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
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
