www.timstinchcombe.co.uk

Home

Site map

  
 

Plan B Model 9 Mixer: mod to reduce bleed-through

On the Model 9 Mixer, if a signal is applied to a particular input channel but not switched through to either the 'A' or 'B' output channels, there may be noticeable 'bleed-through' of the signal to the outputs: the effect is quite variable, dependent on the input channel, and 'Ch. 4' seems to be the worst affected. This appears to be caused by capacitive coupling within the circuit, which then tends to act like a differentiator, and hence the effect is more noticeable at higher frequencies. The symptoms can be partially alleviated by a simple change—reducing the values of the main gain-setting resistors in the circuit. Replace all blue-circled resistors shown in the following picture with 10kΩ values (7 in total):

They are currently all 100kΩ values: reducing them by this factor of 10 will reduce the bleed-through by a similar amount (i.e. around 20dB). As an added precaution I would recommend replacing all red-circled resistors with a wire link, or simply shorting across the bottom of each one with a jumper—the module I have has a tendency to oscillate at several hundred kHz, and this quashes the oscillations...

There is a little more detail in this thread at Muff Wiggler's forum, where there are also reports from several people who have successfully applied the modification. (I originally reported the idea for the mod in this post, with a bit more detail added in this and subsequent posts.)

[Page last updated: 14 Dec 2009]