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Old 20th November 2003, 12:14 AM   #1
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Default Current Drive Power Amp

Once I read an article by Japanese about current power amp. He said that what drives a speaker really is current, not voltage.
But all this time we define audio power amp as a device to amplify voltage. That is like there is sinusoidal 1Vpp input, the power amp is amplifying the voltage to certain value, like 30Vpp.
If the speaker is purely resistive, we can get clean sinusoidal current from sinusoidal voltage, since the load is pure resistive. But since the speaker is Impedance (zr+zl+zc), not pure resistive, the current shape certainly be different from the voltage shape, cause of Zspeaker depends on frequency.
The idea is this. What happens if we make an audio power amp, that sense voltage shape input (like sinusoidal), but gives output of current, with that particular input shape.
Is this what we know as "current feedback" (usually using opamp like Alexander Feedback- Analog Device current feedback power amp), or is it something else?

My idea of current amp is like this. Look at the example schematic below. Let's assume that this is a working amp (not experimental).
The output is taken from the drain junction of SK and SJ. Push-pull amp will have this kind of output, usually taken in the junction of 0.22ohm/5W resistor (after emitors, if it is EF)
To get current sensor, before going to loud speaker, between the junction of those drains before output to sepaker, we put R, like 1ohm, to detect what is the current delivered to the speaker.
Then, this current figure is compared to the voltage signal in the differential input.
This way we don't need the 10k and 510ohm voltage feedback divider, since we are detecting the output current in the 1ohm resistor drop. That is the data for the differential to measure the difference in signal input and current output. Maybe this is "voltage input - current output" audio power amp. But is this possible? What will it sound?
This idea is coming from a statement that said that speakers are driven by current, not voltage.
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Old 20th November 2003, 12:32 AM   #2
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Default Re: Current Drive Power Amp

Quote:
Originally posted by lumanauw
To get current sensor, before going to loud speaker, between the junction of those drains before output to sepaker, we put R, like 1ohm, to detect what is the current delivered to the speaker.
Then, this current figure is compared to the voltage signal in the differential input.

this has been done for many years, going back to vaccum amp days. I have tried it on a couple of amps.

I could not detect any audible diffience for the most part but it did produce (marginally) better bass.

I think one can run a simulation (treating a speaker as a RLC network) using realistic data. I would be interested knowing what "numeric" improvement such a set-up would make and what audible difference it will make.
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Old 20th November 2003, 12:52 AM   #3
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Drivers are current driven BUT most are designed to respond as if voltage driven. Sort of a kludge based on ease of implementation. (easier to work with voltage since we don't have to open the loop (insert a current probe)) to see what's going on. A GC chip with the info on link below would be a easy way to experiment.

http://sound.westhost.com/project56.htm
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Old 20th November 2003, 09:47 AM   #4
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I'm quite confused with the picture attached. Picture A is ordinary voltage feedback power amp. I think the one in the picture B is not current drive power amp, it's still a voltage divider feedback, but using the speaker as upper resistance.
If it is a current drive audio power amp, how does it sounds? Has anyone tried to built schematic B?
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Old 20th November 2003, 10:15 AM   #5
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With a zero resistance loudspeaker voice coil, if we drive it with a VOLTAGE waveform the VELOCITY of the voice coil follows the drive waveform.

If we drive it with a CURRENT waveform, the FORCE of the voice coil on the cone follows the drive waveform.

Having said that, if there were such a thing as a zero resistance (but normal impedance) speaker, I get the feeling that for a sealed box voltage drive would be best, but for a vented box current drive *might* be better.

With a vented system, when the box/port is at resonance the cone would be allowed to reach minimum displacement while still applying the *same* force on the air. This is only approximated in a conventional setup with voltage drive courtesy of the voice coil resistance helping make a psuedo current source.

Not only that, it *may* improve the transient response of the vented enclosure because the constant-force drive would make the cone really get up and move until the (now not as) sluggish box/port got moving and started the reflex bit, damping the cone movement back to normal in the process.
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Old 20th November 2003, 10:23 AM   #6
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SEE:transconductance power amplifier for current-drive loudspeaker?
output stage with high impedance
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Old 20th November 2003, 11:11 AM   #7
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Quote:
Originally posted by lumanauw
I'm quite confused with the picture attached. Picture A is ordinary voltage feedback power amp. I think the one in the picture B is not current drive power amp, it's still a voltage divider feedback, but using the speaker as upper resistance.
If it is a current drive audio power amp, how does it sounds? Has anyone tried to built schematic B?
Your B schematic is indeed a current feedback amp, because the signal fed back to the input is a measure of the output current. Within the capabilities of the amp, the output current is 1.25A per V input voltage: it's a current source whose transconductance is 1.25 Siemens. This is what you were asking about in your first post. How it sounds? Check the other posts in this thread, I have no experience with that.

Jan Didden
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Old 20th November 2003, 02:48 PM   #8
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>With a zero resistance loudspeaker voice coil, if we drive it with a VOLTAGE waveform the VELOCITY of the voice coil follows the drive waveform.

>If we drive it with a CURRENT waveform, the FORCE of the voice coil on the cone follows the drive waveform.

Might be good for an open baffle where sometimes it's hard to find a driver of sufficient Q ........ ? ............ mike
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Old 21st November 2003, 12:38 AM   #9
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So, is it good for infinite baffle speaker placement? There is one application that needs such characteristic, that is when we place a subwoofer in car trunk, without box, just place it in infinite baffle (treating the trunk as a big box for the subwoofer). It has been known for a long time the subwoofer for this application has their own Q, quite different for the subs that is to be placed in a box.
Maybe current drive will help? We can use any subwoofer ignoring characteristic for infinite baffle?
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Old 21st November 2003, 12:42 AM   #10
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>We can use any subwoofer

Not 'any' subwoofer, but perhaps have a better selection.
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