lm386 as preamp.

[Mnvizb]

Hello guys. I've seen Ruby on runoffgroove.com and was quite interested. You can look it up here Ruby

Could this amp be connected to another amplifier (used as preamp for more power and that nice overdrive) instead of speaker? Have any of you tried it? If not, what adjustments should be made (if even possible) ?

Thanks

[Nigel Goodwin]

You can simply attenuate the output of any power amplifier to feed the input of another amp (two resistors).

But it seems pretty pointless doing so, if you want a preamp, then just use a real opamp.

[Mnvizb]

I know that i could use an opamp, but i fell in love with this amplifier and would really love to have this nice overdrive and some more power. Maybe you could tell me how to connect those resistors and how to calculate the values? Will it be something like power sink?

[gootee]

Look up "voltage divider".

The two resistors go in series with each other, from your 386's output to ground, and you take the new (attenuated) output signal from between the resistors, i.e. the voltage across the resistor connected to ground.

The original output voltage gets divided between the two resistors in proportion to their values.

To get 1/4th as much voltage as the original signal, you could use 3k and 1k, or 30k and 10k, or 6k and 2k, or any pair with a 3-to-1 ratio of resistances. Note that 3 + 1 = 4, so taking the voltage across the "1" is 1/4th of the total.

You get to and have to choose in what range the R values should be. Too large and you pick up more noise. Too small and they overheat and start on fire, or the previous amp can't drive them well.

A potentiometer is a ready-made variable version of a voltage divider.

You might need an opamp configured as a unity-gain buffer (i.e. output tied to negative input), after the voltage divider, if you're powering another amp with the new output. But you can try it without one, first.

[mjf]

QUOTE Maybe you could tell me how to connect those resistors and how to calculate the values? Will it be something like power sink?

.....you can add a (trim)pot at the output as a volumecontrol.......as shown in the other examples/schematics. perhaps you try out 1k......10k

[Mnvizb]

I picked 4k and 10k for the voltage divider, connected them properly (wikipedia) and for testing i plugged in 9V battery. Checked the voltage of 9V battery - 9,12V, the voltage after divider was 8,85V What's wrong? According to calculations (both mine and the ones I found in internet Circuit Design Tutor) i should get about 2,5V.

mjf,
If tried adding pot how would i need to set up properly? Should I just set it at some value (to give like 0,15W instead of 0,9W(which I get from lm386)).

[mjf]

have you built this amp and does it work?
there should be no dc - volts at the output.........

suggestion: build in a res at the output (1k.......10k or so ) so that the dc - path is closed.
if the output of the ruby is too big for the power amp,change to a trimpot at the output...

[mjf]

......example

[Mnvizb]

I've built it, tweaked it a bit. Now the only thing I want is a line out (which I would also use as headphone output)

[mjf]

.......the lm386 is a (cheap) headphoneamp. you can use it with your headphone - if it is too loud ,build in a res in series (optional, a few ohms).