500 series vs. Rackmount mic pres

My experiance is that it has more to do with the PSU of the 500 rack than anything. Not all are created equal. Vs a rack mount pre the designer(hopefully) made the PSU good and up to their standards. Otherwise if you make a 500 series anything you're at the mercy of whatever the user plugs it into.

Could be a good unit. Could be a bad one.

So as long as you have one of the good Racks(Purple, CAPI, TAC, Lindell, RND,ect) then no, I don't think it matters.

500 pre’s in racks with an outboard supply probably have a lower noise floor than a rack with PSU built in , sheer proximity factor. Also why people like cloudlifter type devices, the gain is not near the PSU.

As an EE doing analog for a living, my feeling is this:
1. Does the mic pre have input and/or output transformers? If so, can they fit those transformers into the 500 series form-factor or are compromises required? It is my understanding that API generally uses same transformer for preamp regardless of 19 inch or 500 series…but I know for a fact that at least one company has significantly different transformers in a 500 series version of the preamp, and I can tell a difference in low-end behavior; I’d bet it’s measurable (but I did not take any measurements)

2. Does the circuit topology used have any major trade-off with +\- 16V (IIRC that’s the 500 series spec) or could some modest incremental performance benefit come along with even higher supply voltages (distortion or otherwise)? Example: my memory is that quad eight and newest RND preamps in 19 inch format use +\- 24V… I can’t imagine that being anything other than a carefully crafted design choice: it costs more to implement high voltage circuitry in almost every case.

3. There are design implementations that can make the supply voltage difference much less impactful.
(A) Some companies use compact DC-DC converters (I’ve seen at least one switched-capacitor AKA charge pump implementation) to locally boost the internal supply voltage for a subset of circuits in the 500 series version - as long as switching-noise and ripple are handled appropriately, no big deal.
(B) Another common approach is modifying a “single-supply” 24V circuit (using capacitor coupling) so it operates with +\-16V using the negative supply as its ground-like reference, basically like 16V - -16V = 32V supply. In at least one case I’ve noticed that the designer put an RC circuit in the “ supply path” (Like old vacuum tube amps) so that with the I*R drop the supply voltage “seen” by the circuit at audio frequencies is still 24V.
(C) “fully-differential” signaling is used by the designer, which effectively doubles the allowable signal swing for the same supply voltage. This is routinely used in chip design (single-ended circuits are the exception) to take advantage of matching, operate with cell phone battery voltage of 3.7V and below, and to improve THD since 2nd harmonic effectively canceled in these circuits. I believe Buzz Audio designs (still using discrete transistors) do this.

In the end, there are too many variables to simply state that 19 inch vs. 500 series differences “matter “ or “don’t matter”. Since we’re not all going to get to talk with the original circuit designer, etc…and even if we did: We just have to use our ears :-)

Update (a few days later): the above points are assuming that the 500 series power-supply in use is capable of providing the preamp unit's or units' the appropriate amount of current/power at the +/- 16V and with minimal interaction between channels (channel "isolation")! All bets are off if supply isn't operating properly.

Most of the early issues have been sorted out and 500 series units being "considerably power-starved and weaker" is mostly in the past when comparing apples to apples.

Or you just run a big old DDA / Midas console supply all trimmed out, never heard the words starving for current.

I don’t have those specific models but do have the BAE 1073D (500 series) and their rack mount 1073 and don’t hear any difference at all.