Simple line mixer circuit

Discussion in 'The Projects Forum' started by reevery, Jun 10, 2009.

  1. reevery

    Thread Starter New Member

    Jun 10, 2009
    I was wondering if I could ask for opinions on my circuit. I am trying to build a two stage line mixer circuit, and I have attached the circuit diagram ?@I have made based on what I can find on the internet. I have made the notes below and would appreciate some feedback on the circuit. For simplicity I have not jointed the grounds together but presume I need to, and have not noted where a slider would be controlling two pots.

    - 5 inputs are all line-level PC stereo outputs, and are all volume controlled by a 10k pot.

    Stage 1 mixer:
    - Two stereo feeds mixed together (1 and 2) where they will be outputted and later fed to another volume control.
    - Two further stereo feeds mixed together (4 and 5) and fed to stage 2 where they won't have any further volume control.
    - One remaining feed (3) is just volume controlled here but not combined with anything else yet.
    - At this stage of Stage 1 I want to output to two headsets (marked SPKR 2 and SPKR 3) - with the mixed feed of 1 and 2 in one ear of each, and the feed from 3 in the other.

    Stage 2 mixer:
    - This stage combines everything - there is no volume control on the 4 and 5 feeds as this will have been done in stage 1, but I have put another series of 10k pots in to control whether or not 1, 2 and 3 can be heard. This output is going to a pair of speakers (rather than headsets).

    I realise this is complicated and because of this and the number of lines I am concerned I may need to improve the circuit. Other than general comments about my approach, I have the following specific questions:

    1) If I use car speakers for SPKR 1, would I need to amplify this output? At the moment it would be going to active computer speakers which would have an amplifier in them.
    2) I am expecting some loss on this, which is acceptable, but without spending lots of money is there anything I can do to reduce this, particularly if a circuit is faded out (as I expect in stage 2 for a fair time feeds 1, 2 and 3 will be? [NB the simple solution is to introduce a fourth speaker set and to separate 1 and 2 completely, should I consider this?]
    3) Inputs 1, 2 and 3 are essentially mono feeds, is there any point at which I could or should consider them to be mono or is it best to consider them stereo so that they can be fed into the left and right circuits appropriately?

    I appreciate your time and expertise and comments would be gratefully received.

  2. beenthere

    Retired Moderator

    Apr 20, 2004
    You will need some op amps to buffer the signals, or they will get padded down to nothing by all those resistors. Googling "audio mixer circuit" brings up many ideas, like this one -
    Last edited: Jun 10, 2009
  3. reevery

    Thread Starter New Member

    Jun 10, 2009
    I had googled and found some simpler circuits that looked suitable, the problem was I wasn't sure whether I needed anything more. I have added some opamps as I think they would work but not very sure about the circuit - does it look more appropriate now?

    Two questions specifically: Do I need to put an op-amp on the two lines not being mixed before they go through second stage mixing; and as these should in theory be line-level inputs I presume I don't need to pre-amp them?
    Last edited: Jun 23, 2009