Pre-MT Aston Origin noise fix

[jaymz168]

Hi all, starting a new thread here from the discussion in SamsaGregor's thread on the same mic.

I bought this mic back when they first came out and it's always been fairly noisy, leaning towards low freq noise and popping/crackling. I've got other mics so I never really addressed it but my understanding is that at some point there was some sort of replacement program for noise issues, possibly the capsule. So the first thing I did was email Aston about a replacement capsule and Music Tribe replied with a service center that I could buy parts from. I hadn't realized that they were bought out by MT. I ordered that part nearly two months ago and according to the service center they haven't even heard back from MT much less received a part yet.

So in the interest of isolating the issue I replaced the capsule with a polystyrene capacitor of roughly the same value (68pF cap vs 64pF capsule) and fired up the Quantasylum boxes. I've been getting into mic stuff lately so I got their QA472 mic preamp to feed the QA403. I set the preamp to 20dB of gain to make the microphone's noise more visible. This is the QA472 at 20dB gain with its inputs shorted, eight averages:




And here's the mic with the dummy capsule in place, no averaging :




And an attempt at capturing one of the low freq transients which requires some patience at 128k bins:




And here it is with eight averages which helps to see what's really going on down there:




So we can see that the noise is strongly leaning towards the low frequencies and that there are some nasty peaks which seem harmonically related but not multiples of US line freq. In the other thread @Voyager10 suggested to short across C13 to shut down the front end and this is the result :



The low frequency noise has dropped significantly and most of the visible spikes are line voltage and harmonics, other than that 10Hz spike. So I think the problem is isolated to the front end which isn't surprising given what I've read about the LSK170. If I plug it into my MOTU M4 and listen to it the noise is much less audible, more of a white noise, and the popping/crackling is gone. And it's definitely running because if I tap hard on the mic body I can still hear it.

I'm not an EE, or even a college graduate but it's my slow season so I've had plenty of time to dig into the various threads on Schoeps, Dorsey mod, JFET selection, and Wurcer's articles in Linear Audio. I've got some SK170A, LSK189, and J201 here and 2SK660 from eBay out for delivery. The seller has been around for over a decade, thousands of transactions, and nearly 100% positive feedback but I'm still skeptical whether they'll be real. We shall see. If a simple JFET swap and maybe rebiasing does the job then we'll have a quick and easy way to sort out these mics. FWIW the newer MT version of this mic uses SST201.

I'm also planning on using this thing as a playground to test out some mods. It's got a 67-style capsule and I've got an RK-47 here so I might see how I like that flavor. Maybe also attempt to flip the capsule bias polarity so that the diaphragm isn't a dust magnet but there isn't much clearance inside the headbasket...

I'm also intrigued by the mod replacing the lower bias resistor with a diode, I have some expensive picoamp diodes here to try out for fun.

More updates soon!

 

[Voyager10]

I wouldn't rush to replace the FET itself, just yet. The '8 averages' picture shows there's some periodic signal there, which isn't your common-or-gardem thermal noise or shot noise.

If shorting out C13 makes the noise go away, the next thing I'd try is a short across the capsule/68pF capacitor, so the 'left hand' end of C1 is grounded. That will take most of the high-impedance circuitry out of the equation.

Keep us posted, whatever you discover :

 

[jaymz168]

Hi, thanks for your help! I've been running around all day so I haven't gotten around to much but I just tried your suggestions.


Here's the output with C13 shorted and the 'capsule' shorted. I gave it a listen on the MOTU again and the character leans much more towards white noise whereas before it had more of a pink/red noise tilt, very low end heavy.

And I just got a chance to open up the eBay order:

They were sold as NOS and don't look like pulls. The markings do look to be lasered and don't come off with 99% IPA. I don't have any acetone handy to try that out. My Peak DCA75 Pro seems to think they're depletion mode MOSFETS but I presume the gate diode is confusing it. The traces seem to match well with the datasheet:



If anyone knows how to accurately set the scales on the Peak DCA software please let me know. This is the best I could do, it insists on zooming both axes for some reason.

 

[Voyager10]

Here's the output with C13 shorted and the 'capsule' shorted.

Ah, perhaps I was unclear, I was interested in 'capsule shorted' but C13 not shorter, so that the FET is receiving power but its input is coupled to ground via C1.

 

[jaymz168]

I wasn't sure which you meant and C13 was still shorted so I went with that haha. Here's only the capsule shorted:



It's back to having the original quality, a pink noise sound with a hollow sounding peak reminiscent of inside a passenger jet. With C13 grounded and the front end turned off it had more of a white noise quality despite the tilt shown in the graph in the first post.

The amplitudes vary a bit but the frequencies remain the same. The QA403 doesn't go down to 10Hz but I suspect we'd see a spike there as well. I also have a four channel Siglent scope if you think that would be helpful to poke around in the DC world. It's just an 8-bit scope though so I don't think we'd even see any of this.

I also took the liberty of swapping C1 with a C0G ceramic, same as they had, but it's the same. Strangely odd order harmonics (if we consider the theoretical 10Hz spike that we can't see as the fundamental) were lower at first but returned to a similar distribution as before. Turning it off and on again shows similar behavior.

 

[Voyager10]

Thanks, that's very useful. It looks like we've narrowed it down to one of the components in the (non-shaded) area below:


(From your original schematic at https://groupdiy.com/threads/aston-microphones.89398/ ).

It's unlikely to be C1, if you swapped it anyway and the noise didn't change. The most likely suspects (IMO) are C12, C13, C15 or the FET itself.

If you're able to replace C13, C14 and/or C15, pretty much any cap 22uF or higher will do, at least for testing. You can also test with C12 removed altogether, it's not doing that much in the circuit.

If you're replacing the FET with a different type, R5 will need to be changed to set the bias current. (As a first approximation, you'll want to bias it so that the FET drain is at the same voltage as in the original circuit. A 10K or 20K multi-turn trimmer is useful, if you have one). This can be quite a lot of faff, so personally I'd try the capacitors first.

 

[jaymz168]

I feel like we're getting close here! One of the first things I did back in the other thread was to replace C13, C14. I've got some Panasonics in there now so they should be fine.

I should say that I do have one question about the schematic that I drew up. When I removed the bypass caps on the outputs I got mixed up and I'm not actually certain the order of C8/C18 and C7/17. That is whether the 1n or 22n are first with respect to the output wires. Embarrassingly I actually lost track of which one was which when pulling them to confirm values out of circuit. The common Schoeps circuit shows 22n before the inductor/ferrite so that's what I put in there. But IIRC in bypassing the smaller value is usually first? I don't think it would make much difference but maybe there's something there?