Audio Systems: Mono Amp single output, audio telex, mono amps

Question
Hi
I originally asked this q of Jerry Mael, who is not now avail. You may be able to help.

I am playing around with two mono amps. They are PA type amps, 250 watt and 120 watt. Made by Audio Telex(now called Australian Monitor).

The 120 watt has 6 switchable speaker outs, the 250w has only one. I can split the stereo signal to send one channel to each amp. But the speaker setup is tricky. Currently have two pairs of speakers with an A/B switch and I use it to run A + B. The 6 switchable amp can output 4 or 8 ohms(set with a jumper) or 100v.

I can send 4 or 8 ohms to each of the two on the right channel. But the amp with single out I need to send to the other two speakers and it only outputs at 4 ohms.

The question?? How to join the two speakers to one output and what happens to the impedence.



Thanks

Jerry's reply

John



Because It would depend on the impedance of the speakers and how they are wired.



Give me a list of the specifics for each speaker and I will try to help you.

......



The speakers are 8 ohms. They are Wharfdale 415s and ALH model 6.



I currently run stereo through a small marantz amp and run the 2 pairs of speakers thru an A/B switch.



I have been using one of the mono amps for guitar etc thru computer and Tascam US-122. Using only one speaker.

Just want to try the two amps to see if better for music.
Also not sure what effect on quality the A/B switch has.


Thanks

John  

Answer
I kind of got lost in the explanation, but here is the rule.

ONE:  Speakers in parallel will cut the resulting impedance in half.

TWO:  Speakers wired in series will double the impedance.

THREE:  Amplifiers should never have an impedance hanging on them that is less than what the amp is rated for.

FOUR:  It never does damage to an amp when the impedance hanging on its output is greater than the rating.  If the power rating is 100w at 4 ohms and you put an 8 ohm load on it the amp will smile and say thank you for not working me so hard.  Will it alter the sound quality? No.  But it will reduce, slightly, the amount of power you can deliver into the speakers.   But, because the higher impedance usually allows the power supply voltages inside the amp to ride a little higher, the amount of power reduction is not so much.

When you use the A + B, you are putting the speaker loads in parallel.  So, the amp sees a load that is half the impedance of the two loads individually. If the speakers are 8 ohms, in parallel you would have  4 ohms.

Hope this helps you figger (that's how we spell it down south!) it out.

C