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dlyxover
Old Croc Joined: 14 June 2007 Location: Liverpool Status: Offline Points: 1502 |
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This project is slowly progressing, sorting out a CNC run of 4 of these cabs now for the passives.
I fired up REW to make my impedance measurements, not used it before so was a learning curve but set up the sense resister and calibrated and was getting consistent readings as below. 8" 1" I was going to put a zobal network on, am I right in thinking I want to add it to the driver then remeasure to get the impedance at the crossover points? |
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Teunos
Old Croc Joined: 23 November 2008 Location: The Netherlands Status: Offline Points: 1799 |
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Zobel networks are generally used to compensate for the increasing impedance due to inductance of the coil rising. When the impedance is non-flat this gives asymmetry in the crossover which messes eith both phase and frequency response. Zobel networks can mend this so in that respect you are correct.
Try it both with and without though, if the eventual differences are small then dont do it as it always sucks up power. |
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Best regards,
Teun. |
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odc04r
Old Croc Joined: 12 July 2006 Location: Sarfampton Status: Offline Points: 5482 |
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You know the impedance curve of the box above resonance now so you can calculate that as an impedance with the base resistance subtracted, then work out your zobel components to flatten it. After you build the filter then yes, if you measure the impedance again then above resonance you should see a flat line, ideally purely resistive with frequency. This will help simplify your xover design, but always bear in mind the actual acoustical response of your drivers. Getting a flat electrical response is one thing, but it will be multiplied by the acoustic TF to form the final response. And then you have all the fun of impulse response, directivity, power dispersion etc if you want to go a level deeper. Sorry if you knew all that anyway! |
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Teunos
Old Croc Joined: 23 November 2008 Location: The Netherlands Status: Offline Points: 1799 |
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here is an example of a drivers impedance i once did for a passive crossover, 2'' driver crossed to a CP25 with a simple RC zobel network. In this case the zobel network definitely made the crossover much easier.
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Best regards,
Teun. |
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dlyxover
Old Croc Joined: 14 June 2007 Location: Liverpool Status: Offline Points: 1502 |
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Upon looking at your plot Teunos I'm thinking my calibration mighty be out.
on my 8" plot its not hitting 8ohm anywhere, same with the 1". Do I need to subtract the base resistance? A Odc04r mentioned I presumed the measurement on the plot is the actual measurement. |
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Teunos
Old Croc Joined: 23 November 2008 Location: The Netherlands Status: Offline Points: 1799 |
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I think it is not your calibration, i think it was more sort of my calibration. This driver which is presented here is actually a 6.7 Ohm Re driver so my reference resistor was probably slightly off, or my gain settings. It was more to prove the point of how well an extremely zobel circuit can work. Note though that the use of zobel circuits is heavily discussed and its use not always stimulated. Acoustic response will eventually be more important then the electrical response the amplifier is presented with and since both are interlinked... well, let's just say there are pros and cons for using them. Also note that i experimented with these things in the past, and i have moved on to all actively driven loudspeakers with no passive elements, except for protection such as LF protection caps and bulbs. Passives are always a bitch when delay is introduced by means of a HF horn so i tend to stay away from them. Also good filters with proper wire thickness inductors etc. are not by any means cheap whilst amplifier power is, and is only needed once if multiple types of rigs/cabinets can be driven by the same amprack. I'd rather go fully active with amplifiers in the cabinet (a lá powersoft modules) than go back to passive.
Edited by Teunos - 19 June 2015 at 3:20pm |
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Best regards,
Teun. |
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odc04r
Old Croc Joined: 12 July 2006 Location: Sarfampton Status: Offline Points: 5482 |
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Remember that what is usually quoted is the DC resistance, so if you take the plot down to nearer 0Hz it may get closer to 8 ohm. Also you have loaded the driver into a cabinet now and that will affect the electrical load it presents to the amplifier. I recall off the top of my head measuring a Volt 8" in a reflex cab not too different from yours in terms of volume at about 11 Ohms nominal a couple of years back. From the amplifiers point of view this is an easier load to drive. If you take the driver out and measure it you might get closer to the nominal impedance. If you are concerned about the calibration of your rig try measuring the impedance of a resistor measured with a multimeter and see what you get. Even a cheap multimeter should get to near 0.1Ohm accuracy if it is calibrated ok. For best result measure the sense resistor with a good DMM but not everyone has access to that. When I say subtract the base resistance I mean that from about 200Hz and up the total resistance is made up of the DC coil resistance and then the frequency dependent impedance. Depending on how you calaculate the Zobel you will probably need to enter those separately, although they are both a component of the total reactance. Just one depends on frequency and the other is a constant. Edited by odc04r - 19 June 2015 at 4:29pm |
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odc04r
Old Croc Joined: 12 July 2006 Location: Sarfampton Status: Offline Points: 5482 |
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Quote != edit...
Edited by odc04r - 19 June 2015 at 4:28pm |
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dlyxover
Old Croc Joined: 14 June 2007 Location: Liverpool Status: Offline Points: 1502 |
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The measurement was mad with the driver in the cab, have re checked the calibration is correct.
odc04r I didn't know the impedance would rise when in a cab kinda make complete sense now you say it. Teunos im overly mad on passives but these are going to be use a delay/infill boxes probably with a sub, if they were just on their own they would be driven actively but compromises as always. Need to get some components ordered and see what happens |
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odc04r
Old Croc Joined: 12 July 2006 Location: Sarfampton Status: Offline Points: 5482 |
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If you consider the whole model of a cabinet, it's electro mechanical. You have the resonances of the driver, then you superimpose on them a volume of air which also acts a damping spring (in the case of a sealed box or vented well above port tuning). So an in-cab impedance shift from free air is not too surprising. If it didn't change at all it would imply that the cabinet had no effect on the driver motion which would not be ideal!
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T-Bone
Registered User Joined: 27 December 2005 Location: Tampa, FL USA Status: Offline Points: 292 |
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Is there a way to just increase the baffle size and increasing the vent size to accomodate a Faital 10" PR300 neo was just curious since I also have a pair looking for a nice cabinet design like this?
http://www.faitalpro.com/en/products/LF_Loudspeakers/product_details/index.php?id=101040110
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BASS, how low can you go!
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dlyxover
Old Croc Joined: 14 June 2007 Location: Liverpool Status: Offline Points: 1502 |
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odc04r
It makes sense when you lay it all out but i never made the connection. t-bone When i get some time till try and sim your driver and let you know when modifications need to, would be a complete redesign form my 8" cab
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