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Thread: resistor parallel over compression drivers

  1. #1
    Senior Member frank23's Avatar
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    resistor parallel over compression drivers

    hi,

    I have built a set with passive crossover that I like the sound of very very much. One of the key features in the crossover is the parallel resistor over the +/- from the 2420 driver. As this is only about 1.2ohm, this damps the impedance peaks of the driver/horn combo a lot. A series resistor of about 6ohm brings the nominal level to about 8ohms again so the amp doesn't see any low impedance. I can do this because I don't need the voltage sensitivity of the driver.

    Yesterday I connected my newly acquired 2440/2382 combi to my active m552 crossover and it didn't sound as good as my passive 2420 setup by far. It might have something to do with the fact that I only had about 20uF of capacitors that I could put between the amp and the driver for DC protection. The 20uF makes a 1st order crossover itself in the 500-1500Hz range [about, I don't have my calculator at hand]

    Now I have also done the "parallel resistor trick" with the 2440 and I like the sound much better. I think this has to do with the impedance peaks that the 2440/2382 combi shows at about 800Hz, which is right in the crossover range of between 500Hz and 1500Hz that was created by the 20uF capacitors for DC protection that I tried out.

    And the 1.2ohm in parallel effectively shorts the driver resonances, like a bad tube amp would do, so it's also damped more.

    The sound is much more sophisticated now, with much less "ssssssss" spitting out. And more balanced because the impedance peaks are no longer there because of the 1.2ohm in parallel. So the voltage driven crossover curves created by the active crossover, arrive at the 2440 as they are intended.

    greetings, Frank

  2. #2
    Senior Member Guido's Avatar
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    JBL often worked with 7.5, 10, 15 or 20 Ohms parallel resistors.

    The 1.2 Ohms are a little low IMO.

  3. #3
    whgeiger
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    Overload

    F23,

    The 1.2-Ohm shunt resistor is drawing a lot more current than the driver. All this does is load down the amplifier and generate extra heat (from the shunt resistor and power devices of the amplifier). To reduce driver sensitivity, use an L-Pad [1] with a series and higher shunt resistance. To suppress an impedance peak, place a Zobel circuit [2] across the voice coil terminals.

    Regards,

    WHG

    References:

    [1] Attenuation Networks (Rod Elliott)
    http://sound.westhost.com/lr-passive.htm#6.0

    [2] Mid Range and Tweeter Impedance Compensation (Rod Elliott)
    http://sound.westhost.com/lr-passive.htm#3.1.2

  4. #4
    Senior Member Ian Mackenzie's Avatar
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    Quote Originally Posted by frank23
    hi,

    I have built a set with passive crossover that I like the sound of very very much. One of the key features in the crossover is the parallel resistor over the +/- from the 2420 driver. As this is only about 1.2ohm, this damps the impedance peaks of the driver/horn combo a lot. A series resistor of about 6ohm brings the nominal level to about 8ohms again so the amp doesn't see any low impedance. I can do this because I don't need the voltage sensitivity of the driver.

    greetings, Frank
    Interesting ideas, it helps if people can read.

    The possibility of direct amp connection and bi amp would yield even better dynamic damping without losses.

    Ian

  5. #5
    whgeiger
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    ?

    Quote Originally Posted by Ian Mackenzie
    Interesting ideas, it helps if people can read.

    The possibility of direct amp connection and bi amp would yield even better dynamic damping without losses.

    Ian
    IM,

    The ad hominem ‘opener’ adds nothing meritorious to the conversation. However, the fact remains, that for a given acoustic output, less current is required if the shunt resistance is only 'seen' at driver resonance, or if an L-pad is required; a shunt resistor of higher Ohms should be used (for the same reason). This is also true if the Zobel is retained in the bi-amp setting. Also if this tack is taken, a series capacitor should be introduced for driver (tweeter) protection. So, it is not necessarily true that all low impedance passives will be eliminated by use of a dedicated amplifier even though level setting and crosover filtering functions may be beneficially moved out of the power delivery circuits.

    Regards,

    WHG

  6. #6
    RIP 2011 Zilch's Avatar
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    Frank: I'd recommend you buy or build an RTA if you are going to work with this, and a reliable means to measure impedance, as well.

    It can't be accomplished by whim or fiat....

    Note: My M552 has nasty on/off transients.

  7. #7
    Senior Member Ian Mackenzie's Avatar
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    I have recently done my own tests in this area and aside from the effects of damping and zobel networks, the effect of active crossovers and series capacitors in the signal path can have a marked effect on sound quality

    It seems to me that while theory is a good starting point, an empirical approach to design is often what is required in reality. Less is more tends to be the outcome...

    I have sent a pm to Frank 23 to discuss his experiment further.


    Ian

  8. #8
    Senior Member frank23's Avatar
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    I agree with the less is more philosophy.

    Of course a zobel network can be used, but it would work for one frequency only [so you'd need more in parallel in case of the 2445/2382 combo, see pic, a 2360 horn is even more difficult], is difficult to tune, creates phase shift. And a resonance frequency can change with ageing and use, so you'd have to review the tuning periodically to be sure that it is still "spot on".

    For me, the parallel resistor has much less components, easier to predict, easier to implement and can be used as a attenuation circuit at the same time.

    ----------

    For now my passive setup still sounds better than the active setup, I think this has to do with 3 things:

    1 I like 1st order [passive] crossovers, this gives less phase shift than 4th order [active] crossovers, and the crossover is less sudden between drivers

    2 my passive setup has paper-oil capacitors and kimber cable, while my experimental active setup has to work with MKT left-over capcitors and bad cable and of course, more active components [the M552 is of course in between]

    3 I optimized my passive setup over the years and this active setup only has been "in business" for 3 days

    greetings, Frank
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  9. #9
    aust-ted
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    Dammed caps

    Quote Originally Posted by Ian Mackenzie
    I have recently done my own tests in this area and aside from the effects of damping and zobel networks, the effect of active crossovers and series capacitors in the signal path can have a marked effect on sound quality

    It seems to me that while theory is a good starting point, an empirical approach to design is often what is required in reality. Less is more tends to be the outcome...

    I have sent a pm to Frank 23 to discuss his experiment further.


    Ian
    Hi Ian

    You got me thinking. I use a trick taught by an old sound engineer to protect my mids & tweeters (2450/ 2404) in my actively xover'd system - banks of polyprop caps, with a freq cut-off well outside the pass band in series with the drivers, to protect against DC and other stupid things I might accidently subject them to.

    Do you think these could affect sound quality?

    Regards
    Ted

  10. #10
    Senior Member frank23's Avatar
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    Hi Ted,

    How large are your combined capacitors for each of the 2450 and 2405?

    Polypropylene capacitors mostly are of a sufficient quality. I like paper-oil caps myself, but they can be very expensive and big [I've got some that are the size of a 2405 and provide only 4uF each].

    If you need large values to keep them outside of the passband, polyprops can be a good alternative.

    But DC protection can also be done through transformers I think, like the outputs of single ended [tube] amps that are also used to downconvert the tube's voltages. Then you wouldn't need caps.

    Greetings, Frank

  11. #11
    Senior Member Ian Mackenzie's Avatar
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    Very interesting discussion brewing here chaps.

    Ted,

    Yes, have you tried charge coupling those caps at this stage....nine volt battery with 2-3 megs of series resistance?..worth a try.

    Frank,

    I don't like scewing with Zobels in the mids and highs, they effect not only driver but amp performance as they are a reactive load. Low impedance damping is far better and this is well documented. If a driver has a bad resonance and its audible, think about an alternative driver/horn.

    I have been playing around with the 4343 style monitors with Ed over the past few months and we have pondered over the active versus passive thingy.

    Our conclusions thus far are that for results in terms of mid and top end sound the quality of the active crossover is critical. The bi amp thing offers 2 key benefits, high damping of the woofer and less clouding of midrange (where the active crossover offers AAA performance).

    Unless you have AAA performing active crossover the passive crossover offers better absolute HiFi. We have found the popular Ashly crossover and the JBL 5235 crossover while clinically clean they loose vital fine details and transparency which can be the determining factor of mid fi and hi end.

    The good idea is a simple Passive RC filter before the amp with a low source impedance pre out if you only need 6 db per octave filter slope. Otherwise discrete class A buffers are the go...I just do not think chips cut the mustard (hot english mustard!) ..particularly where pistonic devices like compression drivers are used...they are ruthlessely accurate and reveal all manner of harmonic distortions...

    If you are careful and have muted started up and DC protecton on the amps, a direct connection to the amp is the best. Only in PA applications is a dc blocking capacitor an absolute must. Also its much easier to have a small value hot cap in the amp input to filter out of band frequencies

    regards

    Ian

  12. #12
    Senior Member frank23's Avatar
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    Quote Originally Posted by Ian Mackenzie

    If you are careful and have muted started up and DC protecton on the amps, a direct connection to the amp is the best. Only in PA applications is a dc blocking capacitor an absolute must. Also its much easier to have a small value hot cap in the amp input to filter out of band frequencies

    Ian
    living on the edge!

    well I would sure like to try it that way, but I measured the DC offset on my amps and it wasn't zero, so how many millivolts of DC offset can these 2440 drivers work with? If it were a cone-mid-driver I would connect them directly without any caps, so maybe I am being too carefull. Of course direct connection would be the best, then the whole dampingfactor of the amp would be available.

    Frank

  13. #13
    Senior Member Ian Mackenzie's Avatar
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    I doubt if 50 millivolts would be an issue but is the amp ac or dc coupled at the input and does it have unity gain at dc or not? If it is dc coupled and has gain at dc it is unwise to dc couple the driver.

    Hmm.... .I take no responsibility.

    Frank just Looking at the big picture Are you utterly convinced the shunt makes this much difference..was the level exactly the same?

    Or is it the active crossover slopes and extra electronics.

    Even 0.5 db variations are quite audible .

    Ian

  14. #14
    Senior Member Guido's Avatar
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    Quote Originally Posted by Ian Mackenzie
    .....it helps if people can read.....
    Sometimes keeping it factual helps even more

  15. #15
    Senior Member Ian Mackenzie's Avatar
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    Quote Originally Posted by Guido
    Sometimes keeping it factual helps even more
    Pity our ears are sometimes the only instruments sensitive enough to register the facts..what is actually happening in reality!

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