Making a Stereo System using TWO Mono Bridged Amplifiers

Discussion in 'The Projects Forum' started by Carlos Almeida, Feb 2, 2015.

  1. Carlos Almeida

    Thread Starter Member

    Jan 15, 2015
    35
    0
    Hi Everyone (again),

    I have a question that Google doesn't seem to be able to reply despite my intensive searches, so since everyone here has been tremendously helpful in my other two threads, I will once again rely on your knowledge to help me figure some things out.

    As always, my budget is tight, my Electronics knowledge is not very extensive but with 1h or so of researching a specific subject I'm capable of understanding enough to engage myself in a discussion and although I have good soldering skills for minor things, my understanding of circuits schematics is very limited, so reading one is kind of like reading Spanish (I'm Portuguese, so I can understand the majority of it but the probabilities of making a bad interpretation is very high).

    Here it goes:

    1.) I have an Audio Decoder that I scavenged from and old radio, it's working properly and the Audio Output is a 3 pin connector (R out, Common Ground and L out)

    2.) I have 2 (two) Class D Stereo Amplifiers that are Bridgeable, they can support 2 x 8ohm (min 6ohm) speakers in Stereo and 1 x 4ohm Speaker in Mono (Bridged), it's also working properly and the Audio Input is the same as the Audio Decoder, a 3 pin Connector (L in, Common Ground and R in).

    3.) I have 2 (two) 4 ohm speakers that I want to connect to the Amplifier(s), for testing purpose I've assembled one amplifier in Stereo with both 4ohm speakers, the amplifier runs smoothly with lower volumes but as expected starts to warm up when the sound is raised, so this is an option that I've discarded.

    Objective: Create a Stereo system using both amplifiers, each amplifier Mono bridged to one 4ohm speaker, and the input sound coming from one single 3pin audio source.

    Problem I'm facing: If I merely split the Audio Output in two, both speakers will be in Mono and therefore I will end up with a Mono Audio System, which is not what I want.

    Questions: 1.) If I only feed the Left(or Right) Channel Output and the Common Ground to one of the amplifiers, will the amplifier still bridge to 4ohm Mono without losing power? My guess is no but it's worth asking (can't test it because the speakers are currently not at my disposal for another couple of weeks).

    2.) If I Split the Left (or Right) Channel in two, and wire it to both R and L Audio Inputs on the Amplifier, plus the common ground to it's given place, will the amplifier be "tricked" and make the Mono Bridge with only Left (Right) Channel ?

    So pretty much that's it, I have a problem that I really can't work a way around it, mostly because I'm ignorant on the subject and never before worked with 3pin stereo systems (this also poses a trouble when connecting a potentiometer but I want to go step by step).

    I'm at work right now so I can't draw any "schematics" of both questions I placed above, hope my description is sufficient for the time being, will try to draw something soon and place it here for ease of reference.

    Thank you very much to everyone.
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,415
    3,354
    When a stereo audio amp is bridged there is a switch on the amp that allows you to do this. Read the user manual of the amp showing how to do this. The purpose of the switch is to introduce an inverting stage so that the L and R channel outputs are now out of phase.

    The user manual will tell you which of the two inputs (L or R) to use for the single input signal. You find out how to do this by trial and error.

    Set up both amps for bridged operation.
    Then feed the L signal into one amp and the R signal into the second amp.
     
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  3. tcmtech

    Well-Known Member

    Nov 4, 2013
    2,032
    1,622
    Just set each amp up as its own channel. Send the Right out signal to the L and R inputs of one amp and the Left out signal to the L and R inputs of the other.

    Then once thats done crank it up and annoy your neighbors for me!

    55440083.jpg
     
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  4. Carlos Almeida

    Thread Starter Member

    Jan 15, 2015
    35
    0
    Hi Mr Chips,

    Thanks for the prompt reply, getting the original manuals is going to be near impossible, got the amps for a few months now and the "papers" are nowhere to be found. However it is a very common amp so it shouldn't be hard to find, TDA7492. By giving a quick look on ebay it says it's Mono BTL and for what I can remember, I don't think there are any switches (will confirm later today). On the other hand, the schematics of my amp (posted below, sorry for the quality but it was the one I found on a quick search) seems that to work on Bridge mode I only need to connect the Audio Input in the correct order? Seems odd but the circuit looks to work like that (although my "circuits reading" is far from good...).

    [​IMG]

    Will have to dig a bit deeper to understand this schematics, will follow up with comments later today.

    Thanks a lot
     
  5. Carlos Almeida

    Thread Starter Member

    Jan 15, 2015
    35
    0
    That's exactly what I'm planning to do, AC/DC style all the way.

    So, pretty much the schematics below would put one amp with L Channel only and the other with R channel only? (sorry, not so good on paint)

    Amplifier & Decoder Wiring.jpg

    Assuming that all I have to do to put it in mono is connecting the Speaker the right way (according to the guy who sold me, Positive Wire Speaker connected to L- on the Amp and Negative Wire Speaker connected to R+ on the Amp), I should en up with two Mono amplifiers that together will give me a nice Stereo "melody" ?

    Thanks everyone
     
  6. Carlos Almeida

    Thread Starter Member

    Jan 15, 2015
    35
    0
    Ok, so just found some more info about my D amps, to bridge them into mono I need to do a PBTL (parallel bridge tied load?). Unfortunately for me, the information about PBTL across the internet is minimal and most of them is based on circuits schematics (which I don't understand most of it), after extensive research I found a chinese seller on ebay that has a picture apparently showing how to do it, however I don't read chinese so can't be 100% sure that it's correct, can anyone give me a hint if the connection in the below picture is viable ?

    TDA7492 PBTL.jpg

    The input part of it seems very similar to the sketch I made on my previous post (thanks to tcmtech) and the output seems a simple bridge between the + and -, wouldn't that have the same effect if I connect both L+ and R+ to the Speaker + and the L- and R- to the Speaker - directly?
    I really need some feedback here otherwise I'll probably blow the amp chip :D

    Many Thanks everyone
     
  7. MrChips

    Moderator

    Oct 2, 2009
    12,415
    3,354
    I do not see how you can bridge that amp.
    Use two speakers instead.
     
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