Just Picked up new Donor mic

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bnwitt

Active member
Joined
Jul 23, 2014
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36
I just picked up a new Chinese mic I intend to gut and rebuild and my preference is to make it either a U67,U87 or a C12.  Here is a link to what I purchased:

http://www.ebay.com/itm/MC700-studio-large-diaphragm-condenser-microphone-w-switchable-characteristics-/301214465415

I'm looking for opinions as to which of the three mic circuits mentioned above (or any other one which will shine on male vocals and acoustic instruments) is best suited for this body.  I don't care if it is a tube or FET mic but I really need a good vocal mic with multi patterns.    This body has three switches.  It would be great if someone has a PCB kit that would fit this body.  Thanks in advance for any and all advice.  I'm new to mic mods but have been a long time CET way back into the vacuum tube days and can handle a soldering iron and PTP or circuit card construction with ease.

Barry
 

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Boy, I can hear my voice echo in this thread.  Hello, hello, elo, elo lo, lo, o, o.............  Can someone tell me where I might post this question and actually have it answered or at least commented on?  Wow!  The silence in here is deafening.
 
I think the silence is a sign of a complexity of the answer to the question. Don't know about the kits, but using your creativity you can probably fit any of the mentioned circuits in that body. What you decide to build depends how much you're willing to invest time and money. Cheapest option is probably U87. You could use the capsule and and maybe also the transformer. For U67 you have to do much more but you can still use the capsule. Though it's hard to say if it's a good capsule or not. For C12 sound you have to buy Tim Campbell or Beesneez CK12 copy. Of coourse you can can build any circuit you want with the stock capsule, but it's the capsule that dominates how the mic sounds, not the circuit.

What I'd do is to build a something like the so called Royer mod and then look for the U67 schematic for how to utilize the switch for the pattern control. 
 
As far as doing a tube mic in this donor body, the first thing is to work out if you can replace the mic body's binder insert from the current 3 pin (fet mic amp) to a multi-pin insert, like 7 pins, as needed by a tube mic.

If the binder insert can't be replaced, then you are looking at a rebuild of the mic amp but remaining as a solid state type.

I am awaiting a donor now of similar type. If I can replace the binder insert with a 7pin, then I will rebuild as a tube mic. Otherwise it remains a fet type.

Assuming I can change the binder to a 7 pin, then I will do it as a tube mic - the body is a mid-size one

ie. 50mm not 60mm like a u47 body or 40mm like a c12 style body.

It is also 3/4 length, not full length like a long body 47 or C12 and not 1/2 length like a small 47.

I already have a C12 style variable-pattern rebuild, so I am  going for a u47 tube kind of thing with some modernisation.

In this case, I'm going for :

- Beezneez K7 capsule with 3 switched patterns and point-point wiring
- fixed bias from seperate well filtered heater supply under heated to taste :)
- pentode-in-triode-mode  eg. EF86 or similar, possibly a smaller length type tube like EF95
- low-ish ratio output transformer Cinemag 2461nico  6.5:1
- turret board power supply, with brute force filtering of each of heater (say 6Vdc) and B+( say 100Vdc)
- 7pin gotham cable

A G7 is similar but uses self-bias, regulated heater and lightly filtered B+ as do many other mics.

----

So - the possibilities can be endless.

As far as pcbs go, not sure. You would need to find some that can be made to fit. Not impossible but would require some research.

Good luck with - I start mine soon. It should wind up taking a 200usd mic to around a 900usd mic when all is said and done.

Here's my donor - an apex480.

I Like it because :

- cheap : a good 47 type body is nice $$$ compared to this of 200usd with shockmount
- it is wider than a typical apex 460 (a nice C12 type)
- longer than a u47 small body
- has a pattern switch at the mic and will not use a variable pattern at the psu
- captive nut shock mount rather than the lever-tightened band which is more common
 

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Thank you for the replies Pasarski and Alexc.  I do realize that my question is a loaded and deep one.  I'm hoping for a modern U47 for this one too but we'll see if the body is capable of that.  I'll do some pic posting of it when it arrives and I take it apart.  A U67 would also be a good addition to my mic cabinet. 

I just picked up two of the MXL V250 mics on the $49 stupid deal of the day that I am considering doing the Microphone Parts .com MP-V57 Multipattern PCB Kit upgrade on. I'm also going to replace the capsules.  Is anyone familiar with that PCB mod's circuit and which historical or current microphone circuit it is similar to?  Any idea which of Matt's recommended capsules (RK-47 or RK-7) would do better on Acoustic guitar, male vocals female vocals, banjo, mandolin or fiddle.  Not all from a single capsule but which one leans towards any of those sound sources.  I know Banjo is the odd man out of those mentioned sources.  I probably should ask Matt that question as well.  Again, thanks for the replies gentlemen.  Alexc, I hope you will post your progress on that 480 as you go.

Barry
 
It's certainly a lot of fun playing with cheap donor  mics .. like many mxl mics.

I have an old mxl v67 which is stock - it's a very nice mic as is  :)  No changes, although I should check the FET biasing one day.

Me - I just triggered another. This time, an Apex 471 (among many other names!) SDC

It is a fet capsule impedance converter and a mini pentode-in-triode mode transformer driver.

Quite cheap, in my terms, as delivered down here in the southern most parish of the known world.
Around 200usd. Cardioid.

I'll just check it out in a bit of detail for kicks ...

ie. look at the transformer driver circuit on the test bench, then with the fet-capsule circuit connected. Make sure it isn't doing something unexpected.

Kind of addictive really ...  c12 rebuild, fet rebuild, 47 attempt :)  and now a SDC.

:)

Anyway - awaiting the apex480 to see if I can binder-insert mod to 7pin.  If not, thence to await someone to have alctron 47 at a reasonable (meaning CHEAP  :) prices.

CHeers

 

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So, assuming I can replace the binder in the 3/4 length fet apex, I will start by making it tube and compatible with this Apex 471 tube sdc psu.

It's prolly  a simple zener HV and a 3xx regulated heater, but will serve without mods while I begin my tom foolery in the big apex  :)

Once I have the big fet mechanicals sorted out on the sdc apex psu, thence to be building a 47 style brute force custom psu.

When that's done, I'll replace the capsule with the .... BeezNeez  :)

I'm keen to test whether miniscule plate currents are really the way to go, as many seem to be.

I'm just not happy with less than 1ma thru a tube. Ever.  Period! 

Unless, of course it is being vari-gm'd  (moo-d) into submission.

Only one more I can think of now to mess with. A ribbon   

:)
 
I received the Recording Tools microphone.  Here is a photo of the mic body
 

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Here is a photo of the mic with the body sleeve removed.
 

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Here is a photo of the back PCB with the tranny
 

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Here is a photo of the switchboard PCB
 

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The mic has an interesting interconnection scheme.  The tranny PCB plugs into the switch PCB and the other body PCB plugs into the back of the tranny PCB.  The Boards are tapered being 1.5 inches across the bottom and 1.6875 across the top with an overall length of 3.25 inches
 

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Here's the guts of the apex480 I mentioned earlier.

It sounded reasonably quiet and mostly decent, with a somewhat top-py sound, as expected with it's cheap k67 type cap. Not a bad sound at all, and all the switches seemed to work!

For anyone who is interested, Brian Fox describes his mods for this at his site.
http://www.foxaudioresearch.ca/FARFET47.htm

(I'm not associated with Fox Audio Research but I enjoy studying his technical treatments of many things microphone and mod related - Thanks Brian  :)

---

2 pcbs : one the fet impedance converter + bipolar transformer driver, the other pcb is the psu with some voltage multiplication of phantom to provide capsule polarisation voltages.
 

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The Apex480 is

- approx 48mm inside the mic body at the top end, and same at bottom end inside.
- approx 70mm of usable length in the tube, allowing 10mm clearance below the switches pcb

- the 2 rails are 37mm apart on the inside surfaces  at the top end of the mic - there's prolly 5mm from that to the mic body tube on each side.

They taper down at the bottom end where the base cup binder assembly is.

So - a reasonable amount of length and diameter to work with.

It was an easy discombobulation process - the binder insert came away easy - I have a 7 pin on the way.

So - hacked out the innards.

I'm going to use the fet+bipolar -> traffo guts as a hiZ guitar input on another cheap 'channel strip' thing in conjunction with some pultec eq and api-style make up amp.  :) 
 

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and here is the basic new scheme with an unshielded transformer, pentode tube shown for approx sizing.

I'll be starting the build off as a platform to try a few different approaches :

- compatibility with china patternless PSU
  -> self biased at first
  -> fixed bias using heater or maybe some diodes

and then decide whether or not to build a brute force filtered psu box.

Am using

- nos Jan GE 5654  (ef95 equiv) 7pin midi (1.5" tall) pentode in triode mode
- cinemag 2461nico 6.5:1
- modified Oliver Archut/u47 circuit

The 5654 tube should give me around 5K anode resistance - nice and low so as to drive the low ratio traffo with reasonable bottom end.

Bias looks something like  -1.0V grid-cath for 8mA at 90V plate.

Seems like a lot of current don't it ;) Compared with 550uA or so for a real deal u47.
I'll fool around with it, but I like to bias the way I like.  8)

Using such low current must have advantages I guess
  -> noise ? (maybe), reduced anode resistance ? (who knows), lower ripple B+ ?

I'll stay with the stock capsule until the last thing and then put in a beezneez K7  :)

I think it should better my previous mic, an apex460 stock with reworked circuit+tube.

That one came out pretty good even with the BeezNeez generic 67.

This new Apex480 stock was much more toppy than that rebuilt mic.
Was OK but no where near the deepness and richness. 

So  - there it is.

Now working on an upper circular tube socket disc from extra think phenolic board.
That will house the socket and some hiZ parts.

I'll probably have a lower circular disc for the transformer mounting with cable-ties.
That should provide space for mounting the low Z parts and the coupling caps.

I'm not much of a cap rolling type - I'll find some ones that fit, prolly yellow tubular ones. :)
 

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alexc said:
Here's the guts of the apex480 I mentioned earlier.

It sounded reasonably quiet and mostly decent, with a somewhat top-py sound, as expected with it's cheap k67 type cap. Not a bad sound at all, and all the switches seemed to work!

For anyone who is interested, Brian Fox describes his mods for this at his site.
http://www.foxaudioresearch.ca/FARFET47.htm

(I'm not associated with Fox Audio Research but I enjoy studying his technical treatments of many things microphone and mod related - Thanks Brian  :)

---

2 pcbs : one the fet impedance converter + bipolar transformer driver, the other pcb is the psu with some voltage multiplication of phantom to provide capsule polarisation voltages.

Have you seen this thread?
http://groupdiy.com/index.php?topic=47569.msg598836#msg598836
I was going to post more about circuit like that and what you can do with them.
 
Thanks Gus - yes I do remember reading that thread with interest .\:)

I'm just refreshing myself now. I am interested in fet ldc too - I almost kept this one for mod'ing the fet circuit and with a m7 type capsule.

------

But this one - she's all in pieces now - awaiting some parts !

I do like the fet circuit, however : simple and with a couple of twists and twerks.

So I'll re-plant it in a suitable channel strip - kind of like a super cheap 1081 style box.

The circuit worked mostly well and the traffo is not bad at all.
 
bnwitt said:
I just picked up a new Chinese mic I intend to gut and rebuild and my preference is to make it either a U67,U87 or a C12.  Here is a link to what I purchased:

http://www.ebay.com/itm/MC700-studio-large-diaphragm-condenser-microphone-w-switchable-characteristics-/301214465415

I'm looking for opinions as to which of the three mic circuits mentioned above (or any other one which will shine on male vocals and acoustic instruments) is best suited for this body.
FIRST buy another of these mikes and check that the 2 sound the same.  If they are different, keep the 'better' sounding one and do your surgery on the other.

That way you at least have a chance of objectivity in your 'improvements' and supa dupa stuff hand carved from Unobtainium by virgins.  Its actually a very small chance cos if you've spent loadsa $$$ & time modding something, you can't be objective unless you conduct Double Blind Listening Tests.
__________

BTW, the really serious mike makers do this too.  You never take apart a good sounding sample until you build another which sounds the same or better.
 
What .. never ?

No, never!


:)

I guess that's probably true for pro builders - we less than so,  have the discretion to do whatever the hell we wants to!

Double blind objectivity ??  vas is das?

Life's way too precious to waste on lengthy deciding which of two things is 'slightly better'.
Frankly I'd have to be paid to do that.

ANd I don't recall any mention of this oft-spoke-talk of unobtainium  virgins that which do you speak.

This is about making cheap mics into something else, generally cheaper than any other option one has! Whilst still using good parts.

I believe by and large, if you use good parts and are not too dim, then the end result should be very good.

Objectively good you might say.
 
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