Bass guiter buffer/headphone amp

[stigonbass]

Hi, this is my first post / box, so sorry if I seem to be a bit slow on this, thank you for your patience.

I am trying to build a bass guitar signal buffer that will output a clean signal for the amp, and drive a pair of headphones for monitoring.

While I can build a circuit that produces (what sounds like) a clean feed for the amplifier, I get distortion on the headphones.

The basic circuit is like this...



Clean feed (black lines) works fine to bas amp.
Headphone feed (blue lines) underlying distortion

I have tried various combinations including taking the feed from the headphones from Node #2, taking left and right headphone feeds from just one op amp, reducing the headphone input and increasing the gain, however I can't seem to lose the distortion on the headphones.

If I connect the headphone feed to the amp no distortion, if I connect headphones to the amp feed distortion.

I appreciate my circuit may be a bit too simple and I may have missed some obvious things but I am new to this so anything constructive would be appreciated. 

Stig.
8¬)

[PRR]

What is the DC voltage on the headphone output? (You have DC blocking caps on the amlifier output, you may need a cap on the phones output.)

Even so, plain unspecified "opamp" may not get loud and clean in headphones. More info needed.

Did you look around to see how other designers have approached this? (As they said in Texas 130 years ago: don't eat your own cattle if you can get someone else's.)

[stigonbass]

Sorry, I did say I was bound to mess it up a bit.

The op amp is a TL084.  I am splitting a 9v battery voltage using a LM358p op amp as a voltage follower to give me +/- 4.5v (no shown in diagram), I have also tried it with 2 batteries to give me 18v (+/- 9v).  This does not seem to make any difference.  Have checked and balanced the voltages using a multi-meter, so I am assuming I get the full sine wave.

Have tried introducing caps to the headphone output but this cuts out the signal altogether.  I understand that they block DC voltages but not very sure on how they effect audio signals.  I am using non polarised caps in the audio circuit.

Have tried different headphones.

Have looked on this forum for headphones did not find anything that met my needs (although it may be there somewhere).

Will do some research into headphone amps and post more info if I can.

I just want to be able to hear what I am doing especially when playing a fretless bass and don't have a proper monitoring system.

Can I measure the DC output using a standard multi-meter? 

Are there other op amps better suited to this task?  I have tried an LM324 which uses a 0v ground instead of a negative voltage but it was worse.

Thanks for your comment, hope the extra info helps you understand my attempt at a circuit, look forward to further posts.

[GibsonGM]

Hi, welcome to the forum!  The NE5532 will make a better headphone amp, as well as the tried and true LM386. And there are others, too.

There are a billion examples to be found on Google etc. ! 

[duck_arse]

something is missing from that link, Sir M.

and welcome. doesn't MXR make a headphone amp? and hasn't that been reverse engineered?

https://www.diystompboxes.com/smfforum/index.php?topic=128335.0
https://www.diystompboxes.com/smfforum/index.php?topic=118981.msg1109191#msg1109191

folow the links, ignore the problems.

[GibsonGM]

Thanks, DA...the link doesn't want to be copied from that (rather mundane) site to here.  So screw 'em....yes, there are probably 100 of these headphone amps out there!    The MXR is probably a great choice.

[PRR]

The web is full of plans. More than half are crap.

_I_ like the LM386. (Favor the JRC brand.) Ample power for 32 ohm phones, or even two 32r ears on parallel mono.
https://www.gadgetronicx.com/headphone-amplifier-circuit/
https://www.gadgetronicx.com/audio-amplifier-circuit-diagram-iclm386/

Here is a fine example of opamp. Note it suggests a '5532, more oomph than TL0xx, but is marginal on 9V ('5532 will crap-out at 7V). For bass you might want to double the uFd of the output caps, say 470uFd.
https://www.redcircuits.com/Page119.htm

[stigonbass]

Thanks for the all the support and the food for thought.  Will do some further research and post how things go.

I have a bag full of LM358p chips, would they be any good for this?  I am using one for my power supply but don't know if they would be suitable for the audio circuit.

Stig.
8¬)

[PRR]

> LM358p ..... good for this?

If you plug your ears with expanding silicone foam, and keep the volume on zero, maybe.

The LM324 (and many daughters) is perhaps the worse *audio* opamp made. Not always, but when loaded and driven through its crossover zone, you will hate it.

[Rob Strand]

I have a bag full of LM358p chips, would they be any good for this?  I am using one for my power supply but don't know if they would be suitable for the audio circuit.

The LM358 chips have very poor drive capability.

If you *really* wanted to try an LM358 you can add a transistor booster like this,
http://www.generalguitargadgets.com/pdf/ggg_mxr_hpamp_sc.pdf

R2 and R3 can be replaced with a fixed resistance.

For the LM358 it's probably best to operate the LM358 output stage in class-A (ish) by moving the
point where the opamp connects like this.    Only look at the output stages here,
the rest of the circuit should be kept like the GGG circuit,
https://codecwired.blogspot.com/2017/08/lm358-audio-amplifier-circuit-diagram.html

Some notes: (referring to ggg ckt)
- you can improve the output swing by adding a 47uF cap across the outside of both diodes.
- Do not removed C6. 
- If you have problems with oscillation move the C6 connection from the output of the transistors to
the output of the opamp.
- You should make the circuit with a gain of 3 to 10.  Wiring these boosters
   as a buffer is not recommended for newbies, as they can oscillate.
   Put a divider at the input to tune the level.

It's much easier to use an LM386 with a divider at the input.

If you go with a NE5532 design you should add at least 47 ohms in series with the outputs
to prevent the NE5532 clipping due to the opamp current limit.