UCD problem

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#1
I would appreciate advice from someone familiar with UCD400 modules. On all 3 modules in my amp, the two 1.1K resistors near the top of the UCD400 board are very hot. I measure 22 volts across them and that, if I'm right, is about 0.4 watts which seems too much.

Is a schematic available?

HG supply with 60 volt rails. Otherwise, the modules work OK.

Thanks a LOT for any help...
 
#4
If the 400 modules are like the 180 modules those resistors are there to drop some voltage for the op amp power supply transistors. According to your measurements there is about 20mA flowing through each resistor. Some of that goes to bias the zeners and the rest goes through the transistor base and collector. I don't know what is causing that much current draw, but it's either the op amp circuit or the zeners are being biased with a lot of current. You'd need to trace out the circuit to see exactly how everything is connected.

Assuming the zeners used are 15V types and 0.7Vbe drop across the transistors;
60-22=38V
38-14.3=23.7Vce across the transistor
Pdiss=(20mA)*(23.7V)=0.474Wmaximum dissipated by the transistors assuming little is used to bias the zeners and little is used for base current. These must be SOT223 or TO220 types to safely handle that kind of dissipation.

Just thought of something, do you know if your modules have the op amp biased into class A? I remember reading about the option to do this on some UcD modules.
 
#5
The modules are the latest stock version with the AD8620 option.

The resistors are connected to the main voltage rails. I will take an older module to work tomorrow and trace some of the circuitry to see if I can figure anything out.

I do seem to remember seeing a schematic somewhere.
 
#7
Kinda off-topic... would it help to remove these resistors and connect a clean regulated supply here say +/-24V to feed the onboard regulators or is it better to remove the transistors and directly feed the front-end with +/- 12V?
 
#10
Recently, I use 2 UCD400s with SPS80, I got a problem, I check the SPS80 Voltage(without UCD400 connected), it is +(-)59.5V, but when I connect 2 UCD400, the voltage become +64V, -55V. UCD400 still work well, the music was well. Is that any problem? Thanks.
 
#11
Recently, I use 2 UCD400s with SPS80, I got a problem, I check the SPS80 Voltage(without UCD400 connected), it is +(-)59.5V, but when I connect 2 UCD400, the voltage become +64V, -55V. UCD400 still work well, the music was well. Is that any problem? Thanks.
Click to expand...

That's due to the difference in current consumption from both rails in the UCD amplifiers (to be fair, also in other Class-D modules including our own coldamp BP4078). In typical Class-D architectures, the negative rail draws considerably higher current than the positive rail. This can produce some unbalance in the supply rails, as it only measures the total voltage across both rails and tries to keep it constant, so there can be some offset with respect to GND.

There is no problem in a slight voltage unbalance, (provided that no rail reachs the module overvoltage trip point). It won't produce appreciable output DC offset or similar.
However, for your peace of mind, there is a simple solution: just disconnect the bleeder resistor that ensures minimum loading of the SPS80 in the negative rail (RL-), and if necessary, add some more loading to the positive rail so the current seen by the supply in both rails is similar.

The unbalance gets higher as you connect more modules to the same supply, as the difference between rails consumption increases.
 
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