What voltages and currents are present on amplifier outputs? - diyAudio
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Old 9th March 2010, 08:20 AM   #1
M5Sime is offline M5Sime  United Kingdom
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Question What voltages and currents are present on amplifier outputs?

Hello all,

I need to design an amplifier switching system (Home Cinema and HiFI amplifiers to the same set of L&R Front HiFi speakers).

I realise all the caveats of not being purist HiFi
I also do not wish to make something that is going to be switched when anything is 'active')

Question is - For a typical 100W per ch amp into a typical 8 Ohm speaker load, what are the typical and peak

Voltages (V)
Currents (A)

Clearly I am looking for the highest quality lowest loss DPDT switches that will remain in-circuit for the duration of the system in state A or B.. So I need to work out what power rating of switch I need.. Most of course are either rated at 230 or 110 VAC or 12/24 VDC and get fewer as one approaches 16A+ capabilities

Appreciate any thoughts

BR

Simon
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Old 9th March 2010, 09:17 AM   #2
jaycee is online now jaycee  United Kingdom
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100W/8ohm amps typically have 50V peaks at their output... the used supply is usually 55v and there is some loss in the output stage.

The key thing is that speakers are not simply resistive. 8 ohm impedance speakers can dip to 4 or even 2 ohms at certain frequencies. Assume the worst. You will want 16A switches. 230VAC rated switches will be fine but make sure they have hefty contacts. To avoid wear on the contacts, don't switch the speakers while the amp is working with a signal.
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Old 9th March 2010, 09:28 AM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
switches do not like switching current. They erode/overheat/wear out.

If you organise a temporary mute on the source and/or the power amp then you can switch quite high current using a switch that is rated similarly and get long life from the contacts.

I have seen it suggested that a high power signal relay benefits from two sets of contacts. A high power pair to switch the power off and on and a high quality pair that make last and break first to cause minimal audible effect.

A typical 100W amplifier is usually passes an average level of -20dB cf. maximum power, i.e. about 2.8Vac into 8ohms.
This results in average currents of about 350mAac.
The amp playing like this will pass regular transients that are >+6dB above the average level and often pass transients >+10dB
Transients exceeding +15dB are relatively rare but do depend on the music source.


If the amp were asked to deliver peak transients then expect 40Vpk and 14Apk to pass to the speaker.
Switching during these high power moments would quickly wear out any switch/relay.
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Old 9th March 2010, 09:41 AM   #4
M5Sime is offline M5Sime  United Kingdom
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Default Currents

Hi Guys,

Thanks for the quick comments...

The actual amps I am talking about are a set of bi-AMPed Exposures running at 75wpc (but clearly good HiFi 75w of real muscle)

The other is a 50wpc (but could be upgraded later to 100w pc) Home Cinema AMP...

So, typical voltages and currents would all be within the specs of a 230VAC 16A rated switch and making the firm assumption that these will NEVER be switched when live, then actually the contacts handling the typically varying loads mean a variable power draw though a fixed connection of contacts..

Fully agree on the nastiness of switching power fed contacts..

I am searching for the lowest resistance spec switches and ideally was not going to choose the fattest power handling switch unless neccesary..

Is the consensus then:

Average V = 25-35 VAC
Average A = 5-8A??

Peak V = 55VAC
Peak A = 15A

Rates at 100wpc

???

BR and thanks again

Simon
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Old 9th March 2010, 09:50 AM   #5
AndrewT is offline AndrewT  Scotland
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yes, that looks about right. A mains switch that can handle 16A.
How about a touch operated mute switch that feels you touching the switch knob and mutes all the power amps automatically, to allow effectively zero crossing switching?

You may need to look at switching the speaker return/ground to prevent big loops forming from source through both sets of poweramps back through the speaker return cables.
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Old 9th March 2010, 10:16 AM   #6
M5Sime is offline M5Sime  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
You may need to look at switching the speaker return/ground to prevent big loops forming from source through both sets of poweramps back through the speaker return cables.
Thanks Andrew,

I plan to switch all connections individually and not group any of the return/grounds..

230VAC 16A rated it is then..

Cheers

Simon
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Old 9th March 2010, 05:18 PM   #7
cbdb is offline cbdb  Canada
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If your switching speakers for movies, wont you be switching them cold? Then no need for a mute cct.
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Old 9th March 2010, 05:22 PM   #8
M5Sime is offline M5Sime  United Kingdom
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Quote:
Originally Posted by cbdb View Post
If your switching speakers for movies, wont you be switching them cold? Then no need for a mute cct.
Hello

Yes - I will ONLY be switching when the amplifiers are turned off. It is a teeny bit more complex as the Exposure setup is a true bi-amp and the Home Cinema amp is not.. Just a matter of some cabling though...

I am searching now for the highest quality switches I can find with the right ratings (in terms of lowest insertion resistance and silver/gold contacts etc etc)

I do not need huge inrush current support etc and other 50,000 switch cycles as these will happen more rarely and never with current flowing

BR

Simon
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Old 9th March 2010, 06:42 PM   #9
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Quote:
Originally Posted by M5Sime View Post
making the... assumption that these will NEVER be switched when live
Can't you just avoid doing this? Switch the signal at low power, i.e. before the amp(s)? Double up the speakers?

Anything that CAN go wrong, WILL.

w
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Old 10th March 2010, 07:22 AM   #10
M5Sime is offline M5Sime  United Kingdom
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Quote:
Originally Posted by wakibaki View Post
Can't you just avoid doing this? Switch the signal at low power, i.e. before the amp(s)? Double up the speakers?

Anything that CAN go wrong, WILL.

w
Sadly not as the whole point is that I do not want to double-up on the speakers. Leverage the HiFi front L&R speakers for home cinema when needed.

There is very little chance of anyone switching with loads running
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