Cap Voltage Ratings

Hi gang, I have a wonder about capacitor voltage ratings in a mic. I'm building an M49 PTP and there are four .01uF caps. I noticed on Poctop's build he uses four 400V rated caps at .01uF.

Mightn't the voltage rating affect capacitance/performance?

I've read the rule of thumb is to use caps at 2/3 their rated voltage for best performance. And that to use an overly large cap could adversely affect capacitance, and the performance of same.

So is this silly to worry about, or should I find .01uF caps that are about 3/2 higher than the circuit section they are in?

In the M49, that means two of the .01uF caps would be at 75V, and the other 2, not sure, but certainly nowhere near the 400V.

I guess ultimately, does it matter, and will this affect the sound? I even read on the Neumann forum that they were sure to pick capacitors with the right value and "appropriate voltage ratings."

Thanks for your thoughts.

Mike

The capacitor voltage rules you're using apply more to electrolytic caps, and much less to film or ceramic caps. What's much more important is to pick the right dielectric. This will have a direct and predictable effect upon the sound quality. I'm not sure where these caps are in the circuit and what kind of voltage is on them - is there any signal component? Is it purely an RFI stopper? Is it used to couple the front and back side of the capsule?

If you could post a schematic or a link it would help!

Thanks. It's the M49 schematic, but the forum won't let me post it here (guess some things still aren't working?)

Here's a link to it: http://elektrotanya.com/PREVIEWS/63463243/23432455/neumann/neumann_kondensator_mikrofon_m49c_sch.pdf_1.png

I know that C1 should be a polystyrene, and I've heard that Neumann actually used ceramics for some of them.

I've never built an m49 circuit, however I can tell you that the capacitance value will not be affected by the voltage rating....  it is wise to choose caps well above the minimum voltage rating, if you going too far though the dialectric will be thicker (to deal with the increased voltage rating) and the sound 'might' suffer.....  but I'm not convinced you'd hear the difference between  200v & 400v caps... esp in the positions you're talking about.

I'd just choose high quality caps with appropriate dialectric like McGuire said.... and if it was my mic, I'd stay away from ceramics.... personally, I'd go for polystyrene for the 1000pf, and either film or poly or pio for the rest of the lower values... tant for the 25uf, and film or pio for the 1uf...  of course if you're going for historical accuracy, that's another thing....

Best of luck!

Thank-you Seeker.

I would use 160V rated C1 as minimum as also for all 10nF.

ln76d said:

I would use 160V rated C1 as minimum as also for all 10nF.

Click to expand...

Yes makes sense. I was thinking of using Cardas Clear caps for the .01uF - uses copper foil.

Don't waste time for cardas hocus-pocus.
Caps? Mustard caps
Am using it mostly at this position.
Btw. If you are not using switch S2 for "true cardiod" option, use 470pF for C1.
You will get little bit better sensitivity.
For 5840/EF732 tube R7 from 1.6-1.8k should be much more proper.
Another thing, try some good carbon resistors, especially for R7 and R10.
For the output cap MP works like a charm, another option is MKT, but definately i wouldn't go for MKP (any kind from typical to audiofool range). You may consider change value from 1uF to 0.5uF - usual works better in tube mikes which am making.

Many thanks Ln76 for the tips! 

I'll be using a real AC701, a Neumann k49, AMI Bv11.

I do have a Bosch MP 1uf  but I'm concerned about using a 40-50 year old cap...

Having trouble deciding and sourcing the resistors; I'd like something close to what would have been used. I have googled and googled and can't find the brand/exact types of the high z resistors etc.

Would they have all been carbon Comp, even the high meg resistors? I have done TRW glass carbon glaze resistors st 100M but not sure if I should use them.

Thx for your thoughts!

Mike

No, no, no

Not carbon composite type, these are totally different.
Check PM for what am talking about
You shouldn't have any problem with glaze resistors.
R3, R7, R10 are only important. For high impednace sections try TRW, For the rest (voltage divider) use 1% low noise metal film.
I never had troubles with old MP like bosh, hydra, siemens etc.

ln76d said:

No, no, no

Not carbon composite type, these are totally different.
Check PM for what am talking about
You shouldn't have any problem with glaze resistors.
R3, R7, R10 are only important. For high impednace sections try TRW, For the rest (voltage divider) use 1% low noise metal film.
I never had troubles with old MP like bosh, hydra, siemens etc.

Click to expand...

Thanks very much Piotr!

I tried PM'ing you back but your box is full. I appreciated you sending me all those links!

So why do you suggest the carbon films are good - they were not used in the originals, right? Do they have the same kind of sound/effect as carbon comps do?

I never heard of the Ero caps; they sound very interesting.

I'm still a bit concerned about using 40-50 year old parts in a "new" mic build.

I got a Bosch MP 1uF cap from Andreas, and it tests OK. Would you use that cap or prefer to use another as the x-former coupling cap? I can get a VCAP OIMP that will fit, but of course this is a modern cap.

Thanks for your thoughts; very helpful!!!

Sorry

I cleaned my inbox

They were used in original, not exactly the same brand, but the same type. They works uber fine in tube microphone circuits as also are low noise. You should notice difference between carbon and metal film in mentioned circuit positions. Especially plate resistor.
First versions of M49 used mostly siemens and elap wirewound resistors. next versions like "C"  had beyschlag, siemens and other carbon resistors. Somehow am not a fan of carbon comps in tube microphones, sometimes i use it for FET mikes, like schoeps topology.
Here are some pics of resistors inside M49:
http://www.twin-x.com/groupdiy/albums/userpics/Neumann_M49_017~0.jpg
http://repforums.prosoundweb.com/index.php?topic=36393.0
http://s2.postimg.org/yfk9opz6h/photo_2.jpg
http://s23.postimg.org/nutw3ttsr/photo_4.jpg
Most of tube mikes which i build use NOS parts. Years ago, when i started tube microphones adventure, i used mostly components from audiophile market. Then i started buy NOS parts. I did many tests and comparisons.
There's a significant difference. Somehow the same circuits sounds much more "real" with the old parts inside. More natural.
That kind of sound which you will find in most of good old mikes.
For the output cap you can always make A/B test. I tested that way many capacitors. You need a switch, wire and few caps.
Old MP always win.
Bosch is one of the best what you can use for C5.
ERO is Roederstein.
http://www.vishay.com/landingpage/50year/images/roedersteindiagram.png
These, which link i sent you, are similar in type and design to wima used in M49.
For every part of the circuit you can make tests
You wouldn't regret of using "40-50 year old parts in a "new" mic build".
Believe me

Piotr, man, you are a treasure dude! I can't thank-you enough for your awesome sharing and ideas!

I'm going to look into what you sent me. I'll pm you later, and thx!

By the way you mentioned wire wound resistors; were they non-inductive? I'd worry about screwing up the audio...

Here's one I saw on the bay what do you think?

http://m.ebay.com/itm/122100778655?_mwBanner=1

BBFW are carbon film resistors not wirewound as stated in the auction.
It's a carbon film around ceramic tube.
I had 200M and 300M from these, they work as it should be.
Completely no problem with noise.
Usual are pretty big.

Awesome man! I finally got home from work so I've been checking out the photos and info you sent me man - thanks!!

It looks like original NOS Wima TFM's come up for sale on eBay sometimes; I just found some but so far none of the correct capacitance.

So I'm looking at the photo you linked to and it shows one of those .01uF caps rated at 10v. Looking at the schematic that must be C8, since that comes off the 4V heater circuit.

Which leads me to believe not all the .01uF caps should be the same voltage rating - do you agree? The schematic doesn't put the voltage ratings on the capacitors either.

Half the fun to me is sourcing the parts; now you got me all interested in using NOS stuff, which I hadn't honestly even considered!

Thanks man!

Hey LN,

I just saw these - Wima TFF caps - I think Neumann used them as well as the TFM's. What do you think of these?

http://www.ebay.com/itm/WIMA-TFF-0-01uF-400v-VINTAGE-FILM-AND-FOIL-CAPACITOR-4-PIECES/302115562254?_trksid=p2045573.c100033.m2042&_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D38530%26meid%3D48ffc2e7bd9e4d1fbceb4789dcbf06f6%26pid%3D100033%26rk%3D2%26rkt%3D8%26sd%3D371785831634

WIMA TFM and TFF is good choice, i even looked for 10nF WIMA for you but forgot look for 0.01uF in the title
Neumann use it in many mics as also other WIMA caps.
10V rated is electrolytic C9 for cathode bypass. Probaby also WIMA.
Years ago WIMA made many different electrolytic caps.
Rather all caps with the same value were same voltage rated -  simply  cutting cost.
These days caps were little bit pricey than now.
Yeah, i like sourcing parts also

Hey terrific! Yeah I'd love to get the real deal parts for this if possible, and I have you to thank for turning me onto this!!!

I'll let you know how it goes...best to you man!  8)

Mike

No need to thanks
Make great microphone and be happy with the results!
Waiting for the news about the progress