Crossover appears to be off on PE amplifier

[owdi]

I took some nearfield measurements of my subwoofer today, and found the reason I haven't been happy with it. The lowpass crossover point appears to be completely off. Here's the modeled response:

Blueprint 1001 in .6 cu. ft. sealed (qts=.8) w/ 160hz lowpass 2nd order xover.



Measured response. The lower curve is with the amplifer crossover set at 40hz, the higher curve is when the crossover is set at 160hz.



This is bad, and it explains why I've had such trouble integrating the subwoofer with my new HiVi towers. The amplifier is the Partsexpress 250watt plate amplifier, without bass boost. No matter what I set it to, I can't get the 100hz crossover frequency I need.

Anyone know if I can modify the crossover in the PE amplifier?

Dan

[Volenti]

Do you have any way of electricially measuring the output of the amp? to confirm that the X-over is screwed up?

You could also try running the sub full range off one channel of your mains amp and do the nearfield measurements again, see how that compares to when it's run off the plate amp.

[owdi]

Originally posted by Volenti
Do you have any way of electricially measuring the output of the amp? to confirm that the X-over is screwed up?

You could also try running the sub full range off one channel of your mains amp and do the nearfield measurements again, see how that compares to when it's run off the plate amp.

I need to buy a large resistor before I can measure the output of the amp. I will try running the sub full range off my receiver, but I'll have to take apart the subwoofer first.

Anyone else measure output from a subwoofer powered by a PE plate amp?

Dan

[bwaslo]

If you are only measuring the frequency response, you don't need an output load resistor on the amp. Any decent amp made within the last 30 years will be stable without a load resistor, and should have the same response as when loaded. Only the maximum clipping level should be affected.

You do want to put a series resistor from the power amp output to your soundcard input to avoid blowing the sound card input. Something in the range 22k to 68k should do.

The curves you publish could be due to the response of the sub driver and box. Both the sub and the amp/crossover are involved in making the complete frequency response.