Driving Magnepan MG QR 1.5 from transconductance / high output impedance amp?

[padamiecki]

Hi!

I know how hungry Magnepan’s drivers for the current are, I know about their impedance and resistance, and general efficiency but I wanted to try something different, against all of the recommendations:

I connected them to transconductance chip amp with ~300 Ohm output impedance, this means the damping factor for Meggies in this case was ~0,01!!!

My first impressions were: strange sound…

But then I found good bass, extremely good mids, delicate and detailed highs and listening scene became holographic with unlimited depth.

If you can please give your impressions with similar amps, you can easly build one in a half of hour: http://www.diyaudio.com/forums/chip...-watt-transconductance-current-amplifier.html or http://www.diyaudio.com/forums/chip-amps/270956-lm1875-transconductance-amp.html

Magnepan’s drivers has (in general) flat impedance curve in audio region so maybe this is the explanation of “high impedance amp” good sound?

 

[bwaslo]

If the impedance of both the speaker and the amplifier output are flat, then there shouldn't be any difference (in frequency response) from driving from a low impedance amp.

 

[RogerGustavsson]

The Magnepan speakers are really not very current hungry. Most of them are close to resistive loading 4-5 Ohms. If you have 200 W amp, you will not see that many Amperes.

 

[sreten]

Hi,

Any amplifier with 300 ohm output impedance will have a damping
factor of 37.5 into 8 ohms, and some serious issues with a normal
loudspeaker impedance profile. But the not the Maggies which
are near flat resistive and don't care much about current drive.

rgds, sreten.

 

[ErikdeBest]

I was just about to ask a similar question, but related to the NEO10 and NEO3 drivers played from a transconductance amplifier.

interested how this thread evolves, so subscribed!

 

[padamiecki]

The Magnepan speakers are really not very current hungry. Most of them are close to resistive loading 4-5 Ohms. If you have 200 W amp, you will not see that many Amperes.

I agree with classic VFA or CFA. But with my trasnconductance amp, watt meter showed ~25W at normal/higher listening level what was enormous level for dynamic loudspeakers for whom I did not see more than 1W! ... but to be honest with the speakers of efficiency 96dB/m...

 

[padamiecki]

Hi,

Any amplifier with 300 ohm output impedance will have a damping
factor of 37.5 into 8 ohms, and some serious issues with a normal
loudspeaker impedance profile. But the not the Maggies which
are near flat resistive and don't care much about current drive.

rgds, sreten.

Dear Sreten!

for me, maybe I am wrong, in the case, DF = Zload/Zsource = 8 Ohm/ 300 Ohm = 0,02(6)
for Maggies; 2,7/300 = 0,009 where 2,7 is HF speaker resistance

Regarding those serious issues, maybe it exists but I experienced lots of loudspeakers showing only light sound coloration with transconductance amp.

But why Maggies are playing completely different having flat impedance/phase vs frequency?

I guess that 10uF cap parallel to LF woofer from the crossover has big impact on high output impedance of transc amp?

 

[padamiecki]

I was just about to ask a similar question, but related to the NEO10 and NEO3 drivers played from a transconductance amplifier.

did you try them already with transc amp? What are your remarks?

 

[ErikdeBest]

Hi Pawel,

not yet - I am still building the speakers. I would say it doesnt matter a lot, as the impedance of the NEOs is rather a DC resistance
http://medleysmusings.com/bohlender-graebenar-neo10-planar-transducer/bg-neo10-impedance-comparison/

I also (just) read this, from http://www.current-drive.info/faq/25#ankkuri8

Do panel speakers suffer from EMFs?
In electrodynamic (i.e. magnetic) panels, the proportions of motional EMF and inductance EMF in the total voltage of the speaker are much smaller than in voice coil drivers, due to lower flux density and the coilless structure, which is seen as nearly flat impedance. Hence, the magnitudes of the EMF-derived currents relative to the total current remain minor, and electrodynamic panels exhibit much less of the adverse effects due to voltage driving than the usual speakers do.
Electrostatic speakers, in turn, are totally free from actual electromotive forces since there are no magnetic fields involved. ESLs, however, have their own corresponding motion-related effect, that manifests itself as plate current directly proportional to the diaphragm velocity. This motional current, however, does not cause any distortion or impairment as long as it is supplied from a low-impedance signal source, keeping the voltage intact.
Open speakers are also free from any cabinet noise leakage through the diaphragm. Thus, there are found definite reasons for the reputation these systems enjoy, taken the available amplifier technology. (This is not to deny that directivity properties may also play some role.)


regards, Erik

 

[sreten]

Dear Sreten!

for me, maybe I am wrong, in the case, DF = Zload/Zsource = 8 Ohm/ 300 Ohm = 0,02(6)
for Maggies; 2,7/300 = 0,009 where 2,7 is HF speaker resistance

Regarding those serious issues, maybe it exists but I experienced lots of loudspeakers showing only light sound coloration with transconductance amp.

But why Maggies are playing completely different having flat impedance/phase vs frequency?

I guess that 10uF cap parallel to LF woofer from the crossover has big impact on high output impedance of transc amp?

Hi,

Damping factor = Zsourse/Zload.

The maggies will be different if the impedance is not totally flat.

rgds, sreten.

 

[SY]

Hi,

Damping factor = Zsourse/Zload.

No, you have it upside down. Zload/Zsource.

https://en.wikipedia.org/wiki/Damping_factor

 

[pieter t]

No Sreten,

Pawel is right: DF = Zload/Zsource.

 

[padamiecki]

crossover freq?

Well, what I have heard so well is probably the crossover frequency? C1, R1 are Zobel parts... the rest is MG schematic and 0.3 Ohm resistor is for gain setting.