Simple Amp for active subwoofer

[Spiro007]

I am looking for a simple cheep amp to make my subwoofer active.


I have designed this very simple amp using darlington transistors such as BDV64 bdv65. but would like to use mosfets instead for a more rugged design.
can any one help?

maybe modify the design to use mosfets instead of bipolar.

[MikeB]

Hi !

Your circuit is a bit too simple, you have no biasing for outputstage
at all. Even if it's "only" for a subwoofer, you can't work without it.
You could try at least 3 diodes between the bases of the bjts.
For mosfets this issue is even bigger, they have a much bigger
threshold for biasing.

Mike

[ingrast]

How much power are you planning to handle?

What impedance driver do you have in mind (4 - 8 ohms)?

Do you have an actual Subwoofer output available (e.g. DVD player) with channels mixed and filtered down?

As a matter of fact a chip amplifier will probably be actually simpler to build and cheaper to boot!!

Rodolfo

[acenovelty]

http://www.partsexpress.com/pe/showd...number=300-792

$89.95 US

ready to go.

Prosit

[Spiro007]

Ok i have corrected the schematic. it works fine in the simulation. the output transistors are actually going to be BDV64 BDV65
but the problem is that i really want to use flat pack transistor and also keep the cost down. so the BDV64/65 are onyl about $4 australian each the whole PCB and components will only cost about $15 austalian thats about $11US.
I am going to biulding this amp this wekend just for fun but if anyone can help with a similar design using mosfet , i will go for that design. because the BDV64/65 do not have a good Safe operating area compared to similar priced mosfets.
Any help would be apreciated as I have never designed or biult a mosfet AMP.

[MikeB]

Hi !
One more thing: You also should place a 100kohm resistor from
base of input-bjt to ground, and add a inputcapacitor (~4.7uF mks
or 10-22uf electrolytic). Or you might get some strange inputoffset,
depending on the subwoofer-out from preamp.

Did you zoom into crossoverregion ? (In oscilloscope)

Mike

[Spiro007]

Thanks Mike
I have added the input resistor and capacitor as you mentioned.
Actually i have just made the PCB, it is ready for drilling I will know how it sounds over the weekend I hope.
here is a picture of the PCB. the transistors mount flat on the bottom of the board so that the board and the transistors mount onto the heatsink easily.
board is very small only about 9cm x 4 cm or less than 4 inch x 2 inch.
if any one is interested i will post more info after a tial run.
I was hoping to get close to 100 w RMS into 8 ohms with mosfets but I will probably only get about 70 w RMS with the darlingtons.

[Spiro007]

Also the new schematic

[kinser]

Spiro007,
My name is David and im 14 years old,
I like your amp design, its seems pretty simple
I have some questions:
1.Can I use mjw21195/6 on the output?
2.Can the amp handel 4 ohm load? If it can then how much power will it deliver?

Thanks,
Kinser (David)

[Spiro007]

Hi david.
Thats some nice transistors you have there. I bought a kit amp using those devices and it sound very good.
The amp i designed doesnt have enough current to drive the mjw21195 devices , but if you added an extra transistor to each of the mjw21194/5 's it would probably work.
i have attached a pdf to explain. but since the mjw21194/5 are quite expensive i would experiment with some cheeper devices, also the sound quality of my amp is only good upto about 1khz.