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Saturday, 2 February 2013

Amplifier Speaker Load Box 16 ohm For Direct Recording

I wanted to use my Bugera 6262 cranked up a bit so I thought I need to design a quick load box so I can unplug the speakers and put the line out into my recording interface and use software based cabinets.

There isn't a layout per se but I've provided a schematic and pictures - it is a very simple design so you should be able to follow it easily enough.
you will also notice that it uses 2 100watt 33 ohm resistors in parallel which will allow you to use amps upto 100watts or in my case (130watt) an amp not turned all the way up

you need twice the power rating on the resistors as indicated on your amp - ie if your amp is 50 watts you need 100watt resistors.


  1. Thanks for this Paul!
    Would it be possible to mod this to be variable for use as an attenuator?

    1. it's possible yes but you'd need some heavy resistors and you wouldn't be able to go lower than about an order of magnitude higher than 16Ω
      EG the 22K resistor would have to be at least 1k so it wouldn't affect the load resistors

    2. So the 22k can go down no lower than 1K (1 watt or higher?), and reducing it's value will give a hotter signal at the output?

  2. Thanks for the info !
    One question, for 8 Ohms output impedance I'll use 2 16 Ohm resistors. Anything else ?

    1. I forgot, I need to change the 22K resistor for a 11K to get the correct 8K resistence (according to my quick calculations)

    2. I mean 8 Ohms resistance

    3. try not to get hung up on calculations - when a resistor is a couple of orders of magnitude higher than others it is in parallel with then it won't affect the lower resistance. I know calculations are spitting out one number but if you measured it with your DMM you'll just get the lowest figure (in this case 8ohm) with maybe a tiny bit of deviation at the most

    4. Yep, you are right, thanks :)

    5. no worries. it's easy to go overboard with the calculations in a hobby like this and I just urge that after anyone does calculations (and you should when necessary)but always check with a meter because sometimes real world doesn't quite match

  3. Quick Q on this old circuit of yours...

    Resitive load boxes usually tend to lose high frequency content. Is this the reason why you used a capacitor/resistor circuit on the output?

    Also... can I easily connect an 8Ohm amp to this circuit? Should be possible, no? The heat dissipation might be higher though.

    Unless there is a way to create a "deluxe" version with a (heavy duty) dip switch to switch from 16Ohm (2x 33Ohm parallel) to 8OHm (2x 22Ohm parallel). But I'd probably need diodes to force the currency into one direction so that the other resistor array is ignored, no? The switch would be between the Input Ring and the first resistor array if I understand that right, and the diodes (should I even need any) post the resistor array (heatsink) in question.

    Or would that mess up the circuit?