Source for C37A /C800 style mic capsules ?

[MatthewF]

I believe MatthewF did this circuit, but adapted for a 5840. However, he calculated the voltages, so hopefully has them somewhere.

I did indeed build a mic with one of Dale's Sony style capsules and a 5840. The circuit topology in my mic is identical to that of the C37, which is actually just a textbook cathode follower. I changed resistor values to accommodate the use of a standard Chinese tube mic PSU (150v B+). The 5840 is handy if you want to simplify the PSU as it can tollerate a much higher heater to cathode voltage than the 6AU6. (Note that in the original C37 PSU the heater supply is referenced to 1/2 B+).

Before I built the mic I attempted an analysis of the original C37 circuit and PSU, which can be found here:
http://www.groupdiy.com/index.php?topic=153.msg383471#msg383471
I was always a little perplexed by the 100v polarisation voltage I arrived at. When I built my mic I took on board Dale's comments in the above thread and wound up with 60-odd volts on the capsule.

Do you have the schematics for the older non-G C800 also? Or is it the same circuit (apart from the pattern switch)?

This scan on Gyraf's site suggests that the G and non-G versions of the C800 were quite different - the former being a cathode follower design like the C37, whereas the latter is a plate out circuit:
http://www.gyraf.dk/schematics/Sony_C800_Article_03.gif

Matthew

 

[maxwall]

Can anyone share what output transformers they chose to use ?

I think its a 10K:600 on the original C37A

 

[MatthewF]

I used this: http://www.sowter.co.uk/specs/3678.htm. I reversed its connections, using its secondary as my primary and vica versa, and configured it with a 3:1 ratio. In this configuration it would be specced as 5K:600. I remember checking the ac load line with a 10k load (roughly what the tube might see if a 1k impedance mic pre were used) and thinking it looked OK. I've not made any AC measurements on the mic, but it sounds fine and quite clean at the moment. More testing to come!

 

[Gus]

something to read that might help
http://www.tubecad.com/2005/June/blog0049.htm

 

[mushy]

Thanks guys for the awesome info.

Two questions.

What is it about the Sony and Altec capsules that make the CF an appropriate circuit?

MatthewF- would you be willing to share more of your findings with the 5840? I'm not a total toob noob. I have a pretty good handle on the hows but I'm just wrapping my head around the whys. 

 

[MatthewF]

Hey Mushy, sorry for the big delay!

My findings with the 5840 in this circuit are somewhat preliminary. So far I can say that the NOS Mullard unit I used has no noise issues whatsoever. I can check later on what resistor values I used to get a reasonable operating point with the 150v supply. I've only had a brief chance to test the mic on front of a rock vocalist, my initial impressions were that it was quite clean and tight sounding. (Sorry for the mostly useless adjectives - this stuff is always so hard to describe in words!)

I currently have the tube hanging from the PCB, using its leads for physical support. This is obviously not a great mechanical solution and is also, I believe, giving rise to some microphonics. I'll damp the tube down and affix it more securely, then report back.

What is it about the Sony and Altec capsules that make the CF an appropriate circuit?

I don't know the answer to this, but it might be worth noting that the C37 seems to be popular in certain studios for loud sources (I've seen photos of them used to close mic toms in a rock drum setup). If the capsule is well suited to this application, the CF circuit might be a good choice, providing potentially greater headroom and avoiding unnecessarily high output level. (I'm not, however, suggesting that Sony designed the mic with this in mind - that's anyone's guess, and the placement of the O/P trafo in the PSU would have necessitated a CF circuit anyway.)

Matthew

 

[Gus]

I think solid state is overlooked.

http://www.gyraf.dk/schematics/AKG_C414_old.GIF
The direct coupled EF lets one adjust the NPN emitter current independent of the Jfet source current if the hfe is high so you could make the resistor a lower value for more emitter current this is limited by the phantom supply.  Also being a follower design you do not need much supply voltage.  You do need to make sure the input signal is not too "large" for  the jfet.

http://www.gyraf.dk/schematics/AKG_C422.gif
note capsule voltage from one channel and circuit supply from the other

http://www.gyraf.dk/schematics/Sony_C38B.pdf
look at the circuit and note the 4.7VDC across the 3.9K

 

[mushy]

You guys are the awesome.

Hey Mushy, sorry for the big delay!

No worries.  I'm in no hurry. Thanks for the response.  I'd be very interested in those resistor values. Seems like a good option.

I think solid state is overlooked.

Also a good point.  As a matter of fact I have a Rode NT2 here that is screaming out for a capsule swap.  I was thinking of throwing this C800 capsule in there and seeing what happens. The beauty of having a capsule and no preconception of what to do with it is that there's room for experimentation. 

 

[MatthewF]

Thanks for those links and pointers Gus, certainly food for thought. I guess the 4.7v across 3k9 you pointed out in the C38 schematic, implying 1.2mA idle current in the output BJT, suggests the C38 head amp will be unlikely to run short of headroom. With that in mind, I think I understand your comment about the benefits of a solid state design - the qualities I mentioned above can be realised with transistors, removing the expense and extra complexity of an external PSU and NOS tube.

If anyone has had the chance to compare the sound of the C37 vs. the C38 I'd be interested to hear any comments - perhaps the negative feedback inherent in the cathode follower design prohibits the valve from adding much character to the sound.

Matthew

 

[desol]

Hey guys. Here's a couple shots of my C38, in case anyone's interested. This is a first issue mic; not sure if it is the same capsule as the 37. The inside of the headbasket is lined in a white foam type substance which may or may not have something to do with it's high spl handling. I know the headbasket design was changed at least once. The mic does seem pretty hard to distort come to think of it.

I haven't tried a C37 for comparison, having said that, this mic sounds quite nice and rich up close. Not a hyped sound at all, which i like. Omni sounded pretty good the couple times i used it.


Here are the shots:

 

[Out of the Blue]

Had c47 for awhile. awesome mic! I'm also wondering where to find a capsule for the c37/a.

 

[tardishead]

So we need a flat response capsule. But less sensitive than M7 K47 CK12 etc.
Are there no capsules out there that fit this bill??

I have read a lot about Chinese K67 type capsules with brightness - is this pretty much all they manufacture nowadays in terms of LDCs??

Can a Chinese capsule be used in this circuit with a HF roll off - if so where is the best place to put this and what range would the component values be?

 

[MatthewF]

I finally got around to opening my mic up and checking resistor values, it seems I used 68k and 3k3 in the cathode, with 200M feeding the capsule / grid bias. B+ is 150v. From what I can remember I had around 0.7mA idle current in the valve - it could probably idle a bit hotter but I'm not sure there would be much benefit - headroom shouldn't be lacking. Its probably worth mentioning that the valve is a little underheated - my PSU puts out 6v and those horrible Chinese made cables tend to drop ~0.5v.

Here are a couple of pics of the insides, including detail of the recently added tube damping which is still to be tested properly...

Cheers!

Matthew

 

[Rossi]

Looks like a t.bone SCT700 body. I recently worked on one of those and found out that the black rubber of the capsule mount is more or less conductive. If you grounded the backplate, that doesn't matter. But if you have the diaphragm at ground or direct to grid, you may run into noise problems.

I got rid of the rubber and used hot glue to connect top and bottom metal pieces (they kind of button into the rubber). The hot glue I used doesn't really stick to the metal but more or less forms a plastic piece similar to the rubber piece, but not conductive. Looks kinda ugly, but it works. I prefer to have the diaphragm at ground or almost ground potential, so it does not attract dust.

 

[rodabod]

Rossi, did you notice any effects from usin the leaky capsule post? Noise or loading effects?

I actually made that capsule mount with Matthew, and we use Araldite epoxy glue to fix the mounting ring which should help to insulate. Also, there is the plastic saddle that the capsule sits on to help too.

 

[Rossi]

I worked on the stock capsule and a 12AT7 plate out circuit. When I connected the capsule center terminal direct to grid, I had noise problems, and it turned out that the capsule mount was slightly conductive. If I rememmber correctly, it was in the kilo ohms range. With the hot glue connected mount the mic is very low noise.

I think there was someone in the MK7 thread who had noise problems, and Max mentioned the rubber mount as a possible culprit. Seems like the black rubber contains carbon.

If I remember correctly, the screw which attaches the plastic holder with the upper metal piece touches the (stock) capsule. But that may not be the case the way you mounted the Dale capsule.

 

[MatthewF]

That's interesting info, thanks Rossi. It is indeed a cheap'n'nasty Chinese made donor mic. This one came with no branding at all (not even a mark to identify which side is the front!)

I do have the diaphragm grounded, with the backplate connected direct to grid (which is at ~60v). I've not noticed any noise issues so far though. That may be, as Roddy suggested, due to the epoxy and plastic saddle insulating the capsule from the rubber post (I don't think the screw in the post touches the brass ring in which the capsule sits).

Was the noise issue you had very obvious or was it quite subtle? I need to spend some more time testing this mic so I'll pay attention to the noise floor.

Matthew

 

[Gus]

I like the reuse of the PCB.

 

[Rossi]

Was the noise issue you had very obvious or was it quite subtle? I need to spend some more time testing this mic so I'll pay attention to the noise floor.

Matthew

If your mic is low noise, don't worry about it. The noise issue was pretty obvious, at least to me. If you have a meter with a wide enough range, you could measure the resistance between grid and ground. It shouldn't be significantly less than the grid resistor.

 

[bernbrue]

I finally got around to opening my mic up and checking resistor values, it seems I used 68k and 3k3 in the cathode, with 200M feeding the capsule / grid bias. B+ is 150v. From what I can remember I had around 0.7mA idle current in the valve - it could probably idle a bit hotter but I'm not sure there would be much benefit - headroom shouldn't be lacking. Its probably worth mentioning that the valve is a little underheated - my PSU puts out 6v and those horrible Chinese made cables tend to drop ~0.5v.

Here are a couple of pics of the insides, including detail of the recently added tube damping which is still to be tested properly...

Cheers!

Matthew

Hi Matthew,
would you mind posting the schematic for this mic? Is it the C37 circuit?
regards
Bernd