Mic Preamp Topology

[sarcanon]

There is a YouTuber who produces generally very good tutorials and video and audio reviews on the consumer/DSLR end of the spectrum. However, he just did a comparison of solely the audio recording quality of various devices which piqued my interest. Putatively, it compared consumer-level recorders like the Tascam DR-60D and Zoom H6 to professional gear, for which he used the 744t as his supposed test case. However, halfway through the video, he discloses he didn't actually have a 744t for comparison, so instead he used a Sound Devices USBPre2 on the premise that the preamps "are exactly the same".

Now, I immediately took exception to that claim, and tried to determine what lay behind it, since generally this reviewer is very scrupulous and not prone to making patently false statements. So it turns out that SD in their marketing literature for the USBPre2 do state that it "shares" the same preamp topology with the 744t. Now my understanding of basic electronics is negligible bordering on non-existent, but I'm guessing that a "shared topology" is not the same as "exactly the same" circuitry. A comparison of the specs of the two devices, particularly the THD and equivalent input noise, seems pretty clear evidence that these devices do not share their preamp circuitry.

But what I was hoping someone might explain to me is the difference between the topology and circuitry. Is topology simply a general architectural scheme that can have significantly different implementations? Can two devices share the same topology and yet have vastly different technical specifications and qualitative characteristics?

[Scott Smith]

The simple answer is yes. There are many mike pre's that share the same topology, but have very different components, as well as varying powering arrangements, which have a huge impact on the sound quality. (For example, a mic pre with +/- 12 volt rails will obviously have less headroom that a preamp fed from +/- 18 volt rails).

I don't have a full schematic for the 744T and USBPRe2 mic preamps, but while they may share similar design concepts, I think that Jon may have upgraded the USB2 preamps a bit since the time the 744T was brought to market. At the very least, there have been quite a few advances in chip technology, (as well as some different packaging options).

Attached is a rather interesting design by Studer from the early 1980's.

--S

Studer 961 Mic Preamp.pdf

[sarcanon]

Excellent. Thanks for clearing that up for me.

Cheers.