Volume control for kit amplifier

[robm]

I just built this little 13.5W amp from jaycar http://www.jaycar.com.au/productView.asp?ID=KJ8076&keywords=kj8076&form=KEYWORD

It works a bit too well as the volume is a bit overpowering.  I have just put this amp in line with the woofer in the cabinet and am running a 10" car 4 ohm sub with the amp.  I would like to be able to control the volume of the sub separate to the mid and tweeter .

I have purchased a 100k potentiometer and thats where my electrical knowledge runs out.  The bloke in Jaycar said i'd need a 100k resistor before the pot - with and without the resistor, the sub makes a hideous buzzing noise as soon as connected (without any input signal) whereas without the pot at all its perfect (just a really low hum - almost inaudible).

I have the pot in line before the amp using the signal from the + speaker wire inputting to the amp.  Input is tab 1, output tab 2 and earth tab 3.

Is there somewhere i've gone wrong or do i need a different resistor somewhere.

I have a few 15k resistors and read somewhere it makes better volume control if you put this between output and GND on the pot - made no difference to the buzzing.

Thanks

Rob

[ajlaird]

Can you provide a schematic? I think this would help us to see what is happening.

[beaky]

I have the pot in line before the amp using the signal from the + speaker wire inputting to the amp.  Input is tab 1, output tab 2 and earth tab 3.

silly question but have you also connected the " - "  speaker wire from the pin to the earth or " - " on the amp?


also do your experimenting with a cheap mp3 player, connect it to the amp first and get it working before hooking it up to the pinball, that way if you have made a mistake or there is a fault in your amp and it is throwing voltage out the input you won't go damaging your sound board on your pin.

what are you using for a power supply?

and as ajlaird said a schematic would help 

[robm]

I've just whipped up a schematic using some fancy CAD software.....

I am wondering if my problem lies with the "-" speaker input - does this need to go to the earth for the potentiometer?

Yes,  did some mucking around with an old radio 1st.

Currently i am using a 12v battery sponsored by work as a power supply, but intend to use the 12v source at the coin door interface board (from an Indiana Jones - some other guy has done this (http://www.edcheung.com/album/album06/Pinball/ij2.htm - at the end of the page).

**Just looked at my diagram and the pot numbers should be reversed ie: 3 at the top and 1 at the bottom**....dodgy software!!

[Marty Machine]

Aaahhh the old "PencilCAD" i use that too sometimes hehehehe

Have you confirmed the 12V from the coindoor is CLEAN DC? it might be very unregualted DC (lots of AC hum on it).

Grab a multimeter, and measure the 12V using the "AC volts" setting, and hopefully it wont read anything much above 0v.

MM.

[beaky]

I've just whipped up a schematic using some fancy CAD software.....

I am wondering if my problem lies with the "-" speaker input - does this need to go to the earth for the potentiometer?

Yes,  did some mucking around with an old radio 1st.

Currently i am using a 12v battery sponsored by work as a power supply, but intend to use the 12v source at the coin door interface board (from an Indiana Jones - some other guy has done this (http://www.edcheung.com/album/album06/Pinball/ij2.htm - at the end of the page).

**Just looked at my diagram and the pot numbers should be reversed ie: 3 at the top and 1 at the bottom**....dodgy software!!

first try disconecting the ground from the pot to the cab earth: if this solves your problem then try hooking the pin you had from your pot to cab earth to the spk -
make sure that you have the 100k resistor in line  because if you dont when you turn the pot to the volume low side you will be  shorting the output of your pin amp to ground and will probably fry some components on the sound board.

Remember electronic components are like a genie in a lantern, once the smoke has escaped neither of them will work

[robm]

Marty - at the moment i am still using power from a 12v battery - so i know the input power is fine.

Beaky,

Disconnecting earth made no difference, but connecting earth from pot to the speaker "-" input reduced the buzz a fair bit....but not enough - when sound is playing you can't hear much, but when it stops the hum is still quite noticeable.

[Marty Machine]

Getting smoke out of a chip is EASY, getting it back in is HARD.   

MM.

[Marty Machine]

Looking at your diagram again, are you taking the audio from the cabinet "speaker" wiring? (as you labelled "cab speaker signal).
This would be too loud to feed 'into' the jaycar amp, overdriving and passing any minor hum thru as LOUD hum....

MM.

[robm]

Yes i was wondering if that was the problem - which is why i put the potentiometer in line - but maybe this is not enough decrease in signal?

If this is the case i guess the two options are

1) convert speaker level signal to line level (i have a line level convertor)

or

2) try to pick the signal up off the audio board before the pre-amp - would have to check to see if this is possible.

[ddstoys]

If you have a speaker line to line level id throw that in and see how you go

[beaky]

If you have a speaker line to line level id throw that in and see how you go

I agree, it should also have some extra filtering in that line level convertor, i had a quick look at the kit and there is very minimal filtering, so any noise would be amplified as well

[Marty Machine]

2) try to pick the signal up off the audio board before the pre-amp - would have to check to see if this is possible.

Yep, you definately need to tap off the audio (line level) that is feeding 'into' the pinball amplifier.

This signal preamp signal would be 100's mV (milli-volts) upto 1volt at most, and the speaker drive output would be peaking up around 12volts (based on it running on a single 12v supply).

12v is far too much level to put into a preamp input.

If you have a diagram of your pinball soundboard, we can show you where to tap off from....

MM.

[Geoffh]

These guys sell specific subwoofer and sound kits for pinballs

Here are some of their diagrams if of any help

http://www.decoratorsupply.com/pinball/installation/index.htm