Great AKG C414 EB Mod

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mbf90

Member
Joined
Jun 6, 2015
Messages
14
I have had the mod bug pretty badly lately and an idea for my AKG C414 EB P48 struck me tonight. The mod turned out well enough that I thought I would share it as I have not read about anyone else trying this particular one, and I think some people would enjoy it.

My C414 EB P48 had already been modded by Jim Williams when I bought it used. The existing mods were upgraded, low noise transistors and better capsule coupling caps. I took this a step further after I bought it and replaced all of the tanalum capacitors with high quality, modern electrolytics. This led to a more detailed sound and the elimination of what Jim Williams calls a spitty and gritty sound that tantalum capacitors can add ( I now agree).

Tonight, I decided to try something that came to me after looking at the schematic today, randomly. I noticed that the fet used for the voltage gain has a source resistor but no bypass capacitor, creating degenerative local feedback. I decided to add a bypass capacitor on the hunch that I would get a fuller, more tube like sound. This ended up being the result, exactly. The sound is now fuller and smoother. Adding a bypass cap for the source resistor increases overall thd. Some may not like this. However, it is pretty much purely second harmonic. Whereas, the degenerative feedback from an unbypassed source resistor lowers overall thd, but the nice sounding second harmonics are actually what is drastically lowered and high harmonics are actually created. The mic sounds more like a high quality, tube mic now with a richer and rounder midrange and top end and fuller sound. It was a little tough to get the capacitor to fit. I had to be a bit creative. I used a 100uf low esr, Panasonic FM cap I had on hand and was able to fit it on the board.

If you want your 414 to sound a bit more flattering, this is a great mod. It will work with the original EB and the EB P48 as they have almost identical pre amp circuits.

Matt
 
mbf90 said:
I have had the mod bug pretty badly lately and an idea for my AKG C414 EB P48 struck me tonight. The mod turned out well enough that I thought I would share it as I have not read about anyone else trying this particular one, and I think some people would enjoy it.

My C414 EB P48 had already been modded by Jim Williams when I bought it used. The existing mods were upgraded, low noise transistors and better capsule coupling caps. I took this a step further after I bought it and replaced all of the tanalum capacitors with high quality, modern electrolytics. This led to a more detailed sound and the elimination of what Jim Williams calls a spitty and gritty sound that tantalum capacitors can add ( I now agree).

Tonight, I decided to try something that came to me after looking at the schematic today, randomly. I noticed that the fet used for the voltage gain has a source resistor but no bypass capacitor, creating degenerative local feedback. I decided to add a bypass capacitor on the hunch that I would get a fuller, more tube like sound. This ended up being the result, exactly. The sound is now fuller and smoother. Adding a bypass cap for the source resistor increases overall thd. Some may not like this. However, it is pretty much purely second harmonic. Whereas, the degenerative feedback from an unbypassed source resistor lowers overall thd, but the nice sounding second harmonics are actually what is drastically lowered and high harmonics are actually created. The mic sounds more like a high quality, tube mic now with a richer and rounder midrange and top end and fuller sound. It was a little tough to get the capacitor to fit. I had to be a bit creative. I used a 100uf low esr, Panasonic FM cap I had on hand and was able to fit it on the board.

If you want your 414 to sound a bit more flattering, this is a great mod. It will work with the original EB and the EB P48 as they have almost identical pre amp circuits.

Matt

Have you rebiased fet? Have you tested mic on high SPL? What is THD level after mod? Mind sharing audio clips before and after?
 
mbf90 said:
.....
..... the fet used for the voltage gain has a source resistor but no bypass capacitor, ....

.....It will work with the original EB .....
Interesting..... Looking at the schematic for the C414-EB I have, I can't spot any FET "used for voltage gain", in my schematic is only a FET used as a "gain"-less impedance converter.

Could you please be so kind and post the schematic (part) you are talking about ?
 
I don't really see what you mean.
If you bypass the source resistor (2.2K.ohm) with a capacitor, you short out the signal to the NPN transistor following the FET...
Maybe some explanation is needed!
 

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Umm... Guys... If you read the actual post (not just the title) a bit more carefully...  ::)

mbf90 said:
I have had the mod bug pretty badly lately and an idea for my AKG C414 EB P48 struck me tonight.

<ahem> P48
 
Khron said:
Umm... Guys... If you read the actual post (not just the title) a bit more carefully...  ::)

<ahem> P48

Still a question about what they did. look at page 7
http://www.barryrudolph.com/recall/manuals/akg414eb.pdf

First stage is a cascode and the JFET source resistor is bypassed
2nd stage JFET is a CC device(note the gate is "AC grounded" by the 10uf) in the emitter leg of the output emitter follower
The 6.81k along with the gate voltage at the "top" of the 40.2k and the JFET characteristics sets the current

mbf90
The eb and eb p 48 have different first stages the EB is a two device follower with bootstrapping of the JFET drain to keep the drain to gate voltage somewhat constant. The Vbe of the NPN and the 2.2k are also a kind of CC because the Vbe drop does not change all that much.
The p48 has a cascode first stage with voltage gain, however there is an always in circuit 15pf cap that is part of setting the  gain of the first stage the other caps by it reduce the gain as they are switched in. The 2nd stage is a EF (with the high pass filter section)with  CC in the source leg.
 
Khron said:
Umm... Guys... If you read the actual post (not just the title) a bit more carefully...  ::)

I read the actual post carefully - you too ?  Then you should have seen this:

If you want your 414 to sound a bit more flattering, this is a great mod. It will work with the original EB and the EB P48 as they have almost identical pre amp circuits.

Khron said:
 
Ah, I see. Thanks for explaining it. I see the fet acting as a current sink for the emitter follower. So, I guess I bypassed the ccs source resistor.  I’m still trying to learn more about solid state electronics as I am used to tubes. It kind of looks like a totem pole output. What is the implication in bypassing the source for the ccs? It still sounds good. Also, I had read that both the eb and eb p48 were fet gains stages into an emitter follower, but clearly the difference is more nuanced than that. Sorry if I confused anyone.

Matt
 

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