Topic: Noisy preamp UCX
I recently bought a UCX (new to me).
Through a bit of bad luck, it's taken me a couple of months to realise mic input 2 is noisy :-(
Not only is the general noise floor elevated but it's exhibiting random crackling and hiss.
Some things to note:
- With/without phantom power makes no difference.
- When shorting pins 2 and 3 the random crackling all but disappears but the spectrum remains somewhat different to input 1
- I didn't have a 30 or 150 ohm resistor to hand, but the noise/crackling is audible above the signal level with an SM57 connected, so it is far too high to tolerate in real-world use.
I assume mic input 1 is ok (mostly just exhibits hiss) but without knowing 'what's normal' I don't know whether it's performing to spec.
To illustrate the difference the plots below are taken for input 1 (left hand plot) and input 2 (right hand plot) unterminated and at max gain ‘65’ - I know that's not representative of anything normal but it shows the issue. I've added audio examples below.
Input 1: https://sndup.net/5ckt
Input 2: https://sndup.net/98g5
True Peak of input 1 is about -46 dBFS and input 2 is about -18 dBFS which more or less lines up with Total Mix metering.
Apart from any ideas on likely component failures, I guess it would be nice to know if the performance of input 1 looks right (unterminated, gain cranked)? Would be great if someone’s able to run an equivalent plot with the free version of SPAN (default settings but with 65536 block size).
I did notice some other ‘off’ things with the UCX whilst testing input 1 – like the 12 kHz component in the last plot. It was only present when terminating pins 2 and 3 and seems to be related to sampling frequency. (e.g. 12 kHz @ 96 kHz Fs, 6 kHz and 18 kHz components @ 48 kHz Fs). These were admittedly low in level, but slightly audible. This added some doubt to my mind that my unit’s not healthy in other ways but I guess these could be design quirks!