iSK Little Gem internals
« on: August 13, 2019, 04:43:02 PM »
I recently got myself a pair of iSK Little Gems, so I thought I'd take a picture of the internals and upload it. The input capacitor and all other components are through hole, and the transistors appear to be 2N5401s. That zener is 9v1. The PCB is a bit cramped, so I'm not sure what mods could be done to it. It's worth noting IMO that you'll probably have to clean the back of the PCB because there was quite a bit of leftover flux.

The capsules have the slotted grilles like a lot of the Chinese ones, and they also have a layer of foam, so there may be another grille under the foam.

Another interesting thing is the contact pin in the body retracts quite a bit.

Again, I'm using my phone's camera, and it's not great in normal home lighting.
« Last Edit: November 18, 2019, 09:02:09 PM by Icantthinkofaname »


Khron

Re: iSK Little Gem internals
« Reply #1 on: August 13, 2019, 05:05:38 PM »
JFET's something like the ones in BM800's and such, 2SK596 or something?
Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

Re: iSK Little Gem internals
« Reply #2 on: August 13, 2019, 05:12:09 PM »
JFET's something like the ones in BM800's and such, 2SK596 or something?
How much would that reduce self noise if at all? The mics have an advertised 24 dBA but the noise is actually pretty usable IMO, and I don't know how much comes from the capsule.

Think there's space to add a capacitor for high frequency roll off? Ideally I'd only want about 3-4db attenuated so it doesn't mess with the omni capsule too much if it's actually flat like they say (which I doubt). High end rise starts after 5k on cardioid, after 4k on super cardioid.
« Last Edit: August 13, 2019, 05:15:19 PM by Icantthinkofaname »

Re: iSK Little Gem internals
« Reply #3 on: August 13, 2019, 06:13:26 PM »
You can do the typical Schoeps EQ thing, for me that made the mic too dark. YMMV.

http://www.audioimprov.com/AudioImprov/Mics/Entries/2014/2/8_EQ_Pt.3_-_Transformerless_Mics.html

Re: iSK Little Gem internals
« Reply #4 on: August 13, 2019, 07:01:42 PM »
You can do the typical Schoeps EQ thing, for me that made the mic too dark. YMMV.

http://www.audioimprov.com/AudioImprov/Mics/Entries/2014/2/8_EQ_Pt.3_-_Transformerless_Mics.html
Thanks for the link. I'm a bit unsure about doing it just because I don't want it to be too dark.

For "professional" use I'm planning to order a couple new MK-012 hyper cardioid caps, but I'd like try to mod the Little Gems as well to see if I can make them sound just a bit better. I could probably just EQ them in post as well because I'd still like a bit of the high end emphasis to remain.

Re: iSK Little Gem internals
« Reply #5 on: August 13, 2019, 07:34:54 PM »
You just trim the 500k timer to your liking. The pot adjusts attenuation level. Or use lower value capacitor for roll off to start at  higher frequencies.

Gus

Re: iSK Little Gem internals
« Reply #6 on: August 13, 2019, 09:21:47 PM »
I don't see a DC to DC converter section.

Is there anything on the other side of the PCB?

Are the capsules electrets?
« Last Edit: August 13, 2019, 09:25:13 PM by Gus »

Re: iSK Little Gem internals
« Reply #7 on: August 13, 2019, 11:15:48 PM »
I don't see a DC to DC converter section.

Is there anything on the other side of the PCB?

Are the capsules electrets?
Nothing on the other side, the capsules are electret. The iSK Pearl is also electret but it's quite a bit flatter sounding. Prefer it's sound to other cheap SDCs like the Rode NT5. Unfortunately the Little Gem capsules (at least the cardioid and super cardioid) aren't very neutral, but they were only $50/pc with the capsules.
« Last Edit: August 14, 2019, 04:23:50 AM by Icantthinkofaname »

Re: iSK Little Gem internals
« Reply #8 on: November 07, 2019, 05:59:11 AM »
Bumping. The mics have a self noise of 24 dBA which is quite audible to me, especially considering they're also not as hot as other mics (self noise and frequency response sounds close enough to my ears, and both of mine output about the same volume). If I'm replacing the transistors, would the 2SK596 help reduce self noise? Or is it likely the capsule is the culprit? I know my MK-012s have the 2N5401s in them and they're not nearly as noisy as the Little Gems. They're certainly usable mics for $50 each in any case, I'd just like to clean up the noise if it's at all possible because that's the only real flaw they have.

rogs

Re: iSK Little Gem internals
« Reply #9 on: November 07, 2019, 08:39:45 AM »
In the case of the standard BM800 microphone I've found the main source of noise was from the Zener diode..... much more significant than noise from either the FET or the output emitter followers.
The addition of a second simple single order low pass filter made a significant improvement to the noise figure - something like 8 or 9dB better, as I recall..
I see what appears to be a 9v1 zener diode on the photo of your PCB.... is there a decent low pass filter attached to that zener output?....
www.amx.jp137.com - A DIY RF condenser mic project


Re: iSK Little Gem internals
« Reply #10 on: November 07, 2019, 09:28:24 AM »
The two black capacitors before it are 47uF, I'm not sure about the gray ones, but the next black one is 10uF 50v. Would I be able to check the gray ones with a multimeter?

Edit: My multimeter doesn't measure capacitance, so I ordered one that did. I'll update the thread when it arrives and I try it out.
« Last Edit: November 07, 2019, 10:22:36 AM by Icantthinkofaname »

Re: iSK Little Gem internals
« Reply #11 on: November 07, 2019, 10:16:00 PM »
In the case of the standard BM800 microphone I've found the main source of noise was from the Zener diode..... much more significant than noise from either the FET or the output emitter followers.
The addition of a second simple single order low pass filter made a significant improvement to the noise figure - something like 8 or 9dB better, as I recall..
I see what appears to be a 9v1 zener diode on the photo of your PCB.... is there a decent low pass filter attached to that zener output?....
Forgive the stupid question, but that zener diode is what provides the electret capsule here with the required 9v right? Or converts the 48v DC into the 9v the mic is spec'd for?

Re: iSK Little Gem internals
« Reply #12 on: November 18, 2019, 02:59:41 AM »
Could improving the capsule bias voltage help? Or because this is an electret mic is the capsule voltage unchangeable?

RuudNL

Re: iSK Little Gem internals
« Reply #13 on: November 18, 2019, 04:01:42 AM »
There is no bias voltage, since the capsules are electrets.
They look more or less the same as the Samson CO2 microphones.
I had them for a day but I returned them immediately, because they were too noisy for me!
(They sounded like I was standing near to Niagara Falls...
There is a solution for every problem!

http://www.vansteenisaudio.nl

Re: iSK Little Gem internals
« Reply #14 on: November 18, 2019, 05:08:20 AM »
There is no bias voltage, since the capsules are electrets.
They look more or less the same as the Samson CO2 microphones.
I had them for a day but I returned them immediately, because they were too noisy for me!
(They sounded like I was standing near to Niagara Falls...
Yeah, the Little Gems are noisy too. I'd like to try to fix that, but I'm pretty clueless on what to do. I picked them up though because they have interchangeable capsules and they were cheap enough.

I didn't expect the capsule voltage to be adjustable, since electret capsules are prepolarized, just thought I'd ask if there was anything that can be done there.
« Last Edit: November 18, 2019, 05:22:27 AM by Icantthinkofaname »

rogs

Re: iSK Little Gem internals
« Reply #15 on: November 18, 2019, 05:45:06 AM »
Forgive the stupid question, but that zener diode is what provides the electret capsule here with the required 9v right? Or converts the 48v DC into the 9v the mic is spec'd for?

The zener is fitted to regulate the pre-amp voltages, to compensate for differences between  48V phantom power supplies. 
If it is not well decoupled, it can create noise into the pre-amp circuitry. This is the dominate source of noise in BM800 tpye microphones - as I mentioned above.
Whether it is true  with your mic circuit as well I'm not sure... I've not seen a schematic of the iSK Little Gem published anywhere?

The low pass filter added into  the signal path - Henry's circuit mentioned above - can actually add more noise into the signal path rather than remove it, so that's probably not a good solution...

A copy of the iSK Little Gem schematic would help here...
www.amx.jp137.com - A DIY RF condenser mic project

Re: iSK Little Gem internals
« Reply #16 on: November 18, 2019, 06:15:56 AM »
The zener is fitted to regulate the pre-amp voltages, to compensate for differences between  48V phantom power supplies. 
If it is not well decoupled, it can create noise into the pre-amp circuitry. This is the dominate source of noise in BM800 tpye microphones - as I mentioned above.
Whether it is true  with your mic circuit as well I'm not sure... I've not seen a schematic of the iSK Little Gem published anywhere?

The low pass filter added into  the signal path - Henry's circuit mentioned above - can actually add more noise into the signal path rather than remove it, so that's probably not a good solution...

A copy of the iSK Little Gem schematic would help here...
I'm not sure there's a schematic floating around for the Little Gem/CM10. The Little Gem is kind of hard to get a hold of since it's out of stock a lot (the Neewer NW-410 and LyxPro SDPC-2 are the same mic though), and I don't know how to trace a circuit to make a schematic.

Here's the back of the PCB if that helps anyone.

« Last Edit: November 18, 2019, 08:56:58 PM by Icantthinkofaname »

analogguru

Re: iSK Little Gem internals
« Reply #17 on: November 18, 2019, 03:07:29 PM »
..... I don't know how to trace a circuit to make a schematic.
But you know how to take (and post) pictures.... especially from the other side of the pcb.

....
They look more or less the same as the Samson CO2 microphones.
....
The circuit appears to be very similar to the CO2.... also to the Takstar 6100, The Sonotrad BL-7, who all have the nonsense of a 27k between E-C (instead to the base) of the output-transistors.  Someone made a mistake during the cloning-process and the rest is following blindly like lemmings.
Would be interesting to find out if this is another of the lemmings.

BTW, I wouldn't be surprised if you find a Transound electret inside the capsule.

RuudNL

Re: iSK Little Gem internals
« Reply #18 on: November 19, 2019, 03:13:44 AM »
....
The low pass filter added into  the signal path - Henry's circuit mentioned above - can actually add more noise into the signal path rather than remove it, so that's probably not a good solution...
....

I fully agree with that! Although it is stated that this modification would not increase the self noise of the microphone, my experience is different. Adding series resistors to the base connections of the output transistors will increase the noise level!
With 10 K there is an audible difference in self noise. Although this modification is sold (for too much money!) by a well known seller of 'microphone parts', I would never use it. If you need HF rolloff (for a K67 style capsule), there are better ways.
There is a solution for every problem!

http://www.vansteenisaudio.nl

Re: iSK Little Gem internals
« Reply #19 on: November 19, 2019, 03:57:42 PM »
I fully agree with that! Although it is stated that this modification would not increase the self noise of the microphone, my experience is different. Adding series resistors to the base connections of the output transistors will increase the noise level!
With 10 K there is an audible difference in self noise. Although this modification is sold (for too much money!) by a well known seller of 'microphone parts', I would never use it. If you need HF rolloff (for a K67 style capsule), there are better ways.
Would an EQ rolloff cap actually change the self noise though? I know a lot of the self noise is in the high frequencies, but if it's caused by the circuit and not necessarily the capsule would it help? I've been interested in adding something to roll off a bit of the high end anyway, so if it will cut down on self noise that's a bonus.


 

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