Re: OPA Alice: can 1 gig resister be replaced with a 750M ?
It's going to work, but not as good. I have a friend who is an electronics expert who said that even 1G resistor is actually not too good and it is recommended to use even higher value resistor. It has something to do with overal noise etc. I'm not sure as I am no expert, just that I've been told.
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Re: AOMs-5024 in enclosures from China
nice inexpensive solution.
which aom 5024 version is yours? I've got a pair of 5024 with naked metallic front side with some pinhole perforation on it. I know there is also aom 5024 with felt or foam-something covered front side. I do not know how they compare regarding their sound and frequency response and I wonder.
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Re: True tree ears or DYI binaural dummy head.
Casey - My wonderful, funny wife thought of this even before I finished putting everything together.
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Re: True tree ears or DYI binaural dummy head.
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Re: True tree ears or DYI binaural dummy head.
Sounds great! But it's not done until it has a couple of googly
eyes (helps with HF dispersion). :-)
On 6/8/24 13:03, pmfalcman wrote:
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Sample recording from my garden.
Birds, a bit of LF hum (on and off) from neighbour's lawn mower
and me, at the end, walking around the mic.
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Re: True tree ears or DYI binaural dummy head.
I love it!? Best Regards,
Jules Ryckebusch?
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On Jun 8, 2024, at 15:03, pmfalcman via groups.io <pmfalcman@...> wrote:
?Sample recording from my garden.
Birds, a bit of LF hum (on and off) from neighbour's lawn mower and me, at the end, walking around the mic.
https://on.soundcloud.com/v3Wc9pAuJqq5Lvhx9
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Re: True tree ears or DYI binaural dummy head.
Sample recording from my garden.
Birds, a bit of LF hum (on and off) from neighbour's lawn mower and me, at the end, walking around the mic.
https://on.soundcloud.com/v3Wc9pAuJqq5Lvhx9
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True tree ears or DYI binaural dummy head.
Inspired by tree ears concept and having it tested in few setups I decided to make a permanent fixture.? I found myself a roughly head-sized log, bought silicon ears from Ali as well as a connector box with XLR sockets and a front plate. I set the ears in the log (router is handy for this and even hand-held gives nice, clean results), drilled the log through, filed a flat square at the back and drilled a ca. 20 mm hole in the base to set it up on a tripod. Electronics as simple as it gets: AOMs 5024 and Simple P48. Capsules are set in 3 cm long tubes recessed into the head, not mounted directly in the ears. It's heavy, though not as heavy as KU 100 (3,5 kg!). It looks funny. I like funny. But it works quite nicely. I think. I embraced unevenness of the log - human heads are not even, regular, symmetric nor perfectly shaped either.   
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Re: Small Voltage Across FET-less Electret Capsule?
Le 08/06/2024 ¨¤ 12:56, sergio_logic via
groups.io a ¨¦crit?:
The capsule needs no current to operate as it does not have a FET.
That was not clear in your post.
The question is,
does it have to have a DC voltage of 0 across the capacitor, or
will having 1 volt affect the bias enough to impact the response?
The equivalent voltage of an electret mic is close to 100V, so 1V is
negligible. t should be less than a fraction of dB.
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Re: Small Voltage Across FET-less Electret Capsule?
The purpose of that 1Meg pot is to bias the FET properly. I do this in my "Pimped Alice" circuit. The addition (or subtraction depending on polarity of the capsule pre bias) is negligible?for operation. Typically the "equivalent" voltage that biases the capsule in an electret is about 200V or so. You are good here.?
Jules
I am a newbie in terms of mic building and modding, I have studied electronics in university, but have done only digital design for about 15 years now. I am trying to design (for fun) a different preamp to fit into a t.bone EM700. I like the response of the capsule, but would like a bit more level out of the mic (about 10 dB more than the original preamp offers). I have considered just plugging a FetHed Phantom in between, but based on some images I found and based on the schematic of the regular (which I have confirmed by tracing one I have), I believe that it will forward the equivalent of ~15V phantom to the mic, which is not what it is designed for. So here I am, dusting off some old skills and learning new ones. The capsule is FET-less, the original preamp has a FET and a 1G resistor in the input stage. I like the Schoeps input stage, pictured below, including the ability to adjust Vgs (I have done some simulations with different FET models, and having a pot there is very useful). The effect of the adjustment is that the DC value of the gate voltage is now set by the split of the 1M pot, which may set Vg at let's say around 1 volt. This effectively attempts to polarize a pre-polarized electret capsule by 1 volt or so. Will this impact the response of the capsule? Of course I could mitigate this by adding a series capacitor (probably a 1 nF polypropylene cap between the capsule and the gate/ 1G resistor), but do I really have to? I am grateful for your replies. While I know my way around transistors and electronics in general, how a pre-polarized (electret) capsule will behave under these conditions is a mystery to me. 
-- Best Regards,
Jules Ryckebusch
214 399 0931
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Re: Small Voltage Across FET-less Electret Capsule?
There was some discussion a while back about deliberately adding external polarization to an electret to try to influence its SPL handling.? So probably no big effect with just one volt.
-Scott
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On 6/8/24 05:56, sergio_logic via groups.io wrote: The capsule needs no current to operate as it does not have a FET. In fact, if I connect it to an oscilloscope with 10x probes (10MegOhm input impedance) and speak loudly into it, I see almost 200 mV pk-pk. Electrically, it¡¯s just a (probably 20-30 pF) capacitor (permanently polarized by the electret material).
The question is, does it have to have a DC voltage of 0 across the capacitor, or will having 1 volt affect the bias enough to impact the response?
--
---- Scott Helmke ---- scott@... ---- (734) 604-9340 ----
"I have ceased distinguishing between the religious and the secular,
for everything is holy" - Joe Henry
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Re: Small Voltage Across FET-less Electret Capsule?
The capsule needs no current to operate as it does not have a FET. In fact, if I connect it to an oscilloscope with 10x probes (10MegOhm input impedance) and speak loudly into it, I see almost 200 mV pk-pk. Electrically, it¡¯s just a (probably 20-30 pF) capacitor (permanently polarized by the electret material).
The question is, does it have to have a DC voltage of 0 across the capacitor, or will having 1 volt affect the bias enough to impact the response?
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Re: Small Voltage Across FET-less Electret Capsule?
"This effectively attempts to polarize a pre-polarized electret
capsule by 1 volt or so."
No. The available current is in the order of nA, so the capsule
will not work.
You need to power the capsule as it is intended AND add a
coupling capacitor. Failure to do so would result in completely
disturbing the headamp's FET operating point.
Le 08/06/2024 ¨¤ 12:07, sergio_logic via
groups.io a ¨¦crit?:
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I am a newbie in terms of mic building and modding, I have studied
electronics in university, but have done only digital design for
about 15 years now.
I am trying to design (for fun) a different preamp to fit into a
t.bone EM700. I like the response of the capsule, but would like a
bit more level out of the mic (about 10 dB more than the original
preamp offers).
I have considered just plugging a FetHed Phantom in between, but
based on some images I found and
based on the schematic of the regular (which I have
confirmed by tracing one I have), I believe that it will forward
the equivalent of ~15V phantom to the mic, which is not what it is
designed for. So here I am, dusting off some old skills and
learning new ones.
The capsule is FET-less, the original preamp has a FET and a 1G
resistor in the input stage. I like the Schoeps input stage,
pictured below, including the ability to adjust Vgs (I have done
some simulations with different FET models, and having a pot there
is very useful).
The effect of the adjustment is that the DC value of the gate
voltage is now set by the split of the 1M pot, which may set Vg at
let's say around 1 volt.
This effectively attempts to polarize a pre-polarized electret
capsule by 1 volt or so. Will this impact the response of the
capsule?
Of course I could mitigate this by adding a series capacitor
(probably a 1 nF polypropylene cap between the capsule and the
gate/ 1G resistor), but do I really have to?
I am grateful for your replies. While I know my way around
transistors and electronics in general, how a pre-polarized
(electret) capsule will behave under these conditions is a mystery
to me.
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Small Voltage Across FET-less Electret Capsule?
I am a newbie in terms of mic building and modding, I have studied electronics in university, but have done only digital design for about 15 years now. I am trying to design (for fun) a different preamp to fit into a t.bone EM700. I like the response of the capsule, but would like a bit more level out of the mic (about 10 dB more than the original preamp offers). I have considered just plugging a FetHed Phantom in between, but based on some images I found and based on the schematic of the regular (which I have confirmed by tracing one I have), I believe that it will forward the equivalent of ~15V phantom to the mic, which is not what it is designed for. So here I am, dusting off some old skills and learning new ones. The capsule is FET-less, the original preamp has a FET and a 1G resistor in the input stage. I like the Schoeps input stage, pictured below, including the ability to adjust Vgs (I have done some simulations with different FET models, and having a pot there is very useful). The effect of the adjustment is that the DC value of the gate voltage is now set by the split of the 1M pot, which may set Vg at let's say around 1 volt. This effectively attempts to polarize a pre-polarized electret capsule by 1 volt or so. Will this impact the response of the capsule? Of course I could mitigate this by adding a series capacitor (probably a 1 nF polypropylene cap between the capsule and the gate/ 1G resistor), but do I really have to? I am grateful for your replies. While I know my way around transistors and electronics in general, how a pre-polarized (electret) capsule will behave under these conditions is a mystery to me. 
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Re: OPA Alice: can 1 gig resister be replaced with a 750M ?
Yup, your understanding was right.? But of course, you could still first try 750 MOhm and only if you're not happy with the LF noise, then try to get 1 or 2 GOhm resistors. The human ear is not very sensitive to low frequencies, so maybe you won't even notice.
While I agree there are quality concerns when byuing stuff from Aliexpress, especially concerning audio ICs, I found some good quality GOhm resistors there that I'm happy with.
1GOhm:
2GOhm
But these are not at all cheap, though...
The cheap and popular TE Connectivity part is the RGP0207CHK1G0.
Jan
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Re: OPA Alice: can 1 gig resister be replaced with a 750M ?
@jp8
If I understand you correctly, the sum total of noise in the audible band is reduced with a larger resistor value, so I should try to get 1G if possible.
Unfortunately there's some availability issue where i am. So getting 1G is a bit of an extra amount of work. For aliexpress, I am concerned about quality. I am not very experienced so I want error potential to be low. so i think im gonna go with the 750M, and hope the extra noise wont be noticed by me.
thx again
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Re: OPA Alice: can 1 gig resister be replaced with a 750M ?
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Re: OPA Alice: can 1 gig resister be replaced with a 750M ?
Yes, you can, like Jules already said. But not only will the LF cutoff point move to a higher frequency. LF noise will also increase, because with every doubling of the resistance, the resistor's noise that will be shorted to ground through the capsule capacitance will double as well. The resistor's Johnson noise increases by the squareroot of the resistance, so the net effect is still a noise reduction.
1 G or higher resistors are abundantly available from Mouser an Digikey. A popular one is an axial type from TE Connectivity. Or search on Aliexpress. The smaller types on Aliexpress are mostly radial types. The axial types mostly large high voltage versions.
Jan
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Re: OPA Alice: can 1 gig resister be replaced with a 750M ?
Yes a 750M will work. Anything above 200-300M will work. ?Actually 100M will work ?you start to get a 3dB down point in the audio range below 200M ? Best Regards,
Jules Ryckebusch?
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On Jun 5, 2024, at 14:22, denysrud via groups.io <denysrud@...> wrote:
?I cant get a 1 gig ohm resistor but i can get a 750M. I can also series 2x 500M but that will complicate things and be difficult.
>https://www.jlielectronics.com/content/OPABoardsREV_1.pdf
Here's a schematic for reference
thank you
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OPA Alice: can 1 gig resister be replaced with a 750M ?
I cant get a 1 gig ohm resistor but i can get a 750M. I can also series 2x 500M but that will complicate things and be difficult.
>https://www.jlielectronics.com/content/OPABoardsREV_1.pdf
Here's a schematic for reference
thank you
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