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Old 18th January 2015, 11:38 PM   #1
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Default Bi-amp for single mono speaker?

Hi, I have a stereo tube amp that can be run in mono mode. 25 watts per channel. I am building a single coaxial speaker to be used by itself (eminence beta-8cx and asd-1001). Can I run one channel to the woofer and the other channel to the tweeter (with high pass and low pass filters)? I have heard that tube amps shouldn't be run without a load on one channel - would this be okay since there is a load, even though it wouldn't be the same load?
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Old 18th January 2015, 11:52 PM   #2
kevinkr is offline kevinkr  United States
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It will be fine, just use a fairly large film cap in series with the tweeter to assure that no low frequency stuff is sent to the tweeter by accident. (Cap should be sized so that it passes more than one octave down with less than 3dB of loss to avoid interfering with your intended XO frequency.)
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Old 19th January 2015, 12:10 AM   #3
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Thanks for the reply. But yowser, I'm getting the feeling that I may be biting off more than I can chew here. I'm planning on ordering from parts express. If anyone can suggest specific parts to make this work, I'd be very grateful. I emailed eminence about the right crossover to select for this setup, and they recommended:

Eminence PXB2:2K5CX 2-Way Speaker Crossover Board 2,500 Hz

I'm not sure if that would be what I need now that I'm going to bi-amp. Maybe I need a low pass filter set to 2500 and a high pass filter set to 2500 and the capacitor you suggested?
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Old 19th January 2015, 01:54 PM   #4
Bigun is offline Bigun  Canada
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Welcome to the 'mono club' - I use a SE tube amp to drive a large mono speaker. It's a full range Audio Nirvana so I only use one channel of the amp. I see no issue at all having the other channel unloaded providing you don't feed it any signal - it's a Class A amplifier and without a load it will simply draw an idle current through the tube. It's wasted heat though.

I've no experience with line level cross-overs so I can't help on that part but would suggest you consider whether you need to put a low pass filter on the input to the woofer channel and a high pass filter on the input to the tweeter amp. If you can drive the main cone full range that would avoid the need for a low pass filter. And as mentioned, a simple cap in series with the input along with some shunt resistance to ground on the input would form a high pass filter for the tweeter amp. You may want to adjust the corner frequency of the filter by adjusting the shunt resistance or both the capacitor and resistor. Your amp likely has a shunt resistor on the grid of the input valve that you'll have to take into account.
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