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Old 12th November 2010, 05:17 PM   #1
knutn is offline knutn  Norway
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Default Full range Open Baffle with Current Drive

I think it's time to do something different. I want to try Current drive instead of voltage drive. I also would like to combine this with a full range solution avoiding cross over networks. And why not try to combine this with open baffle. All three items have their pro's of their own, so why not combine them?

I am fully aware of the problem with current drive; we will have to consider the driver impedance, and especially the resonance in the low end. But since the problem with OB is the lack of descent bass reproduction, with current drive we will have a bass boost for free. Suitable drivers could be Tang Band W4-1320SJ for a small room/office and W8-1772 for a living room.

Unfortunately I only have some experience with amplifiers and filtering, but not with speakers. So I would like to know if someone have tried the combination of full range open baffel with current Drive. Or has someone raised the output impedance of their amplifiers to extend the low end with OB?
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Old 12th November 2010, 07:06 PM   #2
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For an OB, W8-1808 is probably better suited. There is another active thread discussing driver selection.

The idea of using the OB roll-off to naturally EQ the bump at resonance of a driver with Qm>1 is a very good one (but not new). Careful matching of baffle with driver will be needed.

dave
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Old 12th November 2010, 08:14 PM   #3
el`Ol is offline el`Ol  Germany
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The 8" Tangband drivers have very strong machanical damping, I don't know whether influencing electrical damping by current driver will change so much. Supravox 215-2000 would be nice, but expensive. And current drive won't liberate you from building big baffles, like these U-shaped bending plywood constructions.
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Old 13th November 2010, 09:15 AM   #4
knutn is offline knutn  Norway
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Quote:
Originally Posted by planet10 View Post
For an OB, W8-1808 is probably better suited.
Why is that? With current drive, a low Qm is preferable. W8-1772 has Qm = 1.02 and W8-1808 has 1.33. Are there other parameters that are more important?
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Old 13th November 2010, 03:13 PM   #5
el`Ol is offline el`Ol  Germany
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Quote:
Originally Posted by knutn View Post
With current drive, a low Qm is preferable.
preferable for sealed or BR, but not for OB
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Old 13th November 2010, 04:59 PM   #6
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You may already be aware of my article on current driving
full range speakers. If not, check it out at either

FIRST WATT HOME or Pass Laboratories

It works pretty well with OB's, and I demonstrated this with
the new SEAS full range in an OB at BAF two weeks ago.

It does generally need some compensation - some resistance
in parallel with the driver (or in series with a voltage source
amplifier - you don't necessarily have to use a current source
amplifier) to set the bass character and a high frequency
RC network (zobel) to adjust the upper mid and top.

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Old 13th November 2010, 05:43 PM   #7
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Quote:
Originally Posted by knutn View Post
Why is that? With current drive, a low Qm is preferable. W8-1772 has Qm = 1.02 and W8-1808 has 1.33. Are there other parameters that are more important?
Measured Qms a bit higher than that.

It is a juggling act. Qms>1 causes a lift in the bass with a current amp, which you try to counter with the appropriate baffle width (then filter as per Nelson).

From my listening, the 1808 is the better sounding driver (and has less ripples in the impedance curve) But i personally wouldn't use either.

dave
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Old 13th November 2010, 07:19 PM   #8
knutn is offline knutn  Norway
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Default Q and compensation

So If I understand you right, it is not a first goal to look for drivers with low mechanical Q. It is, as Nelson says, maybe necessary to compensate for the high resonant impedance somehow. But to my knowledge it is easyer to compensate for a low than a high Q. I would think that for a DIY project, an active compensation (here I am talking about a partly compensation with a series resonant circuit, realized with e.g. a JFET as the active component) would be the way to go. Since I don't have the bank account as packed as Nelson , I would not afford a full range SEAS driver. My choice of TB drivers is also a question of economy.
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Old 13th November 2010, 07:36 PM   #9
knutn is offline knutn  Norway
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Default OB and Current Drive

Having read the excellant article of Nelson, http://www.firstwatt.com/pdf/art_cs_amps.pdf, it seems to me that for sealed enclosures, we would anticipate a bass lift that can be taylored by a passive or active compensation network. In addition it may be necessary to soften the high end of the frequency spectrum. But if I understand speaker theory right, the OB should need less compensation at the resonant frequency - but current drive would have a great impact on amplifier overloading (due to the high resonant impedance), or?
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Old 14th November 2010, 12:10 AM   #10
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Both cases will give you a bump. Here's a curve of the
SEAS nearfield in an OB with a 16 ohm source and a 0.1 ohm
source.

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