audio inversion for amp in bridged mode - Page 1
Forum section: electronics / projects

author: lilshawn
author_rank: Regular Contributor
timestamp: April 04, 2024, 08:35:48 pm
content: so i'm working on a project to replace 2 amplifiers in a sound system and I have some questions regarding the input board.. and i'd like a sanity check, if y'all don't mind.the audio input to this device is balanced audio, (L+, L-, R+,R-, and GND). it is converted to a unbalanced signal (L+, R+, gnd) on an input board, and then after processing, is output to a 2 channel amp for left and right speaker audio. straight forward enough.now, my understanding of the circuit i've attached, (from the original equipment manufacturer) is that this part of the circuit exclusively used to drive a subwoofer with an additional 2 channel amp that is configured in "bridged" mode for additional power.so it seems the left and right signals are combined, then converted to unbalanced audio... then split... with one signal path being run through an opamp to be inverted from the other. then each signal goes out to a 3rd and 4th channel (CH3 / CH4) to the second woofer amp, with (I THINK) the CH3 signal being the "regular" audio signal and the CH4 signal being the "inverted" signal. is this correct in my understanding?so my chosen amp has the following simplified diagram (also attached... and from the amp manufacturer) for what you need to do to run the amp in bridged mode (called BTL mode by the amp manufacture), I assume the triangle with the "-1" would be an indication of an inverted audio signal being placed into the input of channel 1 and a regular signal being placed into channel 2, with ground being ground.now, it seems to me that I can just take this CH4 signal and feed it into channel 1 of my amp and CH3 into channel 2 of my amp, attach ground as well, and wire the output of the amp accordingly to the woofer.does this seem correct? this seems too easy. I feel like I'm missing something.

author: moffy
author_rank: Super Contributor
timestamp: April 04, 2024, 10:40:31 pm
content: It looks like you connect the (L+,L-), (R+,R-) outputs go to the main amp inputs and the input to the Subwoofer processing board. The single ended outputs of the Subwoofer board CH3, CH4 then go into CH1 and CH2 of the BTL amp respectively, as you stated, with grounds of course connected.

author: donlisms
author_rank: Frequent Contributor
timestamp: April 05, 2024, 03:45:58 am
content: Starting at the end... an amplifier that directly supports bridge mode generally has a switch that splits one input signal into the non-inverted and inverted signals that drive the two channels. All done for you, integrated nicely into the amp.If the amp doesn't offer bridge support, assuming balanced inputs (and I always do), all we need to do is wire up a Y connector with pins 2 and 3 swapped on one branch to accomplish the same thing. Very simple. (Unless, of course, there are some unbalanced elements stirred into the configuration, e.g., pseudo-balanced outputs.)With unbalanced signals or amp inputs, you're on your own to find a way to split the signal and invert one branch. And it looks like you indeed already have that.I assume everything's fine with the crossover board. I would prefer an approach with balanced inputs and outputs: left and right in, left and right mains out, subwoofer out. Balancing is good in a sound system.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 05, 2024, 04:38:59 am
content: You can just passively mix the two signals using resistors and have each side drive each input. Add capacitors to ground for low pass filtering. Or if that's not steep enough of a filter, add an active filter on each line, only needs one opamp per line.If common mode rejection is important, adding a single opamp and 4 resistors to the input mixer (connected such that the opamp tries to make the sum of the signals equal to zero) achieves that. Or a 1:1 transformer connected as an autotransformer, even relatively cheap ones can work well.

author: lilshawn
author_rank: Regular Contributor
timestamp: April 07, 2024, 07:29:26 pm
content: thanks for the input guys. it's been loads of help! I've looked into some of the things mentioned in your posts... and to google some of the audio terms i was not super familiar with. (beyond subwoofer box design and the associated audio dynamics surrounding them...not too much) But for sure, I have been using what i've learned to continue with this project. I will be finding time to be working on this again the next coming week.thanks again for the input!

author: lilshawn
author_rank: Regular Contributor
timestamp: April 07, 2024, 07:38:35 pm
content: Quote from: donlisms on April 05, 2024, 03:45:58 amit does support it... it just seems... it implements it... kinda... really weird... mainly having the user have to manually configure the input to a push pull kind of input, opposed to the "above the 0v rail DC bias" kind of input... and they take care of the driver balancing on the output side. i feel this could have just been a jumper/switch on the amp doing both automagically so you didn't need additional circuitry, but it is what it is... at least i already have an inverted signal so i don't need that extra bit... but i guess it keeps another couple parts off the replacement amp board if you don't know if your end user will require it or not... so i guess just have the ability to "if" kind of setup.thanks for the post!

author: Terry Bites
author_rank: Super Contributor
timestamp: April 08, 2024, 04:19:34 pm
content: Balanced Line Driver ICs are ideal for generating bridge drive eg THAT 1606/1646 consider application specific line recivers too. Far less bother.

author: donlisms
author_rank: Frequent Contributor
timestamp: April 08, 2024, 06:37:27 pm
content: Balanced driver - interesting idea!As to the amp, yup - is what it is. It's surprising they didn't just add the small amount of circuitry required.Most of my experience is live sound, big amps, and it's quite normal to use bridged amps for mains as well as subs. It's an easy way to get the most from each amp.Best of luck on your project!