"Charged Coupled" crossovers?

[Steve Schell]

Rob, the DC polarizing voltage and AC signal voltages are both imposed on the biased capacitor plate, and its potential at any given instant is their sum. This situation is very similar to the DC biasing voltage applied to a vacuum tube grid. The audio signal is superimposed on this potential, and when the signal swings more positive than the grid is biased negative then the grid potential becomes positive in relation to the cathode and the grid draws current from the cathode. If DC and AC are so different and cannot coexist in the same circuit then how does grid current occur?

[Robh3606]

If DC and AC are so different and cannot coexist in the same circuit then how does grid current occur?

Hello Steve

I didn't say they couldn't coexist. All I am saying is they act differently in the same circuit. If they didn't act differently how would you get an amplfier to work?? Once you set-up the DC bias on a transistor it just sits there and does nothing. It takes an AC signal to get things going and the AC signal is what gets amplified. Obviously without the DC bias it couldn't work. The DC in this case is the power source and the AC the signal.

When you get the tubes all tuned up and working does the AC signal negate the DC bias?? The DC bias stays as set and power supply does it's best to maintain a constant DC voltages to the tubes and bias voltage all set by the resistors. This happens independent of what signal is passing through the amplifier.

Once you have that capacitor charged its just like a DC voltage source for at least the first 2 time constants. Just like the power supply capacitors in an amplifier. When you run an AC signal through the capacitor will the DC bias just disappear?? The AC may change with reference to ground, 0 volts because it is alternating, but why would the DC voltage change with respect to ground??

If you look at the previous example of say 12 peak or 24 peak to peak you would get what +9 and 12 so +21 volts on the low side you have -12 volts plus +9 so -3 volts. But that's the negative peak AC voltage not the +9 volts DC that has not changed. The AC voltage is riding the DC rail but the rail voltage isn't changing. It's still +9 volts. It can't change any where near as fast as the AC because you have the time constant of C x R which in this case is measured in seconds. Just like in a good stiff power supply. Figure a 10uf times 2.2 meg is what 22 seconds so 66 seconds for 3 time constants. That time constant is only there for a DC voltage.

With an AC signal in the capacitors pass band it looks like a short. To DC it's an open so the AC passes freely and DC is blocked by the capacitor. Once those caps are charged that DC voltage has no place to go. The only way to get rid of it is to manually discharge them, with low leakage in the poly caps they will stay charged for quite some time if left alone.

Well that's my take anyway. What do you think??

Rob

[Steve Schell]

Rob, I think my statements to this point in the discussion are at least basically correct. I believe that you are overthinking what is a fairly simple matter, but I don't have the energy for a prolonged debate. Electrical engineering was never my strong subject anyway. Does someone else want to jump in and help Rob determine if +9VDC combined with -9VAC equals something other than zero volts?

[pierce]

+9VDC and a +/-9V AC signal (aka 18V P-P aka 6.4V RMS) would equal a 0-18V signal. you can call that what you want, AC and DC are just names and abstractions.

its all volts, its just that some of the volts are changing, and others are constant. if you add a constant to a changing value or function, you get a changing value whats got a different offset.

[Zilch]

http://www.audioheritage.org/vbullet...34&#post130034

197 posts in that thread alone, 2.5 years ago, 14 pages.

[We REALLY need to get past this.... ]

[Allanvh5150]

As I understand it the input signal peak to peak voltage would have to be twice the polarizing voltage, or 18V in the case of 9V bias, before the signal would swamp the bias. One direction of swing will momentarily increase the bias while the other direction will decrease it. If the input signal happened to reach or exceed 18V PtoP, all that would happen would be that one side of the signal peaks would be subject to the zero crossing nonlinearities that have been avoided with the bias. It is doubtful that this would be audible. In fact, if any of the speakers I listen to regularly receive an 18V swing I don't want to be anywhere close!

18V P-P is only 20watts RMS into 8 ohms. My TV has bigger amplifiers...

Allan.

[Allanvh5150]

Rob, I think my statements to this point in the discussion are at least basically correct. I believe that you are overthinking what is a fairly simple matter, but I don't have the energy for a prolonged debate. Electrical engineering was never my strong subject anyway. Does someone else want to jump in and help Rob determine if +9VDC combined with -9VAC equals something other than zero volts?

Yup. I will jump in and agree with Rob. 9VDC doesn't combine with 9VAC. The DC sits there and does basically nothing but hold the capacitor above OV so it always has a charge on it. The AC voltage simply flows past with the impedence of the capacitor decreasing with increasing frequency. At DC the capacitor presents a very high impedence. The AC and DC ALWAYS remain separate. If they could somehow combine I have the feeling that the DC would flow through the capacitor as well. Probably not a good thing. I cant even imagine what would happen if the DC bias on the grids of my tube amps somehow combined with the drive voltage.....

Allan.

[Allanvh5150]

+9VDC and a +/-9V AC signal (aka 18V P-P aka 6.4V RMS) would equal a 0-18V signal. you can call that what you want, AC and DC are just names and abstractions.

its all volts, its just that some of the volts are changing, and others are constant. if you add a constant to a changing value or function, you get a changing value whats got a different offset.

hmmmmm.......I think 9VDC is actually 9V RMS. Calculations made for DC use basic Ohms law. Calculations only take into account Volts, Current and Resistance and Power is thrown in for good measure. AC calculations, on the other hand, take into account Frequency and reactance. AC and DC are totally different animals and never the twain shall meet!

Allan.

[Robh3606]

Does someone else want to jump in and help Rob determine if +9VDC combined with -9VAC equals something other than zero volts?

Hello Steve

Well from my standpoint you are trying to oversimplify what's happening and not taking the time to think it through.



Rob

[4313B]

[We REALLY need to get past this.... ]

I'm way past it to the tune of a dozen already built and several dozen left to build. I flat out can't build them fast enough. They rock! I'm quite glad Greg saw fit to hook me up.

He wants to make it clear though that he really doesn't care whether or not anyone coats their diaphragms or biases their networks. They aren't some kind of requirements for the general public. These are things he does and these are things he's shared with me. If I'd known the end result I'd never have mentioned any of it on a public forum.

It could have been pretty fun, instead it's been kind of a drag.

[jblsound]

I'm way past it to the tune of a dozen already built and several dozen left to build. I flat out can't build them fast enough. They rock! I'm quite glad Greg saw fit to hook me up.

He wants to make it clear though that he really doesn't care whether or not anyone coats their diaphragms or biases their networks. They aren't some kind of requirements for the general public. These are things he does and these are things he's shared with me. If I'd known the end result I'd never have mentioned any of it on a public forum.

It could have been pretty fun, instead it's been kind of a drag.

I think people can tend to over think way too much, and not take it for what it is. C-C is a huge improvement to the basic crossover. Period. It works.

If anyone doubts that, try my previous suggestion. Setup speakers for near field first with the basic and then w/bias and hear the difference.

[pierce]

hmmmmm.......I think 9VDC is actually 9V RMS.

RMS is normally used on AC voltages, it stands for Root Mean Square. for a DC, or unchanging constant voltage, RMS would be somewhat meaningless, and in fact, any DC offset (bias) is usually removed from a signal before measuring its RMS voltage.

For a pure sinusoidal AC voltage, the RMS voltage is equal to .707 of the peak voltage (eg, 1 over the square root of 2)... So, a 18 V P-P sine wave, which goes from +9V to -9V, would be about 6.4V RMS (9/sqr(2)). This represents the integral of the absolute value of the voltage over time, eg, the average of the absolute value of the instananeous voltage, and is what you would get if you full wave rectified the AC signal (aka ABS(V)), and then filtered it for DC.

Calculations made for DC use basic Ohms law. Calculations only take into account Volts, Current and Resistance and Power is thrown in for good measure. AC calculations, on the other hand, take into account Frequency and reactance. AC and DC are totally different animals and never the twain shall meet!

AC follows Ohms law too, V=I*R, and watts law (P=V*I), etc. Its just that AC is constantly changing in time, and some of the Impedances (those with non-zero Reactance, ie capacitors and inductors) are dynamic dependent on frequency, and introduce time delays where current lags voltage, and so forth, which leads to interesting things like resonant circuits from which oscillators, filters, etc can be derived. Z = (R + j*X), Impedance is a Complex number if reactance (X) is non zero. A purely DC world would be very very boring


side note: none of this has anything to do with how 'charge coupled capacitors' based crossovers sound to the ear... this is a purely pedantic discussion where I'm trying to correct what I percieve as some common misconceptions about fundamentals of electronics. I tend to take a Physics approach to all this, rather than that of a practicing electronics designer, and I think Ive stated my exact same point ni about 5 different ways so far in this thread, and I should probably let it rest

[Zilch]

18V P-P is only 20watts RMS into 8 ohms. My TV has bigger amplifiers...

It's the first watt that counts.

[Especially when running compression drivers.... ]

[Steve Schell]

4313B, I'm pleased with the discussion thus far, and if it exposes more folks to the charge coupling concept or leads to a better understanding then so much the better. Some seem to be quite skeptical, which is okay. Based on my experiments and listening I'm quite biased in favor of this technique, +9 volts at least.

[Zilch]

Timbers discussed experimenting with higher bias voltages.

Bottom line: "Unnecessary."