In the mid-1970's Philips designed their Motional Feedback (MFB) speakers of which I have an old pair, the model 22RH544 early version. These active speakers are three way with an active crossover splitting the input to feed an amplifier for the woofer and one for the midrange and tweeter via  passive crossover. The motional feedback feature is essentially a servo circuit where the low frequency signal is altered by a piezo electric detector that measures the woofer movement, giving a lower frequency response than the small closed boc would otherwise allow.

I bought my 544's second hand in the late eighties and fed them from a preamp. They have been in a spare bedroom for the past ten years or more but drew my attention again last year. I had already downloaded the user and service manuals with the circuit schematics and specsI had planned to service them this winter as a project to see how they sounded before and after but the recent wet weather gave me an earlier opportunity to start on this indoor pastime.

So I plugged them in on the bench and awaited any worrying signs like nasty smoke or noises, luckily they were fine. Next step was a low power audition where I noticed they were pleasant to listen to but lacked a little conviction on the bass. As one woofer was moving more than the other I guessed all was not entirely well so time for a full service.

The service manual explains the bias levels for the LF and MF/HF amps and how to set the MFB gain. Before proceeding with adjustments I thought it worth inspecting and testing some of the key parts. Being 40+ years old I suspected some components would be past their best, notably the old electrolytic capacitors. I let the amps power down and opened the first one up today. I unsoldered one cable going to the main smoothing capacitor, a dual can 2 x 2,350uF 63V so I could measure its current values. Even a quick capacitance reading of under 200uF with a lowish dc resistance of 20k makes the situation clear. Similar experience on the LP amp's output decoupling cap. My assumption is most of the other 20ish electrolytics are goners too but I stopped testing at that point.

I may well order the parts in a few months to replace all the electrolytics and the passive MF/HF crossover parts too. Right now the 544's are back in storage To be continued...

I have clipped a few magazine articles about the MFB concept and the various production models. I found online resources over the years including a very interesting Dutch website, unsurprising given it was Philips of Eindhoven who manaufactured them.

I have built up a parts list for replacing the various electrolytic capacitors. The ones in the MFB circuit boards are standard values and not exotic in any way. The circuits are single supply rail with dc blocking caps between the various stages. I will probably go for 63 volt electrolytics throughout based on previous advice from Graham.

On my early version of the 22RH544 the crossover frequencies are 500 Hertz ('active') between the two amps and 4 kiloHertz ('passive') crossover spliitting the mid and high frequency drivers. In the later versions this changed to 3.5 kiloHertz, not sure why but I will probably replace the polyester film caps and old inductors in the passive crossover so it's worth investigating. There is a impedance correction filter in parallel with the midrange driver. I'll be measuring the driver impedances and responses so I can check the crossover component values are right for the current driver specs.

These MFB speakers are active and don't use op amps. Their circuit schematic is classic 1970's amp design i.e. transistor stages like those Graham Slee has been explaining elsewhere. That has definitely helped me understand how the Philips' circuit works although I am not intending to make any significant chnages, just restore it to original spec with replacement components.

Thanks Richard, that's an interesting website and I did look at their modern take on the classic LS3/5A design too. Tempting but I really don't need a set of speakers in my living room!

When I get going on the 544's I'll probably start by replacing the preamp/filters and amp capacitors and hear that before swapping out the passive crossover parts where your tip will be very helpful. I tend to buy speaker crossover parts from Wilmslow and it never hurts to have an alternative with the range Falcon have put on at sensible prices.

I popped my mid-seventies Motional Feedback speakers on some stands this evening and am still listening! They are a pair of the 544's with the HF cut circuit and still running well with the original capacitors. Eighty quid in the late eighties!

Has anyone got or had a pair?

That would be interesting too. If you have a pair that work, you could use the service manual* to fix then and fine tune the servo feedback which is (ta dah!) the special feature of the MF series.

*available online or I can e mail you it if that would help?

I didn't get round to the recapping but did remeasure the supply caps and got similar readings in 2021. Then I listened ti them and found they were working!

Several hours ago I serviced my 544's. In each case a good hoovering with a soft brush came first. Visual inspection and physical stuff like unplugging replugging connectors.

Power up and 10 minutes warm up before adjusting the two amps' bias. Both had the mid/high amp bias close to correct whereas the woofer amp bias was low. Leave and recheck.

Then I set up the test signal and adjusted the MFB servo gain. The test was quieter than I expected and easy enough once my analogue signal generator settled on 125 Hertz.

I am now two hours into checking my work with some music. Currently I have Who's Next playing. It just reached Won't Get Fooled Again. Great as always and just older than my MFB speakers. Philips have earned more of my respect!

I should think so too! Otala did tons of research for them in the '70s - something lost on the back scratching hi-fi press.

The fact both active speakers are still working well 45 years later with the original parts inside is very impressive. That's worth respect as much as the excellent R&D Philips were known for and demonstrate in the circuits and drivers here.

To be fair I have looked after these speakers. They were the flagship model when my father worked for Philips in the seventies but too expensive even with staff discount. We had various bits of their kit including a music centre. So I was delighted to pick up a pair of MFB speakers second hand.

 

Marvellous what they achieved with just six transistors (per channel, power amp), and I'm hoping my six sounds good this week, because if it does, your wish will have come true. 

From what I've read your new amp is coming along very well indeed Graham.

Out of interest here is an example of the amplifier stages inside the RH544:

Clipping mullimeter leads onto the top of R670 and bottom of R673 allows you to measure the dc millivolts across 1 ohm, which translates into the bias current in milliamps with no music playing. In fact I shorted the input before this. As I wrote earlier the amp should be allowed to warm up before the measurement and then R665 can be used carefully to set the bias current flowing through the output stage.

I have changed the passive crossover frequency back to 4 kHz by replacing the 4u7 polester capacitors in my RH544's with polypropylene 3u3 capacitors. 3u3 was the original value and I have read the change was made to adapt to a different Philips tweeter. I used REW to measure the raw driver responses and their combined response with the crossover. Moving up has evened the midrange response and further improved the sound I am hearing.

Here is a photo of the inside of a motional feedback speaker, with a multimeter being used to check the bias.

Note the lack of IC's, it's all transistors in here.

Precious speakers, I wonder how many have survived?

Ian

Yes I agree Ian. I have modified and built lots of stereo and music equipment over the years. I enjoy getting back to hand soldering discrete components as I did in my youth.

Using a soldering iron and desoldering pump, I swapped out the 4u7 (yellow, violet, green) PET's you can see on the bottom left of the PCB. I kept the old ones which I think are Mallard 'tropical fish' when I fitted the 3u3 PPL's instead. I swapped out the yellow 6u8 Zobel cap for a PPL at the same time.

The good news is mine use standard electronic parts for transistors, resistors, and capacitors. When I serviced them reecently I hoovered the PCB's and cleaned up some solder splash. Couldn't see any dry joints which is good news, although I suspect issues like that would have become faults for repair.

I fitted phono input sockets in the late eighties as it allowed the use of readily available cables rather than DIN connectors although we did have DIN cables around.

There are still quite a lot of MFB speakers about but not many in the UK from my experience. These are the only ones I have seen. I bought mine second hand in Sudbury, Suffolk in the mid-eighties so suspect they had belonged to someone working for Philips in Colchester nearby. I know Philips made a lot of MFB speakers in the seventies in this style of rounded black cabs and the some taller brown wood ones in the early eighties with flat cones. So the classics like the RH532, RH544 (mine), RH545 (studio monitors which I think were used in mixing 'The Wall,' are still about and treasured as you say. Mostly in the Netherlands for obvious reasons.

There's a very good MFB site with forums full of useful MFB data that I found several years ago. There are posts on there from UK owners so I am not alone!

Inevitably (for me at least!) the next modification is coming along. I think I will replace some of the old electrolytics, starting with the power supply reservoir caps and the dc blockers on each amp. All 63V types for good reasons that Graham has explained elsewhere on our forum.

And I haven't forgotten why... I have been playing a lot of music on my 1970's active speakers.

It is raining outside so a lazy day of listening to music is tempting me! No I will get some exercise...  Maybe I could play another album first...

Trivially I also fitted a C14 socket on the older speaker that had an original 2 pin socket. I had converted the other speaker years ago when I lost the 2 pin mains lead in a house move

I listen at moderate levels so I am going to lower the LF extension by changing the rumble filter frequency. It has been written about on the MFB Freaks site so why not give it a try?

I checked the gain needed and amp power was sufficient. The Qtc is about 0.8 as the box ratio alpha is about 3, so response falls at about 11dB/octave. Fc is 77 Hz so 2 octaves down is 19 Hz. At 21Hz this would need over 10 Watts to match 1 Watt at 100 Hz.

It sounded fine and I tried some tracks with sub-bass. Then a 20 - 120 Hz sweep, no distortion but door and furniture rattled.

Reading service manuals helps. I found Philips had changed the negative feedback circuit to cut back the treble when they changed the tweeters, fitting a larger 5n6 capacitor to rolloff the treble when they put a new tweeter in. Having testing the FR and THD on the AD0140's in my 544's I understood why they wanted to dial them back. But I had put  modern fabric dome tweeters in that were well matched and quite smooth so I thought an experiment was in order with C561.

There is a HF cut control in the input stage of my 544's which operates from 7kHz so I knew I could reduce the treble later if I needed to. I tried 1nF, 4n7, and 2n2 PPL plus the original 5n6 ceramic. I went with 2n2 PPL as a good compromise on the FR and I liked the sound best. Another cheap modification I am enjoying.

You can have too much of a good thing which is why I went for 2n2 not less. Some guitar amps have a presence control that decreases the NFB signal with a capacitor in series with the pot so it is the treble that is boosted. It will be slightly  louder. The sound gets richer or less clean, turn it up and turn your guitar down slightly.

The fine MFB Freaks website has people going down to 680nF so I may yet try 1nF. See what I have in my spares box!