Home » Sponsored » Pi Speakers » Low Pass
Low Pass [message #91156] Thu, 14 November 2019 07:39 Go to next message
Barryso is currently offline  Barryso
Messages: 66
Registered: May 2009
Viscount
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?
Re: Low Pass [message #91162 is a reply to message #91156] Thu, 14 November 2019 10:56 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 17598
Registered: January 2001
Illuminati (33rd Degree)

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.
Re: Low Pass [message #91167 is a reply to message #91162] Thu, 14 November 2019 16:44 Go to previous messageGo to next message
Rusty is currently offline  Rusty
Messages: 281
Registered: May 2018
Location: Kansas City Missouri
Grand Master
I've recently wondered too about a 12db crossover device having gotten a couple sub drivers recently. I've considered the mini dsp, but for the cheaper 2x4 the output is only 0.9 volts which probably wont drive the amplifier to full volume. Moving up to the HD 2x4 will give 2.0 volts, but is double the price, $205. Thus being a budget minded fellow. I wonder if this product would fill the bill. It has a low level output and a mono sub output. Wonder if this would accommodate 2 subs.
https://www.parts-express.com/rolls-sx45-stereo-two-way-mini-crossover-w-sub-output--245-1184
Re: Low Pass [message #91168 is a reply to message #91167] Thu, 14 November 2019 17:39 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 17598
Registered: January 2001
Illuminati (33rd Degree)

The 100Hz at 12dB/octave (2nd-order) slope recommendation is specifically for flanking subs, which are a type of multisub arrangement. Distributed multisubs are usually run lower and with a quicker slope, often 60Hz at 4th-order or 80Hz 4th-order. The best setups run both, with flanking subs up front by the mains and distributed multisubs at the other end of the room.
Re: Low Pass [message #91175 is a reply to message #91168] Sat, 16 November 2019 08:32 Go to previous messageGo to next message
Rusty is currently offline  Rusty
Messages: 281
Registered: May 2018
Location: Kansas City Missouri
Grand Master
That's my aim, flanking subs. So I'll give that Rolls crossover a try using the stereo low pass. For distribution subs using plate amps, whats the best method of running a signal to it? Doesn't running a long interconnect cause problems? I wondered if I should keep the plate amp that has a linkwitz slope close to the preamp output for that reason. And run speaker cables to the sub.
Re: Low Pass [message #91176 is a reply to message #91175] Sat, 16 November 2019 11:13 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 17598
Registered: January 2001
Illuminati (33rd Degree)

Very good. Please report back and tell us your experience with the Rolls crossover. I am always looking at options for flanking subs, so I'd like to grab one and check it out.

As for distribution, obviously balanced pairs are best, but the devices we're talking here are unbalanced. And you're right that unbalanced lines are more susceptible to noise, so length can be a potential problem. Of course, you can always keep the amplifier close to the preamp and crossover, and run the high-level balanced speaker output lines the distance. Then all you have to worry about is the resistance of the speaker cable, and that's rarely a problem in a home.

Another option is wireless connections. I wouldn't use an inexpensive wireless connection for mains, but I've used wireless distribution for surrounds and subs and been happy with it.
Re: Low Pass [message #91177 is a reply to message #91176] Sun, 17 November 2019 09:09 Go to previous messageGo to next message
Rusty is currently offline  Rusty
Messages: 281
Registered: May 2018
Location: Kansas City Missouri
Grand Master
That's an interesting aspect. Does a bluetooth transmitter receiver have any issues with a lag time with signal? Or interference with wifi? That would be a nice option to running speaker cables from one end of the room to the other.
Re: Low Pass [message #91181 is a reply to message #91177] Sun, 17 November 2019 14:51 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 17598
Registered: January 2001
Illuminati (33rd Degree)

The ones I've used haven't had any problems with interference or latency. Those kinds of problems would be show-stoppers to me.

You can always give one a try and if it doesn't satisfy, return it. That was my approach. I tried hanging some surrounds and connecting them with a little Rocketfish transmitter/receiver system. I figured it it suffered from dropouts or didn't sound good, I'd send it back. But it worked just great.

Re: Low Pass [message #91186 is a reply to message #91181] Sun, 17 November 2019 17:58 Go to previous messageGo to next message
OutOfSpace is currently offline  OutOfSpace
Messages: 4
Registered: June 2017
Esquire
One trick I read about and have used for guitar cables is to used balanced cable (microphone wire with two conductors + shield). The trick is to only connect the shield at one end, ideally the end with the best ground. This way, the any noise would get grounded and the signal would be cleaner. Not balanced cleaner, but better than carrying the signal on the shield, which is where the noise would be induced.
Chris
Re: Low Pass [message #91187 is a reply to message #91186] Sun, 17 November 2019 18:39 Go to previous messageGo to previous message
Wayne Parham is currently offline  Wayne Parham
Messages: 17598
Registered: January 2001
Illuminati (33rd Degree)

I agree on both counts, the use of balanced pairs when possible and the attention paid to reducing the likelihood of ground loops, which is why you only connect the shield at one end.

But sadly, neither is an issue for Rusty, who has devices that use unbalanced RCA connections.

And I wonder about Barry, who started this thread. His problems were mostly impedance-matching and lossy inline filters. How's it going there, Barry? Did you find a solution that worked for you?
Previous Topic: New(ish) 4 pi's
Next Topic: Three Pi and Three Pi Sub Plans
Goto Forum:
  


Current Time: Thu Dec 05 18:54:02 CST 2019

Sponsoring Organizations

DIY Audio Projects
DIY Audio Projects
OddWatt Audio
OddWatt Audio
Pi Speakers
Pi Speakers
Prosound Shootout
Prosound Shootout
Smith & Larson Audio
Smith & Larson Audio
Tubes For Amps
TubesForAmps.com

Lone Star Audiofest