need suggestions for mini amp

[Feifer]

Hello everybody. I'm looking to make a small amp to be run off a 9v battery (or wall wart) to make an analogue synthesizer I'm also building be a little more portable. I was looking at a couple based on the LM386 like this one...
Tiny audio amplifier using LM386 that I already threw in the proto board, but it doesn't sound very clean. I don't want any (or much) distortion and unhooking the cap and resistor from pins 1 and 8 to make it clean just doesn't make it that loud. Of course, the synth isn't done so I'm only testing it with a microphone.

Any ideas to make this sound descent? Should I try something like this..
Little Gems and use 2x 386, or do something completely different? Also, any suggestions about pairing it with a small speaker would help. I'm still pretty new at this.

Thanks

[Rembrant]

This is very clean. I use one everyday. Will definitely power small Ipod type speakers no problem.
HeadBanger Headphone Amp Construction Kit

[paulb]

The Headbanger is basically what you've already got. You probably need a pre-amp if you are using it with a microphone. The LM386 should sound okay for what you're doing. If you have access to an oscilloscope, check if it's oscillating.

[Feifer]

Thanks for the replies. I should have stated that it only needs to be mono, not stereo.

Are you saying that it's only quiet because of the mic, and hooking it up to the synth will be plenty loud? Also, why test if it's oscillating?

This is what it's getting hooked up with if it's relevant...Music From Outer Space

[star882]

If you're planning to regularly run it from batteries, you'll want hybrid digital for longer battery life.
QuickView - MAX9759 3.2W, High-Efficiency, Low-EMI, Filterless, Class D Audio Amplifier
Digital Amplifier Solutions - Analog-Input Class-D Speaker Amplifiers - TPA2039D1 - TI.com
Just two of the many choices available. Both will run from a regulated 5v or 3.3v power supply or directly from a single cell lithium ion battery.

[Feifer]

What's a good place to get these chips? My normal haunts of jameco and mouser have powerfully small selections when it comes to these.

[AndrewT]

Originally Posted by Feifer

This is what it's getting hooked up with if it's relevant...Music From Outer Space

what is the quiescent current draw of the WSG. That must be added to the quiescent current of the power amp.

BTW,
1W into 8r0 requires 4Vpk and 500mApk.
I suspect that 1W from a 9V battery is impossible or at enormous distortion.
What is the sensitivity of the 8ohm speaker you plan to use?

32ohm, or better still, 300ohm headphones that can produce >100dB from 1mW would be more suitable for portable use.

[Feifer]

Hey Andrew, thanks for the reply. Unfortunately I don't know the draw of the unit. I'll just have to test it once it's put together.

Correct me if I'm wrong (and I probably am, as I am quite new at this) but would a 16ohm speaker at 1w require 5.7vpk and 356mApk? You said higher resistance is better, but why? Is it to get the current down, and if so, is it for the sake of the amp or the battery? Would it be possible to put in two 9v in parallel to give more available amps?

I was thinking of building the amp at the bottom of this page...Little Gems
It says it'll produce about 1W, but I don't know if that's with or without the 10uF caps for the overdrive. I won't be using those because I want it clean. So the voltage gain will be 20x for each chip, plus something from the FET (I'm assuming). Would this be less than 1W? And could I run a 4ohm speaker with it like is says?

Sorry for all the questions at once, but I'm eager to learn.

[AndrewT]

Hi,
the reason for efficient drivers is to reduce the need for high power output.
Unless you go to switching amps then high power output generally equates to high quiescent current = short battery life.
Ultra efficiency comes from headphone drivers. 100dB/mW is almost common, particularly in the higher impedance headphones.

As a general rule the smaller the speaker, the less efficient it is.
An 8inch FRD could be 96dB/W @ 1m.
Whereas a 2inch full range driver might be 80dB/W @ 1m, i.e. it needs 20times as much power to reach the same SPL (volume).

[Feifer]

So I got a response on the specs of the synth I'll be using. The output is generally 600-700 mV but can spike at 1.5V and the current draw is about 4-6 mA. This isn't enough to drive a speaker alone so I'll still need the amp, but the datasheet for the LM386 says the input voltage should be +/- 400mV. Is there someway I can attenuate the output before it goes to the amp? And how?

Also I would appreciate it if someone would check my math on this. Lets say I can get the output down to 0.3V. To find the power output when I set the amp gain to 10 would be (0.3*10)^2 divided by the speaker impedance, lets say 4ohms, would = 2.25W. Is this right?