Issues arround the Renkforce E-SA9

Hi folks!

Here's my experience with the Renkforce E-SA9:

My Renkforce E-SA9 was disappointing with an unmistakable, overtone-rich mains hum in the speakers. In addition, a broadband noise could be heard in the right channel, which was superimposed by a quiet rhythmic rattling. Depending on how I routed the speaker cable past the amplifier, the hissing and rattling became louder or quieter. At this moment I couldn't think of using the amplifier with external boxes for a flat screen TV, which is what I actually bought the part for.
So, I decided to open the device and to take a look. You can quickly find a circuit diagram on the Internet. For example here:

https://asset.conrad...llic-dunkelbraun.pdf

The hum was not caused by a wiring or layout error, but by the toroidal transformer itself: The transformer comes the preamplifier board much too close and spreads its magnetic field there. This can be remedied by placing some aluminum sheet between the transformer and the preamplifier board. To do this, I used tin snips to cut four small sheets of 1mm pure aluminum. This shields like a 4mm thick sheet of pure aluminum. Since aluminum shields magnetic fields with eddy current losses and this works the better the higher the frequency of the magnetic fields is, the very low-frequency components of the transformer field are hardly shielded, but the higher-frequency components are almost completely shielded. The annoying hum becomes much quieter and appears much less aggressive, so that it is no longer annoying.

The hiss and rattle on the right channel turned out to be HF oscillation. When dismantling the amplifier, it had noticed that the thickly painted upper part of the housing had no electrical contact with the amplifier. This means that it could not function as a shield and therefore the speaker cable could capacitively stray couple back to the amplifier input and cause RF instability.

But that was not the main reason for the instability. The main cause was the 2k7 resistor, which is connected between the inverting inputs of the LM1875 via the "surround" switch. Strictly speaking, it is the long cable that runs across the entire housing to this switch. And at one LM1875 it is even connected directly to the inverting input, which is an absolute no-go in amplifier construction. The inverting input is the most sensitive point of an amplifier. A wire connected to the inverting input acts like an antenna and any capacitively coupled interference into it and injected noise appears highly amplified at the output of the amplifier. The safest way to make an amplifier oscillate.

After I removed all the wiring, including the 2k7 resistor, the hissing and rattling is gone.

Another unpleasant feature of the amplifier is the loud power-up pop. Although the power amplifiers receive a symmetrical power supply, the preamplifiers - for completely inexplicable reasons - unfortunately only receive an unbalanced one. And since the electrolytic capacitors in the signal path first have to charge themselves to around 10V when the amplifier is turned on, this results in a very loud switch-on pop, almost a bang. A conversion to a symmetrical power supply would be possible, but (since I need the tone control) this would require much deeper changes in the circuit. At this moment the only remedy is to turn the volume down to zero before switching on.

Kai