The 500 Rack

I decided to go into the 500-series world but which rack?  There are so many to choose from.  I wanted:

  • External power supply (and a good one)
  • Full 19" rack width (not a "lunchbox")
  • Nice to have : some sort of patching or routing options

I settled on the Black Lion Audio PBR-8 


This rack seems to be getting some good reiviews.  Black Lion has a great reputation as a company and it has a cool little TT patch panel to allow totally flexible routing changes.

First device kit

For my fist device, I wanted a preamp - and I wanted to build it!  But which kit? Again, so many choices.

I decided on a Sound Skulptor MP573.  This kit claims to be based very closely on the Neve 1073 Mic pre.  That sounded good. It's a dual stage discret op-amp design with input and output transformers. 


The kit arrives in a small box with everything included.The documentaiton is online and I found the assembly guide, parts lists and setup guide to be extremely accurate. The PC boards are bare and you get nicely organized bags of resistors, caps, transistors and diodes. You also get all the mechanical chassis bits and the transformers. I never printed out the documentation and instead assembled it by viewing the docs on my phone. They did provide a printed parts list so I was able to cross them off as I went along.


A few hours in, I realized I was short one 1K5 Resistor. Oh OH!  Obviously, I put a 1k5 where something else was supposed to go. I took some close up photos of the board with my phone and went to bed for the night. While listening to a podcast, I reviewed all the photos and Ah HA! I spotted the errant 1K5.  Back to the shop and made the fix. Then I hung it up for the night and finished the build the next day.  All in all, I'd say I spent about 10 hours.  I could build another one much faster now.






Once complete on the bench (after about 10 hours build time), It was time to smoke test it and do some initial tests and calibration.


I crossed my fingers and powered up the rack. I half expected smoke and minor explosions, but there was nothing. No SMOKE!  There is one small calibration step to do and the device needs to go in and out of the rack a few times before the final final assembly.  So far, so good!


Finally fully assembled on the bench with the cover. They pack a lot of stuff in this space!


Finally installed in the rack and powered up for a several hour burn-in (smoke test)


So how does it sound?  Is it worth it? Is it really that much better/different compared to the preamps in my focusrite 18i20?

I admit I'm new at this esoteric mic pre stuff. Most audio nuts feel that the preamp has a HUGE impact on the sound. I'm witholding judgement on that until I get some hours experience on different material. Of course, after all of that hard work, I wanted to believe the difference would be huge! This is a very limited shootout - one instrument, one set of gains on the preamps. Just wanted to get an overall picture.

Short version is - yes, I like it better and I will go to it first. But, I won't feel too sad about whatever else ends up going through the focusrite amps. The MP573 does sound very good. It is super quiet and clean even at very high gain levels. This is also a nice shoutout to Black Lion for making such a clean/quiet power supply for the rack.

I didn't push either preamp; and a preamp like this should really start to show its true color when you push it into a bit of saturation. For this test though, I wanted to operate both preamps in their linear range without exploring the saturation characteristics too much. One downside to this preamp is that there is no output attenuation control. Having that allows you to push the gain stages and transformers without overloading the downstream digital input (think of it like a "master volume" control on a guitar amp). 

The two closest matched mics I have are the two SM81s. Not my first choice for guitar but I wanted two identical mics to record simultaniously from about the same point. So I set them up and recorded some acoustic guitar. One is feeding the MP573 kit and the other is feeding an input on the Focusrite 18i20.


I did two passes. Swapped the mics/pres on the second pass so I could partially rule out differences between the mics. I recorded acoustic guitar with the mics about 12 inches from where the neck joins the body.

There are four files as a result

  1. SM81 A, Preamp A
  2. SM81 B, Preamp B
  3. SM81 A, Preamp B
  4. SM81 B, Preamp A

No, I won't say which is which. I will say that they are all level matched as carefully as I could. There is absolutely no processing on any of them and they are exported as 32bit floating point files.  No dithering on my end! 

Want to make it move obvious?

I did another test this time with a single mic alternately plugged into the MP573 and directly into the Focusrite. I recorded two passes - one on each preamp and lined up the two in time.  To make the difference more obvious, I looped one small section.

So, here are two files of a small looped section. Volume matched within a couple 1/10s dB.  One obviously sounds much better. It's not even subtle.

I am very pleased with the amp. It would be a bit more scientific if it were a blind test (and *I* didn't know which is which).  I'd also have to conclude that the focusrite amps are not exactly awful (which is good, because until I build more kits, I have a lot more of those!). I am sure that subtle as the differences are, that if I tracked everything in a project through the kit, the cumulative effect would be much more apparent - of course it would be impossible to "shoot that out".