MR sound boards amp, spectrum and sound contents

**erv** · 03-31-2007, 11:49 AM

just a few thoughts about how MR boards work. Not to teach a class about electronic and acoustics, but I've studied them a lill bit closer when designing buttered toast V2, to get a rough idea of their features and specs.
Here is what I know :

- audio amp : pretty ugly electronic solution. Single bipolar NPN transistor, emitter to the ground, speaker between positive of supply and collector of the transistor. Got a board with no sound ? guess what : probably the speaker is dead and in the worst case went in short circuit or so and burnt the output transistor. This transistor is easy to find on the board, you have 6 identical ones one behind the led strip connector. The audio amp transistor is just like those ones, 3 pins in triangle, impossible to miss the orientation / polarity. Somewhere in the middle of the board, close to the speaker wires.
If like me you use an external luxeon driver (like mine or corbin's one), you don't need anymore the one used to switch the LED strip. Savage one from the 6, and replace the audio amp transitor. Generally the have enough pin length so that you can cut the terminal, unsolder the rest of the wire, clean the solder pad and solder back the replacement transistor.
If you are using the board to power the luxeon, buy a BC547 NPN transistor at your local electronic shop. Inexpensive. Less than 1$ to repair a 45$ board, it's worth the effort.

Using a single transistor to make an audio amp is awfull cause you cannot have a symetric driving of the audio wave (you need 2 transistors to do so, it's called a pushpull) and moreover some current is flowing into the speaker even during zero crossing. However, to avoid depleting the batteries when off, the signal driving the output transistor is set to 0.
Although the solution is ugly (but damn cheap), sound is correct, sound contents has been tuned to match the audio amp solution.

- spectrum and digital resolution
quantification of the signal was easy to find out with an oscilloscope (the "gap" or "steps" between 2 consecutive samples) : the output Digital to Analog Converter (DAC) does not have a single reconstruction filter, not even a resistor/capacitor 1st order low pass filter, so the signal is "stepped". Don't hear the difference on the speaker ? so you don't need a filter ! (okay, I'm not blaming them, a cents is a cents, and it's true that you don't hear the difference on a 29 mm speaker, I therefore removed my filter on my V2).
I measured, if my memory is correct, a 14 or 16 kHz sample rate (7-8 kHz sound contents max), and I bet on 10 or 12 bit resolution. 10 bits are required to almost not hearing quantification noise (a little constant sssshhhhhhhhh noise in the sound background). You can hear it on my board (V1, 8 bit sounds). You can barely hear it with 10 bits, and it's better and better when you increase the resolution of the sounds (my V2 is 16 bits).

I patched a little program yesterday to make a spectral analysis of the board, using a little electret mike. I'll post the pictures later. Idle hum of the luke ep VI is around 650 Hz, clearly not the original 50 or 100 Hz of the original movie sounds. Easy to understand, such a little speaker starts at 400 or 500 Hz. Bass requires a big physical displacement of the membrane. Spectrum can be seen to extend up to 8 kHz during the clash.
I'm not at home right now, no FTP access, so I can't post the pics, I'll have to wait until I return from my trip.

Any thoughts, feedback or contribution in this field would be welcomed to increase (our posts count ?

) the general knowledge base about MR boards.
Erv'

**Madcow** · 03-31-2007, 02:24 PM

Hey Erv,
Thanks for your contributions!

A question for you.

You mentioned there might be a way to make a minor adjustment to the MR board that would put out more mA to the LED thus removing the need for a relay to drive the K2.

Details? Is it safe for the board?

Cheers,
MC

**Jay-gon Jinn** · 03-31-2007, 10:13 PM

I'd like to see if that can be done as well, I ave a K2 in my custom right now, and I'm just running it off the board because I don't think I have enough room for a relay in the hilt. Also, do you think there's a way to increase the volume of the sound, or did you say that above, and I just missed it?

**erv** · 04-01-2007, 01:34 AM

MC, I still need to investigate my lead using a MOS FET to replace the 6 bipolar transitors of the MR board. As usual, need some time to try it.
Even with a small transitor, it could drive at least 2 or 3 amps.
What I plan to do is to MOD the MR board to get a nice ramp effect (in 6 steps plus linear interpolation).
mmmmm, enough smalltalk, I'll keep you posted about that.

about the sound volume : no chance to get really more, except if you use an additionnal amp and more voltage. P = U^2 / R
speaker is 8 ohm, MR voltage is about 4.5 V. P = 2.5 W max. But the loud speaker is for .1 to .25 watt not more. Driving more power into such a speaker will just lead to saturation and ugly sound.
Only solution to amplify the sound : resonant chamber. Get rid of the original battery holder of the saber, replace it by 4 AA inline (2 // 2), it will free some space in the hilt and air will be able to flow there. Some additionnal space in front of the speaker helps a lot as well, I use 1 or 2 cm.

the spectrogram pics of the different sounds. Overall scale is 11025 Hz on the X axis

**Jay-gon Jinn** · 04-01-2007, 05:03 PM

Thanks, erv. I actually have one of these boards in my custom saber using the speaker from the store here. I have it located in the pommel, and might try to find a way to make some kind of spacer to back the speaker away from the holes in the bottom of the pommel, using the pommel as the resonance chamber.

Ryma Mara · 04-01-2007, 08:16 PM

I found that using plastic pieces from say the former plastic casing from Mr sabers that used to hold the board and stuff work really well, like the white-ish looking bit of the old bladeholder on a vader work really well for holding a speaker in place and it will also hold itself inside the hilt.

Now I dont know if the id of your hilt is like the mhs's but if its close then just shave teh bit a bit.

**LAN-ED-TUL** · 04-08-2007, 06:01 AM

i use either a piece of pvc pipe or a ring of wood as a spacer to get the resonance chamber effect.

its amazing how much louder a hasbro board and speaker can be with a different way of mounting the speaker.

normally a hasbro in the toy, isnt as loud a a stock MR FX. but placing the board and speaker into a new hilt, like the hielands or a graflex, or a sinktube saber, and setting up the small space between the speaker and the end cap, just makes the world of a difference. the ones i have made up are now louder than a stock MR by far and above.

**Novastar** · 04-12-2007, 01:15 AM

Erv is totally right (well about everything)... but especially on the resonance chamber thing.

You want some space behind and in front of the actual speaker "disc".

Also, remember that soundwaves... uh... are waves! If you want the most reverberation, you want to have quite a bit of "cheese-grater"-ness to your pommel!

If you want the sound to resonate in MANY directions (and not just out and away from the pommel)... drill side holes in front of and behind the speaker.

As a final note, I've noticed that EVERY MR saber sounds different. BUT, every MR saber speaker... is TOTALLY IDENTICAL. Not necessarily the housing... the actual part.

This is clearly because of resonance. Try swapping a Luke batt/spk pack with a Mace one. The "louder" or different sounds follow the saber--not the pack.

If you already knew all that, great.

I didn't until about a year ago...

**Big Daddy Clone** · 09-13-2010, 06:52 AM

If you are using the board to power the luxeon, buy a BC547 NPN transistor at your local electronic shop. Inexpensive. Less than 1$ to repair a 45$ board, it's worth the effort.

Using a single transistor to make an audio amp is awfull cause you cannot have a symetric driving of the audio wave (you need 2 transistors to do so, it's called a pushpull) and moreover some current is flowing into the speaker even during zero crossing. However, to avoid depleting the batteries when off, the signal driving the output transistor is set to 0.

I think I am having this problem.... Will radio shack have the npn I need? Also how do I connect the npn(s) to the board. I'm a guy I need visial diagrams please.