Audio attenuation (JVS2jamma upscaler thingy)

[invzim]

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So researching this a bit, and analog stuff is not my strong side. Took a look at what the namco, konami and riverservice jvs 2 jamma boards do when it comes to audio attenuation.

Observations:
The namco board obviously has some kind of error in mono mode.
All have electrolytic capacitors in-line in some variation, with values corresponding to a resistor.
Using only speaker+ seems ok.

I was planning on doing plain voltage division, but seems all these boards use caps for some filtering stuff. Anyone good with RC circuits/filters that can explain what's going on?

 

[twistedsymphony]

Most of it looks like a simple High-Pass filter http://www.electronics-tutorials.ws/filter/filter_3.html

There are some extra resistors in there so calculating the actual frequency cut-off might be difficult. I wonder if there's something in the JAMMA spec that specifies a supported decibel range.

 

[skate323k137]

Not a technical help, but I find hi low converters (line out converters) work great. I use one hooked to speaker + and - for jamma games in my net city.

Basically both positive inputs on the converter go to spkr+ and both negatives to spkr-. As a bonus the attenuation is adjustable and they're very inexpensive.

 

[invzim]

Can you snap a picture of the inside of one of those converters?

Looking at the schematics, the one by Konami by far seem the cleanest, a cap and a simple voltage divider..

I'm also tempted to drop support for amplified stereo input as it would mean another connector, added user complexity and a pretty limited audience?

 

[skate323k137]

Can you snap a picture of the inside of one of those converters?

Looking at the schematics, the one by Konami by far seem the cleanest, a cap and a simple voltage divider..

I'm also tempted to drop support for amplified stereo input as it would mean another connector, added user complexity and a pretty limited audience?

Sure, I'll see if I can pop one open non-destructively. I'd agree that input for amplified stereo isn't really a huge deal. Most of the time I'm using amplified mono from a jamma edge, or line level stereo.

The one I use is similar to this, minus the remote turn on wire
http://www.bcae1.com/images/jpegs/IMG_8088b.jpg

 

[invzim]

Some nice people told me what the caps were for, and I learned a little in the process

The caps are there to remove DC offset/DC components of the audio signal - or said in another way, removes signals with 0hz frequency (like a DC offset).

skate323k137: no need to open it.. I saw some circuits with transformers, but not going to get into those variants.

 

[l_oliveira]

The power amplifier on some boards, like for example CPS2 and NAMCO SYSTEM 11/12 use a "H-BRIDGE" configuration where both poles of the speaker are connected to the amplifier instead of just one side, meaning each speaker wire will have a opposing power potential.

In monaural mode they will cancel each other as they're generated from the same signal (one of the wires will have the opposite/inverted voltage potential than that of the other wire).

But then the point is moot as those have RCA pins for audio outputs. (exception being System 11)