An Old subwoofer amplifier requires a modification

[lycanlk]

Hi everybody,

I found this sub-amp from an old magazine. Basically it has 4 stages
stage 1 - input buffer
stage 2 - high pass filter
stage 3 - low pass filter
stage 4 - amplification section.

This is the most simplest sub amp i have ever seen. but it has a couple of drawbacks. Actually I dont understand the amplification area but 3 filters are quite simple to me. i want to use my own filters for this. but when I change it the amp is unstable. Can anybody explain the amplification area and any suggestions are most welcome ! I also wants to replace those old drivers and output transistors.

Another major drawback is at stage 1. That is 47K stereo to mono conversion will decrease the input gain when I plug this to my PC. But still I need the stereo input if I try to replace final stage feedback resistors 47k and 1k again the amp will be unstable .. sad sad..

Thank you !

[jaycee]

The output amplification is basically using the opamp as a front end, and then it has a CFP output stage that has gain in order to do voltage amplification.

Not really what I'd call robust enough for subwoofer duty!

[nigelwright7557]

Its not very hi fi, looks like class b so will crossover distortion.

Go for a better amplifier class AB and add the filters on the front end.

[lycanlk]

I agree that adding filters to a class AB amplifier will be better.. but can I do it that simple? You can see it uses only a few components. I just want to tweak this by replacing drivers and outputs to handle more power. But I don't know whether it is possible or not ???

[dangus]

A chip amp using the inexpensive LM3886 could be as powerful, lower distortion, and more robust. And those can be easily paralleled and/or bridged for much more power.

[lycanlk]

I can buy 3 pair of output transistors for that cost
It also needs more components than this.. but I can see its a good solution if this cannot be modified..
TIP 41c and 42c drives works for this and those are highly available.. any suggestions for outputs with that ???

I just need a simple, high power, low cost and low quality sub amp.. there are many high quality amps in this forum so this time I like to sacrifice the quality for simplicity and low cost.. may be this is a bad idea but take it as a very unique requirement..

[lycanlk]

regards,
Lycanlk

[jaycee]

The problem here is the output stage has gain. If you want to make it more powerful, youll have to start adding pairs. As soon as you start doing this, you make a CFP even more difficult to stabilise.

I might be tempted to suggest the use of MOSFET pairs if you really want more current drive - but you will not get suitable lateral MOSFETs cheap. Verticals will require a more complex drive arrangement and will sound dreadful in class B.

[lycanlk]

The problem here is I dont understand the amplification stage and what is CFP ? If you can provide me any source documents to read more about it, pls do.. then perhaps I can modify it myself.. It already has burnt 5 output pairs during tests.. LOL

Regards,
Lycanlk

[consort_ee_um]

See this link for a more sophisticated version of that design ie op-amp followed by gain stage.
A Paul Kemble web page - PW Texan amplifier and the PE Rondo.