DCC Capacitor question

[Anonymous]

QUOTE: Originally posted by yardgoat46

Hello Mark

Did I used to speak to you some years ago on duck hunting forums (Hoosier's, etc)? Or is there another Mark_in_Utah out there?

Jim in New Jersey

Possibly, I used to frequent several hunting boards, but I can't remember visiting a duck hunting board. I'm more of an elk hunter and target shooter.

Mark in Utah

[rrinker]

You are absolutely correct. Capacitance across the rails with DCC will distort the signal for ALL decoders on the track, not just the one with the capacitor.
The capacitor at the speaker is to protect the speaker from damage from too low frequencies. The QSi decoders have more tha one capacitor - there are ones for the speaker as well as a rather large one after the rectifier and voltage regulator to reduce drop outs due to poor electrical contact. This is the one that causes the problems, as there is no current limiting at this point, and a discharge capacitor will look like a dead short until it charges. It is a realtively small matter to add a diode and resistor to slow ths charge action while allowing for the full effectiveness during power drops. On one of the Yahoo DCC groups, Mark Gurries has posted a circuit to provide regulated DC from DCC track power for powering whatever - passenger car lighting, sound effects, camera systems, whatever you might need power for. These circuits incorpoate a large capacitor to maintain the power supply over momentary drop outs, but also incorporate a limiting resistor so that when power is first applied, the capacitor charges slowly and does not have the initial inrush current problem seen with QSI. This is the RIGHT way to build such circuits.
Any capacitor on the speaker leads should not be altered - Soudtraxx decoders have these as well. The speaker capacitor does not cause any power issues.

--Randy

[Texas Zepher]

QUOTE: Originally posted by rainierj
My Q is why put a capacitor in series with the speaker and not between the pickups and the decoder itself. My limited thinking says to me that you want to keep the decoder alive rather than only the speaker?

I didn't see anyone else mention this, so I'm addressing a question within the question here. A capacitor placed (in parallel) between the pickups and the decoder would distort the digital signal by "smoothing" it. Delaying the ON and prolonging the timing before OFF. Depending on the size of the capacitor it could blur one bit into the next and corrupt the signal. Don't use capacitors in a DCC signal even to reduce the flicker in lighted cars, it is a formula for bizzare unexplainable operational behavior.

[Anonymous]

Hello Mark

Did I used to speak to you some years ago on duck hunting forums (Hoosier's, etc)? Or is there another Mark_in_Utah out there?

Jim in New Jersey

[Anonymous]

There are two possible reasons why you'd put a capacitor between a speaker and it's source.

One is to block DC from flowing through the speaker, which could cause problems to the source (DCC chip in this case) or the speaker. Speakers do NOT like DC flowing through them.

The second reason is to block lower frequencies from passing through the speaker. This is done on speaker system crossover networks to prevent tweeters from being hit by all the energy from the bass.

Putting a capacitor in series with a speaker will not likely reduce the buzzing or hum from the DCC system.

Hope this helps,

Mark in Utah

[nedthomas]

A capacitor in series with a speaker is to block DC from reaching the speaker. If the capacitor is bi-polar (not marked + on one end and - of the other) this is what it is used for. Bi-polar capacitors are electrolytic capatitors that will pass AC.

[Derailedtoosoon]

[#ditto] Your answer is correct Overdurff, capacitors are filters attached to speakers as well as when used with bridge diodes on power supplies.

[Anonymous]

I believe that the capacitor is functioning as a "filter" to prevent line hum and decoder signals from reaching the speaker, instead of the wanted sounds put out by the decoder itself. Because of the relatively large size of the capacitor it is probably impractical to mount it on the decoer itself. Placing the capacitor across the terminals would effectively make it a reservoir and increase the voltage/amps, which is not required in this application.
Hope my thinking and explaination are correct.
Will

[rainierj]

I have only been exposed to soundtraxx sound decoders, and have read up a little on BLI and MRC. My Q is why put a capacitor in series with the speaker and not between the pickups and the decoder itself. My limited thinking says to me that you want to keep the decoder alive rather than only the speaker?
[?]