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Sterling ST79 - Monoprice Observation #3007663 09/11/19 09:05 PM
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A couple of years ago, I nabbed (3) Monoprice 600850 microphones on EBay for $80 total. The 800650, now discontinued, is a large, heavy, "lollipop", multi-pattern microphone. It looked like a good mod platform. They have been in the closet for a while and I pulled one out today to look at the mod possibilities and the circuit board looked somewhat familiar. It seemed to resemble the boards in a Sterling ST79 that I had been working on. Upon comparison, the boards and components were identical as was most of the metalwork. They are basically the same microphone with different head baskets.

Not surprising for Chinese imports, I guess, but, it was interesting to see two very different looking microphones that are identical internally. They both have 8-micron gold-sputtered, dual-diaphragm capsules, and I am assuming the transformers are also the same.

Now, what kind of mod should I do... ?


Mark Karney

StudioMicZone.com
Norwest.net
Re: Sterling ST79 - Monoprice Observation [Re: StudioMicZone] #3008329 09/16/19 09:10 PM
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matthew mcglynn Offline
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Mark,

I bought one of those as well, for the same reason. I posted some notes from my analysis of it here: <http://recordinghacks.com/microphones/monoprice/600850>

In a nutshell, it's a three-pattern FET condenser. The audio circuit has an output transformer. The DC circuit (on the second PCB) is a standard Chinese implementation of the oscillator found in the Schoeps CMC5, with a second voltage doubler on the output -- needed to hit the +120V required for Fig-8. Typical of cheap Chinese stuff, the whole thing was built with forgettable parts.

A signal injection test revealed no in-circuit EQ, which suggests that the stock 32mm K67ish capsule is not a great choice (unless you like bright microphones). A K47, M7, or CK-12 style capsule is likely to work better with this circuit. Whether you can sandwich a 34mm capsule into the 32mm rubber harness is another question. I guess you could fabricate a mount for a 34mm capsule; that wouldn't be terribly hard if you have some CAD experience and a 3D printer.

You could measure the transformer's turns ratio using the procedure here: <http://recordinghacks.com/2017/08/29/how-to-measure-transformer-ratio/>

You could also inject a swept sine wave to check for frequency response and distortion characteristics. If it is a low-ratio transformer, it's probably fairly clean. If it is a higher ratio than about 5:1, chances are it's going to exhibit some funky artifacts, especially at low frequencies.

Whether it's worth replacing the transformer depends a lot on what else you plan to do with the mic. I don't personally think that capsule will ever sound good. The circuit could be upgraded with nicer parts, but unless you have a schematic (or want to trace the circuit to make your own) you won't really know what you're changing. The challenge with a lot of cheap mics is not just that they're built with cheap parts, but that they commit topology crimes too. Those problems can be difficult to find or fix without a schematic.

Had this mic not disappeared from the Monoprice catalog so quickly, I would have cooked up a retrofit mod kit for it. It's a solid piece of metalwork. You could keep the DC board and switch board, and just replace the audio PCB and transformer. I'd pursue that rather than get hung up on esoteric capacitor upgrades of the sort held in such high esteem on lesser forums. ;-)


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Re: Sterling ST79 - Monoprice Observation [Re: StudioMicZone] #3008471 09/17/19 06:51 PM
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Thanks for the info, Matt. I probably would do the mod only if I could come up with a microphone that had a "different" sound from my other mikes. Right now I have 5 mikes that have been modded with the RK-12 capsule and 5 with the RK-47. (Not to mention the mini K47.) So I am fairly well equipped with those capsules. I thought that maybe with the 8 micron K67 style capsule I might be able to come up with something that sounded different from the rest of the locker. Bright isn't a problem as I have quite a few neutral and darker mikes.

Maybe I'll try and trace out a schematic. It seems to be a 2SK170 followed by an emitter follower into the transformer. At least they used a non-polar capacitor between the emitter follower and the transformer.


Mark Karney

StudioMicZone.com
Norwest.net
Re: Sterling ST79 - Monoprice Observation [Re: StudioMicZone] #3008541 09/18/19 02:22 AM
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matthew mcglynn Offline
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It might just be the V67/2001 circuit in disguise, then. IIRC, those mics typically don't have a DC multiplier, whereas the 600850 definitely does. But the audio circuit might be familiar.

Your collection is missing a properly-EQ'd K67. That recipe yields audibly different results than the other capsule types: flatter through the upper mids, with no presence peak above that.


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Re: Sterling ST79 - Monoprice Observation [Re: matthew mcglynn] #3008579 09/18/19 01:47 PM
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I think you are right about it being very similar to V67/2001 circuitry. It's the same topology and some of the parts values match up. It shouldn't be too hard to trace out. I was thinking of re-biasing/ replacing and rebiasing the FET. Also, I was watching a video of a guy who replaced the emitter resistor of the follower with a J508 constant current source which he said improved the circuit considerably.

I do have some nice K67 mikes in the locker. I have a pair of Studio Projects C1s with your EQ boards in them, A Mojave MA-201 fet, a Mojave MA300, and an old Equitek E300 which I really love on upright bass. I also have a single-sided RK-67 on the shelf waiting for a home. There are a couple of possibilities where that can go.


Mark Karney

StudioMicZone.com
Norwest.net

Moderated by  matthew mcglynn 

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