Post
by emrr » Fri Feb 12, 2021 3:03 pm
I recently finished up two projects using Meanwell PD-2515, both with audio input and output transformers.
One was 990C based make-up amps for a pair of passive EQ's, no trace of noise heard, virtually nothing registering on a spectrogram. Gain roughly 16dB to get back to unity. No additional filtering before the onboard filtering around the op amp.
On this one I learned the lesson that some isolation is required on the op amp input when certain amounts of LC interaction is present, specifically old school 'K' filter high pass networks. Those would shut the amp down within a few seconds. I experimented with a couple of methods, and this being a 600 ohm passive EQ, a 2.3dB resistive 'T' pad was all it took to fix it.
The following is rambling discovery process, sorry. An edit and buildout of my initial post.
The other is a set of API type preamps. I don't hear anything, but the spectrogram at max gain of 60dB is interesting. There is a spike around 19.2kHz, a hump centered on 30kHz, and depending on how the unit powers up, a hump at 1.7kHz, which is not always present. I just discovered 1.7kHz will go away if the 3 channels not under test have their gain reduced from max....or power cycling the whole thing...or....any number of other actions. ....and there's a 13.5kHz hump popping up.... OK, I'm in the weeds here, the 19.2K is highest at -99dBFS, the 30K is similar. The 1K7 is no higher than -104dBFS. There is background of evenly spaced frequency bars throughout the audio spectrum, all below the primary hum elements and the couple of spikes. The hum elements are unchanged with power on or off, so I'm ignoring that as environmental, they're all well under -116dBFS with 60dB gain. If I disconnect the AC cable entirely, 60 is unchanged but the harmonics disappear. This isn't something I'm used to seeing, and I'm rarely looking at audio preamps at max gain with switching supplies, it may be entirely normal for the supply type, as it's all so much lower than what I'm used to. Again, I don't 'hear' any of it.
The preamps have 100mfd filters after diodes and 1000 micro-henry inductors. The audio input transformers are (supposedly) 60dB shielded, and are mounted on the other side of steel frames relative to the Meanwell, which is in itself mounted inside a Hammond cast aluminum box that's physically distanced from the inputs. The preamp furthest from the PSU with highest sequential shielding has no less noise floor than the one closest. The output transformers are open frame Jensen 600:600's, and I do see a bit more of the 60Hz in the one closest to AC input, it's probably 6 inches away. From the Meanwell there's 50Ω/100mfd in each power leg before the preamps.
This appears to be excellent noise performance with 60dB gain in a semi-antique product.
Best,
Doug Williams
Electromagnetic Radiation Recorders
