R/E/P Community

Please login or register.

Login with username, password and session length
Advanced search  

Pages: 1 [2]  All   Go Down

Author Topic: C414EB noisy filter switch  (Read 4819 times)

klaus

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1659
Re: C414EB noisy filter switch
« Reply #15 on: December 20, 2016, 01:37:25 pm »

The two low cuts are effected by a couple of tiny Tantalum capacitors (.1mfd.) They are always suspect in this mic, and I routinely replace them. Mechanically not easy, as they sit right in the center of the circuit board.

Schematic with position of caps here:
http://recordinghacks.com/microphones/AKG-Acoustics/C-414-EB
Logged
Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

jackreynolds

  • Newbie
  • *
  • Offline Offline
  • Posts: 18
Re: C414EB noisy filter switch
« Reply #16 on: December 20, 2016, 06:14:04 pm »

It looks like the tantalums may have been replaced with Wimas in the picture on recording hacks. They are two little green 35V tantalums in the mic on my bench.

http://cdn.recordinghacks.com/images//mic_extras/akg/C414EB-exploded.jpg

Would you recommend replacing like with like ie tantalum, or substituting a film cap?

On the schematic, i'm seeing two 0.1uF polarised caps, rather than 0.01uF, which appear to be part of the RC network for the filters.

As you say, it looks like quite a mission to get into the centre of the board there, but i imagine with some care and patience it won't be impossible.

Jack
Logged

Kai

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 314
Re: C414EB noisy filter switch
« Reply #17 on: December 20, 2016, 07:03:03 pm »

Would you recommend replacing like with like ie tantalum, or substituting a film cap?

On the schematic, i'm seeing two 0.1uF polarised caps, rather than 0.01uF, which appear to be part of the RC network...
Either would work, if you use film caps there might be a slight sound change, because a significant amount of (LF) signal voltage is dropping at those, even in filter bypass.
Explanation: The characteristics of a condenser will become audible if there is a voltage present between both sides of it, so it acts as a frequency dependent resistor.
If this is the case depends on dimensioning and topology of the circuits.

Be sure to use the right value, best measure the ones you have taken out.
When calculating the x-over frequency the values given in the schematic (0.1uF) are correct.
Still it's possible that those of your specific microphone are different, as it might not be 100% coincident with the schematic.
Logged

klaus

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1659
Re: C414EB noisy filter switch
« Reply #18 on: December 21, 2016, 12:25:56 am »

Quote
there might be a slight sound change, because a significant amount of (LF) signal voltage is dropping at those, even in filter bypass
I don't believe so. There is a hard bypass before the caps, so I am not clear how the cap material would affect the sound in the flat setting?


On the schematic, i'm seeing two 0.1uF polarised caps, rather than 0.01uF

You are correct. It's 0.1µF.  I have corrected the value in my original post.
KH
Logged
Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

Kai

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 314
Re: C414EB noisy filter switch
« Reply #19 on: December 21, 2016, 02:48:00 am »

I don't believe so. There is a hard bypass before the caps, so I am not clear how the cap material would affect the sound in the flat setting
You are right, the trick is that the BC413 bias is switched off in bypass, so the whole filter is out of circuit.
This leads to the idea that even the BC413 might be faulty.
Logged

jackreynolds

  • Newbie
  • *
  • Offline Offline
  • Posts: 18
Re: C414EB noisy filter switch
« Reply #20 on: December 21, 2016, 06:18:20 am »

The two tantalums are marked +1 35V, but they look rather tricky to replace, so if there is a chance it could be one of the transistors, which do you think would be the most sensible to replace?

Or is there a way of testing the circuit to ascertain which parts are faulty?

It seems like replacing old tantalums is a good idea anyway, but I would rather not change the sound of the filter too much.
Logged

Jim Williams

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 575
Re: C414EB noisy filter switch
« Reply #21 on: December 21, 2016, 11:30:47 am »

Wima MKS 02 2.5 mm mylar caps will sound smoother than the tantalums, plus you never have to worry about those parts ever again. They have a 2.5 mm lead spacing and should fit easily. Schoeps used those in their mics.

Also check the input coupling cap to the jfet gate, they sometimes used a ceramic disc cap, bad choice. Swap that out to a polystyene film or a Wima FKP-2 polypropylene film cap. That makes a huge difference in the resolution of that mic.
Logged

klaus

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1659
Re: C414EB noisy filter switch
« Reply #22 on: December 21, 2016, 02:21:22 pm »

Let's stick to the thread's focus: troubleshooting of a specific problem.

The input coupling capacitor or considerations what components sound better or worse have no bearing on what the poster reported: noise, but only when the filter switch is engaged.
Logged
Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

Kai

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 314
Re: C414EB noisy filter switch
« Reply #23 on: December 21, 2016, 04:03:17 pm »

...the poster reported: noise, but only when the filter switch is engaged.
  It's very clear now that the problem lies within the filter circuit. If I had to do it, I would change the caps first, then the transistor.
Logged
Pages: 1 [2]  All   Go Up