Reducing volume in a MRC decoder speaker via a resistor.

trnj

it is possible to solder a resistor to one of the speaker leads to decrease the volume?

Theoretically a qualified Yes.  All audio amplifiers work most efficiently when feeding a specific load impedance. Some are more tolerant than others of mismatched impedances.

If you put in a series resistor equal in value the nominal impedance of your speaker (8 or 16 ohms perhaps - check the speaker) then volume should be halved or more. The problem is that there is a finite chance that the decoder will not like this. 

The best solution is using an fixed value "L-pad" instead of a single resistor. ' problem is it involves some light electronics theory and probably experimentation to get the right values.

So there is my non answer. Hopefully someone else has already tried the series resistor and will simply say "yes, it works".

Karl

PS: Could there be a secondary CV for sound?

 It may work to a limited degree, but speakers and amplifiers work on impedance, which is not the same as resistance.  Adding a resistor will cause an impedance mismatch and could cause the decoder's amplifier to overheat and burn up, or could cause the speaker to fail.

I just checked the documentation that comes with an MRC Sounder, and there is no CV for overall volume, only for each individual sound.  Poor engineering.

trnj

is possible to solder a resistor to one of the speaker leads to decrease the volume?  If so, what would be a good recommended value for the resistor? 

 John

Mebane, NC

As the others have said, audio amplifiers are designed to handle a certain range of impedances without burning up or overly distorting.  Most speakers are in the 4-16 ohm range, and (what the others didn't say) most audio amplifiers have output impedances in the hundreds of ohm range.  Maximum power transfer takes place when the impedances are matched, but putting out max power may take down an audio amp.

Some ESU sound decoders are set up for 100 ohm speakers.  Most sound decoders can handle 8-32 ohm speakers.  I don't know of any that won't choke on 4 ohm speakers.

So yes, adding a resistance equal to the impedance rating of the speaker will approximately half (to a little more than half) the power to the speaker, and hence the volume.  This is probably safe - but it is not guaranteed to be safe.  If you are using an 8 ohm speaker, add an 8 ohm resistor.  Problem is that the power rating of the resistor has to be equal to the amplifier output rating - which likely means a 1 or 2 watt 8 ohm resistor.  Don't cheat on the power rating, because the amplifier is actually going to send a little more power than it did before with twice the total impedance.

The other problem is that our hearing is not linear when it comes to volume.  Halving the power to the speaker does not make the sound half as loud to our ears - it will be a perceptible difference but not half as loud.  To get half as loud, you need to get in the 1/10 the power to the speaker range.

Something else you can do to lower volume is reduce the speaker efficiency.  Reduce the size of the speaker enclosure, or take away some of the sealing of the enclosure.  The drawback of this method is that you take away some of the bass response from our already bass-challenged speakers and decoders.

hope this helps

Fred W

What would happen if you just put a piece of electrical tape over a portion of the speaker to muffle the sound a bit? 

Adding a resistor will cause an impedance mismatch and could cause the decoder's amplifier to overheat and burn up, or could cause the speaker to fail.

Adding a resistor shouldn't harm the decoder or speaker, since it would reduce the amount of current flowing in the circuit, regardless of the impedence mismatch. Reducing the impedence probably would blow the decoder, but increasing it should reduce the volume. The problem is finding the right amount of resistance to add. The problem might be in the decoder. I have tried to reduce the volume in MRC decoders, and I find little or no difference when programming 0 or 3. It still sounds the same to me. Also the decoder will only put out one watt into 8 ohms. If you add an 8 ohm resistor, the decoder will only put out 1/2 watt into 16 ohms of resistance.

hobo9941

Adding a resistor shouldn't harm the decoder or speaker, since it would reduce the amount of current flowing in the circuit, regardless of the impedence mismatch. Reducing the impedence probably would blow the decoder, but increasing it should reduce the volume....Also the decoder will only put out one watt into 8 ohms. If you add an 8 ohm resistor, the decoder will only put out 1/2 watt into 16 ohms of resistance.

Actually, no.  Maximum current flow, and hence power transfer, is achieved when speaker impedance matches output amplifier impedance.  In our case, nominal output amplifier impedance is typically in the 100-500 ohm range, and speaker impedance is nominally 8-100 ohms.  Increasing the resistance of the speaker circuit may increase current, not decrease it, by creating a closer match to the output amplifier impedance.

In the old days of tube amplifiers (and sometimes with discrete transistors with good heat sinks) we used audio transformers on the amplifier output to better match the impedances.

Impedances are nominal because both are frequency dependent.  The impedance of a speaker, which is inductive, goes up with frequency.  An integrated circuit amplifier will usually be capacitive, and it's output impedance will go down with frequency.

The safest bet is to add a resistor equal to the nominal speaker impedance.  That almost halves the power to the speaker, and probably keeps you in the safe range of the output amplifier.  The better solution - not easily available in this case - is to control the gain of the output amplifier.  MRC apparently failed to provide enough gain control range in the software (CVs) and responding hardware.

Fred W

 I reduced the volume on an MRC sounder by putting a 10ohm resistor in series with one of the speaker leads and a 10ohm resistor in parallel with the speaker. It reduced the volume at the highest setting to about what is was at the lowest setting.

 It may or may not work for other MRC decoders,  I found a recommendation on the MRC Yahoo group to wire the resistors. It recommended other values but the 10ohm was the closest I could  find so I tried it and it seems to work. I've run the locos for a few hours with no ill effects. 

I've used a couple of the MRC sound-only "Sounders" and had a similar problem. It seems to be more of a problem with the diesels than the steam one though. Since one of the diesel Sounder's four sound choices was an SD-70, I moved it into my Kato SD-70 with a 1/2" round speaker and enclosure. It seemed OK, I actually moved the volume up to 1 and it seems fine. It's too bad in a way as the sound is pretty good, but using the 1" speaker that came with it (in an F unit) was just way too loud for my taste.

davidmbedard

 There is only one sure-fire way to tame an MRC decoder......it involves a hankerchief and a hammer......and the side effect is a grin on your face.

David B

 No need to waste a perfectly good hankerchief - just put on some safety glasses and hammer away.