MTM Impedance Issues

matt09 · 8th August 2006, 11:09 PM

I have two nice pairs of speakers for use in an MTM cabinet.
I know really the pairs of speakers should be the same, but they aren't as one is slighty better as bass with a bigger internal section, whilst the other midrange - high speaker has a smaller propertion of the cabinet...

ANYWAY, My problem is the one pair of speakers is 8 ohm , while the other pair are 4 ohm. I want to driver these speakers in parallel with a good amp, however this of course would result in totally different volumes of the different drivers as well as resulting in a horrible 3 ohm load that my amp cannot cope with.

I know adding in a 4 ohm resistor is not really the way, but I cannot see any other way of doing it.

Is there a way to get these speakers at equal levels?

infinia · 9th August 2006, 01:32 AM

Quote:

Is there a way to get these speakers at equal levels?

No, not really. Not with a single amp anyways.
Think 2.5 ways in MMT alignment, IMO MTM is overrated and almost passe'.

Andy G · 9th August 2006, 02:13 AM

don't know about MTM's being passe ... but in this case yes, a TMM 2.5 is the go.

Use the one with the better bass response as the .5
You should provide separate compartments for them.

Also note that they can still be arranged in an MTM format if desired, you just can't use the drivers wired directly together in series or parallel because of the mismatched impedances.

infinia · 9th August 2006, 02:23 AM

MTM's require higher order filters to get a halfway decent polar pattern at least 18dB/oct acoustic. When you start to see that MTM alignment everywhere and at a price point you just know is not done proper, then it's passe'.

matt09 · 9th August 2006, 10:50 AM

Hello,

Thank you everyone for your replies, this may sound stupid but I am fairly new to the business, so what is an TMM 2.5 ? Obviously I understand the 'TMM' is just the '2.5'. I take it your talking of some sort of crossover type circuit? What does this TMM 2.5 do? does it result in equil-ish volumes from both types of drivers? and does it mean that the total of the 8 ohm and 4 ohm driver is greater or equal to 4 ohms so I can still power them with one amp?

Thanks.

matt09 · 9th August 2006, 12:09 PM

anyone?

Cal Weldon · 9th August 2006, 02:08 PM

Generally a 2.5 way is two identical and often small drivers connected in parallel with one of the two being cut off at a lower frequency. This allows the mids from the second driver to come through but allows only the lower frequencies from the cut off one. It's like a poor mans BSC for multiple drivers.

The tweeter is then crossed over normally.

I have used this set-up with two of the CSS WR125 and a dome. I simply added a coil.

I'm not sure you can implement this well with two different drivers though. You may want to convert it to a three way.

matt09 · 9th August 2006, 02:12 PM

Hello,

I have tested the crude adding a resistor in to make it 8 ohms. I increased the ohms between 1-4 until they both sounded identical.

I know this is not a proper way of doing things, but how good if a way really is adding in a resistor?

It sounds very good with my calibration of using my ear at different freqencies and they both sound equal.

it this acceptable although not mathematically correct?

sangram · 10th August 2006, 09:14 AM

Just an idea...

Why don't you use one 8 ohm and one 4 ohm driver in each channel, and do a proper 3-way design instead?

If done right you may be able to get the benfits of a 2.5 way (lower impedance at bass frequencies) with a simpler crossover.

IMO the first XO point would be somewhere around the baffle step frequency.

The problem with adding a resistor is you change the Qes as well as the Fs, apart from dissipating a lot of power. Depending on make, you may be introducing distortion as well, and will start getting more and more affected as the speaker is driven harder, specially at bass frequencies.

matt09 · 10th August 2006, 10:34 AM

Hello,

Thanks for you reply, yes I was going to put an 8 ohm and 4 ohm in each cabinet. Please remember I am new to this stuff, what is the baffle step frequency? If I were to introduce a 3 way crossover would that not mean that the the other 4 ohm high quality speaker would only be used from 500 hz up or something like that? or would the crossover be at the point be up to the frequency where the bass apeaker is better?

8th August 2006, 11:09 PM	#1
matt09 diyAudio Member Join Date: Aug 2006	MTM Impedance Issues I have two nice pairs of speakers for use in an MTM cabinet. I know really the pairs of speakers should be the same, but they aren't as one is slighty better as bass with a bigger internal section, whilst the other midrange - high speaker has a smaller propertion of the cabinet... ANYWAY, My problem is the one pair of speakers is 8 ohm , while the other pair are 4 ohm. I want to driver these speakers in parallel with a good amp, however this of course would result in totally different volumes of the different drivers as well as resulting in a horrible 3 ohm load that my amp cannot cope with. I know adding in a 4 ohm resistor is not really the way, but I cannot see any other way of doing it. Is there a way to get these speakers at equal levels?

9th August 2006, 02:23 AM	#4
infinia diyAudio Member Join Date: May 2005 Location: Californication	MTM's require higher order filters to get a halfway decent polar pattern at least 18dB/oct acoustic. When you start to see that MTM alignment everywhere and at a price point you just know is not done proper, then it's passe'. __________________ like four million tons of hydrogen exploding on the sun like the whisper of the termites building castles in the dust

9th August 2006, 02:08 PM	#7
Cal Weldon Speakerholic diyAudio Moderator Join Date: Jan 2004 Location: British Columbia	Generally a 2.5 way is two identical and often small drivers connected in parallel with one of the two being cut off at a lower frequency. This allows the mids from the second driver to come through but allows only the lower frequencies from the cut off one. It's like a poor mans BSC for multiple drivers. The tweeter is then crossed over normally. I have used this set-up with two of the CSS WR125 and a dome. I simply added a coil. I'm not sure you can implement this well with two different drivers though. You may want to convert it to a three way. __________________ Next stop: Margaritaville Some of Cal's stuff \| Cal Weldon Consulting

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9th August 2006, 02:13 AM	#3
Andy G diyAudio Member Join Date: Apr 2002 Location: Newcastle, Australia	don't know about MTM's being passe ... but in this case yes, a TMM 2.5 is the go. Use the one with the better bass response as the .5 You should provide separate compartments for them. Also note that they can still be arranged in an MTM format if desired, you just can't use the drivers wired directly together in series or parallel because of the mismatched impedances.

9th August 2006, 10:50 AM	#5
matt09 diyAudio Member Join Date: Aug 2006	Hello, Thank you everyone for your replies, this may sound stupid but I am fairly new to the business, so what is an TMM 2.5 ? Obviously I understand the 'TMM' is just the '2.5'. I take it your talking of some sort of crossover type circuit? What does this TMM 2.5 do? does it result in equil-ish volumes from both types of drivers? and does it mean that the total of the 8 ohm and 4 ohm driver is greater or equal to 4 ohms so I can still power them with one amp? Thanks.

9th August 2006, 12:09 PM	#6
matt09 diyAudio Member Join Date: Aug 2006	anyone?

9th August 2006, 02:12 PM	#8
matt09 diyAudio Member Join Date: Aug 2006	Hello, I have tested the crude adding a resistor in to make it 8 ohms. I increased the ohms between 1-4 until they both sounded identical. I know this is not a proper way of doing things, but how good if a way really is adding in a resistor? It sounds very good with my calibration of using my ear at different freqencies and they both sound equal. it this acceptable although not mathematically correct?

10th August 2006, 09:14 AM	#9
sangram diyAudio Member Join Date: Sep 2002 Location: India	Just an idea... Why don't you use one 8 ohm and one 4 ohm driver in each channel, and do a proper 3-way design instead? If done right you may be able to get the benfits of a 2.5 way (lower impedance at bass frequencies) with a simpler crossover. IMO the first XO point would be somewhere around the baffle step frequency. The problem with adding a resistor is you change the Qes as well as the Fs, apart from dissipating a lot of power. Depending on make, you may be introducing distortion as well, and will start getting more and more affected as the speaker is driven harder, specially at bass frequencies.

10th August 2006, 10:34 AM	#10
matt09 diyAudio Member Join Date: Aug 2006	Hello, Thanks for you reply, yes I was going to put an 8 ohm and 4 ohm in each cabinet. Please remember I am new to this stuff, what is the baffle step frequency? If I were to introduce a 3 way crossover would that not mean that the the other 4 ohm high quality speaker would only be used from 500 hz up or something like that? or would the crossover be at the point be up to the frequency where the bass apeaker is better?

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