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Old 24th December 2012, 01:40 PM   #1
pdm4606 is offline pdm4606  United States
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Default x'over for a sub?

I want to have a crossover for my sub. I have a Dynaco PAT-4(mod) preamp feeding a
Yamaha M-50 power amp. I have my sub being fed by using a "Y" between the pre and power amp. Is there a reasonable x'over to use to prohibit the lows from the M-50?
Something that I would insert at the "Y" point to keep the lows going to the sub where they belong. The impedances between the pre and power are at around 50k. Or should I say the power input is at around 50k.
I would like to x'over at 100hz to 140 hz.

Paul
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Old 24th December 2012, 01:49 PM   #2
Mooly is online now Mooly  United Kingdom
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I've moved this to Analogue Line Level as it seems more a "line level" type crossover you are after
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Old 24th December 2012, 02:21 PM   #3
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Hi Pdm4606,

I am not very sure if this will serve your purpose or not but I think you could have a quick look at this post and the successive ones that Apex kindly shared with us.

Mic, Line, EQ... Preamps

All the very best.
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Last edited by csom; 24th December 2012 at 02:25 PM.
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Old 24th December 2012, 09:56 PM   #4
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Quote:
Originally Posted by pdm4606 View Post
I want to have a crossover for my sub. I have a Dynaco PAT-4(mod) preamp feeding a
Yamaha M-50 power amp. I have my sub being fed by using a "Y" between the pre and power amp. Is there a reasonable x'over to use to prohibit the lows from the M-50?
Something that I would insert at the "Y" point to keep the lows going to the sub where they belong. The impedances between the pre and power are at around 50k. Or should I say the power input is at around 50k.
I would like to x'over at 100hz to 140 hz.
A simple "Y" made with only wires will give you mono sound through the M50 also, so I assume you're including some interchannel isolation.

To calculate the value of a series capacitor with a rolloff frequency of F, we use the formula C=1 / 2piFR, and here R is 50K (close enough). So for 100Hz, C is about .032uF if I did my cypherin' right.

All good fortune,
Chris
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Old 25th December 2012, 07:11 PM   #5
pdm4606 is offline pdm4606  United States
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Quote:
Originally Posted by Chris Hornbeck View Post
A simple "Y" made with only wires will give you mono sound through the M50 also, so I assume you're including some interchannel isolation.

To calculate the value of a series capacitor with a rolloff frequency of F, we use the formula C=1 / 2piFR, and here R is 50K (close enough). So for 100Hz, C is about .032uF if I did my cypherin' right.

All good fortune,
Chris
I should have explained better.
I have 2 "Y"'s connecting the 2 channels. One for each channel.
The x'over would have to be of course a 2 channel suggestion or I will build 2 .
Thanks for the quiry;

Paul
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Old 25th December 2012, 08:01 PM   #6
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If I'm understanding correctly, what you want is a high-pass filter for the L and R speakers?
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Old 25th December 2012, 09:21 PM   #7
pdm4606 is offline pdm4606  United States
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Default high pass

I guess it would be a high pass going to the L and R speakers being fed by the Yamaha M-50.
That would direct the lows to the sub and keep the highs above 100hz to 120hz going to the Yamaha M-50.
I can't believe with all the misc. stuff available that there isn't a box that will do a high pass for the M-50 and a low pass for the sub. I just feel that a simple "Y" cable splitting each channel as the only method of crossing over for the sub. to be too simple. Doesn't that produce phase shift? Certainly (?) creates an impedance miss match due to the two circuits on each channel sucking up the signal from the Dynaco pre.

Paul
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Old 26th December 2012, 01:46 AM   #8
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There's nothing inherently wrong with simple. The Y splitter won't create any phase shift - that's what an actual crossover will do. The Y puts the two amps in parallel, meaning that current will be divided but voltage remains the same. There is no impedance mismatch to be concerned about, with the possible exception of the input impedance and/or sensitivity of one amp being much greater than the other.
There's more than one way to do it... a cap on the input of the Yamaha for a purely passive solution, or an active high-pass filter, or a crossover of the type used for bi-amped systems (which will likely steepen the sub's cutoff). From your description it seems like your sub is active with its own summing/filter circuitry.
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Old 26th December 2012, 06:50 PM   #9
pdm4606 is offline pdm4606  United States
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Default thanks

Quote:
Originally Posted by sofaspud View Post
There's nothing inherently wrong with simple. The Y splitter won't create any phase shift - that's what an actual crossover will do. The Y puts the two amps in parallel, meaning that current will be divided but voltage remains the same. There is no impedance mismatch to be concerned about, with the possible exception of the input impedance and/or sensitivity of one amp being much greater than the other.
There's more than one way to do it... a cap on the input of the Yamaha for a purely passive solution, or an active high-pass filter, or a crossover of the type used for bi-amped systems (which will likely steepen the sub's cutoff). From your description it seems like your sub is active with its own summing/filter circuitry.
You are right. It just seems so simple to have it hooked up like it is now.
Sometimes our hobby makes the most simple complicated.
I was sure there was a better way. All I ever read was the hookup as is now.
I was sure there was a problem.
The JBL sub has an adjustable x'over point and level and phase switch. All issues covered.
The M-50, of course, has level adjustment for input. Right now the both levels are at mid for the sub and the M-50.
Thanks;

Paul
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Old 26th December 2012, 07:08 PM   #10
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Hey Paul, take a deep breath, it should be pretty simple.

It sounds like your JBL sub already has crossover, so that's taken care of.

But of course you want to cut some bass and power out of the satellite speakers. You can:
- Use some active crossover to feed the M50 only, which is great, but I'll leave to others to reply.
- Make/buy a passive crossover in-line with the M50. This is just like designing a "cookbook" passive crossover for a speaker, "cookbook" assuming the impedances are resistive.

Some points to consider, once you have decided your crossover frequency*.



1) A steeper slope will reduce the cone motion of the speakers more. But, it may cause more phase problems.

2) A steeper slope will NOT reduce the power being transmitted through the crossover. That is because although a steeper slope cuts more power below the crossover frequency, it actually lets through more power right above.

So really, I'd say you can probably just use a series capacitor. You can get premade little things from Harrison Labs called "F-Mods"
Crossovers
but you need to do some actual checking. My experience is when they say 12 dB/octave, it is really like stacked 6 dB crossovers and therefore not at the frequency they said.

Do measure your amplifier's actual input impedance. It can easily be different from spec. You need to measure versus frequency and with different gains.

Or you could just get a pair of 100 Hz F-Mod, plug them in, fiddle with the sub crossover, and enjoy!

*If you need help with that, you need to post your speaker and sub model numbers, room size/layout, and positioning within the room. I'd put that in a different thread in the subwoofer forum for more replies, and link back to this thread.
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