ADK version II - mod options

Hi all,

I've had this ADK since I bought it new in probably 2002. There are no version markings, which according to my research and email conversation with Larry, looks to be a version II.

Apparently the version II (vI was a non-production prototype) was their attempt to recreate a U67 with all Chinese parts. The vIII was the same with higher quality Japanese components).

What are my mod options with this? Would simply upgrading the passive components be a worthwhile upgrade? If this were you (and you just bought a house, so funds are tight) what would you do?

Also, if anyone has a schematic, that would help as well.

Cheers,

- GZ

In lack of a schematic, some internal photos might provide a starting point, i'd imagine

Without pictures or schematic it's a reading tea leaves.
More engagement and surely someone will help
Correct me if i'm wrong, but this is FET mike like Hamburg 67?

Apologies for not attaching these in the first place.

Yep, I do believe this is a FET 67 affair.

Thoughts?

Aaaaand the other side.

If this is not the schem, it's extremely close.

Yes, it looks like it.

This circuit is really nice to work with it.

Check what transformer you have in some they used oep but i believe here would be typical Chinese 1,87:1 or 2:1.

So what do you want to change in microphone? In case of sound of it of course.

Thanks, I'll check the transformer tonight.

I'd just like to take the edge off slightly, make it a tiny bit warmer.

Ok so here you have capacitors in audio path.
Marked blue are for frequency response.
To change HF response you will need change C3 220pF for higher value.
To change LF response you will need  change C1 and C8.
About that - later.
Do you have option to set the bias of the FET transistor?
You will need generator and oscilloscope!
Another thing!
Check all capacitors values and make description on your photos.
Use "paint" or similar program
Then we can start fun

Also you could check both transistors models.

I have a scope and a tone generator app on my iPhone. I've used it to send test tones through tube amps for circuit tracing with an audio probe. Would that work?

I'll have a look at the transistors and cap values tonight.

Thanks!

I don't know how phone scope and generator works, but it's worth to try
Here's a basic mod schematic.
For C1 best would be wima mkp 100nF.
For C8 you can try few values.
I will try 1uF for the start wima mks or evox rifa mmk.
If there would be weaker low end response try 2.2uF but 1uF should be enough.
C5 and C6 you can aalso change for some better mkt or mkp capacitor.
As you can see, there's no C2 and R5. You will need to use potentiometer to set the bias.
After those changes and tests you can work with C3.
I would start with 270pF but also you could test 300pF and 330pF.
Wima MKP/FKP are pretty good for that and small sized. You can use also ceramic C0G/NP0 or polysteryne.

Thanks lm76d! I got some values last night, though I couldn't quite make out c8, and wasn't able to get to the transformer (the screw holding the XLR connector wouldn't move). I think there must be another way to get to it, but it looks like I'd have to disconnect the entire frame.

Here are my values:
- D2: K170
- D2: A1015
- C1: 10n
- C3: 220p
- C5: .47uF
- C6: .047uF
- C8: 4.7uF Tant
- T1: unable to confirm

So it's almost the same mike as on schematic
You have additional hpf circuit which isn't present on schematic but it was also used on some mxl, audix and other chinese condensers.
You don't need to change FET and bipolar transistor.
For bias potentiometer try 10k or 20k trimmer.
To see transformer you will need take off the frame, but i'm 100% sure that it's typical chinese 2:1 (1,87:1) which you can see in many microphones. If you will find some nice transformer with similar ratio you can always change it but the basic mod should be huge step anyway. Also cheap

Perfect! Looking forward to hearing the difference. I did notice that the trimmer pot on the schem was replaced with a fixed resistor value. (R7, I think).

Also, I forgot to clarify in my last post, I have an actual o'scope, but just a signal generator app. Is there a resource you can point me to for biasing with the scope, or alternately, an ideal bias voltage I should be targeting?

Cheers, and thanks for your help!!

If you already removed R5 and C2 and connected R4 to the FET Gate!
Disconnect capsule from R4 and apply 1kHz  - connect generator through capacitor (best would be 47p-100pF) and  monitor sine wave after C6 or C8. You need set potentiometer  to get both halves of sine wave equal. Try to get maximum level from generator where you can still keep undistorted sinus on both ends.
Is it clear? I'm sleepy as hell

Perfect, very clear! Now please go get some well deserved rest!

Cheers!

A friend of mine asked me to check a cheap SE3600 SE Electronics, which is using exactly the same schematic..
When he starts to sing on the mic, the sound dissappears, then reappears

http://groupdiy.com/index.php?action=dlattach;topic=62304.0;attach=41529;image
the voltage at the fet's drain is about 16v, when the sound dissappears, this voltage drop to about 3v, then rises again to ~16v

I tried several things in vain (transistors replacement, cap and resistor replacement arount the fet), but still didn't try to replace the 1G resistors

Any idea is welcome!
Thanks

Change capsule for test or clean it.
For me it looks like capsule fault.