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Old 3rd May 2012, 09:47 PM   #1
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Default Below 20 Hz

Hi.

Im planing on building a fan powerd subwoofer (similar idea to the TRW-17 rotary subwoofer, but different design) useing valves to direct air preasure. Its probably going to have a verry low frequency range, im aiming for 1 to 20 Hz.

I have most of the design done for the fan and the valves, but I have run in to a problem. I need something to drive the valves. I will either use an electric motor, or, preferably, I will use a magnet and a voice coil.

If I am to use a voice coil and a magnet like in any speaker I will need an amplifier. From what I heard most amps dont deliver much below 20 Hz. I have seen some rather expensive amps that deliver down to 5 Hz, but for this project thats not low enough. I need something which goes down more or less all the way to DC.

The reason I would rather use a convensional amp and a speaker is mostly because I want to play audio through it, and with a motor controling the pitch I have no idea how to conect it to an audio source. Its also because I think a convensional voice coil would give a better frequency response, maby raising it up to 20 Hz.
The speakers I have now can handle down to about 25 Hz at a high volume, and down to 16 Hz with a hearable signal, so to I need to make sure there is not a scilent gap around 16-20 Hz between the ordinary woofers and the fan powerd woofer.

So If anybody knows of an amp with a good frequency response below 20 Hz I would be happy to hear about it. Or if you have any other valuable input on the project I would very much like to know about it.

[sorry to the mods if I put this in the wrong part of the forum]
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Old 3rd May 2012, 10:17 PM   #2
sesebe is offline sesebe  Romania
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The solution to your problem is relatively simple, almost any amplifier DC coupled without servo can do this. I used TDA7294 to make a modulated power supply (12V DC + 1V AC) for some tests. For TDA7294 you need to bypass the input capacitor and the feedback capacitor.

My question is what signal you feed in and from what equipment to have useful DC components ?????
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Old 3rd May 2012, 10:25 PM   #3
sreten is offline sreten  United Kingdom
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Hi,

How can it be a fan subwoofer if you are using "valves" to direct
air pressure ? That simply isn't how fan subwoofers work ....

Down to <1 Hz is easy with a DC servo amplifier.
You've got a reply chain down to 1Hz except the power amplifier ?

You understand down to 1Hz may be measurable and not a problem
for a fan subwoofer, but due to loudness curves for hifi near irrelevant ?

rgds, sreten.

I suspect bad distortion issues with whatever is planned, if it works.
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Last edited by sreten; 3rd May 2012 at 10:29 PM.
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Old 4th May 2012, 01:10 AM   #4
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I used to work for an oil exploration company that used hydraulics to drive the sea or ground in a sweep from .5-20 Hz. They had a hydraulic pump powered by a 1500 hp CAT diesel engine. They had a solenoid hydraulic valve driven by - oddly, a tube amp. This was built in 1976 before there was any semiconductor better than the 2N3055 (selected). They used 4 parallel 5881 tubes to drive the hydraulic valve. It was a rip when they were testing this thing out in the parking lot. They would lift a whole truck with the hydraulic cylinder, then drive the ground with the weight of the truck sweeping from .5 to 20 hz. They had a very primitive TTL sine wave oscillator using 32x1 PROM IC's I believe for the waveform.
The main limiter on the DC if an amp is not the output stage of a power amp, but the capacitor on the input stage. If you are going to go down to DC you have to start worrying about offset voltages and nulling. Mostly in audio we use 8 pin dual op amps with no nulling function. There are 14 pin op amps that have nulling inputs to make the output stable at DC.
There are scientific amps that are specified out to DC but they are expensive.
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Last edited by indianajo; 4th May 2012 at 01:20 AM.
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Old 4th May 2012, 01:51 AM   #5
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"motor controlling the pitch"...

There are some Danley designs using belts/cones or fans directly from a modulated rotary DC motor shaft that oscillates at low audio frequencies. But most high-power fan-type subs now are constant-speed and modulate the blade pitch instead. That way you get a lot of audio power from relatively modest amplifier power. The big constant-speed fan motor is acting like a mechanical amplifier.
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Old 4th May 2012, 02:01 AM   #6
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direct-drive fans where you modulate the fan blades have a lot of problems. You may need a huge array of expensive motors and fans to accomplish much, and a bank of amps too.

Seems to me the trick is coming up with a good way to modulate the fan pitch quickly. Usually some kind of linkage, a cross between a car-clutch's throwout bearing and engage/release fork and the linkages on an airplane or helicopter prop. But if you can use some kind of linear actuator like for the arm on the big old disk drives, and mount it on the spinning shaft, that could be good. Some kind of piezo-electric blade material is another possibility. So then you start wondering how to get the power and/or signal to the rotating mechanism, and a brush commutator doesn't seem like a great idea.

So how about a standard constant-speed motor, but have it spin another set of coils to power the on-shaft balde actuator...you could modulate the field coil that couples to the second set of coils, to control the blade pitches..
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Old 4th May 2012, 09:37 PM   #7
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Quote:
Originally Posted by indianajo View Post
I used to work for an oil exploration company that used hydraulics to drive the sea or ground in a sweep from .5-20 Hz. They had a hydraulic pump powered by a 1500 hp CAT diesel engine. They had a solenoid hydraulic valve driven by - oddly, a tube amp. This was built in 1976 before there was any semiconductor better than the 2N3055 (selected). They used 4 parallel 5881 tubes to drive the hydraulic valve. It was a rip when they were testing this thing out in the parking lot. They would lift a whole truck with the hydraulic cylinder, then drive the ground with the weight of the truck sweeping from .5 to 20 hz. They had a very primitive TTL sine wave oscillator using 32x1 PROM IC's I believe for the waveform.
The main limiter on the DC if an amp is not the output stage of a power amp, but the capacitor on the input stage. If you are going to go down to DC you have to start worrying about offset voltages and nulling. Mostly in audio we use 8 pin dual op amps with no nulling function. There are 14 pin op amps that have nulling inputs to make the output stable at DC.
There are scientific amps that are specified out to DC but they are expensive.
This sounds alot like what im planing on building. Is there anywhere I could find more information on this?
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Old 5th May 2012, 12:55 AM   #8
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Quote:
Originally Posted by PotatisSlottet View Post
This sounds alot like what im planing on building. Is there anywhere I could find more information on this?
The technology was sine wave sweep seismology, was going obsolete even as I worked there 1975-1977. Eventually the 3 trucks ended up in the back lot parked. They brought back the bladder from sweep boat # 2 and all the diesel engines etc. The hydraulic pumps weren't special, the diesel engines weren't special, I don't imagine the hydraulic cylinders lifting up the trucks over the weight were special. But that solenoid valve that would drive a 5" diameter cylinder from that output of 4 -5881 tubes, that was very special Up to 20 hz, too. I didn't catch where the solenoid valve was made. Vickers, maybe? I have a good memory but I didn't work on it, just was impressed by it. That solenoid valve must have had an energy gain of 100000 or something- way more than a transistor or tube.
My employer was Seiscom Delta, but they were bankrupt even at the time- the bank downtown was running us. I imagine sine wave sweep seismology is a footnote in some old geophysics book in some library somewhere in Texas or Oklahoma. But it made a **** of a show. The windows and garage doors in the building would rattle from 5 hz up. Your stomach would jiggle. Impressive.
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Last edited by indianajo; 5th May 2012 at 01:01 AM.
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