Author Topic: Speaker level to line level converter for a bridged amplifier?  (Read 394 times)

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Offline wnorcott

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Speaker level to line level converter for a bridged amplifier?
« on: September 23, 2019, 06:46:12 pm »
Hello friends. My  question is how to create a speaker level to line level converter for audio.    The twist on it is the speaker is driven by a bridged amplifier, i.e. both terminals of   the speaker are active and neither is tied to ground.   If it were a regular speaker I could  have the choice of either just a simple voltage divider with resistors or else use `an inverting op amp as an attenuator.  But all he designs I have seen for either assume the (-) terminal is tied to ground, which in this case it is not.    It is low wattage maybe 10 watts per channel into 8 ohm speakers.

I was thinking maybe  use a 1:1 600 ohm audio transformer, connect  the speaker terminals to the primary side, and then  secondary taps would mirror the speaker voltage but be electrically isolated from the amplifier, and I could tie one lead  to ground and the other lead be  signal.  Then it could go into an op amp attenuator or just a voltage divider.   It seems like it would work, but will it?  The #1 concern is to not fry the amplifier.   The line level signal from L and R channels would be mixed down to mono and will be low pass filtered then sent to a subwoofer amplifier, so I am not concerned about the high frequency audio, just going to filter all of that out anyway.

Thank you in advance.
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Offline mikerj

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Re: Speaker level to line level converter for a bridged amplifier?
« Reply #1 on: September 23, 2019, 07:55:07 pm »
An audio transformer will work perfectly, provided it has a suitable turns ratio.  You can buy suitable transformers quite cheaply designed for automotive audio installations so an amplifier can be used with a head unit that has no pre-outs.
 

Offline wnorcott

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Re: Speaker level to line level converter for a bridged amplifier?
« Reply #2 on: September 23, 2019, 08:09:58 pm »
Thank you!
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Offline Audioguru again

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Re: Speaker level to line level converter for a bridged amplifier?
« Reply #3 on: September 23, 2019, 11:44:36 pm »
A transformer is not needed to convert bridged to single-ended.
A cheap transformer will have issues with frequency response and distortion.

A bridged output has an amplifier for each speaker wire. Simply select the original output (the other output might simply take the original output and invert it, adding distortion) and use the output and the amplifier's ground. Make suitable attenuator with two resistors.
 

Offline mikerj

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Re: Speaker level to line level converter for a bridged amplifier?
« Reply #4 on: September 24, 2019, 01:18:05 pm »
A transformer is not needed to convert bridged to single-ended.
A cheap transformer will have issues with frequency response and distortion.

A proper audio transformer will introduce negligible issues with frequency response and distortion, and will eliminate any chance of hum from ground loops.
 

Offline L_Euler

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Re: Speaker level to line level converter for a bridged amplifier?
« Reply #5 on: September 24, 2019, 05:28:29 pm »
Line level converters used to be readily available from car audio shops.  Don't know about nowadays.  It was an essential component when upgrading older GM/Delco radios.
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Offline Zero999

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Re: Speaker level to line level converter for a bridged amplifier?
« Reply #6 on: September 24, 2019, 07:56:46 pm »
A transformer is not needed to convert bridged to single-ended.
A cheap transformer will have issues with frequency response and distortion.

A proper audio transformer will introduce negligible issues with frequency response and distortion, and will eliminate any chance of hum from ground loops.
The main advantage of a transformer, is galvanic isolation. I've had fairly good results with mains transformers, just de-rate the voltage by a factor of three or more and keep the current below the rating, so a 120V primary should only be used up to 40V, for audio.
 


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