OPEN SOURCE DIY Mic Project - ORS 87 - Stripped Down u87

[Wordsushi]

100% agree that if your goal is simply to decrease sensitivity (increase headroom), then lowering the capsule polarization is the way to go. You'd have to lower it a lot to get the equivalent 14dB of reduction though (you'd have to go down to ~12V vs 60V to lose 14dB).

My point was that there might be other reasons to use the NFB cap - one would be to change the frequency response and add linearity to the circuit. This is different than using a capacitor in parallel with the capsule.

Oh, I get it now. That makes sense, and definitely something to consider and listen for. Thanks!

 

[Wordsushi]

Simplest i can give you is if you want extra headroom reduce the polarisation voltage. Adding capacitance one or the other way comes with penalties.

A-ha! That I can understand. I'm thinking that at some point, like how they now have aggregated A.I. written summaries of a product's reviews on Amazon, forums should give you that option to get a summary first, or like an abstract on a technical or medical paper.

 

[kingkorg]

100% agree that if your goal is simply to decrease sensitivity (increase headroom), then lowering the capsule polarization is the way to go. You'd have to lower it a lot to get the equivalent 14dB of reduction though (you'd have to go down to ~12V vs 60V to lose 14dB).

My point was that there might be other reasons to use the NFB cap - one would be to change the frequency response and add linearity to the circuit. This is different than using a capacitor in parallel with the capsule.

I am starting to get lost here. Are you guys talking about C3 in this schematic? If so, where do you put it in cardioid only version?

 

[MagnetoSound]

I am starting to get lost here. Are you guys talking about C3 in this schematic? If so, where do you put it in cardioid only version?

It basically goes (via C1 and C4, although it doesn’t have to) into the gate.

 

[kingkorg]

Ok, get it now. I personally don't like the idea of using this cap. I agree with what @OneRoomStudio said, but this thing throws off the whole de-emphasis network, which he did point to. I feel like it's extremely easy to end up chasing your tail. It would be extremely difficult to tell if the changes are due to FR, or THD, or level changes. Also if @Wordsushi decides to take this route, make sure you choose one hell of a stable cap for this role. I don't see the point to linearize the circuit, in Neumann's schematic the ''linearization'' of the circuit by introducing C3 is countered with increased value of C6 to 820p.

It just seems way more intuitive and simpler to tune the caps in the negative feedback for frequency response changes, and reduce the polarization voltage if THD is an issue.

 

[Wordsushi]

Ok, get it now. I personally don't like the idea of using this cap. I agree with what @OneRoomStudio said, but this thing throws off the whole de-emphasis network, which he did point to. I feel like it's extremely easy to end up chasing your tail. It would be extremely difficult to tell if the changes are due to FR, or THD, or level changes. Also if @Wordsushi decides to take this route, make sure you choose one hell of a stable cap for this role. I don't see the point to linearize the circuit, in Neumann's schematic the ''linearization'' of the circuit by introducing C3 is countered with increased value of C6 to 820p.

It just seems way more intuitive and simpler to tune the caps in the negative feedback for frequency response changes, and reduce the polarization voltage if THD is an issue.

Thank you for taking the time to consider it and the ramifications. I remember you saying a while back you had liked some of the mics you had polarized down into the mid 30v range? Ok, so let's say I want to lower the polarization voltage because it is a hot mic, what would you do with this circuit? What would you change and what polarization target are you looking for?

 

[kingkorg]

Thank you for taking the time to consider it and the ramifications. I remember you saying a while back you had liked some of the mics you had polarized down into the mid 30v range? Ok, so let's say I want to lower the polarization voltage because it is a hot mic, what would you do with this circuit? What would you change and what polarization target are you looking for?

You're welcome
You can add this resistor to ground. Using a 300K will halve the voltage and give you 6db more headroom. Use lower value resistor for even more attenuation, or higher for less attenuation. 600K will give you about 30v, 3db attenuation. There is no penalty except for noise when it comes to using lower pol. voltage, you can go as low as you like. I used as low as 15v on one of my k67 based mics.

 

[Wordsushi]

You're welcome
You can add this resistor to ground. Using a 300K will halve the voltage and give you 6db more headroom. Use lower value resistor for even more attenuation, or higher for less attenuation. 600K will give you about 30v, 3db attenuation. There is no penalty except for noise when it comes to using lower pol. voltage, you can go as low as you like. I used as low as 15v on one of my k67 based mics.

Nice. This is an adjustment worth tinkering with. It looks like a good way to address this mod. The KingKorg College of Knowledge is in session!

So perhaps, if this is something I want to integrate into the build as this project evolves, would it be prudent to factor in a 1M trimpot in that spot in order to dial in the desired amount of attenuation? Or perhaps a switchable path so it could be changed from wide open to attenuated when desired?

 

[OneRoomStudio]

Nice. This is an adjustment worth tinkering with. It looks like a good way to address this mod. The KingKorg College of Knowledge is in session!

So perhaps, if this is something I want to integrate into the build as this project evolves, would it be prudent to factor in a 1M trimpot in that spot in order to dial in the desired amount of attenuation? Or perhaps a switchable path so it could be changed from wide open to attenuated when desired?

You could easily add an on-off-on spdt switch with two resistor values - say 400K and 150K for ~5dB and ~10dB attenuation.

 

[kingkorg]

The KingKorg College of Knowledge is in session!

So perhaps, if this is something I want to integrate into the build as this project evolves, would it be prudent to factor in a 1M trimpot in that spot in order to dial in the desired amount of attenuation? Or perhaps a switchable path so it could be changed from wide open to attenuated when desired?

You can do both actually. Or what OneRoom suggested.

 

[Wordsushi]

View attachment 87 vs ORS87 level check.wav
Quick level check in booth between u87ai and this circuit (with 24v zener). I'm starting to feel less concerned about this issue given how I'd use it.

 

[Barry Hufker]

I agree. I don't think its output is much hotter, but it does seem more sibilant...

 

[OneRoomStudio]

View attachment 126333
Quick level check in booth between u87ai and this circuit (with 24v zener). I'm starting to feel less concerned about this issue given how I'd use it.

Level sounds good to me, but I agree with @Barry Hufker that it's a bit more sibilant. You could try to increase C6 a little, but it might just be the capsule having more presence in the 5KHz region (which C6 won't help you with). Sounds pretty darn good for the price though, I'd say!

 

[Wordsushi]

Thanks guys! For sure it needs to be EQ tweaked a bit more, and the capsule in this one is a generic cheapie for testing. I'm going to put a 3u k87 on it later and adjust based on that sound, but yeah, I'm ok with this level, so for now I'm going to backburner any further attenuation adjustments to keep the mic as quiet as possible, but I'll probably build one down the road with switchable attenuation. I really could not be more pleased with how this is evolving.

And just for notes, I used one of those $18 T-13 transformers from Aliexpress. I was told that Desheng makes them.

 

[micolas]

And just for notes, I used one of those $18 T-13 transformers from Aliexpress. I was told that Desheng makes them.

Hi, can you please post the link for the AliExp T13 transformer?

 

[Wordsushi]

Hi, can you please post the link for the AliExp T13 transformer?

https://www.aliexpress.us/item/3256803857398235.html?channel=twinner

My disclaimer: I haven't really used them enough or done enough comparison to give a proper review.

 

[micolas]

https://www.aliexpress.us/item/3256803857398235.html?channel=twinner

My disclaimer: I haven't really used them enough or done enough comparison to give a proper review.

Thank you very much!

 

[micolas]

Yes,
Apparently it's made here:

http://www.zgdes.com/product/902.html

 

[micolas]

A little physical comparison
I don't know about 3U gzt-87

 

[micolas]

My disclaimer: I haven't really used them enough or done enough comparison to give a proper review.

Ali T-13 seems to sound decent, especially for the price
We await comparisons with 3u, UTM, etc.