Using Batteries to Bias crossovers into Class A, how does it work? - diyAudio
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Old 7th July 2008, 09:36 PM   #1
pjpoes is offline pjpoes  United States
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Default Using Batteries to Bias crossovers into Class A, how does it work?

Well some posts in another thread I started got me thinking about the JBL Everest Speaker project, and I took another look at the Crossover Schematic, available on the web site. In the Owners manual, it says that the Mid, High, and Ulta High frequency crossover networks incorporate battery bias so that the capacitors will operate in Class A. I will give the links to both, but I'm wondering how this works. The crossovers are passive high level crossovers, not active ones. I typically think of biasing into Class A as dealing with amplifiers, and so would be done with the opamps, as it effects the switching (Right?). What does biasing the caps into Class A do exactly, whats it mean? How does it work?

If someone who understands what they are doing could present maybe a simplified schematic so I could better understand where these are hooked in, that would be nice. LF2 confuses me most as I don't understand where all the wires are going to. In the manual it refers to this as there proprietary Charge Couple method, and applies a voltage to the twin capacitors used in each position, which creates a voltage difference between the plates, but no DC found at output.

I'm kind of curious to know if this works, it seems simple and cheap to implement if it does work to reduce distortion, but I'm not sure how to incorporate it.

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Technical Document with Schematic
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Old 8th July 2008, 12:12 AM   #2
Salas is offline Salas  Greece
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There was a thread where SY measured electrolytic bipolar capacitors for possible crossover distortion after an Earl Geddes suggested method. He could not find any. So we doubted the bias helping such a hypothesis. I had a suspicion that the capacitor's electrostatic dimensional stability is enhanced instead, but it remains to be checked. There is subjective evidence that biasing bipolar capacitors in crossovers helps sonics.
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Old 8th July 2008, 12:38 AM   #3
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Long ago Walter Jung wrote a landmark paper on capacitors.
I think it was in Audio magazine and may have been titled
Picking Audio Capacitors. He has some measurements of the
distortion reduction biasing 2 back to back caps and I have used
this and it did work for me.
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Old 8th July 2008, 12:43 AM   #4
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Its real easy to make a biased network. This is how you do it. Lets say the original cap was 1uf. To set one up biased for the same value you would wire 2X 2uF caps in series. You then place a 2meg resistor between the caps and tie a + 9Volt battery to the other side of the resistor. The negative on the battery goes to system ground.

What you end up with is a T shaped circuit. With the caps up top and the resistor going down to the + on the battery. It works by the way. All my crossovers are Charge Coupled the same way as Everest. The only issue can be cost as it can get expensive doubling both the capacitor count and the values.

Look at the UHF section instead of the LF-2, it's much easier to understand.


Rob

http://www.audioheritage.org/vbullet...ead.php?t=3555
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Old 8th July 2008, 12:57 AM   #5
paulb is offline paulb  Canada
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Did they really call it 'Class A'?
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Old 8th July 2008, 01:02 AM   #6
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'Class A'

With the capacitor biased you don't have to have the dielectric go through a zero crossing point like you would with just an AC signal. Similar to differences between a Class A vs a Class B amplifier at their 0 crossing points.

Rob
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Old 8th July 2008, 01:07 AM   #7
Salas is offline Salas  Greece
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In this JBL paper the electrostatic dimensional stability suspicion of mine seems to be discussed. They talk about crossover type distortion too - that SY could not find - and I think that there must be some loose terminology problem here. They probably discuss dielectric memory. In such speakers as JBL big ones the amplifier driving voltage that they can withstand will rise far above 9V bias so I can't see how their bias would keep the caps ''class A'' if that was the real problem.
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Old 8th July 2008, 01:12 AM   #8
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" In such speakers as JBL big ones the amplifier driving voltage that they can withstand will rise far above 9V bias so I can't see how their bias would keep the caps ''class A'' if that was the real problem."

That's not DC so the signal still goes through 0 volts. Also the improvement is subtle and audible at low levels during quite passages. You get more clarity

Rob
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Old 8th July 2008, 01:17 AM   #9
Salas is offline Salas  Greece
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Done that. They get cleaner indeed. What do you mean by not being DC? Under 9V or above the driving signal will always be AC. I guess there assumption is that by keeping the cap on a DC threshold it will not reverse charge under that at AC zero cross.
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Old 8th July 2008, 01:20 AM   #10
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There is a 2 meg resistor between the caps and the battery. It's open to the AC signal. It also gives you one hell of a time constant to discharge the capacitor.

Rob
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