Happy new year y’all. Been a good long while since i’ve posted anything besides my radio playlists. One reason for this is that I’m so much busier than I was 12 years ago when I started this blog; between my regular job and my new brewery operation I don’t have two spare minutes to rub together.
The other reason, though, and this is a major factor, is that there is now so much less of a need for what this blog did: displaying technical ephemera related to audio gear and audio production. When I created this blog, almost of all this sort of discussion happened on internet ‘news groups’ and ‘forums’; there were few blogs, or websites, devoted to making this kind of material available on a semi-permanent basis. Now there are 100s, probably 1000s. It was the case 12 years ago that I would, everyday, pull some piece of old paper out of the shop and sure enough: no one had yet made it available free online. That’s just not the case anymore. In the past decade, many many many folks have created resources like Preservation Sound, and nowadays almost every piece of ancient tech lit I find *already* has a home on the web.
Well, that wasn’t the case with these two things. Above is a pair of GATES M6244 broadcast phono preamps. Tried as I might, I could not find schematics for these things; nor could I find an answer to the mystery of why there are two *different* circuits with the same apparent model number.
The two identical units here that I found and partially recapped are, unfortunately for me, the early M6244 which are designed to work the GE “VRII” variable-reluctance cartridge. Now, this cartridge *is* fairly common, but only as a mono cartridge (there is apparently a Stereo version of the cart, but its very rare, i’ve never found one, and its pricey online). Almost every good-quality phono cartridge sold since the 1960s has a 47,000 ohm impedance – a significant difference.
According the manual, in fact, this preamp derives its RIAA high-frequency roll-off spec from the relationship b/w the 6200 ohm cartridge impedance and R3, the load resistor. The low-frequency characteristic is derived from a feedback network between the two transistors.
The M6244B, which I was not fortunate enough to find, is designed to work with a now-standard 47K ohm cartridge. If you look at the schematic, you will see that the feedback network between the two transistors is a bit more complex; this is because in THIS design, both the low-frequency AND the high-frequency compensation occur via this network:
Alright. So here’s a question for all you more-knowledgeable types out there. I’m no engineer. Is there a relatively EZ way that I can modify this pair of m6244 to behave properly with a 47K ohm cart? If these things were point-to-point wired (IE., not a pcb), I would simple re-build them following the m6244B schematic. But that would be too much work with the PCB -style construction -they are too different in design. Any other ideas? For what its worth, before I was aware of this impedance ‘issue,’ I tested the two units with a deck loaded with a garden-variety vintage SHURE cart (spec’d at 47K ohms), and it sounded absolutely fine to me. Any thoughts?
