Preamp with headphones

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jai.leeworthy said:



thanks, i had found this one. the beginner electronic persons one is here:-
[urlhttp://tangentsoft.net/audio/cmoy-tutorial/[/url]


Ok maybe I wasn't clear. A low gain, low power, stereo amp is not what you want.

A guitar is passive like a mic. It needs an amp with a gain of around 20 and a bit of power behind it. Guitars are also MONO!

The links I have posted above is a simple amp made for a guitar that has a standard gain of 20 with a switch for a gain of 200(distortion). The guitar should have it's own volume control so you don't need anything complicated. The amp posted above has enough power to drive a small PC style speaker like one of those belt clip amps you can buy. It will also run headphones just fine. It will run off batteries or you can use a DC supply of around 9-12v.

Forget the Cmoy. It's a great headphone amp but it was not designed for a guitar or any other passive device for that matter.
 
Rembrant said:

Ok maybe I wasn't clear. A low gain, low power, stereo amp is not what you want.

A guitar is passive like a mic. It needs an amp with a gain of around 20 and a bit of power behind it. Guitars are also MONO!


I didn't see this post, sorry. If I change the gain of the cmoy, can I still use it? (even if guitars are only mono. I figure [correct me if I'm wrong] that if I make my own cable, i can distribute the one guitar channel onto both the channels on the cmoy) uh, sorry, can you elaborate on the term passive? how do I power it? does the circuit have to be running through it to power it? what can it amplify?

its just that i'm reluctant to not use the cmoy. its really well documented, i can obtain the parts (which is hard in australia) and it seems simple enough to make. what do you think?
 
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A typical Cmoy opamp will not be able to do both the tasks of providing a gain of 20 and driving a low-impedance load like a headphone, in my understanding. A current-boosted configuration would be better. Look at the National AN-272 for some examples.

If you really want to scrounge, change the gain structure of a typical Cmoy and parallel the two output channels with a single 1 ohm resistor on each output. That will at least be able to muster enough current for the 'phones. Also, wire the headphone socket so the left and right are in series, that will bring up the impedance of the headphones, making them a bit easier to drive.

The best thing to do in this case is a two-stage amp, can be built with a dual opamp and a bunch of other parts. One stage provides most of the voltage gain (say x20) and the second provides a bit more gain (x2) and a couple of transistors to provide the high-current drive for the 'phones. Not very difficult to do.

Don't you have a effects unit or stompbox? Most of them can passably drive headphones as well.
 
The lm386 is the chip for the job. If you look at the first link in my post above you will see that this is the easiest guitar amp you will find!
The thing only has 6 parts! Comparatively, anything else is going to cost more and probably not work as well. This thing is easy.

As far as power goes the LM386 is good for around 350mw. No other chips are needed. The whole circuit is 1chip, 4 capacitors and a switch.

Forget the cmoy or any other elaborate design. :smash:

If you have a local electronics store similar to radio shack they are likely to have all the parts you need in stock. You need one 1/4inch MONO female jack, one 3.5mm female jack and one SPST switch.

The rest of the parts are clearly labeled in the schematic.
 
sangram said:
A typical Cmoy opamp will not be able to do both the tasks of providing a gain of 20 and driving a low-impedance load like a headphone
Those two things are not directly interconnected. The ability to drive low impedances depends on the output current the op amp can provide. The gain depends on the difference between input and output voltage.

The trouble with op amp based headphone amplifiers is that you can either optimize them for high impedance or for low impedance, while headphones come in a range from 8 to 2000 Ohm.

This article shows that the situation is just as diversified at the input of the amplifier. One guitar gives 1.2 V pk-pk, while the other only delivers 50 mV pk-pk. So the first guitar could drive most op amps into voltage clipping with a gain of only 10, while the second would need a gain of 240 for the same output.

If you want to use the CMoy, you need to know the headphone impedance and the guitar's peak output signal voltage to optimize the amplifier for its task. For use with a wide range of headphone impedances and different guitars, you will need more headroom, which the LM386 could provide. Or better still, use an LM1875. It has enough voltage swing for 2000 Ohm, more than enough current capability for 8 Ohm and it could even serve as small practice amp with an adequate PSU and heatsink.
 
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Guess I wasn't clear enough.

There are high current output opamps that have terrible distortion performance at high gains due to bandwidth-limited performance.

The LM386, for example, has a typical THD of ~1% at 10KHz. The output power is indeed commendable, but the distortion is not.

On the other hand a reasonable Cmoy opamp is the OPA 2227, which is rated to an output current of 45mA (this is short-circuit condition, real case will be lower - half?). This is sufficient to drive 32 ohm headphones to 64 milliwatts, which may or may not be sufficient depending on the 'phones, and since this is a guitar, ambient noise levels.

Therefore the recommendation to split the task into two.

A typical pup provides about 400mV, and depending on playing style and guitar setup this can vary a little - the chart pb posted is of note decay, for a single pluck. I assume the guy just wanted to measure how hot the signal really was. Obviously a guitar pickup will provide for over 10dB of headroom, so peaks can hit about 1.8-2V at the input. Here's a quick link for the SD pickup range - the output level 'moderate' normally refers to 0.4V nominal, and the 'hot' about 0.5-0.6 nominal. http://www.seymourduncan.com/comparetones

Normally preamps use a couple of clamping diodes at the input (or two in series) to keep the signal at manageable levels, LEDs or zeners or similar can also be used for small tonal variations.

Typically a gain of 18-20dB should be sufficient for most practice amps, and give you sufficient range for quiet fingerpicking as well as power riffs. The idea of a LM1875 is very good, but watch that volume knob. It has enough power to turn you deaf if you listen to it through 'phones, so use a series resistor and keep the gain very low.
 
Sangram. Distortion is something you want in a guitar amp.
At a gain of 20 the lm386 is quite tame. At 200 it has a nice grungy sound.

While it may not be the best op amp for audio, with a properly designed circuit it can be used as a head amp with lower distortion levels than most portable gear. Lookup head banger amp on google. I have built many of those and they do a great job. No doubt, the designer Steven Lafferty has created a circuit that is able to wrangle this bad boy into submission.

Like I said though, with a guitar, distortion is a desirable effect.;)
 
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Well, I have 4 guitars, and I like distortion only while playing one of them, and that too not all the time - I prefer a tone/clean switch to one constant type of distortion :)

And one percent is not enough, if grunge is what you're going for, more like 15-20% for old-school rock and 60% for trash metal. Trust me I produce tracks as well and I can tell you if you want grunge the *headphone amp* is not the place to get the tone from. Either a stompbox or and amp sim or best of all, a proper amp cranked to 11.

It was just advice, and the perspective was keeping in mind the use of the amp. For tone it's best combined with a good simulator or effects unit anyway, but most of those are capable of driving phones on their own. You'll notice I asked that question, but still don't have an answer from the TS :)
 
can i just iterate that it is *impossible* for me to get my hands on an lm386, or pretty much any other opamp. it's a stroke of good luck i was able to find the dual opamp OPA2134PA, which fortunately is one of the alternative ones for the cmoy.

I don't care what it sounds like, i just want it to work...
say i'm using a piezo pickup rather than magnetic...?
 
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Piezo will work fine into something with a highish impedance. If it's a acoustic with a bridge or undersaddle pup, it needs a preamp with an input impedance of about 200K. The 2134 is very well suited to the task of providing the gain. Use a ~220-270K resistor for the input resistor (Ri) and the typical 47K/15K feedback gain resistors. Bandiwidth limiting is critical here - a 470pF cap is essential to minimise chances of oscillation, tie between the + and - inputs of the opamp.

A couple of transistors will help boost the output current to the point where the headphone impedance is a non-issue. If this is not at all possible, use both the channels of the 2134 and parallel them using a small-value resistor on each output (much like parallel chipamps, except here your resistor will typically be ~1ohm). Also wire the headphone socket so the two channels are in series, this will help reduce the stress on the opamp. It will work that way - I've used a 5532 as a makeshift headphone amp and it survived, so the 2134 will be fine in that role.

You will need a little less gain with that pickup - about 10dB (x3) should be fine for most practice. A typical piezo puts out about 0.6-0.8 V, and acoustics (if you are using one) don't have as much dynamic range at the upper end as a electric, specially if you are sampling the output on the guitar top instead of using a mic. If it is an electric with a piezo pickup (I wonder which one) you will need a bit of headroom on the peramp but the 2134 should be fine. Just use the clamping diodes to keep the input protected against overload.
 
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