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Old 17th August 2003, 11:58 AM   #1
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Default Gainclone with negative output impedance

Diving right into it, negative output impedance can be useful under certain circumstances because it can at least partially compensate for the resistance of the speaker voice coil. If the coil resistance (not impedance) was zero then the speaker would do just exactly what the drive waveform was telling it to do. The fundamental resonance would be flattened out almost completely. The rolling off at low frequencies in a small sealed box would not occur - at least within the cone excursion limits.

What this cct does is measure the current through the speaker and send back to the non-inverting input a voltage proportional to the voice coil current. This voltage is then amplified in the normal way, in addition to the signal, and it is this extra voltage that will be dropped in the voice coil resistance.

I did make up one of these ccts once before in a different amplifier and if you wind up the pot to far it will gradually slide into instability. It is positive feedback after all. But it does work!

All you need to add to your gainclone or whatever is a low value sensing resistor and a small pot. Tonight I am going to try it myself.
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Old 18th August 2003, 07:17 AM   #2
Raka is offline Raka  Europe
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Not for you, Circlotron, but for all the newbies like me, there is an article about this in the ESP pages that could make a good reading.
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Old 18th August 2003, 02:41 PM   #3
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Default Re: Gainclone with negative output impedance

Konnichiwa,

Quote:
Originally posted by Circlotron
Diving right into it, negative output impedance can be useful under certain circumstances because it can at least partially compensate for the resistance of the speaker voice coil.
And the voice coil resistance is not constant, but signal level dependent. Big problem, Eye Knowz....

NI, been there, done that, got the T-Shirt, NDFG.

Quote:
Originally posted by Circlotron
If the coil resistance (not impedance) was zero then the speaker would do just exactly what the drive waveform was telling it to do.
Actually, that is chemically pure bovine excrement. Normal electrodynamic drivers are pure current controlled devices and will do anything but what the waveform applied as voltage tells them. If we convert the system to current drive the voice coil resistance becomes irelevant, as does compression and several distortion mechanisms.

The only "price" to pay is that we now need to handle the mechanical resonance of the driver mechanically (for example apperiodic boxes) instead of hiding the mechanical system resonance behind the "damping factor" magical trick.

Quote:
Originally posted by Circlotron
The fundamental resonance would be flattened out almost completely. The rolling off at low frequencies in a small sealed box would not occur - at least within the cone excursion limits.
Actually, with fully compensated voice coil DCR the driver does NOT have a linear response, it is actually a rising response, but rising at a constant rate and thus easily equalised. Of course, as the voicecoil varies in resitance with signal and as it has also material inductance the resulting system is highly unstable. I shot quite a few bass drivers to the lake of fire and sulphur before giving up.

If you want to contol the cone accuratley at high excursion (LF) the best way is still to have a sensor, for anything with modest movement current drive sufficiently above the resonance is an excellent solution.

I have been around this lark in the 1980's, I ended up with a current feed, inverting circuit IC powered (TDA2030) 5.25" wideband driver (no whizzer) covering 200Hz -10KHz, a voltage feed inverting circuit IC powered (TDA2030) modified piezo tweeter (10KHz to as far up as they go) and dual 8" drivers powered from bridged TDA2030 with current booster transistors.

The woofers used a standard (omnidirectional) Electrect Mike capsula as sensor on each driver to sense the pressure. Nowadays I would likely apply the Linkwitz Mod, but I never really played that system extremely loud, only "very loud", so the Mike nonlinearity seems to have not mattered that much.

That system had the kind of dynamics I had with my Horn System (Sorta Klipsch Cornwall, but using all EV Drivers) with a more "HiFi" sound. It would probably be worthwhile building something similar again...

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Old 18th August 2003, 06:05 PM   #4
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Quote:
Originally posted by Raka
Not for you, Circlotron, but for all the newbies like me, there is an article about this in the ESP pages that could make a good reading.
Yup, he pretty much sums up the use of negative impedance in Effects of Source Impedance on Loudspeakers where he says:

Negative impedance does exactly what it implies - when the load is increased (with a lower impedance), the signal applied to the load increases (i.e. the exact opposite of what normally happens). This results in an intrinsically unstable system, and great care must be taken to prevent the creation of an oscillator.

The performance of this combination is completely unacceptable in every sense of the term. This combination could never be used in practice, for any reason. There is a vicious attack, with the signal doing something at a frequency completely unrelated to the input signal - unrelated in any way that I can determine, at least.

Considerable ringing is apparent (again at an unrelated frequency) when the signal is removed. This is highly visible and audible, and the sound of the attack and decay is grossly inferior to any other combination. The others (using positive impedance) have some character, but it is related to the signal, and makes some sort of musical sense.


se
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Old 20th August 2003, 09:36 AM   #5
moamps is offline moamps  Croatia
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In ETI JULY1991 was published one interesting article (Consort speaker - Peter Roberts) about implementing PFB in speaker signal path. In some condition PFB obviously works.

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Old 20th August 2003, 10:14 AM   #6
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Quote:
As I mentioned in one or more articles on these pages, I have never found a cone speaker that "likes" negative impedance. Horn compression drivers seem quite happy and this is a test that I may now be forced to perform to find out what it does, and how it affects the sound (other than an apparent improvement in bandwidth).
From the link at Rod's home page http://www.sound.westhost.com/z-effects.htm

As I understand negative impedance is especially good for bass reflex woofer, not so good for a ordinary fullrange speaker. It must be carefully tuned!

The "negative" properties is to cancel out the resistance (losses) in the woofer element and the cables. This cancellation makes it easier to control both the helmholtz and the mechanical resonance.
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Old 20th August 2003, 12:36 PM   #7
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Konnichiwa,

Quote:
Originally posted by peranders


As I understand negative impedance is especially good for bass reflex woofer, not so good for a ordinary fullrange speaker. It must be carefully tuned!

The "negative" properties is to cancel out the resistance (losses) in the woofer element and the cables. This cancellation makes it easier to control both the helmholtz and the mechanical resonance.
Actually, as usual poor understanding.

If you redraw (and disaggregate) the circuit you find that NI that completely cancels the DCR of the Voicecoil actually becomes a simple feedback circuit that uses the voicecoil as sensor. The signal from the voicecoil needs to be integrated to give correct response.

But as said, because the DCR is heavily signal dependent and because of the non-linearity of the voicecoil as sensor (BACK EMF) this is a really stupid way of doing it. I tried it way back. It is STUPID.

I repeat, the easiest way to control your woofer is to generate the feedback signal with a cheap modded panasonic electret mike capsula. As SL esimates, the WM60 modded can handle 141db. If you place the Mike close to the cone (say 12.5cm) you have more SPL than @ 1m.

With 141db @ 12.5cm it will be around 123db @ 1m, meaning for HiFi speakers this mike makes a perfectly sensible sensor (few HiFi Drivers produce > 123db/1m).

With 12.5cm distance to the cone you will have 90 degrees phaseshift @ 860Hz. So if you want to control a woofer only (where it makes sense) you will be fine. Then use a suitable Midrange system and feed it with current drive for eliminating compression and drastically reducing distortion.

For a fullrange driver I suspect a realtime laser position sensing system would be needed probably on the voicecoil itself to work over a wide enough bandwidth.

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Old 20th August 2003, 01:19 PM   #8
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You can mount your mic onto the woofer cone (best place would be the edge of the dustcap) with the mic's diaphragm looking sideways.
B&M were the first ones who did this AFIK. A current sub using this is the Manger "Subsonice".
Because you are in the speaker's nearfield (also with 12.5 cm), the 6dB/distance-doubling rule wouldn't exactly apply and the sound-pressure at the microphoine might be even a bit less than 141 dB for 123 dB @ 1m.

If you ground the non-inverting input in Graham's circuit and put feedback to the inverting input you get current drive. Its pros and cons have already led to lively discussions on this forum !

Regards

Charles
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Old 20th August 2003, 07:09 PM   #9
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Konnichiwa,

Quote:
Originally posted by phase_accurate
You can mount your mic onto the woofer cone (best place would be the edge of the dustcap) with the mic's diaphragm looking sideways.
B&M were the first ones who did this AFIK. A current sub using this is the Manger "Subsonice".
Nice to know. Back in the 80's I simply stuck the Mike on a bridge in front of the Dustcap, pointing inwards.... Worked quite fine. Once it was sufficiently debugged it was very stable, unlike any of the voicecoil based sensing systems (NI, Bridge....) which all worked fine untill the weather chaged or you turned up the Volume, upon which occasion thing wend reliably BANG....

Quote:
Originally posted by phase_accurate
Because you are in the speaker's nearfield (also with 12.5 cm), the 6dB/distance-doubling rule wouldn't exactly apply and the sound-pressure at the microphoine might be even a bit less than 141 dB for 123 dB @ 1m.
I know, but I could not be bothered to calculate accuratly, so I just assumed "worst case".... ;-)

Quote:
Originally posted by phase_accurate
If you ground the non-inverting input in Graham's circuit and put feedback to the inverting input you get current drive. Its pros and cons have already led to lively discussions on this forum !
Yes, indeed. On both counts.

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Old 21st August 2003, 02:55 PM   #10
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Default Re: Re: Gainclone with negative output impedance

quote:
Originally posted by Circlotron
If the coil resistance (not impedance) was zero then the speaker would do just exactly what the drive waveform was telling it to do.

Quote:
Originally posted by Kuei Yang Wang
Actually, that is chemically pure bovine excrement. Normal electrodynamic drivers are pure current controlled devices and will do anything but what the waveform applied as voltage tells them. If we convert the system to current drive the voice coil resistance becomes irelevant, as does compression and several distortion mechanisms.Sayonara
1/ Well, yes and no. If the speaker and whatever is driving it both have zero resistance then if the cone becomes difficult to drive at any point then it will try and move somewhat less. This will cause it's self-generated (back?) emf to fall below the driving emf therefore allowing LOTS of current to flow (no resistance remember) causing LOTS of extra mechanical force to be applied.

In theory, this should make the cone follow the drive =voltage= exactly. The resulting drive *current* would be proportional to how hard the cone needs to be pushed to maintain it's correct position to follow the signal at any given moment.

2/ After playing with it for a while, I have decided that this negative resistance thing is a rotten idea!

3/ The electret microphone feedback thing is something I must try. However, for a bass reflex setup I think a good location for the mic would be halfway between the speaker and the port so it hears a bit of both, not from the speaker only as you would do with a sealed system.
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