[thewotan]

Hi there!

I just finished building an AB528. Everything looks good, red LED lights up and all voltages measured as the guide. However, when testing with REW, I get a strange frequency response... There's a 50 dB deviation from 20 Hz to 20 kHz, and phase response also looks around 90º off.

I'm testing it feeding a sweep from REW, just turning the output from REW down (around -38 dbFS). There should be an impedance mismatch, but the same setup with other preamps shows a pretty linear frequency response.

So, my first question is: is this the expected behaviour?. I'm new to this type of preamp and it could be that it is working properly, although it seems a pretty wild deviation to me. And if it is not working good, any ideas where the error could be?. I checked, double checked and triple checked when soldering, and in a visual inspection everything looks good.


Here's a picture of one measurement where you can see the deviation. I'll uploaded all 4 measurements (one on each impedance, and polarity normal and reversed) and a control measurement done on a Tonelux MP5A to Google drive:

https://drive.google.com/file/d/1ESe5evAJkB6fLMiKmljoz0QaRPI-MbvK/view?usp=drive_link

[JPK]

The frequency response looks like you have only one phase connected at the output.
the output is on the secondary of a transformer and it is completely floating. Check that both phases are connected to your converter.

[thewotan]

Hi! 



First of all, thank you for your help. Appreciate it! 



When you say that one phase is not connected at the output, are you talking about B2 and/or B4 on the schematic, after T2?



Or are you talking about the connection between preamp and interface? 



In case it helps, my setup is as follows:



My 500 rack is a Cranborne Audio 500 R8, which is also my interface. The rack slots can be fed via analog input or via USB input. With the Ab528 I have it being fed via USB and then the output of the slot back into the PC to measure with REW.



Apparently, all looks good on the AB528. How could I Check if both phases are connected? 

[JPK]

When you say that one phase is not connected at the output, are you talking about B2 and/or B4 on the schematic, after T2?
Or are you talking about the connection between preamp and interface?
It could be either one.

You can check the B2-B4 connection with an ohmmeter between pins 2 & 4 of the back connector. It should measure around 38 ohms.
You can also check on the ouput XLR between pins 2&3 --> same result.
The issue could also come from the input connection. Can you check tha you receive the correct input signal on TP1, against 0V. The signal should stay nearly constant between 20Hz and 20kHz

[thewotan]

I did a quick measurement between pin 2 and pin 4 of the edge connector and it measures around 4.7 kOhm. I don't see any obvious mistakes on the PCB🤔

Tomorrow I'll take a more thorough look and see if I misplaced anything, or if I have any cold joint.

Any suggestions to what parts should I focus on? 

Thanks again for your help! 

[JPK]

Measure the resistance of the transformer secondary directly between the orange and green wires. Also check that the black and blue wires are connected together.

[thewotan]

Blue and Black shows continuity, but Orange also showed continuity with blue (and black). 


Unsoldering orange and green, there's no resistance between them, so  the transformer is busted, right? 

[thewotan]

Blue and Black shows continuity, but Orange also showed continuity with blue (and black).


Unsoldering orange and green, there's no resistance between them, so  the transformer is busted, right?

I suppose it doesn't matter, but resistance between pin 2 and pin 4 on edge connector is still 4.7 kOhm 

[thewotan]

Blue and Black shows continuity, but Orange also showed continuity with blue (and black).


Unsoldering orange and green, there's no resistance between them, so  the transformer is busted, right?

Sorry, by "no resistance", I meant infinite resistance. 

[JPK]

The output of the preamp is made of 2 coils in series: orange-black and blue-green.
Check the resistance of these coils. If one is infinite, the transformer is damaged.

[thewotan]

Blue - green is infinite😢🤦


Any chance that this was dead on arrival? I don't see how I could have damaged it while soldering it, but truth is I didn't check the transformer before installation

[JPK]

The transformer was probably defective from the beginning.
We will replace it. Please contact me here.