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Old 20th December 2018, 09:07 PM   #231
Armand is offline Armand  Norway
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Join Date: Aug 2005
Originally Posted by esl 63 View Post
I have used ordinary piezo tweeter chrystals with success.
Make sure to put an OP amp buffer close to the sensor since the piezo chrystal has high output impedance.
I am concerned about the linearity of cheap piezo sensors. Do you have any data or measurements?
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Old 23rd December 2018, 07:10 PM   #232
Turbowatch2 is offline Turbowatch2  Germany
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Join Date: Aug 2013
Location: Northern germany
for some reason this thread seems to be hooked to high tech acceleration sensors. Maybe if we could have a look at other sensors too, from the other side, there might be simpler alternatives even better suited? Instead of asking for real data as a sensor in a woofer, a look as how something performs as a transducer could give some hint. So the distortion of a piezo tweeter will be somehow similar to its sensor abilities and they do quite good. Also there are simple coils with diaphragms, moved in a magnetic field, which form microphones with remarkable low distortion. My last idea are two opposite areas that form a capacitor and can be integrated easily in a large woofer with decent voice coil.
Is there a point I have missed that makes these analog devices unfit for the task?

In my opinion nothing would be more convenient, also seen under a DIYS perspective, than a separate, small coil under the dust cap, with a tiny magnet in it, giving a perfect signal for a correction circuitry. A piezo extracted from a tweeter, with added mass should be worth a try, too.

Hunting for lowest distortion might not be the ultimate aim, there is some tolerance for it under 200 Hz if we could use this as the upper limit for a “true woofer”. As well, reduced distortion will always be a side effect of any MFB.

In the past, the need for stable high power amplifier was the most important cost factor of MFB, making it unattractive for commerce.

A long time ago I build some simple active loudspeakers with a resistor in the negative wire. The closed cabinet was 120 liters and extremely well build, the speakers cheap 12” with relatively small magnets, about .7 Qts and 25 Hz resonance, but (for that time) long travel, +-6mm linear. Crossover was 100 Hz 12db/cot The audible result was incredible impressing, deep and dry. The speakers had an 4 Ohms 100 watts rms rating, but the amps, each with a 600W torodial, could be seen short before clipping on the oscilloscope at +- 60 volts. At this power level the sound was still distortion free. So two 400 watt amps where needed for party level sound pressure, at least twice the energy for the plain speaker. Although the membrane was moving in and out like crazy, the woofer had no problems. At that time such strong amps where heavy and prohibitive expensive, not to speak of the second pair, needed for the conventional mid-high two way speakers.

After some problems where solved, the adjusted system was surprisingly stable and worked for a few years, until the woofers foam surrounds fell apart. To achieve final stability I had to use thick wires, ground the loudspeaker chassis and place the amp close behind the speaker. The resistor for feed back and potentiometer for adjusting it where directly behind the speaker terminal, cutting wires to under one meter. Until the amps reached temperature, the speaker cone moved about 1 mm out, but this did not effect the function.

I experimented with a Philips MFB woofer, too, but the sensor was always running away and had to be constantly readjusted. Maybe the amp was not stable enough. Anyway, I did not see much use in a small 7” or so, tortured to give 30Hz or even deeper at low volume.
From my perspective, the Phillips stuff was not the final system, just cheep stuff over developed and much too expensive. Makes more sense making a good woofer better than turning a cheap, mediocre loudspeaker in a good one, like Phillips did.
The Philips speakers did not excel, if size was no factor. They sounded good, but not better than average., no winner at all. There where many passive speakers at that time, sounding much more fascinating.
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