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8 ohm crossover working OK on 16 ohm cones ??
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rogerharris View Drop Down
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    Posted: 06 January 2016 at 7:12pm
This one has me baffled.. but my electronics is patchy..

Was testing some 2 way cone combinations with separate amps, and thought would try a 3 way setup with on old 3 way cheapie crossover (8 ohm type)

But the LF and MF are 16 ohm, the HF is 8 ohm

LF is a fane medusa, MF is from funktion one AX88, HF is phase plug version of a Celestion comp 

But seems to work, actually sounds pretty good when they are aligned dual concentric with.  Better than the Rogers Hifi and Genelec monitors which using a reference.


r

If I put 16 ohm drivers in an 8 ohm crossover I read that its not supposed to work, as crossover points drop, and the crossover loses some db in its cut..

Could it be that the drivers are well matched so the crossover isnt working as well, but its not really that much of an issue in this case ?  
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markie View Drop Down
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Normally when using a crossover with "Wrong impedence" drivers it moves the crossover point(s) so you end up with holes in the  response curve. However the crossover you have only has 6dB slopes anyway so it probably has less effect than a "proper crossover".

A crossover with 6dB slopes doesn't offer much protection for mid and HF drivers but the very reason for that might also mean that the adverse effect of the wrong drivers is not so pronounced.

Must confess I'm not terribly conversant with the science, there'll be an adult along shortly to pull my reasoning apart.


Edited by markie - 06 January 2016 at 7:42pm
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Andy Kos View Drop Down
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It will work, but it will change how the crossover works.
 
Try this:
 
 
pick a frequency, and 2nd order butterworth, and put in 8 ohms - the calc will work out the crossover and plot the response.
 
Then change to 16 ohms, and click the LOWER calc button - this will recalculate with the same Inductors and Capacitors, but with a different load, it will plot the response at 16 ohms - you will see lumps and bumps around the crossover frequency.
 
 


Edited by Andy Kos - 06 January 2016 at 7:45pm
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rogerharris View Drop Down
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Originally posted by markie markie wrote:

Normally when using a crossover with "Wrong impedence" drivers it moves the crossover point(s) so you end up with holes in the  response curve. However the crossover you have only has 6dB slopes anyway so it probably has less effect than a "proper crossover".

A crossover with 6dB slopes doesn't offer much protection for mid and HF drivers but the very reason for that might also mean that the adverse effect of the wrong drivers is not so pronounced.

Must confess I'm not terribly conversant with the science, there'll be an adult along shortly to pull my reasoning apart.

cheers, that explains it.. there was a flat spot, but bit of eq sorted it
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Originally posted by Andy Kos Andy Kos wrote:

It will work, but it will change how the crossover works.
 
Try this:
 
 
pick a frequency, and 2nd order butterworth, and put in 8 ohms - the calc will work out the crossover and plot the response.
 
Then change to 16 ohms, and click the LOWER calc button - this will recalculate with the same Inductors and Capacitors, but with a different load, it will plot the response at 16 ohms - you will see lumps and bumps around the crossover frequency.
 
 

thanks, just whats need for making the right crossover :)
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rogerharris View Drop Down
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problem solved.. according to the calculator, due to the crossover being only first order, then a higher impedance driver doesnt do anything drastic except give a 1.5db boost at the cross point. The high cross point is actually raised a bit higher from 5000hz to 7.2khz.. which is still in the reach of the AX88 driver and phase plug.  So basically nothing too problematic soundwise.
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Always remember, that most crossover calculations are based on 'nominal' impedances of 8 ohms. Drivers can often be 5-7 ohms in their flat usable range, creeping up towards 10 or more ohms at higher frequencies, and can go up to 20, 30, 40 or even higher at resonance. 8 ohms give you a 'rough' idea of whats occurring, but impedance is frequency dependent.
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Originally posted by Andy Kos Andy Kos wrote:

Always remember, that most crossover calculations are based on 'nominal' impedances of 8 ohms. Drivers can often be 5-7 ohms in their flat usable range, creeping up towards 10 or more ohms at higher frequencies, and can go up to 20, 30, 40 or even higher at resonance. 8 ohms give you a 'rough' idea of whats occurring, but impedance is frequency dependent.

the MF unit is low inductance I understand.... although cant see any copper in the pole piece thats visible. But the coaxial aspect turned out to be the most promising aspect here, and might interesting to see how this project compares to the high spec coaxial units you have.  I did use an RCF HF unit for comparison when setting this up, and it could not come remotely close, maybe something higher rated like the B+C coaxial, but then its 1200 a pair and this is a low budget experiment


Edited by rogerharris - 07 January 2016 at 9:36am
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Although the photo isn't very clear that looks like the old Celestion RTT50 tweeter.  The standard tweeter had an integral 8" x 4" horn but then there was a version for the Road series cabinets. The cabinets were plastic and had the horn integrated with the cab design, hence the "Driver only" version of the RTT50.



I still have a few 8 ohm units  around from when I bought a load of stuff from a guy who cleared the Celestion factory. Let me know if you need any more.
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Originally posted by markie markie wrote:

Although the photo isn't very clear that looks like the old Celestion RTT50 tweeter.  The standard tweeter had an integral 8" x 4" horn but then there was a version for the Road series cabinets. The cabinets were plastic and had the horn integrated with the cab design, hence the "Driver only" version of the RTT50.



I still have a few 8 ohm units  around from when I bought a load of stuff from a guy who cleared the Celestion factory. Let me know if you need any more.

thanks, will need a couple if they make it past prototype selection.. sound great without the phase plug.  Do you have any RTT50 horn for these ? might be ideal for the dual concentric config
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The original RTT50 was an all in one unit but the ones I have were made to bolt on to the cabinet. I did have some horns made but you have to graft a small plywood square onto the back to bolt the driver to.

I have 4 horns, one has the plywood square already attached.

I have about 7 drivers but they are in various states of repair. 2 are ready to go but on others  the plastic that supports the terminals have broken. Terminals are still connected to the coil. On some of them the plastic can just be super glued but others need a bit of plastic padding.

If they are of any interest the lot can go for £30.00 plus carriage.
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Originally posted by markie markie wrote:

The original RTT50 was an all in one unit but the ones I have were made to bolt on to the cabinet. I did have some horns made but you have to graft a small plywood square onto the back to bolt the driver to.

I have 4 horns, one has the plywood square already attached.

I have about 7 drivers but they are in various states of repair. 2 are ready to go but on others  the plastic that supports the terminals have broken. Terminals are still connected to the coil. On some of them the plastic can just be super glued but others need a bit of plastic padding.

If they are of any interest the lot can go for £30.00 plus carriage.


Cheers, that lot will be useful, can replace those taken out the celestion cabs.. 

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