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Old 15th November 2001, 05:04 PM   #1
Bill F. is offline Bill F.  United States
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Bottom line: I don't know much about amplifiers. I know just enough basic electronic theory to be dangerous. So I'm hoping some resident amp guru will answer the following naive questions for me:

I'm putting together a line array of around 30 drivers/channel (cheapo PE 269-469). This ought to be about 104dB/W efficient, so I figure 10 watts should drive me out of the room.

I'd also like to build a one-off amp to run these because... (Here's where I swerve into the weeds)... I'd like to wire all the drivers in parallel to eliminate back EMF interactions. In this case it would work out to a nominal load of just over a quarter of an ohm!

Before you laugh (or after you've finished), let me elaborate on the beauty of the concept: Such an absurdly low impedence certainly wouldn't need many volts to drive it. So I got wondering, could I construct a pure current amplifier that would simply use the line level voltage ( about 2-7 volts) from a CD player or DAC to directly drive a bank of MOSFET outputs?

I'm picturing a stepped attenuator on the line for volume control. This chain would eliminate the preamp and the voltage amplifier stages of the power amplifier. Less messing with the signal!

Now, I realize that there's probably a reason why I've never seen an amplifier capable of driving .25 ohm comfortably. What is that reason? Is it merely the grand ol' 4-16 ohm tradition?

Are there issues that arise when output voltage swing is so small compared to current? Are their weird, funky interactions with the inductive load of the speakers at these voltages/currents? Issues of cost/practicality/zodiacal alignment?

What would stand in the way of me doing this?

I'd deeply appreciate any/all input.


Bill

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Old 15th November 2001, 07:02 PM   #2
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Increasing current increases distortion in the
output stage; can't get around that. You'd have
to parallel a bunch of MOSFETS and the power supply
would have to be rated for a lot more current--bigger
transformer with a hefty secondary winding. Even
with MOSFETs, the drive current might be significant.

Wiring resistance would start to become signficant, too.

Seems like a lot of trouble when existing designs are
already into vanishingly low distortion.
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Old 15th November 2001, 07:51 PM   #3
Bill F. is offline Bill F.  United States
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Yeah, the current output to the speakers at 10 watts would be the sq. root of 40A--a little more than 6.5A! Wiring would have to be heavy gauge.

While I recognize some of these practical issues, I'm still trying to figure out what is and is not possible--an exercise in "what if." (Also, I'd really like to wire my array in parallel!)

As for the attraction of the source directly driving the output stage, I was indulging in a little less-is-more philosophy. (However, in this case I guess less impedence equals more MOSFETs! What's that Enstein quote Nelson Pass cited? Make things as simple as possible, but no simpler?)

Anyway, you're saying that it is possible to drive .25 ohm with enough power supply, MOSFETs, and robust build? I understand (theoretically) that MOSFET gates draw almost no current. What about charging the gate capacitance of a large bank of MOSFETs? Would that put an undue load on a typical line-level source?

I'm also still curious if there are any interaction issues with driving speakers this way. Any increased chance of ocillation?
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Old 15th November 2001, 08:13 PM   #4
walker is offline walker  Australia
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Bill, original thought scares people. Anxiety complex over, I donít think that what you are suggesting is likely to be practical or an improvement over current designs, but Iíve been wrong before. There are lots of issues with your design, both with the speakers, cables as well as the amp, I think youíve realised that already.

Just a few issues to add to those already mentioned.
I donít think that the transformer size will be an issue, even though the amps efficiency is likely to be lower than normal. The power supply filtering will have to be massive, I can only imagine what the ripple current would be like. Also with so little voltage you may have to resort to current feedback. There are semiconductors around that will handle the current easily, check out some of the new IGBTs, they may be a better solution than FETs.
You may want to seek out designs for industrial servo amplifiers.

If you parallel drivers (speakers) they should be matched to have the same properties and enclosure loading/dampening, other wise each speaker can get driven by the other speakers as well as the amp. That is, at some frequencies they can start doing their own thing. Even the same model drivers from the same manufacturer can have different properties. Having said that it is often done

I canít wait to hear all the other problems that will come up. If you really what do go ahead please let us all know how it turns out. He who is afraid of failure is unlikely to discovery anything new. Even if you donít build it youíve woken me up.


Regards WALKER

PS Check out Elliot Sound Products's site on speaker impedance.

[Edited by walker on 11-15-2001 at 03:15 PM]
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Old 15th November 2001, 10:42 PM   #5
Bill F. is offline Bill F.  United States
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Well, I guess that's the mixed blessing of being an amplifier neophyte--blissful ignorance! Until I accumulate a sufficient reservoir of knowlege/experience, I'll be both misguided and original.

As to the parallel wiring of speaker drivers, my array is open baffle, so cabinet loading/damping aren't invited to the party. Also, the drivers are not only identical models, but also have matching lot numbers. Short of hand matching, they're as close as I can get.

Correct me if I'm wrong, but doesn't parallel wiring minimize driver interaction, especially in a massivly parallel array? It seems to me that any series wiring would piggyback deviations in resonance and inductive reactance, forcing drivers to modulate each other.

My (possibly flawed) perception is that in a .25-ohm parallel array of 32 8-ohm drivers, the vast majority of back EMF will be dissapated in the amp output circuit (assuming reasonably low output impedence) rather than in any (comparatively high impedence) driver. That was the most attractive part of my .25-ohm amp, you see!

Is this misguided, original, both, or neither?

Bring me back if I'm in the weeds again.
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Old 16th November 2001, 12:18 AM   #6
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Two points occur to me:
1) The current draw is going to be ferocious. Your main power transformers are likely to end up being filament transformers or the like. That current will need to be dumped into the circuit as bias, so depending on what rail voltage you end up with, you could very well end up generating a lot of heat, even if you go class AB or B.
2) You won't have much damping factor from the amp side of things. Something that frequently gets overlooked is that the lower the driver impedance, the lower the damping factor. If you're looking at .25 ohms for the speaker array, you're going to have to have a vanishingly small output impedance in the back end of this thing. That said, the drivers will to some extent serve to damp each other. I hope the cones/diaphragms/whatever are low mass (hence less needful of a damping factor to crack the whip should they get out of line). What kind of drivers are you planning on?

Grey
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Old 16th November 2001, 07:30 AM   #7
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Bill, thankyou for a great thread mate, (look out he's from downunder). I would like to start by restating that I think you are barking up the wrong tree Having said that even if you go ahead and to doesn't achieve the desired results we will have learnt for your endeavours.

I think that paralleling drivers still allows rogues to miss behave but not as much as series connections. If one cone is doing it's own thing and say moving outward, another cone could be operating at the same resonant frequency but 180 deg out of phase. Nett result is that the amp's happy as are both drivers. But don't let this small issue worry you as many a fine speaker has paralleled drivers, (my beloved AR9s included).

Regarding the amp let's stray a little further into the weeds, it's a nice day for it. Have you considered individual, open ended, (without overall negative feedback, relying on the local device feedback) output devices connected to individual drivers. Bingo dampening issue over. I'm scaring myself now. Reduces some of the concerns of the output stage. Don't expect textbook distortion figures, but I rarely listen to textbooks on my system anyway.


Regards WALKER

Here's to the road less travelled!
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Old 16th November 2001, 09:55 AM   #8
hifiZen is offline hifiZen  Canada
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Yikes! Yeah, you're really far off into the swamp on this one! Well, let me put on my hip-waders. Hmmm, lets see.....

Well, you're going to be battling the open-loop output impedance of the amp (read: inefficient output stage = lots of heat). Even after feedback, your damping factor will probably still be less than optimal, as Grey suggests. I would shy away from MOSFETs here, as they do have a high Vgs (potentially high power loss in the output stage, depending on your circuit), and less power handling capability than BJTs. A bipolar implementation might work better... but i'm waffling on this one. BJTs have their limitations too, such as poorer linearity at high current, and demanding more of the driver stage. hrm... this is a complicated equation to weigh. Not to stifle your creativity, as original thinking is always good, but I think this idea poses too many practical challenges... maybe a slight rethink is what's needed:

Have you considered multiple amplifiers to drive parallel arrays of fewer drivers? So instead of say 30 drivers, you might have six banks of 5 drivers, each bank with it's own amp, or at least portion of an amp. This might bring things down to a more manageable level, and each amp wouldn't have to put out 10W, maybe only 2W each. I think this would make the equation a whole lot more manageable...

as far as the current amplifier goes, I'll have to put on my thinking cap sometime... sometime when I can find it. Right now, I should get some sleep.
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Old 16th November 2001, 10:03 AM   #9
hifiZen is offline hifiZen  Canada
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just had a thought... this might be a good application for a class-D amp, or a power-output discrete DAC... if you're up for the challenge. ;-)
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Old 16th November 2001, 11:56 AM   #10
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Just thought I would throw in an IGBT from left feild.
Hmm, not cheap but???

Regards WALKER

Where did I leave the valium.
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