Speaker level to line level converter for a bridged amplifier? - Page 1
Forum section: electronics / beginners

author: wnorcott
author_rank: Regular Contributor
timestamp: September 23, 2019, 06:46:12 pm
content: 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.

author: mikerj
author_rank: Super Contributor
timestamp: September 23, 2019, 07:55:07 pm
content: 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.

author: wnorcott
author_rank: Regular Contributor
timestamp: September 23, 2019, 08:09:58 pm
content: Thank you!

author: Audioguru again
author_rank: Regular Contributor
timestamp: September 23, 2019, 11:44:36 pm
content: 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.

author: mikerj
author_rank: Super Contributor
timestamp: September 24, 2019, 01:18:05 pm
content: Quote from: Audioguru again on September 23, 2019, 11:44:36 pmA proper audio transformer will introduce negligible issues with frequency response and distortion, and will eliminate any chance of hum from ground loops.

author: L_Euler
author_rank: Regular Contributor
timestamp: September 24, 2019, 05:28:29 pm
content: 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.

author: Zero999
author_rank: Super Contributor
timestamp: September 24, 2019, 07:56:46 pm
content: Quote from: mikerj on September 24, 2019, 01:18:05 pmThe 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.