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Impedance matching for 8 ohm speaker

Kian

Member
Hi all,

I am using a multiplexer (MC74HC4052D) to switch 4 different audio sources to 1 output. The input sources have a range between -6V to 6V. When I used an oscilloscope to measure the output of the multiplexer, the voltage is the same as the input.

However, when I connect a 8 ohm speaker to the output, the voltage drops by quite a lot and the sound became very soft. How can I match the output of the multiplexer to the 8 ohm speaker such the the voltage does not drop and the volume is still as long as the original signal?

Thank in advance.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
No you can't, a multiplexer is a high impedance and low power device, you need a power amplifier after the multiplexer to feed the speaker.

Assuming the inputs are from speaker outputs?, you should either use line outputs instead, or attenuators to drop the signals to line level.

Again, assuming the inputs are speaker levels?, you could use relays instead of the multiplexer to switch the speaker to whichever you want.
 

Kian

Member
Hi Nigel,

Thank you for the reply. I basically have 4 Android based video boards with 4 audio outputs (they are usually directly connected to speakers). I only require mono, so I am tapping from +L and -L. I measured the signal and it giving a +/- 6V peak to peak signal for max volume.

Now these 4 audio outputs are connected to the multiplexer. I don't understand what you mean by using line outputs or attenuators to drop the signal to line level.

In this case, I guess, it might make sense to use relay instead. I saw this reference design:
http://winslomb.blogspot.com/2015/10/audio-multiplexer-with-integrated.html

Can I just use this without the amplifier?
 
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Nigel Goodwin

Super Moderator
Most Helpful Member
The point about attenuators is if you wish to use the multiplexer, as you then need an ampifier after the multiplexer, so would need to attenuate the speaker levels down to line levels to feed the amplifier.

How are you physically switching between the different channels?, if a rotary switch is all you're using?, you could use the switch on it's own to select with input you want. Rotary switches don't handle a lot of power, but it should cope well enough for you relatively low power requirements.
 

Kian

Member
I am actually using a microcontroller to switch the audio. I need to switch one of the 4 to 1 output.


The point about attenuators is if you wish to use the multiplexer, as you then need an ampifier after the multiplexer, so would need to attenuate the speaker levels down to line levels to feed the amplifier.

How are you physically switching between the different channels?, if a rotary switch is all you're using?, you could use the switch on it's own to select with input you want. Rotary switches don't handle a lot of power, but it should cope well enough for you relatively low power requirements.
 

ChrisP58

Well-Known Member
The 74HC4052 has much to high of an internal series resistance to use in the speaker line. That resistance, typically 40+ ohms, is forming voltage divider against the 8 ohm speaker.

Also, since the max rail voltage for the 74HC4052 is +/- 5V, it'd be clipping a +/- 6 volt signal.

If you need a computer controlled selector, you basically have two choices.
1) Relays
2) Attenuate the signals before going into the 74HC4052, then amplifying the selected signal again to drive the speaker.
 

kubeek

Well-Known Member
Most Helpful Member
Or option 3, use a completely different multiplexer. There are even some that have less than 1ohm on-resistance.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I am actually using a microcontroller to switch the audio. I need to switch one of the 4 to 1 output.
In which case you could use four single pole (SPST or SPDT) relays, with four driver transistors, and diodes across the relay coils.

There's a basic circuit in my tutorials:

http://winpicprog.co.uk/pic_tutorial_extras.htm

You can get 5V coil relays, assuming your micro supply is 5V, then just write the software to turn on which relay you want.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
What are the pros and cons with using mechanical relays vs solid state relays? I actually have a size constrain and need this to be as small as possible. especially the height of the board.
A solid state relay would be useless, it's NOT really a relay at all - you need proper mechanical relays.

I thought this might work too, but it only handles 3 channels. Most of the similar audio mux handle 2 or 3 channels. I haven't found one that handles 4 channels.

https://www.maximintegrated.com/en/products/analog/analog-switches-multiplexers/MAX4932.html
None of those kinds of analogue switches are any good, they are all high impedance and low level - you NEED good mechanical contacts, and essentially close to zero ohms.
 

jjw

Member
What are the pros and cons with using mechanical relays vs solid state relays? I actually have a size constrain and need this to be as small as possible. especially the height of the board.

I thought this might work too, but it only handles 3 channels. Most of the similar audio mux handle 2 or 3 channels. I haven't found one that handles 4 channels.

https://www.maximintegrated.com/en/products/analog/analog-switches-multiplexers/MAX4932.html
Max4932 can't handle the current needed for 8 ohm speaker after a power amplifier. Absolute max current is 300mA.
Look msg #6, use relays or multiplex the signals before the power amp.
 

crutschow

Well-Known Member
Most Helpful Member
A solid state relay would be useless, it's NOT really a relay at all - you need proper mechanical relays.
What's your definition of a relay that makes an SSR "NOT really a relay"?

You should be able to use a solid-state relay if it's an AC/DC output type that uses MOSFETs not SCRs, such as this for example, rated at 2A, 30V AC/DC with a 0.15 ohm maximum on-resistance.

Edit: If that's too expensive you could use a small mechanical relay, such as this.
The coil does require 30mA @5V to energize, so you may need a buffer transistor to drive it, along with a diode across the coil for transient suppression.
The SSR doesn't need either of those as it requires only 2mA to energize, and can be driven directly from a logic output (with a suitable series resistor).
 
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Nigel Goodwin

Super Moderator
Most Helpful Member
What's your definition of a relay that makes an SSR "NOT really a relay"?

You should be able to use a solid-state relay if it's an AC/DC output type that uses MOSFETs not SCRs, such as this for example, rated at 2A, 30V AC/DC with a 0.15 ohm maximum on-resistance.
The vast majority use SCR's (or TRIAC's), and neither those or MOSFET types don't really count as 'relays' in my book as they are far too limited. Both are merely 'playing' as been a relay, under certain circumstances.

I don't know how well the MOSFET types would work as a speaker 'relay'?, but it certainly won't be as good (or as cheap) as a proper mechanical relay, which is what is always used commercially.
 

crutschow

Well-Known Member
Most Helpful Member
The vast majority use SCR's (or TRIAC's), and neither those or MOSFET types don't really count as 'relays' in my book as they are far too limited
Seems like your criteria is a little limited.
Relay - Wikipedia
https://en.wikipedia.org/wiki/Relay

A relay is an electrically operated switch.

I would think an SSR meets that definition.
I don't know how well the MOSFET types would work as a speaker 'relay'?,
The MOSFETs act as a low bi-directional resistance when on.
I see no reason that they wouldn't conduct an audio signal with good fidelity.
it certainly won't be as good (or as cheap) as a proper mechanical relay
So there's never a problem with contact oxidation of a mechanical relay causing low-level distortion?
 

crutschow

Well-Known Member
Most Helpful Member
As far as I'm aware it's never been done?
Okay.
But that wouldn't appear to be sufficient reason not to do it.
The SSR is just connecting a MOSFET, that's turned fully on or off, in series with the signal, which is basically what all CMOS analog multiplexers do, so I don't see why it shouldn't work.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Okay.
But that wouldn't appear to be sufficient reason not to do it.
The SSR is just connecting a MOSFET, that's turned fully on or off, in series with the signal, which is basically what all CMOS analog multiplexers do, so I don't see why it shouldn't work.
And it's still a feeble attempt to duplicate a simple mechanical relay.
 

crutschow

Well-Known Member
Most Helpful Member
And it's still a feeble attempt to duplicate a simple mechanical relay.
Well, you are certainly welcome to your opinion, which I don;t happen to agree with. :p
 

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