XH-M164 NE5532 stereo preamp with ton control

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Here is the relevant part of the schematic:

In order to change the gain you must change R9.

Whoops, no cap on the wiper - that's going to be noisy when the pot is adjusted as the bias current will be interrupted (and this may only show after a while as the pot wears in).

A golden rule of bipolar opamps - no input bias current though a pot wiper.... You need a DC bias path not through the pot, and a blocking cap on the wiper terminal.
 
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Yes, that product (or very similar versions) have been around on the Chinese selling platforms for 10 years or more. There have been several versions but there are fewer sellers now and there is more cost cutting. They seem to be really cheaped out with parts like those 10mm square pots and minimal filtering of the power supplies too. The MLCC type ceramic caps are OK with audio signals but since the small parts are so closely spaced, the ceramic types fitted may actually be better than big film caps since they are smaller and less prone to radiated noise.

I have that version shown in #15 and I've used it on the bench for general purpose testing of amplifiers for quite a while now. It's not a bad performer when you've checked the OPAMP and cap. quality and replaced any suspects if necessary. It's also great value for money, considering the costs and time for building one from scratch.
 
The MX50SE gain is set by the ratio of R15 and R17. On the bottom of your MX50SE PCB is there silkscreen which gives two gain values and different resistor values?



In general I believe LJM has written about typically using the LJM amplifiers without a preamp.


I go directly from my DAC in many cases.