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Padding a TSSB2555B for drums (or other loud source)
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I've been tinkering with this for a bit and wanted to share. I padded my OPA Board with a 220pF cap across?the 1Gig resistor and measured. I am getting?about 15db padding and it works great on drums and other loud sources. I close miked a snare and Hi-Hat with them. Working on a front facing one to put in a kick drum see how that goes. I guess I could put in a switch?like Mic Sharf has but then I have to file a slot in the BM-800 body lol. I have enough that I am making four permanently?padded ones. Hey: Build your Own Mic Locker! Best Regards, Jules Ryckebusch 214 399 0931 |
¿ªÔÆÌåÓýregarding condenser pads. A compromise solution in a large condenser is to use a dip switch or other small switch internally.? Then you can defeat the pad by taking the cover off and switching - which is still quicker than soldering. I do pattern selection with internal dip switches like that. Some mics (such as the C414B) seem to implement a pad by reducing
the polarisation voltage instead or as well as adding a parallel
capacitor. I'm not sure of the relative merits of the two methods.
On 12/06/2022 15:14, Jules Ryckebusch
wrote:
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There is a debate?on padding with polarising voltage vs capacitance?addition. See page 96 of this:?? They show about?+/- 2-3db for a pretty wide range. We have one member who builds RF mics that don't?use a polarizing?voltage, just?changes in capacitance?to change the resonant frequency of an oscillator.? I hear you on the switches. I don't?plan on changing them once the pad is in place. They will be my "drum mics"?lol.? Jules On Sun, Jun 12, 2022 at 9:26 AM thet <parenthetical@...> wrote:
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Best Regards, Jules Ryckebusch 214 399 0931 |
Some mics (such as the C414B) seem to implement a pad by reducing?Most TSB-2555 circuits do not use any polarization voltage, because the capsule is pre-polarized. That is, in most TSB-2555 mics, there is no polarization voltage to reduce, so the only easy way to attenuate sensitivity is with a capacitor from JFET gate to ground. Condenser mics with externally polarized capsules can use voltage based pads. Reducing capsule DC polarization voltage from 60V to 30V will reduce the mic¡¯s output level by 6dB. Cutting in half again to 15V produces a total attenuation of 12dB.? I have seen some debate that the frequency response of the capsule changes with polarization voltage, but I have not measured any significant effect in my own testing, at least not between ~40V and ~65V. Very low polarization voltages such as 6-15V might well alter the sound of the capsule. Strapping a capacitor from JFET gate to ground, as has been discussed here, is definitely effective, but can add distortion (which might be good or bad depending on your application).? ¡ª matt. |
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hmm that info in that book raises more questions than it answers! Such as what determines the size of the frequency effect for a particular capsule, and what determines the frequency that is affected.
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I'm guessing some capsules are more affected than others with the frequency response change. On 12/06/2022 16:21, Jules Ryckebusch wrote:
There is a debate?on padding with polarising voltage vs capacitance?addition. See page 96 of this: They show about?+/- 2-3db for a pretty wide range. |
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What section are you referring to?
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Le 12/06/2022 ¨¤ 18:46, thet a ¨¦crit?:
hmm that info in that book raises more questions than it answers! Such as what determines the size of the frequency effect for a particular capsule, and what determines the frequency that is affected. |
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On June 12, 2022 at 9:46:30 AM, thet (parenthetical@...) wrote:
That graph is misleading. The voltages pictured are 28, 150, 250. Those values might be appropriate for a small diaphragm measurement capsule. Large-diaphragm capsules such as most people here would be using would collapse at any voltage higher than 70-80V. The actual range of polarization values you might actually use in practice ranges from about 20V to 70V, not 28-250.addition. See page 96 of this: > bksv.com/media/doc/be1447.pdfThere is a debate on padding with polarising voltage vs > capacitance Further, most large-diaphragm capsules vary in response by more than 2dB from one capsule to the next. As an example, Neumann¡¯s spec on the U87 Ai ($3200) is plus or minus 2dB (which is to say, a 4dB delta from one mic to the next). Therefore it doesn¡¯t make sense to focus on a potential 2dB shift in a single capsule¡¯s performance when the second mic you build would by 2dB if not 4-6dB different anyway. (Cheap capsules from Ali Baba definitely differ by more than 2dB from one to the next.) But more to the point, you are not likely to see even a 2dB delta when changing polarization voltage within the range of 30-60V, at least not in my testing. ¡ª matt. |
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So right Matt. They do say that graph is for a 1/2¡± microphone. And that it varies with diameter. But we are not doing 28V to 200V either.
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Best Regards, Jules Ryckebusch On Jun 12, 2022, at 12:45, recordinghacks <matt@...> wrote: |
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section 3.9.1 and especially figure 3.7
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On 12/06/2022 18:39, Jerry Lee Marcel wrote:
What section are you referring to? |
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Not so much misleading as just not very relevant to the sort of mics we usually build here, for all the reasons you say.
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What isn't clear is why the frequency affected is around 8k rather than some other frequency, and what characteristics of the capsule might lead the effect to be smaller or larger or the frequency to be different. I quite agree that it is irrelevant to a typical LDC though, and certainly AKG didn't think it a problem to drop the polarisation voltage on the 414B-ULS from 60v to 20v when the pad is engaged, even though the whole deal with the ULS mic is flat response. Which completely supports your experience that it isn't an issue with normal LDCs. On 12/06/2022 18:45, recordinghacks wrote:
That graph is misleading. The voltages pictured are 28, 150, 250. Those values might be appropriate for a small diaphragm measurement capsule. Large-diaphragm capsules such as most people here would be using would collapse at any voltage higher than 70-80V. The actual range of polarization values you might actually use in practice ranges from about 20V to 70V, not 28-250. |
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Le 12/06/2022 ¨¤ 23:09, thet a ¨¦crit?:
What isn't clear is why the frequency affected is around 8k rather than some other frequency, and what characteristics of the capsule might lead the effect to be smaller or larger or the frequency to be different.Indeed. Also I would have liked to have an explanation why teh capacitance changes non linearly. Of course, increasing bias voltage tends to bring the diaphragm closer to the backplate, which increases capacitance, but why not so at frequencies above 8k or so? Non linearity of capacitance vs frequency is hardly understandable since the dielectric is air, which is almost perfectly linear. |
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On Jun 15, 2022, at 09:14, Mic Scharf <micscharf@...> wrote:
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This is great. And verifies what I was seeing with 220pF I had about 15-16db padding. You are showing 20db with 270pF. Thanks for doing this! Danke! mit freundlichen Gr¨¹?en, Jules On Wed, Jun 15, 2022 at 10:11 AM Mic Scharf <micscharf@...> wrote:
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Best Regards, Jules Ryckebusch 214 399 0931 |
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Ok I'm not sure if this should be a new thread, as it is not about a TSB2555
It is relevant to padding in general and to drum mics though. Yesterday I received a new batch of PCBs including a version of my single opamp mic board. I assembled a mic using one of those 25mm (20mm membrane dia) chunky brass electret capsules, and ran the opamp at 13v (+/- 6.5v) The mic is really clean and crisp sounding. Pic is here: I tried it in a tom, and the output was waaay too high for the mic preamp. With the 20dB pad engaged on the mic pre I could mostly avoid clipping if I didn't play too loud. I also tried some other condenser mics. All the other mics I tried clipped internally if used on a tom without an internal pad - however the opamp mic did not clip internally - which is not surprising since the headroom in the circuit is much higher than in most mics. The issue was purely excessive output level. This suggests to me that capacitative padding at the input of an opamp mic may not be the only or best way to make it compatible with close micing drums. The next things I am going to try are: 1. connecting the mic to a line level input (I will have to rig some phantom power though, and that means more caps in the signal chain) 2. using an alternate version of the opamp pcb that I also received yesterday that allows a transformer output. This was intended for a 1:1 transformer, but if I were to use a stepdown transformer that would act as an output pad, and just possibly do something nice to the drum transients. |
Mic Scharf
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