Good FET for Chinese U87 copies?
« on: February 28, 2020, 01:26:57 AM »
I recently got a couple more BM-600s that I plan to swap the HF roll-off capacitors of. They're definitely noisier than advertised, which I was expecting. There's no way they're 14 dBA in cardioid, because they're significantly louder than my AT4050. So if increasing the corrective EQ doesn't make them significantly quieter (I expect it will, I wouldn't be surprised if it's 14 dBA with the proper capacitance or a properly biased FET, or it could just be the marketing nonsense I think it is), what's a good easily accessible FET to use?

It's gonna be a bit before the mods though, waiting on some 680pF caps (I have WIMA ones coming, and some silver mica ones as well since the WIMAs seemed like a tough fit in my first BM-600, and all the WIMA ones I could find on eBay either came from China or Taiwan, figured Taiwan was a safer bet, the mica ones are coming from Quebec which is the next province over so I expect those quickly).

Something to note in these mics is that while most capacitors in them are film ones, the in circuit EQ ones are ceramic. Is it worth replacing the low end roll off one too? I'd be replacing it with the same capacitance but a higher quality capacitor if I did, because I like the amount of low end in them.

TL;DR They're a bit noisy stock, I'm increasing the HF attenuation capacitance and possibly replacing the FET in both. I'd like FET suggestions.


RuudNL

Re: Good FET for Chinese U87 copies?
« Reply #1 on: February 28, 2020, 02:06:37 AM »
Did you check the noise of the other transistor(s)?
In some microphones I have had here, the transistor(s) played a more important role in the noise contribution than the FET.
Also the zener noise can be a problem.
Never had much FET noise problems with 2SK170, J305, 2N3819, 2SK30 etc.
There is a solution for every problem!

http://www.vansteenisaudio.nl

Re: Good FET for Chinese U87 copies?
« Reply #2 on: February 28, 2020, 02:49:49 AM »
Did you check the noise of the other transistor(s)?
In some microphones I have had here, the transistor(s) played a more important role in the noise contribution than the FET.
Also the zener noise can be a problem.
Never had much FET noise problems with 2SK170, J305, 2N3819, 2SK30 etc.
Actually, looking at Khron's schematic the actual FET is good, I am thinking of the other transistors. I'll have to get the part/model numbers of the other ones.

The other transistor in the signal path is marked A1015. A quick search tells me people replace this transistor when modding some of the Sterling mics.
« Last Edit: February 28, 2020, 04:08:03 AM by Icantthinkofaname »

RuudNL

Re: Good FET for Chinese U87 copies?
« Reply #3 on: February 28, 2020, 04:57:24 AM »
I have replaced many transistors in 'cheap' microphones.
To my surprise, general purpose transistors, as the European NXP BC557B, seem to have a very low noise level, and can be compared with special 'low noise' types. (Or maybe I just had a good batch... :) )
There is a solution for every problem!

http://www.vansteenisaudio.nl

rogs

Re: Good FET for Chinese U87 copies?
« Reply #4 on: February 28, 2020, 04:15:01 PM »
I've used quite lot of BC557Bs and found them pretty low noise...not a lot (if any?) difference from the official 'low noise' version - the BC560B
And it's easier to find the BC557 here in the UK...
www.amx.jp137.com - A DIY RF condenser mic project


 

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