C414 EB P48 build

[Spencerleehorton]

Hi all,

I'm going to try and put together a couple of pcbs for the C414 EB P48, I might just have it fixed on one position first of all as dont have spare capsule which is correct.
Need to be able to have specs of dr1 and dr2 and what specs the txf is or another workaround for this?

 

[Khron]

As I asked in the other thread, how crucial is the "accuracy" of the DC-DC converter? As in, does it NEED to be the exact same one AKG used?

I'd wager the Schoeps-type converter would work just as well (47-220uH range inductors), or even a CD40106 Schmitt-trigger-inverter (as employed by Rode).

 

[justinheronmusic]

Hi all,

I'm going to try and put together a couple of pcbs for the C414 EB P48, I might just have it fixed on one position first of all as dont have spare capsule which is correct.
Need to be able to have specs of dr1 and dr1 and what specs the txf is or another workaround for this?

Spence, I believe that the 2.25:1 ratio transformer I got from Advanced Audio is for their version of the 414. I am also almost positive I read a thread on GDIY that the ratio was 2.25:1.

 

[Spencerleehorton]

Ah Sorry crossed wires there, the C414 EB P48 doesnt have a DC-DC converter,

Ah reading what youve said again i can see youve said the chokes are in the 47-220H range, i have some small ferrites but dont know if i can get this on there? what DCR?

my only problem with this verison is that is uses 2 x Chokes which i dont know the specs for and a txf which i think is 2:1 but has a feedback winding, feedback normally is 22:1 ratio turns to secondary, thats less so if the specs are similar to the U11 version which is close to 2:1 it has 1800 turns the feedback winding would be 81 turns.
I could experiment with this.
Just putting together a BOM.

Also 2SK30 i could replace with 2n5457 right?

 

[Gus]

The transformer does not have a feedback winding. Maybe I don't understand your post.

The transformer is center tapped for the circuits power supply.

First stage is a charge amp cascode (15pf is always in circuit) pads are more caps added to reduce gain
I don't like how charge amps sound I don't understand why people like the fet 47 circuit as well

Second stage is a follower with a CC in the source leg with switchable flat and two high pass setting

The CC setting at T3 will change the voltage to the first stage so for best results.
I would look for voltage readings from a real one
The CC resistor to adjust would be R8 that will help set the circuit up.
If you change R9 it will mess up the biasing in the HPF settings

If you change the first stage fet
You don't want to change R6 because it will change the output resistance of the first stage.
R5 would be a good place to start. I would make a temp fixed and variable to find a good value and then install a resistor(s) of the same value.

Note this is combination bias fixed and source biasing

If you look at other AKG circuits 220uHs are used for RF filtering however the RF caps can be a lower value .01uf

Have you looked at this thread?
https://groupdiy.com/index.php?topic=74723.msg946423#msg946423

If you substitute a fet look at the Vgs and IDSS curves for a hint  for what you might need to change

 

[rockinrob86]

I have a C414 EB P48, and it  is a good sounding mic.  My favorite feature of it is the pattern switching, and it is a go to "other" mic, meaning it has never been a first call for anything (vocal, guitar, drums, etc) but it works excellently as a room mic and always does its job well, even if not as well as some of my other mics.    If it makes sense, I almost find it kind of blandly accurate.  The natural comparison is the U87, which to me sounds more alive/exciting.  I do really like the c414 on acoustic guitars, although my km84's are the first call there.  But if I need figure 8 for the rejection, the c414 is my favorite for a track that needs to blend in. 

Long story short, I would be into building clones of this if they had pattern switching.  Also, AKG sells these capsules for a very reasonable price direct.

 

[Gus]

The switch might be the issue for the pattern switching because of of the need for  high resistance between contacts otherwise it is not a bad way to set patterns

Look at the TLM170 it has something like this

Found SK and SS  2P4T models
https://www.ckswitches.com/product-selection/slide/?ID=SK&ID=SS

 

[Spencerleehorton]

Not really sure what you mean about the switches?
Thanks guys for that clarity on the centre tap, a least now I can wind a txf and this wont be an issue.
Onto the dr1 and dr2, are you saying I could just replace for a 0.01uf cap?
My approach is usually to get the circuit working with whatever I have here to hand then order more specific parts to refine.
The 2n5457 I have here but I'll order some 2SK30s

 

[Spencerleehorton]

And the aim here is to have a good room mic which is natural sounding as well as a good condenser to go on guitar cab.
I've checked and I've got most of the bits, I actually have some small switches which should work well, just need to mount them on a pcb.

 

[Gus]

Another not P48 414 AKG schematic I found had 220uH and .01uF caps for the RF.
The P48 shows .022uf caps
You could do RC time constant for the same frequency or maybe it is 220uH, they are kind of big in the pictures I found of the inside of a P48.

You are dealing with high resistance at the capsule connection and pattern selection section so any conductance between switch connections can cause issues

 

[Spencerleehorton]

Ah so your talking about if the switch shorts or fails?
What do you suggest for a remedy gus?

I have slide switches at the moment and they are pretty small but look like they can do the job.

Still not really understanding what you mean about the 220uH, your dont seem to mention the choke (DR1 AND DR2) ?
I see the 22nf caps but they go into the chokes, can I replace the chokes for something else?
I have frites here which look to be about the same size but I dont know what Henries I could get out of them or what DCR I should be trying for?
This is my first stumbling block!!

 

[Spencerleehorton]

Sorry I get what your saying now, you are mentioning the chokes but at 220uH.
I should be able to easy hit the 220uH with some 0.1mm which is probably what it is, if not I have some 0.08mm.
I'll get all the bits together and start working on the pcbs in eagle

 

[Gus]

The output chokes and caps should be matched for the best CMRR at RF frequencies.

I was posting about the output chokes maybe being 220uH because other versions of AKG microphone had 220uH chokes.
I would ask at forums search for pictures or posts if anyone measure the P48 chokes in the microphone or if there are any markings on the chokes to indicate the value used for the P48.

I found some pictures on the web but I do not see any markings on the chokes.

 

[Spencerleehorton]

Sounds like you have some reservations about the first stage, do you have any suggestions which in your eyes would make this better?

 

[Gus]

I would build it like the schematic first before doing anything else.
People seem to like it from the posts you find on the web, maybe I hear or think I hear something that other don't .

You should be able to use different fets if they are close enough in specs. Keep in mind there seem to be counterfeits being sold.

https://alltransistors.com/pdfview.php?doc=2n5457re.pdf&dire=_motorola
https://alltransistors.com/pdfview.php?doc=2sk30atm.pdf&dire=_toshiba

Something that will help is to breadboard it first to figure out how the set the operating points of the gain stages.

 

[Spencerleehorton]

Only had a little bit of time today but have a v1 of the 1st pcb, will look at the other one tomorrow.
Will need to triple check these!!!

 

[Spencerleehorton]

Have to ask but does anyone have any pictures of the pcbs for the c414 eb p48 please?
I have done my own single layer layout, which is similar but I have had to move a few things around.
Would be good to compare and obviously have it as original as possible.

 

[Erik2345]

Saturn sound..
http://www.saturn-sound.com/images%20-%20microphones/akg%20c414eb-p48%20-%20inside.jpg

 

[Spencerleehorton]

ya, thanks Erik, but ive already got that one and a few other of the component side, need the solder side to get the full layout.
My pcbs are measuring 45mm x 55mm now and the layout im fairly happy with, both are single sided pcbs.
I just need to check all component sizes now and double check layout is correct.

 

[Gus]

Look for the thread that Rossi started in 2007 you should be able to trace it from that and the schematic

Why don't you design your own PCBs instead of copying AKGs?