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Philips MFB speakers

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Fatmangolf View Drop Down
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    Posted: 12 Aug 2018 at 8:20pm
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...
Jon

Open mind and ears, whilst owning GSP Genera, Accession, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.
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Fatmangolf View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 12 Aug 2018 at 8:29pm
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.

Jon

Open mind and ears, whilst owning GSP Genera, Accession, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 17 Aug 2018 at 10:42am
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.

Jon

Open mind and ears, whilst owning GSP Genera, Accession, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 17 Aug 2018 at 10:46am
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.

Jon

Open mind and ears, whilst owning GSP Genera, Accession, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Richardl60 Quote  Post ReplyReply Direct Link To This Post Posted: 17 Aug 2018 at 11:24am
Jon, Jerry at falcon acoustics has a wide range of caps etc for speakers with close tolerance OE spec and reasonable pricing. They have just brought out some New but OE spec LS35as too!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 17 Aug 2018 at 12:07pm
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.

Jon

Open mind and ears, whilst owning GSP Genera, Accession, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.
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