Is One Type of Cap Faster Charging Than Another?

I'm asking this in regard to the neg charge cap that controls the attack and release in a typical tube compressor.  My first thought is I want the fastest charging type (if it exists) to get over that attack lag, but maybe I am asking the wrong question here, but still curious about the answer as is.

That said, I have many types to choose from, and need to pick a good fit, for any reason.

Thoughts?

There is no such thing as fastest charging type capacitor.

The transient response  (charging/discharging) of a capacitor has to do with the capacitance value not the material used to make the capacitor.

Rate of charge/discharge is generally a simple function of capacitance, but in att/release circuits where the caps get charged and discharged with different impedances there can be subtle differences caused by dielectric absorption. Film caps should be free of DA effects, if using electrolytic tantalum is worse than aluminum for DA, but sometimes better for low leakage current.

JR

John, does it means DA effect can affect charging "linearity" (read: curve)  ?

Laurent.

It's a fairly subtle phenomenon but the best way to model DA is like a large capacitor with several smaller RxCs in parallel. If you charge and discharge the cap with different impedances (like fast attack/slow release, the intrinsic Rs in the DA fraction of the cap, cause lag or delayed settling as all the capacitors equalize to each other.

This is far more problematic for sample and hold circuits, where the voltage on the hold cap will change after it has been driven to a new sample voltage by this internal to the cap settling.

For general purpose dynamics processors, this is not a major concern. I once had a situation with an encode/decode circuit for a vinyl noise reduction system that CBS records was promoting. I was designing a decoder and learned that the encoder used a tantalum cap in the time constant circuit. To make my decoder better track the encoder for accurate record playback I also used a tantalum cap in the complementary circuit in my decoder, so they would presumably have similar errors and track each other.

It was a wasted effort on my part as the vinyl NR system was rejected by consumers.

JR 

> I'm asking this in regard to the neg charge cap that controls the attack and release in a typical tube compressor.  My first thought is I want the fastest charging type (if it exists) to get over that attack lag, but maybe I am asking the wrong question here, but still curious about the answer as is.

This is the WRONG question.

The situation you cite, tube compressors, attack time is not about the type of capacitor. Attack time is essentially the RESISTANCE times the CAPACITANCE. Resistance includes output impedance of sidechain amp (in simple limiters, often the ~~3K of the line-amp plates), ~~500 ohms in rectifier diodes, and any added resistance used to reduce attack time. In many simple compressors the rectifier is cap-coupled, which means it is a staircase generator, and will take many cycles to step-up (slower attack for bass, a good thing).

There is also under 1 ohm in the capacitor. However this does not increase the attack time, actually reduces it a hair.

You do NOT want infinitely short attack. It will "duck" on supersonic ticks. Even short attacks mung transients. Do 1mS-50uS, and clip the leftover hair.

If you do want "infinite fast" attack, transistor sidechains do it better.

Since you ask...

> I want the fastest charging type

Discharge is the same as charge.

Asking for the fastest discharge will get you in trouble (at least, if you know what numbers to ask for). The *fastest* caps are used to detonate the TNT around an atom bomb. At least one A-bomb project was detected when the plotters were asking a cap company for some extreme specs and would not talk about their application. I don't know, but suspect, that such caps don't have wires but BIG lugs like a car battery, to dump BIG charge REAL QUICK. (It's not so much the speed, but getting several discharges to happen at the same instant; if one TNT blows a bit before the others it just throws the atomic-stuff out the side and critical-mass doesn't happen.)

Dielectric absorption is a non-issue in tube compressors using any type of film-cap. The whole thing is approximate and loose-tolerance.

> an encode/decode circuit ...the encoder used a tantalum cap in the time constant circuit

Yes, every don't-care rule can be invalidated. Tracking codecs with a Tantalum time-cap is perhaps thoughtless. I suspect with tube-works it would be benign; with touchy little JFETs it could be an audible burble. (But far worse things have been put to/from vinyl.)

I don't recall a CBS vinyl NR? It wasn't to compete with consumer tape, surely?

Gah!  I knew I would get into trouble asking this question.  But that's why I come here, and many thanks for the thoughtful responses.

Still, I need to pick a cap type for this app: any reason to choose one type over another?  I could squeeze a little cheap electrlytic in there or shoot for a bulky premium *audio* grade.  I'm not out to impress anyone by choosing premium components, just want the right one in there.

I'd say, take any film cap you have lying around and be happy.

PRR said:

I don't recall a CBS vinyl NR? It wasn't to compete with consumer tape, surely?

Click to expand...

They were supposedly trying to take vinyl to a higher level, but the standard circuits for playback decoders from CBS were mid-Fi at best. My kit decoder was probably the cream of the crop, while that isn't anything to brag about. The encoder was designed by Urie, and while they used more expensive parts, they did silly stuff like tantalum time constant caps. 

CBS called it CX and I still have a stack of CX encoded records back in my closet. It was kind of like Dolby B without any HF boost, and the premise was that records could be played with or without playback expansion by less critical customers. It turned out the customers were too critical to embrace it with the decoding. 

I guess they were already in trouble since they gave me a free license and a cover article on Popular electronics to get me to do it.

It failed gloriously. I sold enough kits to justify my effort but just barely.

JR

Any reason to choose one type over another?

Click to expand...

As long as it is 1 uF or lower I'd go with polyester film, those are small and cheap. If it's above that value you can start thinking if perhaps an electrolytic might fit better.

Samuel

> squeeze a little cheap electrlytic

If you may want looooong release, a "cheap electrolytic" may self-bleed faster than you intend. Most modern electros are very low leakage when kept charged, but a limiter may sit un-charged for long periods of time.

Pop music will like a quick release to pump-up the average. My old work, pumping was bad. If I screwed-up and hit the limiter, it should probably stay down for the rest of the piece, or at least sneak-back so slow that nobody could notice.

I forget what I used in my very slow-release box.

I like the old green Panasonics but I think they quit that line. I had some vintage yellow poly caps that looked nice.

Just the info I was seeking.  Thank you all.