China mic again..

..after spending some time with my china mics a few weeks back, I decided to see if I could figure out why my two, and my friends 4 all sound and behave different from each other, even though there the same make and same inside...after poking around a little I realiced the FET is biased ( or not biased :? ..) consistently..on the best sounding one the drain sat confortably at 10volts. on the others the draing voltage was all over..from the lowest at 8volt up to 17volts on the one with the least headroom ( drain loadresistors is fed 20volts when phantomvoltage is 48volts )...so I came up with a simple mod to adjust the bias involving replacing two resistors with a trimpot and moving one cap.... here is a link to the mod http://aronnelson.com/gallery/Chinamic-biasmod-pictorial any thought would be much appreciated..and if you find something wrong with it, please let me know so that I dont spread false or bad information...

johan

Very interesting... nice work. What make and model are you working with exactly?

Very good idea, Johan - Thanks!

suggested to the microphone meta

Jakob E.

[quote author="Greg"]What make and model are you working with exactly?[/quote]

Looks like the standard Alctron mic to me (Nady, etc.)...

Peace,
Al.

Yep, in Europe: the company "TSM"

Think about it some more. It is an OK mod it is not what I would do but it is OK. Good to see someone took a meter and checked why things sound different.

I do like to see threads like this.

The following is what I quess the designer of the china circuit had in mind

The "bottom" unbypassed source resistor is part of the gain setting

The "top" bypassed source resistor is the bias adjust.

Now what you did if I traced it right is have a fixed total source R and wiper and leg that goes to the source is the variable cap bypassed bias resistor. I did not see a circuit schematic fragment at the photo site

Now things for you to read up on

what input headroom do you need

what IDSS gives the max preformance from the circuit.

thank's guys.. :grin:

Gus.
you're absolutely right about the gain and "bottom resistor"..I simply chose to ignore that for the benefit of a wider bias seting range.the wiper goes to bypasscap and gate-resistor ( I suppose an alternative, and possibly more proper alternative, would be to only replace the "top" resistor with a trimmer.)

I have to admit i dont even know what IDSS is, so I do have some reading up to do.
the 10volt is simply after looking at the circuits and comparing schems...you can see my tracing at the last pic, up in the corner ( click on the pic to get the full size.)..its VERY close to the front capsule circuit of the U87ai, with the exeption of that bottom resistor. and the neumann schematic says that FET AND resistor are chosen for 10volt at drain, so that's where I got that...anyhow, after doing this, my mic's all sound the same and headroom is improved a lot on the ones that where off more than a few volts....

thanks again..

johan

Johan

You made your microphones better, a good thing.

You did your own thinking and found a nice 'fix" I listed the hints because I believe sometimes more is learned when someone has a hint but does their own learning. Sometimes one comes across something that might be hard to understand thats were this place can be very good.

http://www.vishay.com/fets-small-signal/ssfanp/

this should help you.

There was a good thread here about the Luna microphone it looks like it is gone?

I was expecting more input with this thread.

What I mean is this china circuit is used in a lot of low cost microphones.
Some people like them others don't.

Maybe one of the reasons is the fet used and parts not being adjusted or the fet selected for best operating points.

Here is what I was kind of hoping to see, people that own china microphones with this circuit measuring the drain voltage and noting the drain resistor and source resistor values as well as the fet number and IDSS code.

Hey Gus,

I'll kick it off..

I own an ADK A-51,a chinese made mic that uses the same circuit.

Drain voltage is 21.0V on mine, assuming the Drain voltage is in the same place that Johan measured, R8.

If you give me the R numbers for the Drain and Source Resistors, I'll post my R values.

RE the FET the markings read 'K30AY 5B', I'm not sure what an IDSS code is?

Sorry, mics are something I've never ventured into, so am in the dark a bit.

Thanks again for all your help and all the great info you share.

Cheers

Matt

http://www.semicon.toshiba.co.jp/td/en/Transistors/Small_Signal_FETs/en_20030326_2SK30ATM_datasheet.pdf

Y 1.2ma to 3ma IDSS

Matt check VDC at both ends of R8

Now if the drain is at 10VDC one can remove R9 and C6(mxl V67 numbers) and connect the base to the drain for the output EF part. I wrote about this in the luna thread.

R7 is the top source R Same # in the v67
R6 the bottom source R Same # in the v67
R8 is the drain R Same # in the v67

Thanks Gus,

Anyone care to see what is in there mics? Do most chinese mics tend to use this FET? Would be interesting to know. I wonder how far each co deviates from the same components, if at all.

Matt

Hey Gus,

Thanks. I must be dyslexic or something.. the voltage is 21.0V, NOT 20.1V like I had it before, on the other side of R8 it reads 9.4V, so not quite 10V.

These are the readings I get from the rest of the resitors on either side, the first side being closest to the capsule, he second being the closest to the XLR:

R8 = 9.4V / 21.0V
R7 = 2.3V / 1.6V
R6 = 0V / 1.6V

Cheers

Matt

..well, mine are obviously allready moded, so any voltages is different from a stock mic, but mine (and my friends) has a 2SK170..

..also..I bought at a surpluss store, a number of 12:1 /0.5watt audio transformers ( they were 1Euro each :grin: )..Im thinking of trying to bypass the E.F. and use one of these instead of the one allready in there, just for fun..obviously the gain would have to be changed too, but is it worth the trouble or would it just be a waste of time?...

johan

[quote author="alk509"][quote author="Greg"]What make and model are you working with exactly?[/quote]

Looks like the standard Alctron mic to me (Nady, etc.)...

Peace,
Al.[/quote]

What's the deal w/Alctron capsules? I bought a couple Nady SCM1000's that used to have the 797's, but no longer do. These are 34mm, 16 screws, center terminated, brass washer over clear plastic clamps. Uncentered diaphragms, it seems... At first I thought they might be SoundKings.

rgrds,
Brad

FWIW I too am working on several Alctron mics. I am waiting shippment of many more. I met with Alctron at the NAMM show and took them out to my friends authentic Chineese restraunt. I discussed 797 with them as I purchased many mics from 797 too. 797 they told me is government owned and several of their engineers have moved to Alctron. 797 no longer has dealings with Alctron. Just last week I had lunch at that restraunt and my friend told me of the conversation they had in their native tounge. He said that Alctron is very nervous because neighboring India is taking away some of their microphone business. As a result Alctron is overhauling their QC.

did anyone copy the info on the web page referenced in this thread. i placed the thread in the mic meta so i (and others) could reference it later . it is no longer up and running. if any one has this info or another link could you please post it here? thanks

WOW
I just got an E-mail telling me that there was a reply to this thread. Strange too, because I just got back from the port of Long Beach with a ton and a half of mics from a group buy I put together a few months ago. Alctron ? I have had nothing but bad things with them. I am now working directly with the manufacturer, Some of these mics are not rebranded mics. Some are original design. The brand is TnC. All the model numbers are ACM. For example "ACM-310" ACM stands for "Another Cheap Mic" The 310 is a SDC tube. All these mics that are on my website, nothing is over $100.00.
This all started when I got a C-12 vr and it sucked, took it back and got another, and it too sucked (almost $5K) Then I tried the new telefunken $1.5K, only to discover that it was just about identical to a mic I was importing from China for less than a note in the key of C ($100,00)! True, it had a different transformer. (Lindahl or Cinimag, I can't remember)
This was my third group buy and each one keeps growing. In a few months I will start the fourth GB
I got in a LOT of trouble with the first GB, because all the brand name mic companies started to complain to the manufacturer, and almost shut things down.
What was the question again ? (old timers desease)

[quote author="Johan"].... here is a link to the mod http://aronnelson.com/gallery/Chinamic-biasmod-pictorial[/quote]
Is that link still OK with you all ? Here it's a 404, not found.

Thanks,

Peter

Peter,

404 in the Swamp also.

Peace - Irv