Need help with calculations please.

[Wynand]

First of all, I didn't search. I would not know how to term the search.

I've been looking at this circuit:

http://www.mhennessy.f9.co.uk/preamp/analogue.htm

Specifically the headphone section.

My goal is to make an headphone amp for a guitar amplifier.
The problem I've got is that my headphones are 8 ohms.
To make matters worse I'd like to parallel them to make the circuit less complicated i.e. using only one op-amp channel.

Thus I have a load of 4 ohms.

This circuit looks to me like it should be used for high-impedance headphones. I say this because of the small-signal transistors used and the 47 ohm resistor at the output.

I don't want values (would be nice), but I'd like it if somebody can walk me through the steps to calculate the values.

I won't mind editing the circuit a bit, but would like to keep it simple.

I was planning on using power transistors for the output, something like 2SD525 and 2SB595, which I've got. MJE29955/3055 is also an option(?).

Thanks
Wynand

 

[Wynand]

by the way...

BTW this will be my first circuit using transistors and I'd really like to get into SS circuits and to be able to understand and analize them.

 

[richie00boy]

You could just drop in more powerful transistors such as BD139/BD140 and reduce the 47 ohms to something smaller, maybe 4.7 ohms or even include a switch to allow bypassing to cater for different taste. The rails could be reduced to 9V each or maybe a little lower.

I have designed a headphone driver that is similar to this design, but features some notable improvements IMO. PCB layout is available if you are interested. Just not got round to putting it on my website yet.

 

[Wynand]

I would appreciate in very much. Thank you.

 

[Wynand]

As for the rails, I've already got 15v rails for other op-amps so no need to adjust them.

 

[richie00boy]

It's just that with 15V rails you are simply wasting power as you will never need to go anywhere near that when driving headphones. In fact with 15V rails if you did happen swing enough into your 4 ohm paralleled headphones you would probably break the regulators.

 

[Wynand]

in THAT case....