DIY RF Condenser Mics

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

khalidgul

New member
Joined
Nov 18, 2022
Messages
4
Location
Pakistan
V.5 of the schematic draws less than 3mA from the phantom power supply in total, so even a relatively 'weak' supply should function OK.

Using a DMM you should be able to measure voltages within the following values at these points, relative to gnd (pin1):
• Junction of R11 and C7 - c.35V DC
• R11 (which is a link) - c.20v DC
• Junction of R2 and Q1 collector - c.7.5v DC
• Across both R4 and R10 - between c.2v and c.4v DC, depending on the Vp cut off voltage of the specific FET used. Both values should be the same.

Using an oscilloscope with at least a 20MHz bandwidth, and a x10 probe with an impedance of c.1M and a capacitive loading of c.15pF -
(again reference to gnd) :
• Junction of R3 and T1 primary - between 4.5 and 5v AC p-p at 10MHz
• At each end of T1 secondary - between 3.5 and 4v AC p-p at 10MHz.
(those last 2 readings may be slightly different from each other, but typically less than 250mV difference )

• Measuring the signal at Q4 gate will not give a very accurate reading, because the 'scope lead will load T2 secondary, but will typically be a sine wave between 500mV and 2v AC p-p at 10MHz.

Can you confirm that you are getting readings close to those values? ...
So I want to add replace sdc with ldc(cardioid capsule with 2 cables)on my Sennheiser mkh 416 rf condenser shotgun microphone. Please let me know how to do this, biasing and modification. Happy to pay if somebody is interested. by the way my first post on the group. Just joined it. Thanks everybody. You are lovely people. With love from pakistan
🇵🇰
🙏🏻
🤗
❤️
 

rogs

Well-known member
Joined
Jun 23, 2006
Messages
456
Location
Dorset, UK
So I want to add replace sdc with ldc (cardioid capsule with 2 cables) on my Sennheiser mkh 416 rf condenser shotgun microphone.
First question has to be -- why? Presumably you are intending just to use the electronics, and mount them in a new enclosure along with this LDC capsule?... (There's no way an LDC will fit into a 416 body!).
As I say, I fail to see why you would want to do this, but you do need to be aware of a couple of things....
• Firstly, you need to measure the capacitive value of the existing capsule. Unless your replacement capsule has a very similar value, there's no way you'll be able to retune the inductor cores effectively.
• Sennheiser RF mic capsules mostly have a low tension membrane, and tend to be much more sensitive than conventional LDC capsules.
Those are generally designed to have a significant DC applied across them, and tend to have a higher tension applied to the membrane.
As result, you're likely to find your re-built mic will be a lot less sensitive than the original -- and probably noisier as well.

As I say, I don't personally think this is a very good idea, and I think you'll be disappointed with the results you get.
If you are keen on trying out an RF mic with a LDC capsule, why not have ago at the project being discussed in this thread?.....
(Project notes here: www.amx.jp137.com )
 

Khron

Well-known member
Joined
Apr 8, 2010
Messages
2,639
Location
Finland
I was just googling for alternate sources for these elusive RF inductors. On Spectrum's ebay listing page for their 5u3HH, they mention they're equivalent to the Toko 3334/3337, so i googled for those too, and came across this:


Might this be a valid, but more affordable, option for us EU members? Even though the shipping's not the cheapest.

In the mean time, also found this (although "only" in packs of 20pcs):

 
Last edited:

rogs

Well-known member
Joined
Jun 23, 2006
Messages
456
Location
Dorset, UK
I was just googling for alternate sources for these elusive RF inductors. On Spectrum's ebay listing page for their 5u3HH, they mention they're equivalent to the Toko 3334/3337....
Sadly, I don't think Spectrum's suggestion is very accurate?...
The T1 inductor employed for this project is required to have a centre tapped winding. Neither the Toko 3334 or 3337 have that.
The 'primary' (secondary in this application) turns differential for the 3334 is 7 and 11, with 2 and 23 for the 3337.
As outlined in Baxandall's original paper, the 'bridge' needs to be almost balanced for effective operation, and a non-centre tapped winding would probably make that extremely difficult to achieve.
The only Toko 10K series coils I have found which have a suitable centre tapped winding are the models KACSK3892, KACSK3893 and KACSK3894.
The last of those I have tried out. It comes with an internal 82pF capacitor fitted, which - being ceramic - is fairly easy to remove by a quick twist of a screwdriver.
Sadly though, the inductive value is only around 2.6uH, and that makes it difficult to use effectively with LDC capsules with a capacitive value of less than around 120pF. Not many of those about!

The Royal Mail international shipping costs listed on Spectrum's Ebay page are very odd.... It's cheaper to post to the USA than it is to France (or Finland) ??
All part of post Brexit EU/UK 'sniping' at each other, I suspect ?.. :)

Despite Spectrum's Ebay page suggesting their coils are from China, I'm pretty sure they actually come from India.....
Where to buy this kind of coil in small quantities from India, I've yet to discover....
 

Khron

Well-known member
Joined
Apr 8, 2010
Messages
2,639
Location
Finland
On a more positive note though, have you noticed any sort of difference between the perfboard and the pcb versions, in case you've built one of each with the same component values?

Just crossed my mind whether the groundplane in the oscillator / capsule area has any meaningful (or negative) effect, an/or whether a gap in it would do any good.
 

rogs

Well-known member
Joined
Jun 23, 2006
Messages
456
Location
Dorset, UK
On a more positive note though, have you noticed any sort of difference between the perfboard and the pcb versions, in case you've built one of each with the same component values?

Just crossed my mind whether the groundplane in the oscillator / capsule area has any meaningful (or negative) effect, an/or whether a gap in it would do any good.
I only ever built perf board versions of the early ideas for the project.

Notes and schematics here:



Once you had laid out the PCB, I was not likely to return to my difficult to build - and rather inelegant - solution for the inductor assembly :)

There were 3 important 'events' in the development of this project:
• The discovery of the Spectrum inductors, which were much more suited than the TOKO ones used in the early experiments.
• The creation of your PCB
• The decision to replace the diode demodulator with an infinite impedance detector.

Once I had constructed the first version using your PCB I never felt any need to look 'backwards' to the perf board, as it were.

The ground plane construction seems to work well. It was never likely to make much difference to the audio side of things, and most of the RF was either contained within the inductors assembly, or superimposed on the low impedance output of T2, and it's connection to the FET gate.
It's unusual to have the FET effectively 'grounded' in condenser mic schematics -- we're much more used to FET inputs being high -- or more likely VERY high - impedance.
The ground plane under the inductor bodies serves to virtually complete the grounded casing of the inductors, and the relatively short external RF paths to the capsule and the tuning caps probably have less effect on the system than the external connections to the capsule - and indeed the capsule itself.
Those are of course largely contained within the grounded head basket.

In reality, once the assembly is complete, the inductor tuning will act on the whole RF structure -- PCB ground plane locations, cable routes, stray capacitance -- so that you are effectively optimising the whole assembly by carefully tuning those inductor cores.

As we have discussed at length in this thread, there are a number of different parameters that can have significant effects on how well individual microphones perform. I'm sure I don't understand the subtleties of many of them.
For example, one of the early perf board mics performed way better than others I had built in a similar way. I never discovered quite why....

But the later mics - using your PCB and the infinite impedance detector - outperformed all those early models.

The variations in (apparently) identical Chinese capsules can still spring a few surprises though..... :)
 

reega

Member
Joined
Oct 28, 2022
Messages
7
Location
ontario
V.5 of the schematic draws less than 3mA from the phantom power supply in total, so even a relatively 'weak' supply should function OK.

Using a DMM you should be able to measure voltages within the following values at these points, relative to gnd (pin1):
• Junction of R11 and C7 - c.35V DC
• R11 (which is a link) - c.20v DC
• Junction of R2 and Q1 collector - c.7.5v DC
• Across both R4 and R10 - between c.2v and c.4v DC, depending on the Vp cut off voltage of the specific FET used. Both values should be the same.

Using an oscilloscope with at least a 20MHz bandwidth, and a x10 probe with an impedance of c.1M and a capacitive loading of c.15pF -
(again reference to gnd) :
• Junction of R3 and T1 primary - between 4.5 and 5v AC p-p at 10MHz
• At each end of T1 secondary - between 3.5 and 4v AC p-p at 10MHz.
(those last 2 readings may be slightly different from each other, but typically less than 250mV difference )

• Measuring the signal at Q4 gate will not give a very accurate reading, because the 'scope lead will load T2 secondary, but will typically be a sine wave between 500mV and 2v AC p-p at 10MHz.

Can you confirm that you are getting readings close to those values? ...
Thanks again for you guide points on what to check! Finally got J113 from mouser (image attached of an authentic one .. without the fake Fairchild logo).

Also had to switch my crystal to 12 Mhz for the 55 pF condenser I've got. A little noisy right now but will hold judgement until unit is fully in case etc.
 

Attachments

  • IMG_3651.JPG
    IMG_3651.JPG
    1.3 MB · Views: 0

rogs

Well-known member
Joined
Jun 23, 2006
Messages
456
Location
Dorset, UK
Thanks again for you guide points on what to check! Finally got J113 from mouser (image attached of an authentic one .. without the fake Fairchild logo).

Also had to switch my crystal to 12 Mhz for the 55 pF condenser I've got. A little noisy right now but will hold judgement until unit is fully in case etc.
55pF is likely to be on the low side for optimum results....
I did try fitting a 12MHz crystal in some of my early experiments -- there's a mention of that in one or two early posts in this thread - but I found that the system 'Q' drops off quite lot using frequency that high with these particular inductors, and the noise level rises as a result.

The suggested range of 65pF to 90pF for capsule capacitance - run at 10MHz - seems to give best results.
If you're lucky enough to find a capsule with an even higher value - say above 100pF - then running the oscillator an 8MHz will give an even higher 'Q', with potentially even better results.
The hunt for a suitable alternative 'off the shelf' inductor that will work well with lower value capsule capacitance has - so far - been a rather elusive exercise... but we can live in hope!
 
Last edited:
Top