DC Bias Voltages. GuideLines ?

[Earl K]

Hi All,

(i) Does anyone here have any thoughts/experiences that they'd like to share on how much DC voltage ( & why ) to apply to a pair of "Charge-Coupled ™" capacitors ?

(ii) I realize that JBL settled on @ 9 volts after first trying 18 volts ( or so it seems from the M9500 network schematic ) .

(iii) I "hear" a certain amount of "UHF damping" when using DC biased caps. I wonder if this is resulting from the ratio of "occupying charge ( the battery ) to the actual voltage rating for the cap in use. IE ; JBL typically uses 100 V caps , so the battery charge is "occupying" 9% ( 9 volts/100volts ) of the caps voltage capacity.

(iv) FWIW; in the future I'll be trying some lower bias voltages on my " polystyrene bypass caps" / which typically have voltage ratings of around 63 Volts.




( I'm offline for the day )

[Zilch]

As I understand it, the idea is to move the zero-crossing outside the operating range of signal. At 9 V, that'd be 18 V peak-to-peak, or 12.73 V RMS, no?

Across 8 Ohms, that's 1.6 A, 20.25 W, and thus outside what a UHF driver typically sees. Even if the drive exceeds this, only the peaks are going through the biased (offset) zero crossing, and then, only on half-cycles.

If I thought it mattered, I'd be using a higher voltage to bias the LF capacitors, but it seems there is little benefit to accrue from biasing them at all. 18 V might be appropriate for mid drivers....

[Earl K]

As I understand it, the idea is to move the zero-crossing outside the operating range of signal. At 9 V, that'd be 18 V peak-to-peak, or 12.73 V RMS, no?

Across 8 Ohms, that's 1.6 A, 20.25 W, and thus outside what a UHF driver typically sees. Even if the drive exceeds this, only the peaks are going through the biased (offset) zero crossing, and then, only on half-cycles.

Hi Zilch

- Let me display one ( of my many) , obvious ignorance of the ( electronic ) facts , about what goes on inside a capacitor.

- First off, I really don't fully grasp the concept that you were getting at. ( I do understand the movement of the caps "zero-crossing" away from a 0 Volt resting place but ,,,,

- to this brain, your words ( across a couple of lines ) seem to suggest that a DC biased capacitors' reactive influence ( to any applied AC signal ) now occur within a newly introduced voltage range, specifically, that range which is dictated by the value of the DC bias voltage.

If I thought it mattered, I'd be using a higher voltage to bias the LF capacitors, but it seems there is little benefit to accrue from biasing them at all. 18 V might be appropriate for mid drivers....

- Therefore ( in my paraphrase of the above ); 9 volts of bias ( as translated into usable watts from your calculations ) would offer less usable headroom than 18 volts and 4.5 volts of bias would offer less headroom than 9 volts.

Have I interpreted or paraphrased your words correctly ?


- FWIW, my impression of the usefulness of the DC bias was ( also ) moving the zero-crossing point away from 0 volts / but / ( & here's the difference in my working comprehension ) the reactive workings of the DC biased cap pair still would occur within the portion/percentage/capacity of the caps electrons that aren't committed to holding the newly introduced DC charge. That available headroom ( working capacity ) would be found by subtracting the actual applied DC bias voltage from the voltage rating for the cap . The difference is the available headroom in the pair of caps.

- If you followed my meanderings, you'll see that I think we have two different impressions of what occurs within a DC biased cap. These two perspectives would give the cap(s) in question different operating capacities ( once they are DC-biased ).

- From my perspective; reducing a bias voltage will increase the available headroom of the ( combined ) cap. To my thinking , one effect of a lower bias will be a possible decrease of what I think is a form of UHF circuit dampening .

- So, What do you think ? ( or did I make your head hurt ? )



Not to worry ; I'll be trying 3 volts DC soon enough, whether or not I fully understand why I'm trying it

[Zilch]

Hi, Earl!

We have the same understanding. But consuming 9 V of the 250 V, 100 V or even 50 V available capacitor working voltage headroom is of no consequence when the highest peak voltage ever seen by the capacitor (UHF) is what? 20 V?

Also, you're doubling the working voltage of each by having two of them in series, no?

I've never looked on a 'scope to see what's actually happening there, but I'll do it to see. I'm also interested to know how much the 9 V bias gets whipped around by the audio AC....

[Earl K]

Hi Zilch

We have the same understanding.

Good to know.

Also, you're doubling the working voltage of each by having two of them in series, no?

- ummmm, I think the pair of caps takes on an accumulated / combined voltage rating derived by adding the two ratings together.
"Each" caps rating doesn't really double .

But consuming 9 V of the 250 V, 100 V or even 50 V available capacitor working voltage headroom is of no consequence when the highest peak voltage ever seen by the capacitor (UHF) is what? 20 V?

- My interest is what percentage ( of capacity ) does HF dampening become audible. So , I speculate there is a consequence to be found from using lower voltage caps with relatively high, DC bias voltages .

- Anyways, enough for now ( until I try some different bias voltages ).


Thanks

[Zilch]

I think the pair of caps takes on an accumulated / combined voltage rating derived by adding the two ratings together.
"Each" caps rating doesn't really double .

Yeah, that's what I meant. You say it better.