Amplifier and Speaker Ohms

JimmyB

JimmyB

Now retired - trains and fishing
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My understanding of electronics is limited, so I have to base my knowledge on other people's information, so I hope somebody can provide me with a definitive answer.
I understand that an amplifier should have a speaker of a specific resistance (Ohms) and that a small resistance speaker used over a prolonged time can cause the amplifier to over heat and possibly fail. A case in point being the MLS requiring a 8 Ohm speaker, and although the Watts could be almost anything, less than 2 Watts and you lose volume, more than 2 Watts is a waste, whereas a 4 Ohm speaker could damage the MLS. My question is what is the effect on the amplifier if no speaker is connected at all. Why the question, I appreciate that the MLS volume can be muted with the "TV remote", however for ease I would just like to isolate the speaker, and why no volume, sometimes I like to be courteous to the neighbours (but only sometimes ;)).
Thanks for the responses in advance :)
 
Whilst it is permissible to do this with a MLS card.. It is considered bad practice, generally.

Rather than placing the switch in the speaker circuit wiring, have a simple off/on switch, in the positive battery feed to the card..

Then the soundcard is switched off. - Saving a little power, to increase your runtime.

PhilP.
 
JimmyB said:
My understanding of electronics is limited, so I have to base my knowledge on other people's information, so I hope somebody can provide me with a definitive answer.
I understand that an amplifier should have a speaker of a specific resistance (Ohms) and that a small resistance speaker used over a prolonged time can cause the amplifier to over heat and possibly fail. A case in point being the MLS requiring a 8 Ohm speaker, and although the Watts could be almost anything, less than 2 Watts and you lose volume, more than 2 Watts is a waste, whereas a 4 Ohm speaker could damage the MLS. My question is what is the effect on the amplifier if no speaker is connected at all. Why the question, I appreciate that the MLS volume can be muted with the "TV remote", however for ease I would just like to isolate the speaker, and why no volume, sometimes I like to be courteous to the neighbours (but only sometimes ;)).
Thanks for the responses in advance :)
Click to expand...
Examining the detail of this subject might prove a little complex as "resistance" usually refers to DC but a sound signal is AC of varying frequency. This is why a speaker specification often refers to impedance.
I'll stick to basics. Use a speaker with an impedance (or resistance) equal to or greater than that specified by the amplifier manufacturer. I also strongly recommend a minimum power (watts) of speaker equal to or greater than the max. power of the amplifier. The noise level will be affected in no small manner by the enclosure but in our hobby this is often impractical to alter.
In general choose the largest size that'll fit. As far as speaker quality goes I recommend "Visaton's" offerings.
 
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JimmyB said:
My understanding of electronics is limited, so I have to base my knowledge on other people's information, so I hope somebody can provide me with a definitive answer.
I understand that an amplifier should have a speaker of a specific resistance (Ohms) and that a small resistance speaker used over a prolonged time can cause the amplifier to over heat and possibly fail. A case in point being the MLS requiring a 8 Ohm speaker, and although the Watts could be almost anything, less than 2 Watts and you lose volume, more than 2 Watts is a waste, whereas a 4 Ohm speaker could damage the MLS. My question is what is the effect on the amplifier if no speaker is connected at all. Why the question, I appreciate that the MLS volume can be muted with the "TV remote", however for ease I would just like to isolate the speaker, and why no volume, sometimes I like to be courteous to the neighbours (but only sometimes ;)).
Thanks for the responses in advance :)
Click to expand...
Interesting question. I had not realised disconnecting the speaker was a no no I like the idea of interposing a switch on the positive wire PhilP.

However, if you have the MLS card's remote sensor fitted, an extended cable is available or can be made up, mounted where it is "visible" to the remote control then it is as easy as flicking a switch to turn the sound off on the MLS. See picture of my roof mounted installation solution for my Vale of Ffestiniog. It could also be mounted near a cab window, it's IR sensitive.

20210809_184714.jpg

With my Sierra cards I either use their supplied external sound control, where fitted, or the card itself made accessible so the sound can be turned "off" (mounted just below a lift out coal load). Sometimes having the sound at minimum setting, just so I know all is well with a loco and its consist, is reassuring. I have very considerate neighbours, but yes, I too do not wish to overextend my welcome. Max
 
An open switch (or no speaker connected) is just an infinite impedance. I can't see how this is could damage the amplifier as no current is being drawn. However, not being an electrical engineer, and not knowing exactly how the new classes of amplifiers operate, I'll defer to those with greater knowledge.
 
I agree with everything Neil said, down to the Visiton recommendation, except DELETE the word resistance. You CANNOT measure impedance without specialized equipment, and please don't even bother to use an ohmmeter on your speaker.

Yes, amplifiers CAN be upset with no load. Depends on the amplifier. Good rule: don't do it. Remember disconnecting the output does not remove the driving current to the output transistors in the amp.

Given a car analogy: in certain cases this would be similar to leaving the motor at full revs and just pushing in the clutch and use the brakes to stop the car. As any mechanic will tell you, revving an engine under no load is not good.

Greg
 
Yes there are certainly 2 ways to turn off the sound, one via the remote if you have one the other via an on/off to the sound card. Would need some slight variation to the MLS Card on the wiring diagram as with tha card but not that tricky.
 
dunnyrail said:
Yes there are certainly 2 ways to turn off the sound, one via the remote if you have one the other via an on/off to the sound card. Would need some slight variation to the MLS Card on the wiring diagram as with tha card but not that tricky.
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As well as the MLS/Fosworks remote control you can use either a standard Sony TV remote control or any universal TV remote control set to Sony protocols, 0140. Or so It says in the manual. There is a slight difference in operation between the standard and premium cards. I have both Fosworks supplied remotes. Max.
 
Many thanks for the responses, however I was looking for a solution without the remote, and cutting the power into the MLS does not work, as soon as the motor voltage is high enough the sound is back on; tried this last year.
If as suggested disconnecting the speaker is a no-no, then back to the drawing board.
 
Some good information here. Just to fill in the gaps regarding why you can only drive speakers of a defined minimum impedance.

For simplification, let us assume that we are talking about a speaker at a fixed frequency and hence, impedance. If we substitute this for a DC resistance, we can use some simplified formulae to illustrate the example. (Electrical engineers who understand Boyles Law, look away now, we're using Ohms law and lots of comparative assumptions..)

Watts (in this case the speaker power) = voltage x current. Current = Voltage ÷ Resistance, so assuming the voltage to the speaker remains the same, the current will double if impedance (resistance) is halved. This will increase the heat generated in the output devices, possibly to the point of failure. The power supply might also not be able to supply the current and the amplifier output may "clip" and audible and potentially speaker damaging distortion could result.

The amplifier output stage should be stable enough to cope with the load being removed, so a switch will be fine, but using the remote is technically a better solution and requires less wiring.
 
JimmyB said:
......... cutting the power into the MLS does not work, as soon as the motor voltage is high enough the sound is back on........
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Sorry Jimmy, I may be missing something here but I don't see how that is possible? Surely if you physically disconnect the power to the sound card, it's not going to come on, whatever the voltage....? :wondering:

Jon.

Edit: Ahh, Max's post below now makes it all drop into place.... never having used an MLS card I hadn't realised that it was getting power from two different sources, the main loco power AND a stay-alive battery backup! :)

So, what about a DPST switch wired so that both power sources to the sound card can be switched off together, with just the one switch?
 
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JimmyB said:
Many thanks for the responses, however I was looking for a solution without the remote, and cutting the power into the MLS does not work, as soon as the motor voltage is high enough the sound is back on; tried this last year.
If as suggested disconnecting the speaker is a no-no, then back to the drawing board.
Click to expand...
That's the issue Jimmy - Cutting the feed from the battery's "stay alive" function alone is only half the problem., You also have the separate power input from the ESC, or track, dependent on your method of powering the loco. As you have noted once the variable voltage kicks in your card comes alive, even if a direct battery feed is isolated. QED, you need two switches, to fully isolate the card, unless you can verify it is safe to isolate the speaker alone while still retaining the normal power feeds to the MLS card. Is there a particular reason you want to avoid the use of a remote ? Max
 
maxi-model said:
That's the issue Jimmy - Cutting the feed from the battery's "stay alive" function alone is only half the problem., You also have the separate power input from the ESC, or track, dependent on your method of powering the loco. As you have noted once the variable voltage kicks in your card comes alive, even if a direct battery feed is isolated. QED, you need two switches, to fully isolate the card, unless you can verify it is safe to isolate the speaker alone while still retaining the normal power feeds to the MLS card. Is there a particular reason you want to avoid the use of a remote ? Max
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I have checked with our engineer and he assures me that there is absolutely no problem with switching off the speaker and leaving the soundcard running. We can't speak for all amplifiers but the one on our soundcards can run as long as you like without a speaker being connected.

This is actually quite common on small scale layouts. Some OO scale modellers want sound but don't want to convert from analogue to DCC. A solution they adopt is to install two soundcards, one steam and one diesel under their baseboard with both outputting to a single speaker through a DPDT switch. They then flick the switch over according to whether a steam or a diesel loco is running. So both soundcards are attached to a 12v DC supply to the battery terminals and to the track feed to the motor terminals and are running all the time. But only one at a time is switched through to the speaker.

Just make sure you don't create an accidental short circuit across the speaker terminals because that will definitely blow the amplifier.

Rgerads
Peter Lucas
MyLocoSound
 
Moonraker said:
I have checked with our engineer and he assures me that there is absolutely no problem with switching off the speaker and leaving the soundcard running. We can't speak for all amplifiers but the one on our soundcards can run as long as you like without a speaker being connected.

This is actually quite common on small scale layouts. Some OO scale modellers want sound but don't want to convert from analogue to DCC. A solution they adopt is to install two soundcards, one steam and one diesel under their baseboard with both outputting to a single speaker through a DPDT switch. They then flick the switch over according to whether a steam or a diesel loco is running. So both soundcards are attached to a 12v DC supply to the battery terminals and to the track feed to the motor terminals and are running all the time. But only one at a time is switched through to the speaker.

Just make sure you don't create an accidental short circuit across the speaker terminals because that will definitely blow the amplifier.

Rgerads
Peter Lucas
MyLocoSound
Click to expand...
Peter, many thanks, I would seem in this particular case disconnecting the speaker is the solution.
 
So following Peter's advice, the task is simple, cut the speaker wire and add a switch, however my own foibles, cause more issues, I need to cut both wires and add a double pole switch:


IMG_1194.jpg

The upper switch is the On/Charge, the lower for the speaker
The wiring with everything in situ is no as easy as it should be (or is that just me:
IMG_1196.jpg

The purple wires are the speaker wires, and it works, but next time I may remove the components and reassemble, it could be easier.
 
no problem, electricity is a weird thing.... think of the circuit like pipes, interrupt the pipe anywhere and all flow stops.

the reality is along this "pipe" electrons are hopping from copper atom to copper atom (sort of) so stopping the flow anywhere stops the whole "bucket brigade"...

but no harm no foul.... lots of people here to help. I used to teach basic physics which of course includes electricity. Used to go to elementary school with wire, batteries, switches, etc, and make circuits in my desk.

Greg
 
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