Effects loop: parallel to series for Markbass LM III

The Markbass Little Mark III series of amps sound great but all have the same issue: the effects loop is set in parallel and there is no way to adjust the blend on the signals. Let’s change the loop to series!

What products does this apply to?

As far as I know, the hardware for the following solid state amps is nearly identical:

  • Little Mark III
  • Little Mark 250
  • Little Mark 800
  • Combo Head II

There’s a chance that this also works for the Little Mark Tube and Little Mark Tube 800. Keep in mind that if you want to retain your warranty, you might be better off letting the Markbass technicians do their thing. If not, read on!

Changing the effects loop to series

I have a Markbass CMD 102P which contains the Combo Head II, so the photos below reference that model. Don’t forgot to make the necessary mental adjustments to accommodate the instructions for your own amp!

Fully disconnect and open up your amp (or slide it out of the casing) to reveal the innards:

Markbass Little Mark III effects loop - innards overview
Overview – click to enlarge

In the picture above, I’ve marked two locations of jumpers on the boards with the numbers 1 and 2:

  1. Jumper CN7 with labels “PARALL” and “SERIES” on the rear board
  2. Jumper CN2 on the front board

Find jumper CN7 located on the rear board between the send and return sockets. Use a pair of needle-nose pliers to gently move the jumper over to the “SERIES” position. Don’t be afraid to apply a little force to get it out of the silicone sealant. It should look like this:

Markbass Little Mark III effects loop - jumper CN7
Jumper CN7 – click to enlarge

Find jumper CN2 on the front board between the clip light and the low EQ knob. This time we want to remove the sealant-covered jumper; I usually place it on just one pin so I don’t have a jumper lying around:

Markbass Little Mark III effects loop - jumper CN2
Jumper CN2 – click to enlarge

That’s it, put the amp back together and you’re set!

Why is the effects loop parallel by default?

On Markbass’ support page, they state:

Can I change the effect send/return from parallel to serial?
The effects loop on Markbass amps is factory set to parallel so that your sound is not degraded as it passes through the effects, and also so that if something happens to one of your effects or batteries or cables in the effect chain, you won’t lose your signal. However, if you would like to change the wiring to serial, this can be done by qualified Markbass service technicians only. Contact your local dealer or distributor for details.

Because the effects loop works in parallel by default and there is no way to adjust the mix of these signals, you’re stuck with a 50/50 blend.

By changing the effects loop to series, you’re effectively using 100% of the return signal of your effect chain as input for the equalizer and power amp. Because the effects loop sits between the preamp gain and the equalizer, you can use that gain to modify the input signal for your effect chain.