Refurbish capacitors inside speaker crossovers?

[singslingr]

Even though I lack the, er, capacity to fully understand all the technical details of this thread, I'm nevertheless grateful for the little understanding I've gained from reading it. Many thanks.

[Labarum]

Just browsing around the forum and I notice I have a a question pending:

"To what extent, Alan, are line level (active) crossovers advantaged over passive (post amplifier) crossovers because of the value and type of capacitor that can be used?"

[Pluto]

To what extent are line level (active) crossovers advantaged over passive (post amplifier) crossovers because of the value and type of capacitor that can be used?

The problem with traditional passive crossovers is that they need to operate at a rather low impedance. The drive unit's voice coil will be the loading impedance, typically around 4-10 ohms. Consider a typical, basic, high pass filter - about the simplest filter you could make:

RC high pass filter.jpg

Let's obtain a response suitable for a hypothetical tweeter: the frequency (Fc) is given by 1 / (2*pi*R*C). If we use values of 10nF and 10kΩ the 3dB point of this filter will be 1.6kHz.

You can see that the load needs to be in parallel with R, so unless the load impedance is high compared to R, the very existence of the load will affect the turnover frequency of the filter. Lets assume that the impedance of our hypothetical tweeter is 10Ω - this means that R in our filter will be, near enough, 10Ω - we are stuck with it because that is the impedance of our tweeter. You can see from the formula that if you decrease the value of R by a factor of 10, you must increase the value of C by the same factor to maintain the same result (Fc). So, having reduced R to 10Ω (the figure - like it or not - that we're stuck with) C has to be increased to 10μF to retain our 1.6kHz value for Fc.

So the answer to your question is that passive crossovers, with their inherent need to operate at a low impedance, require large values of capacitance and inductance and these parameters are only achieved by bulk. Such components are therefore large and expensive, all the more so if quality and stability are desired.

An active crossover, by definition, operates at a much higher impedance (because its output load is an amplifier's very high input impedance - perhaps 47kΩ) and therefore the necessary frequency discriminating components can be physically far smaller.

[Labarum]

An active crossover, by definition, operates at a much higher impedance (because its output load is an amplifier's very high input impedance - perhaps 47kΩ) and therefore the necessary frequency discriminating components can be physically far smaller.

Smaller, cheaper, more accurate, less lossy, and with a specification closer to that of an ideal component.

So, in these days when amplification is or ought to be cheap, what is the conclusion?

[A.S.]

Smaller, cheaper, more accurate, less lossy, and with a specification closer to that of an ideal component.

So, in these days when amplification is or ought to be cheap, what is the conclusion?

I think you have to ask yourself who would benefit from going active. The essential point is that it binds the consumer to a particular active [speaker+internal amp] combination. We all know from the vast rhetoric that drags around hi-fi amplifiers like Marley's ball and chain, that the last thing the audio consumer will vote for is less flexibility in his speaker/amp arrangement.

So the argument pro/against active speakers is 1% techical and 99% emotional. If the consumer won't buy it, it's a concept without a market. And that is the only conclusion you need reach.

[EricW]

.... We all know from the vast rhetoric that drags around hi-fi amplifiers like Marley's ball and chain, that the last thing the audio consumer will vote for is less flexibility in his speaker/amp arrangement.

.... If the consumer won't buy it, it's a concept without a market. And that is the only conclusion you need reach.

Mystifying but undoubtedly true, or there'd be a lot more active setups out there, given the technical advantages.

I have sometimes wondered if an active setup that looked more like a conventional amp/speaker combination - that is, with all electronics, including the crossover, contained in a single box that looked like a conventional integrated amplifier, with dual wires out to the speakers - might have more appeal to the audio purchaser. The longer speaker wires might not be technically optimal, but the more "normal" appearance (plus the preservation of the all-important "choice" of the wires) might make it a commercially more viable proposition. Maybe.

[Labarum]

Quite so, Eric. Did someone say choice? When it comes to domestic active speakers I have very little choice, which is a great frustration. I can buy actives in industrial or star wars styled boxes, or I can buy nicely veneered passives. A most unsatisfactory situation. But how to turn the market! And how to beat some sense into consumers.

Now if there were an agreement on a standard for a DAC-DSP four channel amp, at least it would drive all common two ways with crossovers removed, or two way speakers designed to work with the new range of DAC-DSP four channel amps. Program in the crossover parameters, set the room correction, make minor adjustments to taste (like in tone controls!) and there you have a modern choice with maximum flexibility using better technologies.

But I guess you would never get the producers to agree a standard, and you would never remove the ball and chain from the ankles of the hifi enthusiast consumer.

[hifi_dave]

Smaller, cheaper, more accurate, less lossy, and with a specification closer to that of an ideal component.

So, in these days when amplification is or ought to be cheap, what is the conclusion?

My conclusion, having listened to many active and countless passive speakers, is that there are good and bad of each. Some of the worst speakers I have heard were active, so it's obvious (to me) that it isn't the 'be all'.

[mhennessy]

Douglas Self published a good book about active crossovers:

http://www.google.co.uk/search?clien...-_LsaW0QXGh4Ag

http://www.amazon.co.uk/Design-Activ.../dp/0240817389

For those not familiar with Self, he's famous for his no-nonsense approach to audio, and for his rigour and insight - his analysis of the complexities of power amplifiers is really quite something. Yet he has a very accessible writing style that appeals to less technical readers too - have a look at the opening chapter via the preview on Amazon...

As I've said before, active operation is no guarantee of performance: an average designer might not be able to exploit the potential benefits, leaving a product that is out-performed by passive designs from top-class designers. In the pro world, where active operation is much more commonplace, I can think of plenty that don't come close to, say, an M30, and I know that I couldn't improve the M30 by making it active (but can't help but wonder what Alan would do?).

Mark

[Labarum]

I know that I couldn't improve the M30 by making it active (but can't help but wonder what Alan would do?).

Neither could I. For that reason I would not contemplate buying a passive speaker, ripping out the crossover and doing a DIY activation. I would get it horribly wrong.

I am convinced that skilful voicing is essential to any design, and that is more art than science; but if the voicing is of a better basic technology, and by a master, the active ought to pull ahead.

[A.S.]

...the active ought to pull ahead.

Well, an active M30 did exist, and could again. In 1999 I designed such a beast, and we sold a few pairs. But the cost were against us, because at that moment the cheap foreign active speakers started to appear at a retail price below our manufacturing cost. So that was that.

I have all the notes (of course, amongst my PDF archive!) and I went straight to the active M30 folder. All carefully documented, step by step so that, if the market conditions return, we could reactivate that project with ease. I don't want to give too much away, but attached you will find a snap from one page where the tweeter filter (or to use fancy clever clog language, the tweeter's transfer function) in the passive circuit is carefully overlaid with the active circuit. You can see that, for all intents and purposes, the two curves were within the pen thickness from 3-20kHz and the phase around crossover frequency (about 3.5kHz) within just a few degrees. The bass filter was just as close a match.

So, the idea to leave you with is that the passive design should be completed first, and then the active speaker filters cloned from the frequency response shape of the passive filters. How you do that is up to you as the designer but what I did was to overlay the active onto the passive and simulate and then hand tweak until they are as close as possible. There is nothing really magical about active filters; they just do the job that a passive filter does using (many more) smaller components. The weakness of actives is that they need a power supply and that opens up all sorts of issues including electrical safety and - you will never have even thought of this - a huge hike in public liability insurance.

Note: this is not the frequency response of the speaker. It is a plot of the voltage that appears at the tweeter's terminals, which cannot and does not indicate the sound output (speaker frequency response).

>

[Labarum]

So, the idea to leave you with is that the passive design should be completed first, and then the active speaker filters cloned from the frequency response shape of the passive filters.

Can you tell us why you would do it that way, Alan; unless your sole purpose was to mimic a well loved and proven passive loudspeaker?

With all the power and control of a DSP crossover you could manage your proven voicing practices in minute detail, and without the tiresome need to swap components in and out of the crossover board.

Does mimicking the curves of a passive crossover produce the same audio output in the active version; or do the drivers behave differently when driven directly from an amplifier, and without inductors and capacitors close by which must act like mechanical springs and dampers on the drivers?

I agree that active speakers can lead to more rather than less cable-clutter, especially if the master speaker has a multi-input DAC.

However, if the electronics is, as I was suggesting, in a separate box - (ADC)+DSP + DAC + 4 power amps - a four (maybe three) core cable is all that is needed to each loudspeaker.

Could be an extremely neat solution.

[A.S.]

Can you tell us why you would do it that way, Alan; unless your sole purpose was to mimic a well loved and proven passive loudspeaker? With all the power and control of a DSP crossover you could manage your proven voicing practices in minute detail, and without the tiresome need to swap components in and out of the crossover board.

Does mimicking the curves of a passive crossover produce the same audio output in the active version; or do the drivers behave differently when driven directly from an amplifier, and without inductors and capacitors close by which must act like mechanical springs and dampers on the drivers?....

Good question, very conservative answer coming up.

The cardinal sin committed by most or even all DIY speaker makers (and I assume more than a handful of professional loudspeaker designers) is to fail to consider what we hear at our sweet-spot as a inextricable combination of the 'sound' of drive units plus the 'sound' of the crossover. Every drive unit is at its best only with a certain crossover circuit driving it. Whether that circuit is stumbled across by accident or the result of some deep and meaningful (and repeatable) insight is irrelevant: the fact is that you can hurl out of a high window every text book touching on crossover design because not one of them will steer you towards that magical crossover + driver combination.

In fact, you would be far wiser to ignore the text books as if they were never written, get yourself a bucket of resistors, capacitors and coils of the sort of values used in passive crossover filters, a reliable microphone and measuring system and plenty of listening time and your selected drive units. You will need a rod-of-iron willpower to document every experiment to avoid going around in circles. I call it 'The Faraday Method': discipline is much more important than a deep understanding of what is going on at a molecular level. Once you have experience as a reliable observer of circuit topography that sounds best (and worst) then you are in a position to postulate your own mini-theory, and that theory can guide you until such time as a combination predicted as bad sound good or good sound bad. Then you need to revise your theory.

When you no longer need to revise, you have a hypothesis which although not comprehensive, creates a certain working orbit: keep within that orbit and you will not be caught out: stray beyond it, and you go back to square one. Perhaps.

Prof. Faraday and observational physics at its very approachable best, here.