| Low Pass [message #91156] |
Thu, 14 November 2019 07:39  |
Barryso
Messages: 207
Registered: May 2009
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Master |
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Discovered some very weird component matching issues when adding subs to the system.
My plate amp is mono and if you cross it over at 100 hz it sounds odd as mono doesn't work quite right at that frequency. Borrowed a Crown amp and some 100hz 2nd order low pass fmods to use the subs in stereo.
The fmods evidently do something weird with impedance. Simply putting the fmods on the rca plugs (the rca's that will be plugged into the sub amp) dramatically lowers the volume of the main speakers. There's also a frequency shift and the highs and lows seem to just vanish.
This happens before the cable and fmods get plugged into the sub amp. Just putting the fmods on the end of the RCA cable is enough to do it.
The tube preamp has a 200 ohm output impedance going into a solid state amp with a 10k input impedance. The same ugly trick happens when using a solid state preamp with an output impedance of less than 1 ohm. Perhaps it isn't quite so dramatic with the solid state preamp but it's still enough of a change to make the fmods useless.
The borrowed amp is a Crown XLS that has a built in 4th order crossover. The internal crossover is FAR better sounding than the fmods but it doesn't have the preferred 2nd order low pass.
So it's time for plan b. Parts Express has a speaker level crossover that's 100 hz lowpass and is 2nd order. How about a pair of these with a volume pot connected to the speaker outs of a nice Texas Instruments class d amp? Would it make a good sub amp? By putting the crossovers and volume control on the output side of the amp it should bypass the preamp/amp impedance issues.
https://www.parts-express.com/parts-express-100-hz-low-pass-8-ohm-crossover--266-446#lblProductDetails
Has anyone used these or have any idea of their sound quality? Is this a plan or is there a gotcha that I'm not considering?
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| Re: Low Pass [message #91162 is a reply to message #91156] |
Thu, 14 November 2019 10:56  |
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Wayne Parham
Messages: 18793
Registered: January 2001
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Illuminati (33rd Degree) |
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The problems you're talking about are pretty common. They're impedance-matching problems. The load of the passive crossover is interacting with the output impedance of the preamp. And those inline filters are really lossy too, so I don't like using them. I had hoped they would work, but they really don't.
I've found a few easy solutions:
1. The inexpensive second-order crossovers often used in car stereos
2. Amplifiers with built-in second-order tone controls
3. Mini-DSP
Try this out, Barry. It's an example of solution # 2. I think it will work out well for you. It's super easy to do.
Find an amp with built-in bass and treble controls where the low-pass function in the bass control is 100Hz, second-order. You just turn the treble all the way down and turn the bass all the way up, and the resulting transfer function works nicely for flanking subs. It's a happy accident, I suppose. Level-set the amplitude of the subs to match the mains and you're done.
One example is the Audiosource amp100VS. It's just a 50 watt amp, but that works well for people that have 10 watt SET amps for their mains. There are probably many other products of this type. If an amplifier has built-in tone controls that use second-order filters, and if the bass adjustment frequency is 100Hz or so, then the amp will work very well for flanking subs.
I've attached the spec sheet for this little amp. See the link below. You'll notice it lists the bass control as being a second-order 100Hz filter. That's what we want. Nice that the bass and treble controls are on the back too, 'cause that way it's out of sight. They're just set to make it a flanking sub amp.
Look for amplifiers like that.
When searching for amplifiers for your flanking subs, you can always try out a product and send it back if it doesn't blend well. Just open the box carefully and keep all the packing material so you can repackage it properly if it doesn't serve your purpose.
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