[ACM investiupgradifications] All things MICROPHONE

clintrubber

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Let's have a placeholder here already for the bewildering amount of threads that might arise from the TNC-groupbuy
( http://www.groupdiy.com/index.php?topic=27172 ) once the stuff has arrived.
Many months to go, but before we forget...

So let's try to combine the ACM-microphone-related stuff here;
I guess we don't need another case of a thousand different threads about one and the same compreSSLor.... :wink:

In case a new 'wild' thread pops up we might remember this one and redirect the discussion to here.

Idea ?


And I propose likewise for the ACMP-preamps:

http://www.groupdiy.com/index.php?topic=27791

What is it:

* sound ?



Mod-idea ?

* quality of iron ?

* quality of caps ?

* quality of eventual tube ?


etc



Cheers,

Peter
 

clintrubber

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Thanks JDB for adding that. :thumb:

Let's hear it folks, there must be more info & questions around, let's compile them while they're still hot.

Cheers,

Peter
 

tommypiper

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How close is the ACM 6802-T electronics to the famous Apex 460?

I believe the ACM 1200 will turn out to be another Apex 460.
 

clintrubber

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[quote author="tommypiper"]I believe the ACM 1200 will turn out to be another Apex 460.[/quote]
Not familiar with the '460, but if it's indeed like that, would we like it ? (at least for the price)

Bye,

Peter
 

W DeMarco

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Heres a thread concerning ribbon mics and their tranx. Not sure if this would be a place to start but it looks like a candidate. http://www.groupdiy.com/index.php?topic=310
 

Mylithra

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heh.. I have a pair of NOS RCA 12AT7s destine for an ACM 900 and ACM 1200 :)
Might also end up doing the Trafos in them depending on how they end up sounding.
 

mushy

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I would like to propose a CHINESE MIC MOD META. There's twelve or thirteen different mics coming our way and each are going to have their own needs. Also, since the ACM 1200's are the same things as APEX 460's, and there is already so much data about this mod, cross ref info would be most helpful. The meta could also include ribbon sag info and other such problems that have been known to plague these "factory direct" mics. I would be happy to start searching and compiling links. Just tell me where to put em.
 

rodabod

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[quote author="tommypiper"]How close is the ACM 6802-T electronics to the famous Apex 460?

I believe the ACM 1200 will turn out to be another Apex 460.[/quote]

6802 has been traced by Marik here.

Not the same as the 460. Better for modding in my opinion as you don't need to scrap the second triode used in the 460.

[quote author="clintrubber"]
Not familiar with the '460, but if it's indeed like that, would we like it ? (at least for the price)[/quote]

You should like it. Keep the boards inside and drop the second half of the circuit so you have a standard plate-out (so you can now follow a G7, 1/2 a C24, M251, etc. with a few component swaps). Drop the heater to 6V by bypassing the diode on the heater regulator. Many mods and experiments have been documented here and elsewhere.

In my opinion, the biggest improvement you can make with these mics is to fit a good capsule.
 

ChuckD

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OK I got my Mics on Sunday...
Here is my schematic for the ACM-1200.

It is almost the same as the APEX 460 that someone else posted a long time ago, with the new switches added. Please feel free to make corrections here.

NOW PLEASE READ !!

The ground on both small pcbs is a rail along both the edges. On one of my boards the ground was not connected! The solder bump was there and there was a hole from what appeared to be a wire at one time. There was no loose wires so I can only assume it was removed. The only explanation could be that it was removed on purpose and then they are counting on the bump making good contact to the metal frame (which it may or may not) to give a good ground connection. I will be hooking this up much better when I mod the MIC.


acm_1200_source_v2_big.gif



I will post my mods in a bit.


-Chuck
 

ChuckD

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Here is my 1st attempt of an AKG C12 redraw. Please note any mistakes so I can fix them I am unsure about R11 & R9. The main goal was to draw this the same as the ACM-1200 so modding to a C12 would be easy.

Comments?

akg_c12_source_v2_big.gif



-Chuck
 

mitsos

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Hi Chuck,

Thanks for posting that. Im not the one to comment on your schematic, but how is the mic stock?
 

Marik

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[quote author="ChuckD"]Here is my 1st attempt of an AKG C12 redraw. Please note any mistakes so I can fix them I am unsure about R11 & R9. The main goal was to draw this the same as the ACM-1200 so modding to a C12 would be easy.

Comments?

akg_c12_source_v2_big.gif



-Chuck[/quote]

About 200V on a grid? :shock:
 

CJ

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You never heard of Grid Modulation?
Now go to your Piano and play your interpretation of what your 401 k has been doing, MP3 it, and post back.
 

CJ

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You never heard of Grid Modulation?
Now go to your Piano and play your interpretation of what your 401 k has been doing, MP3 it, and post back.

And No Vodka til we hear it.
.
 

rodabod

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Chuck, I'd recommend following what people usually do and copy 1/2 of the C24 circuit as the biasing is via the same method as used in your mic currently which makes it easy to mod. I tried biasing a little hotter with ~2K or so for the cathode resistor.
 

ChuckD

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Here is an attempt to redraw the 1/2 AKG C24 in the same format. YEs you are correct this is the circuit that was copied for the ACM-1200 / APEX etc mics. I left the pad switch in but I think I may remove that. What do you think? Switch removed R1 would go to 100K.

Comments??

akg_c24_source_v2_big.gif



Thanks

-Chuck
 

ChuckD

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So the C24 is "self biasing" with the 200M grid resistor and the 2.7K cathod resistor. The C12 is not "self biasing" so my question is how is it turned on ... or how was this do in the 50's? That was my question about R9 & R11 in my C12 redraw basically.

How does a self biasing circuit effect the sound quality compared to other?


Also I was thinking of using the second switch to switch C8 output cap between the 1uF (C24 value) and 0.5uF (C12 value).


-Chuck
 

ioaudio

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marik, i dont understand: 200v on grid?

the c12 is set up with a "fixed bias". the negative bias voltage comes through a extra wire (pin4, ignore the fat line on the schematics) from the psu.
note the 1kiloohm resistor on ground of the HT supply. it creates a voltage drop when loaded with the microphone, with the point before (on the left of) the resistor being negative relative to ground.

c12schem.jpg
 

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