance of inverting mode.

And you don't really need C5. Or the FET and its capacitors, though I would use a cap before the volume pot because you never know when your source may leak DC.

You do NOT need 18V to drive Walkman/iPod 32r earphones. 6V peak-peak is plenty; why have three times as much costly battery? Only to flog that skimpy chip into making enough current for such phones. May I suggest wiring both ears in parallel and using one LM386 at 9V to drive both? The 100+mW of power is ample in headphones. You will need an output cap, but unlike run-down batteries a cap is forever (well, decades).

It ain't much like Cmoy's original famous plan.

> can't find a place to buy the 2N5089 JFET from, does anybody know what I could substitute it with?

Can you write the minimum required specs for an FET to work in *this* circuit? Yes, that hasn't been covered in class yet; some curriculums never get to pesky practical problems like that.

And yes, I tole you don't need it. But "can't buy that part!" will be a recurring problem.

So: what is the idle and maximum Voltage across the FET? Can you see that it may be around 4V normal and 9V when upset (such as start-up)? What is the current? My abacus sez 1.6mA, though it is not clear it needs to be that high. What is the maximum Gate-Source turn-on voltage? Well, if over 4V and Gate sits at 4.5V then Source is at 8.5V which is too close to 9V to work. You probably want several volts less. It does depend what the signal level is (note this is before the volume control so you are at the mercy of what your source want to spit out). OTOH, you could change R2 R4 to some other bias point. You still want less than 9V gate-source turn-on.

You can hardly find a 9V FET, you'll have to take a 20V-40V rated part (because 9V parts are no cheaper than 25V parts). 2mA Idss is not a hard spec to find. Vto of any common FET is likely to work with perhaps some R2 R4 tweak for best headroom.

[phector2004]

2N5089 is a NPN BJT

Replaces 2N5088 with a bit higher gain, and I think it's lower noise

I've seen them pretty much everywhere online, maybe you're just looking in the wrong store "sections"