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-   -   Amplifier's impedance effect on XO (http://www.diyaudio.com/forums/multi-way/93104-amplifiers-impedance-effect-xo.html)

K-amps 29th December 2006 09:28 PM

Amplifier's impedance effect on XO
 
I have been playing around with some series and paralell crossovers... However in order not to inadvertantly blow my power amp (as I switched wires and caps around) I installed an 8 ohm resistor in series with the amp's ouputs.

What now I am unsure of, and has not been covered in the readings i have done on the forum is the effect of amplifier output impedance on the XO or even the XO point/freq.

In this case the crossover see's the amplifier having an OP imp. of around 8 ohms.... since the circuit (amp + crossover + tweeter) has an 8 ohm in series with the crossover... I cannot but think it affects the XO point.

There is no resistance once the crossover circuit begins (to eliminate L-Pad type variables in the response I am looking for).

What about tube amps?

What do you guys think?

Davey 30th December 2006 12:24 AM

8 ohms is a significant amplifier output impedance. It will most likely skew the transfer functions of your crossover by quite a bit and also add a significant insertion loss. Any crossover design that you might perform with this resistor in place would be invalid.

Cheers,

Davey.

audio-kraut 30th December 2006 12:37 AM

http://sound.westhost.com/biamp-vs-passive.htm


did you read this re actual value of amp. impedance as seen from the speaker/xover system?

K-amps 30th December 2006 02:40 AM

Quote:

Originally posted by Davey
8 ohms is a significant amplifier output impedance. It will most likely skew the transfer functions of your crossover by quite a bit and also add a significant insertion loss. Any crossover design that you might perform with this resistor in place would be invalid.

Cheers,

Davey.

Thanks... how can I factor the XO point or calculate it based on the 8 ohm series resistor. As far as insertion loss, let's disregard that for now...

so lets say a simple 1st order cap HF crossover for a 8 ohm driver at 3kHz is for example 2.2uF, then how will a resistor afftect the XO point (and maybe phase) if placed:

1) Inside the cap/driver loop (this is ez... the Xo will be 1.5kz with about 3db loss) but what about the following.
2) Outside the cap/driver loop.

K-amps 30th December 2006 02:53 AM

Quote:

Originally posted by audio-kraut
http://sound.westhost.com/biamp-vs-passive.htm


did you read this re actual value of amp. impedance as seen from the speaker/xover system?


Nice read, though does not answer te question (unless you want to rid me of using passive networks altogether. ;)

audio-kraut 30th December 2006 05:13 AM

Quote:

unless you want to rid me of using passive networks altogether.
one can always try, can't one:D

I like active....

K-amps 30th December 2006 05:18 AM

Well so did I... till I got audible noise and hum in my system by inserting it in. i have behringer CX-3400.

roddyama 30th December 2006 05:23 AM

If you assume the output impedance of the amp is very low and purely resistive, the series resistance will just be a frequency independent voltage drop. However, as the output impedence increases, and if there is capacitance and/or inductance in the output, it will become more frequency dependent and began to have an affect on the crossover.

K-amps 30th December 2006 05:26 AM

Quote:

Originally posted by roddyama
If you assume the output impedance of the amp is very low and purely resistive, the series resistance will just be a frequency independent voltage drop. However, as the output impedence increases, and if there is capacitance and/or inductance in the output, it will become more frequency dependent and began to have an affect on the crossover.

Thanks.

Will the affect only be attenuation at that freq? Or will it affect the XO point, thats all I need to know, Will it change the XO frequency?

audio-kraut 30th December 2006 05:30 AM

Never had a problem this way. Started with a marchand xover, wenr from there all digital to behringer deq/dcx. Total quiet when no signal.
Never had a hum in active - I ran both three and two way fully active. Neither in analog, nor in digital.
The only problem is the crackling that a few dcx can exhibit - see the thread at diy.


To stick with your question - looking from amp to speaker, the network sees the speaker impedance. The amp impedance does not play a role being below 1 ohm anyawy (maybe not with tubes)

The impedance that the speaker sees through the network towards the amp only influences the back emf, that now can flow towards the amp, no damping factor stopping it.

But from what I can see, that does do nothing to influence the xover point.


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