The Panasonic MF-800 Motional Feedback Speaker-Amplifier System

[mediatechnology]

I have a vintage Panasonic "Ultra Motional Stereo Hi-Fidelity" tube amp from the 1960s made by Panasonic which sold for a brief time at Radio Shack in the mid-to-late 1960s.
I recall my brother buying it on mark-down and that Radio Shack had purchased the excess inventory from Panasonic. It was never in the catalog to my knowledge.
Now I see one listed in the Europe for 600 Euros. My how things have changed.

This amp and I have a history - I grew up with it - and recall hearing Derek and the Dominoes Layla on it for the very first time.
I also listened to the news reports in Dallas about the goings on in upstate New York at the Woodstock Music Festival.

There's not much published on the web about this interesting technology and unit from the 1960s in which Panasonic used electro-mechanical feedback from the low frequency driver.

The MF-800 is a newly developed speaker-amplifier combination which uniquely utilizes a new variable ACCELERATION-VELOCITY type motional feedback system. This system features adjustable damping and frequency response in the low frequency range of the speaker by using a differential circuit in the feedback loop.

The original manual and schematic were printed with a brown ink giving them a historical look even then.


Panasonic MF-800 Motional Feedback System Amp and Speakers circa 1968.


Panasonic MF-800 Motional Feedback System Feedback Loops


Panasonic MF-800 Motional Feedback System Schematic

Panasonic Model MF-800 Motional Feedback Amplifier with EASB-8M2 Speakers (10MB pdf): https://proaudiodesignforum.com/images/ ... Manual.pdf

Update: Apparently Panasonic licensed the Luxman Motional Feedback patent used in the 1962-era Luxman SQ-65.
The Panasonic amp was also called the SA-30 and also known as the National SA-30.

[emrr]

Interesting. Assume you still have the speakers and they still work? Can you get a pic of the sensing coil in the woofer?

[mediatechnology]

Assume you still have the speakers and they still work? Can you get a pic of the sensing coil in the woofer?

I do still have the speakers.
I haven't heard them in maybe 3-4 years.
I need to figure out how the grill frame is attached since that appears to be the only way in.
The cloth is good but the cabinet needs a little restoration.
Once I get to that part and figure it out I'll get a pic of the sensing coil.

The magnetics in this unit look top-notch. There's a picture of one on the 'net where the Panasonic power transformer bears a Lux label.
I have to wonder if the Matsushita factory made parts for - or built - the Luxman units.
Apparently there was a National-branded SA-30 and another receiver that used MFB.

[JR.]

Interesting stuff... nice

I was aware that Phillips messed around with motional transducers on woofers beck in the '70s but not that Matshusita predated that early attempt. Later Danley made a woofer driven by a servo motor so arbitrarily precise, but even he abandoned that technology for recent design.

JR

[mediatechnology]

I think the Luxman SQ-65 MFB predates both Panasonic and Philips but I can't find the 1962-era patent granted to Lux.
There must have been some relationship between Panasonic and Luxman.

Philips and a number of other people online seem to think Philips invented the entire concept but they didn't publish until 1968 with the Patent awarded after that.

[mediatechnology]

Panasonic Motional Feedback Article from the Miami News, October 19, 1964.


Panasonic Motional Feedback Article from the Miami News, October 19, 1964.

[mediatechnology]

AKG BX-10 Reverb Motional Feedback

Was looking at the BX-10 documentation and noticed that it used Motional Feedback in both the drive and return electronics.

Block #7.


AKG BX10 Block Diagram Motional Feedback

[mediatechnology]

From AudioExpress: https://audioxpress.com/article/voice-c ... e-radiator

In the realization of an active loudspeaker, one of the many useful techniques that has been applied is that of “motional feedback” (MFB)—the sensing of the motional output of the transducer diaphragm to monitor the device’s behavior as compared to the reference input signal and realize a difference signal to be used to determine an appropriate compensation for any correctable errors.

While the approach can be applied to compensate for many small and large signals, linear and nonlinear effects, the greatest subjective impact is often derived from the ability to correct for large signal nonlinearities that occur at the overload limits of the transducer.

The technical literature is rich with investigations into applying MFB to loudspeakers, with one of the seminal reference papers being that of Egbert De Boar, “Theory of Motional Feedback,” IRE Transactions on Audio, January–February, 1961. One of the earliest documents describing an actual reduction to practice of a practical MFB loudspeaker was “A Motional Feedback loudspeaker System” written up in the Philips Technical Review (Volume 29, No. 5, 1968) by J. A. Klaassen and S. H. de Koning of Philips Electronics.

Philips is widely credited with introducing the first loudspeaker product incorporating MFB in 1973 (the RH532, known affectionately as David for its small size and powerful sound). But, it was actually Panasonic (Matsushita), applying Luxman’s SQ-65 MFB, vacuum tube, motional feedback electronics in loudspeaker systems in the early 1960s — models: 12P-X3 tri-axial and EAB-8M2 Speaker Systems — that appear to be the first commercial product to incorporate MFB. Those of you who enjoy such things will be entertained by the thorough operating manual, including schematics, operational principles, and factory assembly line pictures, which can be found here.

Try finding an owner’s manual like that with any product made currently. Audio hobbyists were seriously involved with all technical aspects of their equipment in those days.

Of course, as with so many audio technologies, the first inventor did not see his creative ideas realized in production and that also seems to be the case with MFB loudspeakers—for which the earliest patent appears to be US Patent 2,194,175, “Distortion Reducing Arrangement” to Karl Wilhelm, filed just one year after the two famous 1937 patents on negative feedback in power amplifiers, by H. S. Black and H. T. Nyquist were granted.