Hi.

i'm looking for a small pre-amp. Simply to put between a portable cd player, and a pair of these small portable speakers (the sort that are driven by the headphone output on the cd player, and consequently arn't very loud).

It has to be battey operated, and I would prefer not to use op-amps, because presumably getting the negative voltage would waste some power. So a transistor based one would be good, but because of the battery operated nature, I don't want anything that will draw too much power. It only has to drive a pair of speakers with a similar impedence to standard headphones.

Can anyone provide a diagram/suggestions?

Cheers,

Tim

 

It's not really a preamp, more of a small power amp - if you google for headphone amplifiers you should find something suitable.

Opamps wouldn't be a problem, you can feed them off a single supply and AC couple them, you just need two resistors to generate a centre reference.

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whats the definition of a pre-amp then?

yeah, but wouldn't this waste some power?

thanks, i have several headphone amp digrams lying around. For some reason I had in my mind that I didn't want a power amp, and so had ignored them...oh well.

Thanks

 

It's a fairly individual point, I would 'generally' consider a pre-amp a voltage amplifier, and a power amp a current amplifer. But that's a very simplified definition - just looking at a circuit I can tell which is which (usually?).
It depends on the values of the resistors, they only have to provide a small bias current, so can be quite high. You couple the load via a coupling capacitor down to ground.

Perhaps there was a clue in your original question, where you wanted: "It only has to drive a pair of speakers with a similar impedence to standard headphones." 8)

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thanks for that.

lol, we should have a prize for asking the silliest question.

Thanks again

Tim

 

maybe another silly question, but anyway:

are all op-amps compatible? I mean, ignoring pin layouts, could I replace (for example) the op-amp in this with an NE5532?

 

I see no reason why not (although the NE5532 is a lower quality opamp, but still good!), mostly you can just drop a different amplifier straight in place of another, although there are a few exceptiions.

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Hey Guys,
If those little speakers are designed to be powered by the headphones output of a CD player, then surely they cannot handle the power of a power amplifier. They probably just have headphones' speakers in them anyway.
An opamp "power amp" probably won't make much difference in volume level, because it would perform the same or worse than the CD player's output. An opamp can't provide enough output current to properly drive a low-impedance speaker to an arms-length volume level. Didn't the article spec an output power of a whopping 10mW?
Try a real power amp made from an LM386 for each channel and see if those little speakers can handle its 1/2W of output. Its distortion, noise and low frequency response can be improved by using "global" negative feedback as in the "Headbanger" reference in the article.
Another option would be to use a bridged power amp like an MC33119, to operate on a low supply voltage and to eliminate the large output capacitor. It works well in my clock radio (with 20dB of bass boost). My clock radio's 3 inch speaker is probably similar to your little speakers.

 

well i built that amp (http://headwize.com/projects/showfil...=cmoy2_prj.htm) but put a ne5532 in (only because that was what I had lying around). Its awful. I have only tested it with a pair of headphones plugged in, but it doesn't improve the volume a huge ammount, and there is masses of distortion and interference and everything that is bad about amplifiers is personified in what I've just built.

I suspect it may have something to do with the fact that I built it on a small piece of vero board (approx 35x15mm), and I have interference/crosstalk between channels, and perhaps interference from the power (although I am using a PP3 battery, so it should be fairly 'noise-free'). I have built small amps on vero board before, with no problems, so I'm a bit supprised by this.

audioguru, I will look into your suggestions, thankyou.

Tim

 

The circuit looks fine, and there shouldn't be any problems building it on veroboard - I would suspect you've either got a faulty part, or wired it incorrectly somewhere. The power and zero volt connections are perhaps a little confusing, you may have made a mistake there.

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Hey Tim,
This is just a little audio amp, so Veroboard should be fine. It isn't a VHF or UHF transmitter or receiver.
I've built most things on Veroboard, including high-speed logic without problems.
When your opamp current-limits, maybe without enough supply voltage, then sure it sounds awful. Although some opamps can drive a load as low as 600 ohms, most don't like anything less than about 2K.
What impedance are your little speakers?

 

so it isn't the vero board. In my efforts to get the board to a very small size i made a mistake in one of the under board links, but even now i've fixed that, it still sounds the same.

supply voltage should be fine - I'm splitting 9v into +-4.5v, and the op-amp can (according to the datasheet) operate as low as 3v.

i havn't tried it with the little speakers yet - they're not actually mine, so I don't have them with me (I should be able to borrow them today or tomorow) - so I was testing it using a bog standard pair of sony headphones. My multimeter reckons they have a resistance of about 32ohms. I assume this is the same as the impedence. As described in the article, I tried adding 100R resistors to the output, but to no avail. Your mention of a "load as low as 600ohms" scares me slightly - if 600 is low then 32 is extremely low...?

Tim

 

Can you elaborate on how you are splitting 9V into +/-4.5V?

I don't know what all the hub-bub is about, the LM386 is a simple single supply power amp (which was mentioned earlier). 32 ohms? Pffft no problem.

j.

 

http://headwize.com/projects/showfil...=cmoy2_prj.htm is what I'm using, scroll down to see the method for splitting the 9v.

also I'm using an NE5532, not an LM386. I will probably look into using an LM386, infact I alsready did, but not having any LM386s lying around, I thought it would be easier to use what I had already got.

Still reckon 32ohms would be no probs?

 

*In general* a voltage divider is not a good way to "split a rail." The common will float around depending on how the return currents flow. Otherwise, there wouldn't be a market for "rail-splitters."

The LM386 will drive into 8 ohms all day. Get some at Radio Shack.

j.