audio preamp design questions

[rolerbe]

I'm not getting enough volume in my patch together home theater system. So, I want to design a small in-line audio amp to give me some gain (maybe 20 - 30 dB) between the source output and (8 ohm) powered speaker input. The general design seems pretty simple, but, as I'm an amateur circuit designer I have a couple of questions.

I'm looking at the data sheets for the LM386 as the op-amp choice for example. In the app notes, it shows level control via a pot on the input side of the op-amp. I was thinking of doing this on the output side instead based on the thought that I'll get less noise by knocking down the amplified output signal rather than knocking down the small input signal (?)

Any particular reason to favor input versus output side level control?

Do I have to really concern myself with impedance matching on the output or will just maybe a 2Kohm dual (for stereo) pot with the wiper as the pick off point for the output to the speakers do?

Any comments on other key design points to take into consideration would be welcome. Thanks.

[Engineer1138]

I wouldn't use an LM368: its distortion specs aren't very good and it is designed to drive speakers, not for line drive. There are a lot of cheap op amps that perform much better.

[Robert Reed]

20-30 DB is a lot of gain. If the input to the add on amp is anything more than moderately low, you will certainly drive its out put into saturation. As with any new add in design, it all has to start with some values (i.e.-voltage levels, in/out impedances,gain,etc.) around the point of insertion.
If you wish to bypass the math, then the pot prior to the new amp is your safest bet. At the level of audio you are working with, the pot probably won't contribute any significant amount of noise.

[rolerbe]

Good point. I actually have a gain control on the input side already. And, I put a Boostaroo inline which provides 12dB of gain and output volume still seems low (and no clipping). That's what drove the 20-30dB criteria -- basically another 12dB over the current rig (which I will take out).

[MrAl]

Hi there,


I used an LM386 for a guitar amp just for practice, but i wouldnt use it for any serious audio either. As the other posters have noted, the distortion just isnt very good.

They make some pretty low noise op amps these days (made for pre amps not for power amps) suitable for framing

[Dean Huster]

You really need to start by looking at the input specs for the power speakers to see what voltage level they expect to see (and the input impedance) and whether they're single-ended or balanced line input; then check out the same thing with the output of your driving device.

You know, it may be nothing more than the driving device being high-impedance and the input of the powered speakers being low-impedance. You might only need impedance matching.

[rolerbe]

Dean,

I checked r.e. your questions. The spec's for the Gigaware 2.1 Multimedia powered speakers indicate that the subwoofer is 4 Ohm, and the tweeters are 8 Ohm load. Why a 'powered' speaker system would present low impedance to the input is confusing, since there is an integral amp, but I guess this is just to appear to the input like a normal speaker system. So, I thought that might be why the Boostaroo wasn't giving me enough real gain -- maybe designed just for headphones. But no, they list 12dB gain into an 8 Ohm load.

I'm not overly concerned with sound quality, just want to be able to hear my shows. But I have two alternate designs to protoboard out. One with the 386, one with a higher quality op amp for V-gain followed by a transistor current gain stage.

[CeaSaR]

Those numbers are for the speakers themselves, not input. I doubt you'll find specs anywhere for them.
What you are really looking for is enough voltage gain (dB) to drive your computer speakers loud enough
without overdriving them. I'd like to point you to a thread or 2 on audio, but first: when you hook the
system up to a computer, is it loud enough? If not, the problem is with the speakers, not the input. You
might need to upgrade before you are satisfied. Check it out and get back to us.

CeaSaR

[Bob Scott]

So this is not really a home theatre? A bunch of mystery computer parts? A "boostaroo"? So far in all of this thread, I don't see any helpful reference to any specific input or output level of the mystery equipment from the OP.

[CeaSaR]

Are these the speakers? If so, you'll find the info needed in this PDF.
Scroll to the right side and look under Specifications. It says the input sensitivity is:

200 mV for the subwoofer
700 mV for the satellites

If your TV is putting out the standard 300 mV (Line Out), then you'll need a boost.
I still think you'll be underwhelmed by them anyway, even if you get the levels up.

Have you checked them on a computer yet? Let us know when you do.

CeaSaR

[rolerbe]

Problem solved. I breadboarded two versions, one with the LM 386, one with a gp op amp voltage gain stage and transistor current gain stage. Both worked fine. Will probably box up the 386 version as I can get it into a smaller box.

My Gigaware set is similar to those Cesaer listed, but not identical. I couldn't directly check the audio source output as this is in an apartment I took for a new job, away from the house and all my equipment. But either the source was low, or the issue was too much loss from long cable runs and looping through the projector itself. In any event, plenty of gain now, no discernable distortion (to my non-golden ears).

Thanks for the comments!