Mini Mixer Question etc

Hi, hope all is well with you
Picked up a mini mixer off ebay the other day £4, it excepts 1/4 mic and rca inputs/outputs
I thought Id open her up and have a look
I noticed a few things of interest
NO ICs, so does that mean discrete?
The line level inputs were going through the mic channels on the pcb, but there were 4 resistors(parallell). Are these there to change the voltage/ohms when it goes through the mic line?
On the rca out there were only 3 wires for the stereo output, are these 2 x positive and one negative? What would i need to convert to positive/negative/ground so i could put them into 2 x 1/4 TRS
Slowly trying to figure all this shit out
Have a good one and thanks in advance

8 comments

  1. Another of my comments, https://strangelyperfect.tv/69/planned-diy-builds/#comment-14 is about the guts if amplifiers and has lots of links.
    I mentioned this site, http://hackedgadgets.com/2006/07/08/headphone-amp-collection/ as a suggestion for the boxes to use when money is tight and/or you want something special that isn’t a bog-standard box!

    This comment is to help all the people checking this particular posting. It’s become quite popular recently so I’m trying to get all the links info etc all in the one place.

  2. Paul.
    I just came across a site or two that show good examples of DIY electronic construction methods!
    http://hackedgadgets.com/2008/07/02/diy-headphone-amp-built-in-a-cd-rom-case/ is full of wacky things; this particular one says it all in the title.
    Following some links through brings you to http://hackedgadgets.com/2006/07/08/headphone-amp-collection/ is a collection.
    This one, http://diyaudioprojects.com/Solid/IRF610-Class-A-Headphone-Amp/ is the first link in more detail showing circuits and testing procedures and results.

    They all show you what you can do with a bit of patience and a soldering iron to produce good quality stuff with practical uses.

  3. Hey good-lookin’ dude!
    I don’t know what you’re on about with the line level bit. You say the thing has amplification – so that’s all you need to know. The line, because it’s about a volt or so, will be amplified less than the mic inputs as mikes are only putting out millivolts.
    When doing all this mixed levels and impedances thing, all you’ve got to ensure is that the input of one stage does not load the output of a previous one. That’s when you get distortion. Remember also that a microphone or electric guitar (say) are both current generators so they don’t want to be loaded with a low impedance too much. How much? …it’s all relative of course.
    Second thing is to keep the levels up to a reasonable amount or your signal will get smothered into the noise floor. But don’t make the levels too high or you get clipping distortion.

    That’s all you need to remember really.

    As for the valve thing – you know I don’t touch valves… zzzzttt – horrible nasty sparky things that can seriously upset your health!

    However, from memory, most valves within their own types have more or less identical pinouts – but check, obviously! If it has the same pinout and is the same type viz triode, tetrode, double triode, pentode etc, then the surrounding componetry – resistors and capacitors etc, will be setting the bias voltages for the valve and should be okay for a replacement. There’s a lot of tolerance inbuilt into these things – just make sure that the maximum current through the devices is the same.

    Personally, I don’t know why you bother. You’ve got the “valve sound”, just stick with it and wait for your current one to pop and replace it then – they don’t last long!


    You seems to be posting alright, so problem solved there.
    Oh! Jillian doesn’t do numbers!

  4. Jillian Greenwood
    May 29th, 2008 at 10:56 pm Edit Comment 2

    This is really boring haven’t you got anything happening on this site?

    NOT VERY BUDDHIST LIKE

  5. Thanks for your comments, which have been noted. Unfortunately, we cannot reply to each query individually. Have you tried Facebook?
    -Team Perfect

  6. Hey man, hope all is well
    Thanks for all your information
    I tried to take some pics and send to you but the camera wouldnt pick up that close, it was all blurred
    I understand about the whole mixer being unbalanced, but if i was to run a line level into it, wouldnt it drop the level down by 6dB, wich would have to be boosted back up again
    its not a apssive summing mixer as the line level can be boosted up by 26dB and the mic 40dB
    How dangerous is it to change on of the 12ax7 valves on my preamp
    What do I need to know
    I want to change it for a sovtek, which is supposed to be a lot smoother
    Also i cant seem to register for your website, the pic of the code is not showing, maybe a fault on my end, and also when i tried to use the forgot password link, it sends an email to my my email but the ink from that isnt working
    thnanks in advance
    paul

  7. This is really boring haven’t you got anything happening on this site?

  8. Hey good dude!
    At least you’re keeping your costs down 😉 ….
    I’ll try and answer as best I can without seeing the thing close up. I’d still keep your eyes open for a Seck power supply though…

    As you know, you can make a mixer without any active components whatsoever – a “passive summing mixer”. If it’s very old, then it may be like this.

    If it’s newer, then it’s either got little transistors dotted around – which you should be able to see,
    or really dinky ICs, if it’s newer – which are difficult to distinguish from surface mounted capacitors (they are like this in the little pre-amp IN my guitar),
    or the active components are on the underside of the PCB – unlikely, but possible.

    Q. How old is the mixer and does it have a brand name?

    The resistors on the line inputs are probably series connected and will define the input impedance for those inputs. They are probably 10k or 47k or 100k. Check with the colours or notation (magnifying glass?)

    The RCA out is weird. Do you mean 3 wires going to each RCA connector or 3 wires going to a pair of connectors? RCA is an unbalanced connection, similar to the old DIN standard but with different 0db points and impedances.

    I’ll assume that the 3 wires are going to a PAIR of connectors. In that case one will be ground(screen) and the other two will each be +ve (live), for the left and right channels (usually coloured white and red). These could then easily connect to two mono jacks or one stereo jack. On a stereo jack, the tip should be then left channel (or +ve with a balanced connection).

    I hope you don’t mean to have a balanced TRS (jack) output. That won’t work. It looks like the mixer is unbalanced throughout (because the mic inputs are jacks as well and thus probably unbalanced) and to have a balanced output would be pointless.

    Keep plugging away man. (pun intended)

    Check out wikipedia for succinct descriptions of the connectors and proper wiring and colour standards,Here and here

    -rees

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