What is that blue thing and why did I install it?

[Cajonpassfan]

This is not a tongue in cheek question; I truly do not remember
Back in the last century when I did my first sound install, in a dummy PB unit, the only (somewhat) decent speakers available were “Edgeports”, 1x1x.375” metal boxes with vents in one side and a cheapo round speaker inside. They were meant for HOn3, but in N scale, 1” is, and was then, drumroll, 160” or 13’4. Too wide and too tall. Somehow I managed to put the 160” speaker into a 168” tall carbody vertically, with grinding, filing and possibly cussing involved. The speaker barely clears the rails, see pics below, but it worked wonders for its time. I was hooked on sound. About ten years later, I added a second,18x37 mm speaker for a deeper sound. All this was/is driven by an ancient Soundtraxx DSX Alco diesel decoder. A thing of beauty at the time...

Well, it’s time for an upgrade, and I will likely gut everything but the 18x37 speaker. But, out of curiosity, what IS the blue thing? A cap? And why is it wired in series with the speaker wires? I no longer have the old DSX documentation and don’t see anything on line, but I’m sure there is a reason for it.
Thoughts, anyone?

It’s not that important, but it’s bugging me that I can’t remember.
Thanks, Otto K.

[GaryHinshaw]

Definitely a capacitor, and almost certainly serving as a keep-alive to carry you over voltage dropouts.

[peteski]

It is an electrolytic capacitor.  Probably non-polarized. Those are used in series with a speaker analog audio amplifiers as coupling capacitors (to prevent any DC voltage from being passed through the speaker's voice coil ).  This is a standard design feature for analog audio amps (like the ones in home HiFi systems). At least in the older home HiFi, because I have not looked into more recent home stereo amp design.

The early sound decoders likely used that type of analog audio amplifier to drive the speaker, and when a large-value coupling cap was needed, since it was physically large,  it was installed off-board just like what you see in your model.

More recent sound decoders utilize more efficient PWM-based D-class amplifiers where the speaker can be wired directly to the amp's output. No coupling cap is needed.

That is an ancient install Otto!  My oldest install was a Soundtraxx sound-only decoder in a roomy tender of a brass N scale Challenger.  The motor decoder was in the loco.  it was done for a friend. He still has the loco, but with Athearn Challenger being way superior, he is thinking of selling the brass loco.

[jdcolombo]

Capacitor - yes.

Keep alive - no.

In the DSX design the cap was used as a high-pass filter to avoid having accidental DC voltage or extremely low frequency sound go to the speaker and destroy it.  That's why it was wired in series with the speaker (a keep alive would need to be wired to the power input circuit of the decoder, not the speaker).  Capacitors are used in audio speaker crossover networks as high-pass filters, typically in series with the tweeter/midrange.  A choke is used as a low-pass filter to send bass frequencies to the woofer.

John C.

EDIT - well, I see Peteski gave the answer a couple seconds earlier!

[Cajonpassfan]

Dang, you guys are QUICK! Thank you! That makes sense...I can sleep tonight
Peteski, I did one of the brass Challengers as well, still have it, it runs great and pulls better than the Athearns, but I’m thinking of putting it up for sale also. It has a Lenz motor decoder and a DSX in the tender for sound as I recall. Might even have one of these blue things
Otto

[davefoxx]

Last century?!  I hadn’t even gotten to DCC, let alone sound, by that time.  You’ve got me beat by more than two decades on sound installations.  I’m seriously impressed.

DFF

[peteski]

Last century?!  I hadn’t even gotten to DCC, let alone sound, by that time.  You’ve got me beat by more than two decades on sound installations.  I’m seriously impressed.

DFF

I think Otto has me beat by couple years. I think my install was done early in the 21st Century.    And I didn't use the speaker Otto used. Mine was a small oval speaker that fit nicely into the empty coal bin of the tender.  Then I placed a fake coal load over it, made from a piece of black antistatic foam.  It actually looks like coal, while easily passing the sound waves.

And you're welcome Otto!   

[Cajonpassfan]

Last century?!  I hadn’t even gotten to DCC, let alone sound, by that time.  You’ve got me beat by more than two decades on sound installations.  I’m seriously impressed.

DFF

Lol, Dave, don’t be too impressed, it was 1999, not ages ago although it seems like it. Until then I was resisting the idea of DCC. I had a wireless “Catnip” cab setup and rotary block switches and life was good.  It was a visit to a beautiful local HOn3 layout in OC where I got to run a sound equipped C-19 pusher while the owner ran the road engine, and the sound of the two locos thundering out of town got me hooked. Twenty-some years later, I still have a number of locos waiting conversion.
I’ve done a good number of them, but don’t enjoy doing them, to me it’s too much like work...
Otto

[jdcolombo]

I did one of my first installs with a DSX and a Lenz Silver Mini in the tender of a Life-Like (it was still LL, not Walthers) Berkshire back in the early 2000's.  And I remember trying an ESU 3.5 that sucked as far as its steam sound and required a 100-ohm speaker.  It was awful.  Then Soundtraxx came out with its first Tsunami, and I converted the entire Berk fleet.  But it was too big to fit in anything else!

Wow, have we come a long way since then . . . .

John C.

[peteski]

I remember (probably in the '90s) one of my NTRAK club members shoehorning a Dynatrol receiver in a Rivarossi/Con-Cor E-unit.  Command Control in pre-DCC days.

[Cajonpassfan]

I did one of my first installs with a DSX and a Lenz Silver Mini in the tender of a Life-Like...
...
Wow, have we come a long way since then . . . .

John C.

Yes we have, John, yes we have.
Anyone remember one of these? My first venture into sound...didn’t work.
Otto

[mmagliaro]

When I saw this, I immediately assumed it was a filter cap to block DC from the speaker, like some of you have already noted.
But my question is, why is that necessary?  How would DC get through the decoder into the audio outputs?

[peteski]

When I saw this, I immediately assumed it was a filter cap to block DC from the speaker, like some of you have already noted.
But my question is, why is that necessary?  How would DC get through the decoder into the audio outputs?

Max, it *IS* to block any DC from passing through the speaker voice coil  (wasting power, and possibly heating the voice coil).  This type of design was (and still is) typical for most transistorized analog audio amplifier.  AC coupling is done between amplifier stages, and then also at the output of the final power state where the speaker connects).    In the vacuum tube days they used inductive coupling (transformer) to drive the speaker, but in transistor amps capacitors are used more often.  Do a Google search for audio power amplifier and you should see most using this type of capacitive speaker coupling.

Class-A amplifiers for example have current flowing through their output transistor, even with no audio audio being played.

[mmagliaro]

Max, it *IS* to block any DC from passing through the speaker voice coil  (wasting power, and possibly heating the voice coil).  This type of design was (and still is) typical for most transistorized analog audio amplifier.  AC coupling is done between amplifier stages, and then also at the output of the final power state where the speaker connects).    In the vacuum tube days they used inductive coupling (transformer) to drive the speaker, but in transistor amps capacitors are used more often.  Do a Google search for audio power amplifier and you should see most using this type of capacitive speaker coupling.

Class-A amplifiers for example have current flowing through their output transistor, even with no audio audio being played.

I guess I missed the part where it was explained that this thing was just an analog amp, or I would have realized this.
As someone who grew up in the Dynaco kit stereo days, I am familiar with the coupling capacitors you speak of.  Now I see why the board in that train has the blocking cap.

[peteski]

I guess I missed the part where it was explained that this thing was just an analog amp, or I would have realized this.
As someone who grew up in the Dynaco kit stereo days, I am familiar with the coupling capacitors you speak of.  Now I see why the board in that train has the blocking cap.

Most of contemporary sound decoders use Class-D amps (which use PWM to drive speakers directly).  They are also very efficient, so they do not run hot.

Bus some low-end sound decoders (MRC is one, and probably only-one), still use old-school analog audio amp chip (SMD version of LM386), and an analog voltage regulator (both of which generate heat).