Bass-amp speaker: additional winding on voice coil core (Rickenbacker TR120B)

[clintrubber]

This is about a Rickenbacker TR120B bass combo (early '80s), for which I've yet to find the schematic.
Many 'close'/related schematics around, but not the exact schematic (in case anyone.... thanks!)

Interestingly, this bass-combo seems to have a 15" speaker with an additional winding.

Some quotes of long ago from the company-boss Mr Hall:
"I seem to remember that the TR120B had two upgrades from the TR35B: a dual speaker coil and a piezo tweeter. The extra coil was connected to the power amp to provide damping feedback, while the Motorola piezo was able to be switched on and off."

"It's been a long time since I've looked at one of these so my memory is a bit fuzzy. However, as I recall, this was the model that has an additional winding on the voice coil core which is connected to the power amp. It serves to sense where the cone is and adjusts the damping factor of the amp dynamically. If you have the cabinet open, see if there isn't an extra set of lugs on that speaker."

Since these rickresource.com-quotes are from long ago, I'd figure it'd be better to post here @groupdiy.


While this additional coil won't be wired in total mysterious ways, it's funny exactly this amp-schematic is not 'around' or discussed. So I'm curious if anybody has it or knows more?

(BTW #1: This position-sensing makes one think of the old Philips Motional Feedback system for hi-fi speakers, although there another kind of sensor was used.)

(BTW#2: As it seems, the Rickenbacker RB120 is the follow up type of the TR120B, but this schematic (see: RB120 powerstage ) is conventional w.r.t. powerstage & speaker-connection.
And possibly due to the higher power, it neither uses the 'usual' lift-up low-value resistor in series with the speaker to provide some current feedback, as is often done in low to medium instrument power amps for some added 'beef' in the sound.)

Thanks!

 

[C.B. - Boudio]

I think National (/Panasonic) used to have a HiFi line, with something MFB(-ish), in the sixties, still with a tube amp. They may have used a coil instead of a piezo like your case.

The small Belgian company Servo-Sound made, it was then their core business it seems, in the late sixties, also motional feedback loudspeakers (powered speakers, like the Philips ones but not as standalone). Seems they last untill Philips came along. I think they used a piezo as well, but I could be wrong, so maybe it's worth checking out.

I think there could even have been another HiFi brand out there, but I don't know which anymore.

 

[clintrubber]

I think National (/Panasonic) used to have a HiFi line, with something MFB(-ish), in the sixties, still with a tube amp. They may have used a coil instead of a piezo like your case.

The small Belgian company Servo-Sound made, it was then their core business it seems, in the late sixties, also motional feedback loudspeakers (powered speakers, like the Philips ones but not as standalone). Seems they last untill Philips came along. I think they used a piezo as well, but I could be wrong, so maybe it's worth checking out.

I think there could even have been another HiFi brand out there, but I don't know which anymore.

Thanks for the response and additional info, let's see if I can find something on those '2nd coil amps'.

It also made me check which kind of sensor was actually used by Philips for their MFB.
I knew it wasn't an additional coil, but like you said, indeed a piezo.

Here's the first search-result I found, which neatly summarizes it all - in Dutch though, but I see we're in the same language zone

https://mfbfreaks.com/motional-feedback/geschiedenis/motional-feedback-achtergronden/

W.r.t. the Rickenbacker amp, while I'm not challenging the recollection of the RIC-CEO, it'd be a bit of surprise to me if indeed this amp would be using that kind of feedback system and relatively so little talk about it. OK, to be continued.

 

[C.B. - Boudio]

https://www.waynekirkwood.com/image...al_Feedback_Amp_and_Speaker_System_Manual.pdf

The National/Panasonic/Matsushita ones worked with a coil. But it was not an extra winding on the voice coil, rather another coil connected to the front of the woofer, also moving inside another magnet? And this was then brought into the NFB.
Or that's what I understand from it.

It strikes me that they managed to make (kind of) an MFB system with tubes, because at Philips, they 'waited' for the transistor to make it doable.
But it was of course less effective than the Philips or Servo-Sound ways.

 

[clintrubber]

Thanks for that pdf-link, nice info.

I've never had any of those Philips MFB-types myself, but thought to have understood many existing ones may have a broken sensor, (possibly with the nth owner even unaware of it).

OK, if all goes well I may have access to the Rick-amp, can have a look then how things are wired.

 

[C.B. - Boudio]

I've never had any of those Philips MFB-types myself, but thought to have understood many existing ones may have a broken sensor, (possibly with the nth owner even unaware of it).

Never heard of that!
I've been looking out for a set for a while know, but prices are getting a bit crazy imho - although they sound amazing according to seemingly quite a number of people. So this is worth taking into consideration inderdeed!

 

[clintrubber]

Never heard of that!
I've been looking out for a set for a while know, but prices are getting a bit crazy imho - although they sound amazing according to seemingly quite a number of people. So this is worth taking into consideration inderdeed!

Yes, damaged/disconnected/non-operating sensors, it sure can happen I've heard from a few people who were quite into these, but don't know how often actually. Inderdaad at least something to keep in mind and check.

Own hands-on MFB-experience limited to the repair of the DIN-sockets of the MFB-speakers of a friend, at least for those types these were PCB-mounted and needed to be replaced or resoldered. That's all doable.

 

[clintrubber]

FWIW, let's update this thread.



The Rickenbacker TR120B combo has arrived.
But despite the sales leaflet happily suggesting special/advanced/extended wiring of the 15" speaker, there's no trace of any such thing in the actual amp here.
(Nor could it have been, given the number of connectors & wiring between amp-chassis and speaker-compartment).

It may have been a production change, or it's just something that only existed on paper.

Despite this 'fake tech news', amp sounds fine so far though.

 

[C.B. - Boudio]

Ow, that's a little bit of a bummer.

 

[clintrubber]

Ow, that's a little bit of a bummer.

Must admit I initially was a bit surprised to see this 'separate control winding on speaker'-feature advertised, since it suggests tighter bass-response, as in: keeping the speaker-movement better under control.

So that would have been the other way as one would expect. As solid state power-stages for guitar/bass-amplifications tend to use a bit of current feedback as well (a mix of voltage- and current-feedback), this allowing a (hopefully pleasant-sounding) low-hump at the resonance frequency of the speaker.