- Joined
- Aug 6, 2010
- Messages
- 30
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!
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!
