Setting the attenuation of an Extron amp into a pair of Chora 826

[morethinkingcapacityneede]

Hi there. I'm an inexperienced audio person, and have recently set up an Extron XPA 1002 amp (non-plus version, so rated at 60W @ 8 Ohm and 100W @ 4 Ohm) driving a pair of Chora 826 (8 Ohm nominal, 2.9 Ohm min). Actually chose both of these by reading this website, so thanks! (The DAC is the newly release SMSL Sanskrit 10th MK III, if you are curious.)

My question is that about setting the attenuation level on the amp, so that clipping never occurs. The procedure is described on page 13 of the amp manual, https://media.extron.com/public/download/files/userman/XTRA_Series_68-2354-01_C.pdf. Essentially, set input signal and amp attenuation to max, then slowly reduce attenuation until distortion/clipping occurs, at which point increase attenuation until effects disappear, and finish.

My main question is, how safe is this for the speakers? And can the procedure be done with minimal risk of damaging the speakers? The amplifier documentations talks about auto-clipping protection (quoted below). Does this mean the speakers are unlikely to get damaged if the amp limits are reached? Also, I'm still not sure if this particular hardware combination would actually damage the speakers. They are rated to 250W, so can a 60/100W amp ever damage them even if clipping?

"Automatic clip limiter — Detects actual onset of clipping by comparing input and output waveforms. Gain is automatically reduced without audible artifacts to protect speakers from clipping distortion."

Can I get away with testing this without using the speakers? Could I use some high power 4 Ohm or 8 Ohm resistors? Perhaps without the variable impedance the attenuation needed will be different?

If I do decide to proceed with the test, what's the best audio source to use? I guess some kind of frequency reference would be better. Possibly multiple frequencies? Something with a high dynamic range? And what is the best way of detecting the distortion (by ear)? Will the distortion be obvious? The amp has some LEDs which turn on if clipping occurs, is that enough of an indicator?

Any help would be greatly appreciated. Thanks!

 

[Blumlein 88]

Welcome to ASR.

I didn't see a gain spec in the manual. Here is what I'd do.

Set your DAC output to max. Feed it a 400hz -20 db signal. This is 1/10th max voltage for the DAC. Start with the amp turned down. Max attenuation.

8 ohms at 60 watts is just about 22 volts. Since you are feeding 1/10 max with the -20 db signal. Put a voltmeter across your speaker leads and raise the level (reduce attenuation) until you read 2.2 volts at the speaker terminals. If your DAC were using a 0 db level the amps would be feeding your speakers 22 volts or around 60 watts.

This method should be safe, and put you very close.

 

[Blumlein 88]

Ok, I did find the gain spec. Procedure I described will work in any case. Depending upon particulars, I'd expect you end up with attenuation of 6 db or so.

 

[morethinkingcapacityneede]

Thanks very much for the reply.

This makes sense to me. I will give it a go, and report back the attenuation setting I end up with.

 

[morethinkingcapacityneede]

So, I followed the procedure above, and I chose the setting in the photo below. With that particular setting, I was reading a voltage of 1.9V/2V, the next one along gave 2.3V/2.4V. (apologies for the low precision of the readings, I must invest in a better multimeter)

For some reason, this particular instance of the amp does not have a finer grain numbered scale for the attenuation, like illustrated in the manual and photos online (it also does not have the impedances printed for the outputs). In any case, the tick would be end up being between "4" and "6".

I might repeat the measurement once I have a better multimeter, and see what the notches really are.