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Old 27th April 2006, 10:17 AM   #1
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Default mix current drive with voltage drive at LF?

Current drive has been discussed before here. The main advantages are reduced distortion through these mechanisms:

- force proportional to Bxl for current drive vs. (Bxl) for voltage drive
- Le and Le modulation are rendered irrelevant because they will always be small compared to the impedance of a current drive amp
- dito for VC resistance variation due to self heating

The main disadvantage is that Q_e -> infinity, i.e. you loose electrical damping, resulting in a huge FR hump at resonance and possible uncontrolled overexcursion.

A Linkwitz transform circuit could flatten the FR, but due to the high Q involved, the compensation would be very sensitive to parameter variations, say over temperature. The problem of overexcursion is also not effectively dealt with.

Increased mechanical damping comes to mind, but the usual methods (lossy suspension, increased conductivity of VC former, air flow resistances, oil or fat damping) are usually very nonlinear in excursion or velocity or will vary with temperature.



Have there been attempts to combine voltage and current drive so that there is voltage drive around and below resonance and current drive above? In the simplest case the driver is only used above resonance, i.e. a midrange or tweeter. In this case, it might suffice to connect an inductor in parallel to the driver which would shunt voltages generated by resonant motion of the driver.


The other question I have been wondering about is, for an unmodified driver (as for the approach outlined here or the LT approach), there is only a relatively low mechanical loss above resonance. Will there be a significant difference in decay times between voltage and current drive above resonance?
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Old 9th June 2017, 08:59 PM   #2
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I really hestitate to become an ancient archeologist or maybe someone with even higher powers but I want to resurrect this thread. Does anybody here still work on mixed voltage-current transducers driving method?
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Old 9th June 2017, 10:46 PM   #3
Zero D is offline Zero D  United Kingdom
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Interesting dual option approach

I'm Very surprised this wasn't responded to originally ? Be good to see further discussions etc
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Old 10th June 2017, 12:33 AM   #4
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Quote:
Originally Posted by Zero D View Post
...I'm Very surprised this wasn't responded to...
Most of the discussion about current-drive is in the Solid-State sub-forum, search there?

Best wishes
David
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Old 10th June 2017, 12:35 AM   #5
JMFahey is offline JMFahey  Argentina
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Mixed feedback is alive and well in the Guitar World, >90% of SS Guitar amps use it.
I for one, have been using it since 1972, 50/50 mix so amp output impedance is 4 ohms for a 4 ohm load, or damping =1 , to mimic what I heard from my Fender tube amp clones which I had ceased making.
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Old 10th June 2017, 01:17 AM   #6
system7 is offline system7  United Kingdom
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I honestly think this current versus voltage drive debate is one of the biggest time-wasters in audio. And I spent a lot of time building horribly unstable transconductance amplifiers in my greener days.

You can do it either way if you get the impedance flat. The maths says that they both then get to the same place. For sure, valves are fast devices, and I like them. But really it's all about feedback.

These days I get interested in flat impedance in a speaker, though we usually take a beating with the bass resonance, which is easiest with closed box. It's self-evident to me that it makes for an easy load for an amplifier. Which keeps the feedback in phase.

The BBC used impedance correction a lot:
Rogers Loudspeakers › LS5/9

I learned from this classic design. mh-audio.nl - Home

Not hard to estimate the inductance of a driver, or just lift the "Le" figure.

Then you can do nice things. An easy to drive speaker. What's not to like?
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File Type: png Marc FR.PNG (19.1 KB, 213 views)
File Type: png Marc Phase.PNG (19.7 KB, 211 views)
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Old 10th June 2017, 01:45 AM   #7
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I read a fairly cool article from Nelson Pass regarding using current drive, along with EQ, for some of the full range drivers available, but the idea that it was any sort of general panacea for LF was not something he presented.

http://www.firstwatt.com/pdf/art_cs_amps.pdf
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Old 10th June 2017, 11:21 AM   #8
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Quote:
Originally Posted by system7 View Post
I honestly think this current versus voltage drive debate is one of the biggest time-wasters in audio.
I could not disagree more: this might be one of the most overlooked subject, IMHO.
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Old 11th June 2017, 10:02 AM   #9
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Decades ago there was an AES article about current drive combined with motional feedback that only worked around the resonance frequency. They used an extra voice coil as the sensor for the motional feedback. A long time ago, I did some experiments with that concept using a double-coil woofer (meant to be used as a subwoofer). It seemed to work well. You need to include a filter that compensates for the magnetic coupling between the coils, though.
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Old 12th June 2017, 02:19 AM   #10
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Default Current drive articles

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Originally Posted by MarcelvdG View Post
Decades ago there was an AES article about current drive combined with motional feedback that only worked around the resonance frequency. They used an extra voice coil as the sensor for the motional feedback. A long time ago, I did some experiments with that concept using a double-coil woofer (meant to be used as a subwoofer). It seemed to work well. You need to include a filter that compensates for the magnetic coupling between the coils, though.
Hi Marcel,

Was that article by Mills and Hawksford? Distortion reduction of MC speaker using current drive March 1989, and Current drive power amp October 1989. I think the October article has the filter circuit to compensate for coupling between coils. A nifty circuit.

I concluded the 30dB of distortion reduction above resonance must have been due entirely to current drive since the voice coil feedback was rolled of at 500Hz, since there's not much feedback even at 200Hz for distortion reduction, but this was not mentioned in the papers.

The papers can now be downloaded from here as a compilation 38MB.

There are two other interesting papers I have seen. I have put them on myDrive in IansArticles here.
1. Catrysse, Loudspeaker Feedback (Current drive with Cap position sensor), JAES June 1985
https://drive.google.com/open?id=0B3...EFmeHZLbmhKUG8
2. Hsu, Current Driven Speaker MFB (2nd speaker), JAES Jan 1999
https://drive.google.com/open?id=0B3...lo1OUpKX0dKYUE

There's also Hans Klarskov, Speaker Builder, Jan 1990 here
1st page http://www.audiodesignguide.com/doc/...Feedback_1.jpg

And if you are interested in parametric equalisation of speaker resonance using opamps then I also have available:
1. The Sokol parametric equaliser article 'Practical-Subwoofer-Design', Wireless World Dec-1983
https://drive.google.com/open?id=0B3...3FQdW9tbWo4dTQ
2. Jeff Macaulay, Variable Q speaker parametric equaliser, Electronics World, Jan-1997
https://drive.google.com/open?id=0B3...mp5cHBGdGw4WHM

If you are interested in a compendium of references on speaker feedback systems and current drive my Supplement to 'Speaker Feedback' EW May 1996 has about 50 refs with abstracts: https://drive.google.com/open?id=0B3...nJiQWx1NGpjbVk
and the EW 'Speaker Feedback' article is:https://drive.google.com/open?id=0B3...WhaWGMzd2VKbTQ

Last but not least, have you read Esa Merilainen's peer reviewed paper 'Comparative Measurements on Loudspeaker Distortion: Current vs. Voltage Control' under well controlled conditions and at moderate SPL?
It's here and download http://acousticsnew.ippt.gov.pl/inde...d/1780/pdf_255

It was a surprise to me that there was such a large difference in distortion readings with current drive at normal listening power levels of around 1W!! I had previously assumed that speaker distortion becomes inaudible with voltage drive when the cone movement is small, like normal listening level, but Esa shows there is still significant audible distortion at 1W with voltage drive. But at 1W and current drive the distortion reduces to around the threshold of audibility. That has renewed my interest in current drive of speakers (thanks Esa).

Cheers,
Ian Hegglun
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