MK7 - tube mic project

[ioaudio]

that was stupid question from myself  
ef86 you calculated 6,3V/0,2A = 31,5 ohm
for lower noise we want to operate the heater lower at about 5,7V and 0,18A which calculates about the same (as it should).

 

[Kingston]

I've just verified I have access to all parts, except the choke (couldn't find a suitable one in europe) so looks like I have to order that from the states (not always a good thing) and the PSU I'll have the toroid custom made.


And that damn 1.4M resistor. Not even farnell has them (in any form). I don't suppose someone here would be willing sell me a few of those preferably 1% 0.25 - 0.6W 1.4M resistors?

I wouldn't want to put even 1.2M and 200K (or whatever) in series since it's in the high impedance section, and every single millimeter of extra wire counts (as a bad thing).

 

[Jim50hertz]

RS components carries the choke.

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=2508972521

 

[ioaudio]

regarding the resistors for the polarisation resistor driver:
you are right - 1,4Mohm resistors are rare - sorry about that.

use:
1Mohm instead of 1,4Mohm
910Kohm instead of 1,2Mohm

i will update the information, please update your POMs.

 

[bradb]

maybe a stupid question, but if we've already sourced those 2 resistors, do we need to switch them out?

 

[ioaudio]

the whole thing is about +/- 1 volt on the capsule (if you used 1,5M and 1,2M its ok too)
the two resistors form a voltage divider for the polarisation voltage, and it depends on the capsule whether you want 65v or 55v or something in between. again you can use a trim pot, adjust on test, measure the resistance and insert the choosen value.

schematics have been updated with the new values.

 

[matta]

Hi Max,

Out of interest what value did you use for your Bypass cap? You suggested .33, .46,.68 did you try each of these? Any personal preference? I'd assume that the larger the cap the better the low end response?

Cheers

Matt

 

[ioaudio]

it depends on the capsule and on your taste.
the coupling cap is not only a high pass, but it also forms a LC series circuit with the transformers primary inductance, resulting in a peak.
often bigger caps only waste enrgy in lower regions, it occours that the bass is "better" when  you choose a smaller cap.

 

[matta]

it depends on the capsule and on your taste.
the coupling cap is not only a high pass, but it also forms a LC series circuit with the transformers primary inductance, resulting in a peak.
often bigger caps only waste enrgy in lower regions, it occours that the bass is "better" when  you choose a smaller cap.

Thanks Max!

It seems from your proto you went with .68uF? Also I'm struggling to source a 20K/1W resistor here, most popular value is 22K, is this a suitable replacement, or will it offset things a bit too much?

Cheers

Matt

 

[Gus]

Max why did you post that? It was good to read people questioning things

I was hoping people would do some testing/thinking on there own and figuring things out without telling them that.

 

[ioaudio]

gus,

i learned a lot from this forum and i think its good share the knowledge.
i try to give some directions how different parts of the mic circuit work and suggest some values, while trying to explaining why i choose them. i share the schematics so people can maybe learn something from it.
but building and adjusting a mic is a matter of taste, so there is no rule what to use (except your ears) once you understand the circuits basics.
matta, i used 0.33 and 0.68 for 2 different M7s (dale´s and gefell´s) (and meanwhile for a k47 i used 0.47)
22k is ok.

 

[Rob Flinn]

it depends on the capsule and on your taste.
the coupling cap is not only a high pass, but it also forms a LC series circuit with the transformers primary inductance, resulting in a peak.
often bigger caps only waste enrgy in lower regions, it occours that the bass is "better" when  you choose a smaller cap.

Thanks Max!

It seems from your proto you went with .68uF? Also I'm struggling to source a 20K/1W resistor here, most popular value is 22K, is this a suitable replacement, or will it offset things a bit too much?
 
Cheers

Matt

Had you tried 2 x 10k in series ;-)

 

[Gus]

Max I did not mean the post in a bad way.
  Like I posted I was hoping people would try things on there own and figure stuff out.  You posted early on to try different cap values.  Caps don't cost to much so I was hoping people would try stuff.  There is no tube microphone design book I know of so I have done a lot of looking at schematics and asking myself why did they do that, reading books, buying parts building circuits and testers and doing tests.

 

[matta]

Max why did you post that? It was good to read people questioning things

I was hoping people would do some testing/thinking on there own and figuring things out without telling them that.

Max I did not mean the post in a bad way.
  Like I posted I was hoping people would try things on there own and figure stuff out.  You posted early on to try different cap values.  Caps don't cost to much so I was hoping people would try stuff.  There is no tube microphone design book I know of so I have done a lot of looking at schematics and asking myself why did they do that, reading books, buying parts building circuits and testers and doing tests.

Gus,

I tell you what, you send me the parts, I'll experiment  ;D

I live on the ass end of Africa where almost ALL my parts esp. HV parts need to be imported from the US and or the UK/Europe. Our currency is over 10 to 1 weaker than the USD and worse to most other currencies. The caps I need to source come from RS in Europe, I have to buy a minimum of 5 of EACH value to 'experiment'. They are not cheap and I then have to pay shipping, 14% sales tax for the import and then a further up to 10% on import taxes.

The average person lives here on less than 1$USD a day (25 million people to be exact)... so forgive me if I'm frugal about what little funds I have available to me and DIY.

I've never built a mic, but excited about the prospect. I have however built a handful of other projects, have them documented here and spent several hours troubleshooting errors/problems with others. I don't have your "Mightier than thou" knowledge of electronics, but I try do what I can do to help and learn in the process.

I asked the designer what cap he used and WHY and made what I thought was an educated assumption that the bigger the bypass cap the better the low end (turns out I am wrong and it is the opposite way around so I learnt something new from Max).

To Max, thank you, thanks for taking the time out to share and explain things.

I find it strange that even using M7's, i.e the same mic capsules would differ so radically from each other and sound so different with different valued bypass caps.

I took a listen to Martin Kantola's M7 review a day or 2 ago and was amazed to see how different they sound: Neumann Berlin, Geffel, Thiersch and Dales among them, same style capsule, same headamp.

I found the Thiersch and Dales to exhibit a better low end response, but some of the high end detail was not as distinctive as the Neumann/Geffel.

I am assuming that bypass cap choice could be one of these contributing factors.

Cheers

Matt

 

[matta]

Darn you Gus... I just ordered the other caps (330nF, 470nF and 680nF)... I'm just going to have to try sell the excess to recoup some of the costs, but need to HEAR this for myself....

Matt

 

[e.oelberg]

Matta,
I experimented with 0.33, 0.47, 0.68, 1 and 2.2µF using a EF86 and a Dale M7. I have a small electronic shop round the corner with Wimas so Its cheap for me.
Finally a settled with a russian Oil Film cap I once bought for too much ( 5 € ) on ebay. I found 0.47 the best value ! The Wima and the Russian MP sound very different. Wima sounds clear and stiff, The MP sounds more soft but still has more resolution.

anyway.. nobody posts his impressions here, my mic is up and running since weeks ,

nicholas

 

[ioaudio]

gus,

now i got you.

-max

 

[Jonkan]

Im trying to order parts and have a quick question. Does it matter if the pattern selection switch is make before break or break before make?

Also, can i use any old electrolytics for the heater psu filtering? Or is it better to use low esr caps? Im thinking about the 10000uf/10v ones, and the 4700uf/16v ones.

Please excuse me if these are very basic, newbie questions.
/J

 

[e.oelberg]

10V  might be a bit low voltag rating, but try. If it humms replace them with new once low esr

nicholas

 

[ioaudio]

the 10v rating for the cap is for size reasons: only use them if you do not expect more than 10 volt at the place you put them (after the regulator!) under any circumstances (without the mic connected)