Mods for old ADK A51 mics?

[McIrish]

Someone gave me a pair of ADK A51 mics many years ago. They aren't very good, from what I remember. The ones I have, have the solid drills headbasket. not the more open screen type. It has a single internal switch, though I can't remember what it was for, possibly a HPF. I'm assuming the capsule is an inexpensive K67 type. I'm not even sure these are worth investing any time into, but thought I would ask.
Anyone have info on this? I know they made quite a few revisions over the years. Mine has no extrenal switches. You have to open it up to get to it.





On a side note, I bought into the hype (on another forum) for the ADK Cremona 251 T-Fet. I bought one. So far, I haven't found a great use for it. My guess is that all the T-Fets have the same circuit and they just swap different capsules into it. The build is decent enough. The sound isn't anything magical though. I was hoping it might replace some AT4050 mics, but so far... no.

 

[Khron]

Internal photos?

 

[McIrish]

I haven't done that yet. But that would be the first step. Hopefully, I can snap a few photos tonight.

 

[Gus]

It most likely the n channel JFET into a PNP output into a about 1.9:1 transformer with a switchable coupling cap between the JFET and PNP circuit. It might have an EQ network like what you find in a MXL 2001, V67 etc microphone or it might be more like a ST51 circuit without EQ.

The above circuits can be adjusted.

As posted before inside pictures will help

Does it have a 32mm or a 34mm capsule? is the inside like in the following link?

http://recordinghacks.com/microphones/MXL/2001

 

[McIrish]

I have literally done no homework yet. I will get pictures and more info. I was thinking maybe the headbasket is so poorly designed that it may not be worth upgrading the rest. It has to have a nasty resonance due to the design.

 

[Voyager10]

My first thought was that the headbasket looks really naff, and that no amount of tweaking the circuit / changing the capsule will rescue it.

What does it sound like when you tap the grille or body? Any ringing / resonance is a bad sign.

 

[Gus]

I would straighten the grill out and try it.
I have an older Nady SCM 950 microphone that I thought would sound bad with the grill design however it seems to work OK with the stock capsule. I did work on the electronics.

If the ADK A51 has an Neumann U87 like network it can be adjusted for the capsule along with the first stage gain

 

[McIrish]

Some pictures of the ADK A51. Looks like a single sided K67 type capsule. The circuit is probably a FET followed by a transistor to drive the transformer. As you can see, the headbasket is really cheap. Just perforated metal and it does resonate when tapped.

 

[Gus]

Looks like the n channel to PNP circuit.
Is
C1 .01uf
C5 .47uF
C3 220pF

I would fix the dents
almost all microphones like that have a thump when tapped

I don't like the 170 JFET in this circuit a 30 is OK and the ones Neumann uses are better IMO. hint 20 vs 8 vs 4

 

[McIrish]

Looks like the n channel to PNP circuit.
Is
C1 .01uf
C5 .47uF
C3 220pF

I would fix the dents
almost all microphones like that have a thump when tapped

I don't like the 170 JFET in this circuit a 30 is OK and the ones Neumann uses are better IMO. hint 20 vs 8 vs 4

Gus, you are correct on the cap values. The FET is a 170.
The dent on the first picture I posted was a stock internet photo. The pics I posted today are of the actual mic. There are no dents or deformation. It's actually in mint condition.

The transistor is a 2SA1015. I think this circuit is pretty similar to an MXL V67

 

[Gus]

A picture of two microphones I adjusted that use a circuit like in this thread.
One has the stock MXL capsule and the other a 34mm one.
The microphones in the picture have had experiments and other circuits and capsules installed over the years that is why the inner grills are missing.
I would leave the inner grills in.
The white you see is from a "magic eraser" the other microphone has it as well.
I left the tantalum caps.
The easy thing to do is change the circuit to a U87 like by installing a different transformer however I wanted to use the stock transformer as an experiment.
I changed the JFET removed and added some parts rewired a few things and if you want to do it correctly you need to match the RF filters parts and 2.2Ks very well.
You will note there are no "audiophile" caps but that is on purpose
Also note one uses a SMD JFET