The A-138b is essentially a 4 into 1 mixer optimised for audio signals rather than control voltages. the 'b' in the name means the the mixer uses logarithmic rather than linear potentiometers.
For me the A-138b is an essential piece of equipment because it enables me to sum the outputs of up to 4 sound sources (for instance VCO's or noise sources) before sending the combined output to my A-108 Low pass filter module.
With one input employed, and with the channel and master levels set to 10, the volume of the output is the same level as the input signal. in other words, this module will not increase the gain of an input, it will only attenuate it.
When using multiple inputs I find I have to reduce the levels of the individual channels quite a lot, otherwise the output from the mixer becomes to much for next module in the chain. So if I am using 3 inputs, I will use a maximum of about 33% of each channel's volume.
The mixer appears to operate very quietly. It does not introduce much 'hiss' into the sound. Some reviewers have found otherwise , so it may be that my ears are not as sensitive as theirs.
I have rated the A-138b as 5 stars except for features, which I have given a 4 for. The reason for this is that the Doepfer website indicates that with no input into channel 1, the channel 1 level will act as a +5 volt DC offset. In other words, the unit *can* increase the level beyond that of an input signal. However, my unit doesn't work like this. As delivered, all 4 inputs on my A-138b work in the same way. The manual says there is a jumper I need to re-position on the rear of the circuit board to achieve the +5 volt DD offset with input level 1. I think this option should have been made available as a toggle switch on the front panel. It would be useful on occasions when I am only using 3 inputs, but I do not want to have to remove the unit from its case and re-position a jumper each time to achieve this!
Overall, another useful and great value unit from Doepfer, in my opinion.