Dividing voltage when it exceeds 9v?

[Jussi]

Hey,

I have a circuit which has one part which works with 9-18 volts (preamp with JFETs)  and other part which works with 9-12 volts (LM386 power amp). So I have to decrease the voltage for the power amp part of the circuit if its too high.

Is it possible (without too many parts) to make something like that when the voltage is over 9 volts it starts limiting it a little until at 18 volts it is limited to 12 volts? I checked this article about capacitive voltage divider but it would limit the voltage at 9 volts too.

I am doing this because my device will have two 9 volt batteries in series and I want it to work even when the batteries are almost dead - as long as possible - so I can't just have a resistor which would always decrease the voltage by 6 volts because it wouldn't work when the batteries are in example at 5 volts only (10 volts would be 4).

[Seljer]

This is exactly what you could use a zener diode for

Or one of the simple 3 pin voltage regulators (7812 for 12 volts, or the LM317 for adjustable voltage) but those require you have more than a volt of overhead for the regulation to work.

[Mark Hammer]

If you can, try to use somethng like an 8-pack of AA batteries.  This will give you a nice robust 12VDC for the amplifier AND the preamp.  It will be bigger than a pair of 9v batteries, but it won't take up a lot of room.  The improvement in the performance of the 386 circuit will be VERY evident, and likely more evident than any decline in performance for the JFET preamp when working from 12 vs 18v.  Equally relevant, because the power amp will not drain the 8 penlights nearly as fast, you will continue to have a stable 12v for the preamp for a long period.  If the JFETs require some biasing adjustment, you'll appreciate that.  Finally, it makes it a whole lot easier to know when it's time to change the batteries because the whole thing will sound awful.

Black plastic holders for 4 AA cells are usually easy to find than holders for 8 cells, plus you can attach them to a chassis easily since they have flat backs, although a single holder for all 8 has a certain convenience too.

[Jussi]

Yea maybe I should use 12 volts for everything. Though I only found 4 and 8 packs which are parallel connected 

[Mark Hammer]

If the battery holder has a 9v-type clip on the top for connecting, there is no law that says you can not solder the 9v connector clips in series with each other so that the 6v in each holder sum together.

[Jussi]

I meant that they sell 4 packs which has 4 AA batteries in parallel. Putting 2 of those packs in series will be 3 volts. Its weird they are in parallel. Maybe they aren't really - gotta ask

[Mark Hammer]

Really?  That's weird.

Get yourself two of these.  they have the batteries in series.  They may be lined up beside each other, but the connections are in series.: http://www.taydaelectronics.com/connectors-sockets/battery-connectors-holders/aa-battery-holder-4.html

[EATyourGuitar]

these old aria active guitar pre-amps had two 9v batteries in parallel driving the 386 speaker circuit right off the datasheet. only it was intended to just drive cables so I'm not sure if they thought it would consume lots of current or they wanted a constant current source as the batteries die.

[toneman]

You could run 2 LM317 adjustable regulators.  They are 3-terminal devices and cost about 30cents each.  You need a 240ohm resistor and a trim pot of about 2Kohm.  See the national datasheet.  Very very cheap for multiple voltages in the same box.
Or where you want to limit the voltages, say, to a BBD.  Batteries suck!  Errr unless it's for an electric vehicle  :-)

[Jussi]

I asked and the 8 packs aren't really parallel connected (misinformation at webshop) so I went there and bought one. It sounds good with 12 volts. I have three J201s cascaded in the preamp and there is two diodes working as a audio limiter to decrease volume when gain is cranked, they give some nice distortion too. I had a problem with not getting enough clean sound volume without LM386 clipping a lot when cranking my gain knob so I added the audio limiter because I couldn't just have a resistor to ground decreasing the signal always and it would have beeen stupid that you'd have to turn volume down if you wanted some distortion without lm386 clipping.

Some testing to old computer speaker (4 ohm, 20w, 4"):

[Mark Hammer]

For a small speaker in a poor cabinet, it sounds decent.  Put the same speaker in a bigger, more solid, cab and you'll get some better bass and more volume.

The 12V makes a big difference, eh?

[Jussi]

Thanks. Yea, I didn't notice any difference between 18 and 12 volts with the preamp.

Maybe I should socket some of the capacitors on pcb if I need to adjust bass/treble when I've finished the cab...