t. bone SC1100 aka Feilo Z3300 modding, anyone?

[theaterguy]

Hi, recently purchased a second-hand t. bone mic and for the money it's damned good. Now I'm comparing to NT1000 and 414EB.
However I noticed it needs several minutes to warm-up after applying phantom voltage. Otherwise it cuts itself off for few seconds if spoken into very close or loud.
Bad FET or just a feature?
Has anyone modded those, apart from changing the capsule?

 

[Violinist]

Hi, recently purchased a second-hand t. bone mic and for the money it's damned good. Now I'm comparing to NT1000 and 414EB.
However I noticed it needs several minutes to warm-up after applying phantom voltage. Otherwise it cuts itself off for few seconds if spoken into very close or loud.
Bad FET or just a feature?
Has anyone modded those, apart from changing the capsule?

I have the same effect with my sE2200a. When i give the Phantom, it employs 10-15 seconds to be ready.
i treid to understand the reasons, but I didn't found them

 

[jeremynothman]

Hi, recently purchased a second-hand t. bone mic and for the money it's damned good. Now I'm comparing to NT1000 and 414EB.
However I noticed it needs several minutes to warm-up after applying phantom voltage. Otherwise it cuts itself off for few seconds if spoken into very close or loud.
Bad FET or just a feature?
Has anyone modded those, apart from changing the capsule?

I just bought one of these secondhand, marked as not working. It seems to almost work, but has a similar problem with loud signals. It doesn't seem to be condensation. I managed to record it making a strange sound that it made after one of its drop-outs. Also, it makes a thumping sound when changing pattern.

It sounds quite OK otherwise, but seems to lack any seriously big low end, like i'd like to hear it - I'm guessing I'll want to put in a K47, which was my original plan in any case.

@micolas and @Emmathom , I know you just had a mammoth thread, but it got very detailed. In the end, were there any recommendations you would have to this mic, asides from swapping out the capsule for a K47 type? eg. Biasing the FET? Has anyone added a pad to this? (I'd gladly replace the high-pass with a pad to stick in front of a kick drum with confidence).

Anyway, one step at a time... I'd like to fix this!
Here's a link to the noise: https://www.dropbox.com/scl/fi/trxo...e-44.wav?rlkey=rglj8nj3yflzmb85bymsm9ua8&dl=0

This was directly after switching off the high-pass, which caused the audio to cut out, and then what came after was distorted ("That was with the high-pass in"), followed by this oscillating tone. I've gained up the tone around 10dB to make it more audible.
What could it be?

 

[micolas]

I just bought one of these secondhand, marked as not working. It seems to almost work, but has a similar problem with loud signals. It doesn't seem to be condensation. I managed to record it making a strange sound that it made after one of its drop-outs. Also, it makes a thumping sound when changing pattern.

It sounds quite OK otherwise, but seems to lack any seriously big low end, like i'd like to hear it - I'm guessing I'll want to put in a K47, which was my original plan in any case.

@micolas and @Emmathom , I know you just had a mammoth thread, but it got very detailed. In the end, were there any recommendations you would have to this mic, asides from swapping out the capsule for a K47 type? eg. Biasing the FET? Has anyone added a pad to this? (I'd gladly replace the high-pass with a pad to stick in front of a kick drum with confidence).

Anyway, one step at a time... I'd like to fix this!
Here's a link to the noise: https://www.dropbox.com/scl/fi/trxo...e-44.wav?rlkey=rglj8nj3yflzmb85bymsm9ua8&dl=0

This was directly after switching off the high-pass, which caused the audio to cut out, and then what came after was distorted ("That was with the high-pass in"), followed by this oscillating tone. I've gained up the tone around 10dB to make it more audible.
What could it be?

Hi jeremynothman !
It was indeed a very long thread where I helped @Emmathom identify the problems with a faulty SC-1100 and repair/modify it. Because the result was beyond expectations, he decided to make a pair and modified a second one in perfect working order.
In comparative tests with other microphones, including STAM SA-47fet, Oktava 012, WarmAudio WA-84, SE X1S, etc., the SC-1100mod microphone was preferred. In parallel with the very long, public group thread, there was also a very long and intense private conversation with many tests.
The defect was identified as a short circuit between the turns of the transformer secondary. This stock has around a 2:1 ratio.
It was replaced with a 1.5:1 ratio transformer, is what he managed to achieve.
In addition, the 'castration' resistor
has been shorted - R10(3k) , R9 was replaced with a trimpot and the jFET was reBIASed.
*
The stock microphone provides a very high output level.
The changes made it even more sensitive, (probably by about 10dB more). It became the loudest I've heard.
*
The Lo-Cut switch in your case is necessary, because if you use it for kick-drum you must have a frequency response to the bass in the style of U47fet (which is notorious for kick-drum use)
Compare the two FRs. (Blue refers to the proximity effect)
You've probably tested the microphone on voice that don't have the massive bass of a kick-drum, (and SPL ! ), plus the capsule is probably compromised.
*
As I said the microphone is loud, so you need to attenuate at least 20dB
IIRC @kingkorg in the case of a snare measured an incredible level of 20-22V peak-to-peak!
The good news is that you can reduce its sensitivity.
There would be several ways to do this
It depends on your abilities, skills, preferences.

 

[Emmathom]

@micolas
Yes it would be great to turn the low-cut switch into a pad switch in the SC1100 !
But wich mod. to achieve that ? and what kind of pad would you recommend ? lower drain voltage by a R ? introduce a loopback ? something else ?
BR

 

[jeremynothman]

Hi jeremynothman !
It was indeed a very long thread where I helped @Emmathom identify the problems with a faulty SC-1100 and repair/modify it. Because the result was beyond expectations, he decided to make a pair and modified a second one in perfect working order.
In comparative tests with other microphones, including STAM SA-47fet, Oktava 012, WarmAudio WA-84, SE X1S, etc., the SC-1100mod microphone was preferred. In parallel with the very long, public group thread, there was also a very long and intense private conversation with many tests.
The defect was identified as a short circuit between the turns of the transformer secondary. This stock has around a 2:1 ratio.
It was replaced with a 1.5:1 ratio transformer, is what he managed to achieve.
In addition, the 'castration' resistor
has been shorted - R10(3k) , R9 was replaced with a trimpot and the jFET was reBIASed.
*
The stock microphone provides a very high output level.
The changes made it even more sensitive, (probably by about 10dB more). It became the loudest I've heard.
*
The Lo-Cut switch in your case is necessary, because if you use it for kick-drum you must have a frequency response to the bass in the style of U47fet (which is notorious for kick-drum use)
Compare the two FRs. (Blue refers to the proximity effect)
You've probably tested the microphone on voice that don't have the massive bass of a kick-drum, (and SPL ! ), plus the capsule is probably compromised.
*
As I said the microphone is loud, so you need to attenuate at least 20dB
IIRC @kingkorg in the case of a snare measured an incredible level of 20-22V peak-to-peak!
The good news is that you can reduce its sensitivity.
There would be several ways to do this
It depends on your abilities, skills, preferences.

@micolas, could you explain me the castration resistor?
When you suggest the capsule is compromised, do you say that because of the sound cutting in and out (especially with higher SPL)? Did you listen to the slow kind of oscillation noise?

@Emmathom , am I correct in understanding that the two of you designed a new circuit for this entirely? I did see that one of the mics was now on perf-board. Are both now?

 

[Emmathom]

USER=140266]@Emmathom[/USER] , am I correct in understanding that the two of you designed a new circuit for this entirely? I did see that one of the mics was now on perf-board. Are both now?

Nop. One board (pre) was damaged so I made a new one on a perf. board. I kept the DC/DC board which was ok.
The other mic still has its original ones (pre & DC/DC).
The 2 Tx out were changed for Monacor LTR110
We didn't entirely designed this circuit at all : we just remove one resistor (R4 / 3KΩ) and try to bias the jfet correctly and that was all.

@micolas : what about changing the low-cut switch for a -20dB pad ? how to do that... ?

 

[micolas]

@micolas : what about changing the low-cut switch for a -20dB pad ? how to do that... ?

Now I have no access to the SC-1100.
Is the Lo-Cut switch placed and fixed with several solders on the DC/DC board?

 

[Emmathom]

Now I have no access to the SC-1100.
Is the Lo-Cut switch placed and fixed with several solders on the DC/DC board?

The switch is soldered on the DC/DC board but no tracks are connected : one wire is coming from the drain and one goes to the base of the SA1015 thru 47nF (as showned below)
I could leave the 47nF always connected and use the switch for a pad : but how to proceed ?

 

[micolas]

The switch is soldered on the DC/DC board but no tracks are connected : one wire is coming from the drain and one goes to the base of the SA1015 thru 47nF (as showned below)
I could leave the 47nF always connected and use the switch for a pad : but how to proceed ?

@Emmathom ,
OK. C6=47nF can be permanently connected (it can even be increased a little, to 56...68nF, according to taste, for deeper bass). Thus the switch remains available for sensitivity attenuation.
*
To be able to decide which approach has more benefits we need the complete scheme: DC/DC and the circuit for changing the polar patterns ( +/- 60v or 0-60-120v approximately)
I'm afraid you'll have to draw the schematic yourself this time, but I'm confident you'll be able to do it.
*
We have several options:
Or we will reduce the polarization voltage of the capsule (by replacing the Zenner with another one of lower voltage, or by resistive voltage dividers),
either we will connect a 560...820pF capacitor between the diaphragms/backplate, or we will connect a low-value 10...15pF capacitor between the drain and the gate of the jFET (in this case, a de-emphasis of high frequencies may result)
We do not change the BIAS of the jFET.
We can attenuate between the jFET and the BJT but the headroom will remain small, limited by the distortion of the jFET.

 

[Emmathom]

OK. C6=47nF can be permanently connected (it can even be increased a little, to 56...68nF, according to taste, for deeper bass). Thus the switch remains available for sensitivity attenuation.

Yes 56nF ~ 68nF

To be able to decide which approach has more benefits we need the complete scheme: DC/DC and the circuit for changing the polar patterns ( +/- 60v or 0-60-120v approximately)
I'm afraid you'll have to draw the schematic yourself this time, but I'm confident you'll be able to do it.

We're not gonna change polar voltages

We have several options:
Or we will reduce the polarization voltage of the capsule (by replacing the Zenner with another one of lower voltage, or by resistive voltage dividers),

I fear it would rise noise...

either we will connect a 560...820pF capacitor between the diaphragms/backplate,

not convinced...

or we will connect a low-value 10...15pF capacitor between the drain and the gate of the jFET (in this case, a de-emphasis of high frequencies may result)

More conviced by a small cap between drain and gate but how to calculate its value ?
more pF = less level ?
and more pF = less highs ?

We do not change the BIAS of the jFET.

No

We can attenuate between the jFET and the BJT but the headroom will remain small, limited by the distortion of the jFET.

a local feedback as you proposed (drain jfet > pF > gate) seems the best solution
Thank you for your answer @micolas

 

[Emmathom]

The switch is soldered on the DC/DC board but no tracks are connected : one wire is coming from the drain and one goes to the base of the SA1015 thru 47nF (as showned below)

Wrong , sorry ! The switch is mounted on the pre board but acts "alone" > no tracks are connected to it.
In my mod version I cut the board just to keep the switch (it was the simplest way)

This doesn't change anything from my comment above...

 

[Emmathom]

Ok this is what I propose to install : I've got 22pF CBB or 27pF polystyren in stock > will try with these to begin with...
This cap can be soldered directly on the switch.

For normal bandwith the simplest solution is to leave the 10nF + 47nF of the (old) low cut installed but together active...

 

[micolas]

Ok this is what I propose to install : I've got 22pF CBB or 27pF polystyren in stock > will try with these to begin with...
This cap can be soldered directly on the switch.

For normal bandwith the simplest solution is to leave the 10nF + 47nF of the (old) low cut installed but together active...

Philippe be careful not to explode!
Joking aside,
I'm really curious what the result will be.
If it's not ok, go to the U87 approach with a capacitor connected between the diaphragm and the backplate.
*
Worrying about increased noise is not justified in the real world because you are using attenuation on strong, close sources, so in the end the signal to noise ratio is still good.
*
I still insist that you draw the complete diagram.

 

[Emmathom]

If it's not ok, go to the U87 approach with a capacitor connected between the diaphragm and the backplate.

what's supposed the best ? 27pF btw drain & gate OR 1nF (for -20dB ?) btw baclpkate & diaphragme ?
for the second option remember we have 2 diaphragms for 3 patterns : so how to deal with ?

Worrying about increased noise is not justified in the real world because you are using attenuation on strong, close sources, so in the end the signal to noise ratio is still good.

ok if you say so...

I still insist that you draw the complete diagram.

I must open the mic for that and follow the tracks of the DC/DC board > not for now because I have a lot to do :
- build the second 6AK5 with mods (SC4 caps. / 1uF tin foil / Tamura Tx) and compare to the very first one
- finish & test the Jfets pre 2 channels
- do some mod on the mic tube pre PSU's (Quad & Mixed)
all these tasks will lead me to next week I guess...

and then I'll go back on SC1100 with :
- the pad (only on one of the two)
- Arienne K47 flat (again only on one of the two)
- compare the 2 SC1100 (with acoustic guitar & my voice)

 

[micolas]

1nF (for -20dB ?) btw baclpkate & diaphragme ?

remember we have 2 diaphragms for 3 patterns : so how to deal with ?

Why don't you look at the diagram above?
The front diaphragm is permanently connected. Doesn't it make sense to put the attenuation capacitor here?

 

[Emmathom]

Why don't you look at the diagram above?
The front diaphragm is permanently connected. Doesn't it make sense to put the attenuation capacitor here?

ok but what happens to the rear in omni position for ex ? if front is -20dB what about rear ?

 

[micolas]

ok but what happens to the rear in omni position for ex ? if front is -20dB what about rear ?

That's why I said you have to draw the complete schematic.
It may be similar to the one below.
*
(In omni, rear diaphragm is also connected at the input, in addition, it only becomes active by feeding it with the polarization voltage corresponding to the pattern)

EDIT:
A propos, the 1G resistor from the jFET gate to GND is missing.

 

[micolas]

That's why I said you have to draw the complete schematic.
It may be similar to the one below.
*
(In omni, rear diaphragm is also connected at the input, in addition, it only becomes active by feeding it with the polarization voltage corresponding to the pattern)

The SC-1100 may share some parts of the schematic as the above AA CM87/47
(or not)

 

[Khron]

That's why I said you have to draw the complete schematic.
It may be similar to the one below.

Might this be a tiny bit clearer?

http://recordinghacks.com/images/mic_extras/cad/gxl3000-schematic.png