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

[micolas]

@Wordsushi ,
allow me a purely personal observation:
For VO, commercials, jingles, soundtracks for documentary films, etc. , I like your results when you use a microphone from the U87 family, flatter or more modern.
(although as I said before you can sound good on a $1 microphone! )

 

[micolas]

What some of you might be realizing now is how similar the KM84 and U87 circuits are (aside from the deemphasis). They're both just a JFET 'common source' amplifier (with high input impedance) feeding a step-down transformer (for low output impedance), in addition to bias voltage for the capsule. Capsule > impedance conversion > output. Powered by 48V phantom power.

That's it. No magic here.

The signal-path capacitors, as well as any negative feedback/deemphasis allow you to tune the high and low frequency response for the capsule you're using. You can mess with the headroom and gain by changing the FET voltage and bias. You can improve the noise level by choosing a low-noise FET and/or ensuring your impedance areas are floating. Other than that, your capsule will make the biggest difference in sound, followed by the transformer (might saturate or have limited bandwidth if it's too small). These are simple circuits.

Yes.
In an older post I described how I made a cardioid-only u87i with a 2:1 ratio transformer in a $20 Fame donor, removing the BJT and changing a few components.
It has the same output volume as the U87ai!

 

[OneRoomStudio]

Yes.
In an older post I described how I made a cardioid-only u87i with a 2:1 ratio transformer in a $20 Fame donor, removing the BJT and changing a few components.
It has the same output volume as the U87ai!

With a standard u87i circuit, a 2:1 output transformer would give you around a 4k output impedance, which would significantly load down the FET if you were feeding it into most preamps. You might get more level, but you would be seriously compromising the ability of the FET to drive the transformer/preamp and probably have much worse headroom, more distortion, and less bandwidth.

There's a reason that Neumann used a 9.5:1 output transformer. The output impedance of the 2N3819 FET with a 47k drain resistor is around 16k. put that through a 9.5:1 output transformer and you get around 175ohms. A lot of mic preamps are designed to see between 150 and 200ohms from the mic, so that works well. If you want to use a 2:1 transformer, you'd need a circuit with a much lower output impedance, such as a source follower (or an emitter follower like the C37, or a common source FET followed by an emitter follower like a lot of Chinese mics).

 

[Migs 31]

(... I said before you can sound good on a $1 microphone! )

I am yet to see and hear that $1 microphone.
Where is it, who sells it ?
M

 

[micolas]

With a standard u87i circuit, a 2:1 output transformer would give you around a 4k output impedance, which would significantly load down the FET if you were feeding it into most preamps. You might get more level, but you would be seriously compromising the ability of the FET to drive the transformer/preamp and probably have much worse headroom, more distortion, and less bandwidth.

There's a reason that Neumann used a 9.5:1 output transformer. The output impedance of the 2N3819 FET with a 47k drain resistor is around 16k. put that through a 9.5:1 output transformer and you get around 175ohms. A lot of mic preamps are designed to see between 150 and 200ohms from the mic, so that works well. If you want to use a 2:1 transformer, you'd need a circuit with a much lower output impedance, such as a source follower (or an emitter follower like the C37, or a common source FET followed by an emitter follower like a lot of Chinese mics).

I played, simply I reduced the drain resistor (instead of 47k I put 8.2k, the supply to Rd about 20.5v).
I re-Biased the jFET to Ud 10.4v and increased the output capacitor to the transformer.
I think the preamp has 3k input impedance.
The sound was good, a little less bass, but it was suitable for my voice, more 70's.
Can you do the math again now?
Jfet stock: 2SK30a

 

[micolas]

I am yet to see and hear that $1 microphone.
Where is it, who sells it ?
M

This is.

 

[Migs 31]

The name says it all: « Serioux »
I want to hear, to beleive.
M

 

[micolas]

The name says it all: « Serioux »
I want to hear, to beleive.
M

Yes, sir!

 

[homero.leal]

Just FYI, the PCB is not rendered correctly there. Looks like some holes are drilled correctly, but all other layers are scaled down.

Just FIY, PCB is now rendered correctly. Regards!

 

[OneRoomStudio]

I played, simply I reduced the drain resistor (instead of 47k I put 8.2k, the supply to Rd about 20.5v).
I re-Biased the jFET to Ud 10.4v and increased the output capacitor to the transformer.
I think the preamp has 3k input impedance.
The sound was good, a little less bass, but it was suitable for my voice, more 70's.
Can you do the math again now?
Jfet stock: 2SK30a

A 2SK30a has an internal output resistance of around 835k, and with Rd=8k2, the output impedance would be around 8k1. 8k1 through a 2:1 output transformer still has you at about 2k output impedance, which is too high (even if the input impedance of your preamp is 3k). You may like the sound, and that's fine, but it's definitely less than optimal.

To quote Dan Kennedy from the Great River MP-2NV manual:

"Most microphones will work best with the [higher input impedance] setting. Best isn’t always what you want though. With most microphones, the result of using the [lower input impedance] setting will be a roll off of the lowest frequencies and a tilt upwards of the highs. This effect is highly dependent on the particular microphone, and the only way to know for sure is to try it.
In general, the best mics to try changing the impedance selection effect on are dynamics and ribbons. Condenser microphones have built in amplifiers, and the usual effect of running them into a lower impedance than they like to see is just a large increase in distortion and lower headroom, with minimal tonal changes."

 

[Wordsushi]

@Wordsushi ,
allow me a purely personal observation:
For VO, commercials, jingles, soundtracks for documentary films, etc. , I like your results when you use a microphone from the U87 family, flatter or more modern.
(although as I said before you can sound good on a $1 microphone! )

Thank you for the observation. I could not agree more, which is why I'm only interested in 87 style microphones now.

 

[micolas]

A 2SK30a has an internal output resistance of around 835k, and with Rd=8k2, the output impedance would be around 8k1. 8k1 through a 2:1 output transformer still has you at about 2k output impedance, which is too high (even if the input impedance of your preamp is 3k). You may like the sound, and that's fine, but it's definitely less than optimal.

To quote Dan Kennedy from the Great River MP-2NV manual:

"Most microphones will work best with the [higher input impedance] setting. Best isn’t always what you want though. With most microphones, the result of using the [lower input impedance] setting will be a roll off of the lowest frequencies and a tilt upwards of the highs. This effect is highly dependent on the particular microphone, and the only way to know for sure is to try it.
In general, the best mics to try changing the impedance selection effect on are dynamics and ribbons. Condenser microphones have built in amplifiers, and the usual effect of running them into a lower impedance than they like to see is just a large increase in distortion and lower headroom, with minimal tonal changes."

You are perfectly right.
The electronic theory is clear.
It's debated on the forums all the time.

But one of my great pleasures is to break the limits and try the extremes.
Correct, clean sound bores me.
I am a solo guitarist and for a lifetime I have been looking for distorted, colored tones, with anomalies, resonances, harmonics.
I like the voice as well, I like Rod Stewart, Brian Adams, Chris Norman, Richard Marx, etc. I am the one who passes the voice through the distortion pedal, fuzz, wah, flanger, etc.
One of my favorite solo sounds is achieved with a FuzzFace where the second transistor is biased incorrectly and the guitar sounds almost like synth-brass, with rich harmonics, with a prominent octave.
The clip below reflects a bit of what is harder to put into words purely theoretically.

 

[Wordsushi]

When it comes to PCB design for this circuit, what's the consensus on stacking insert points in series on both C5 and C6 for the option of adding a second cap in order to make it easier to add additional capacitance for tuning?

 

[OneRoomStudio]

When it comes to PCB design for this circuit, what's the consensus on stacking insert points in series on both C5 and C6 for the option of adding a second cap in order to make it easier to add additional capacitance for tuning?

I would probably just use sockets for C5 and C6, then it would be easy to try different values.

 

[Papa Tango Charly]

I just don't know if he makes his in-house or he's getting them OEM'd for him by someone else.

I

I just don't know if he makes his in-house or he's getting them OEM'd for him by someone else.

he makes his owne. They are excellent quality indeed! I have build some great mics no comparaison with Ali express. I haven’t seen any of his transformer anywhere else! Or with other brand names. You should try some of his microphones. Great sound very interesting switch available to correct some tonality. Cheers

 

[Wordsushi]

I

he makes his owne. They are excellent quality indeed! I have build some great mics no comparaison with Ali express. I haven’t seen any of his transformer anywhere else! Or with other brand names. You should try some of his microphones. Great sound very interesting switch available to correct some tonality. Cheers

Thank you! That's good to know. I've bought a few of Guosheng's mics and a bunch of his capsules and transformers over the years. The GZT-87 is one of my faves and I've come to the conclusion that it has a touch bit more color than the Desheng generic T-13 from Aliexpress so they are indeed different. The T-13 feels very clean like the Cinemag CM-13113, but with a little less heft, but the GZT is more balanced overall. Looking forward to expanding my search to other transformers during this microphone expedition.

 

[Wordsushi]

I would probably just use sockets for C5 and C6, then it would be easy to try different values.

That's a great idea! It should possibly be noted that a future consideration for PCB's then is making sure the holes for C5 and C6 can accommodate sockets to make swapping easy.

 

[micolas]

I've bought a few of Guosheng's mics and a bunch of his capsules and transformers over the years. The GZT-87 is one of my faves and I've come to the conclusion that it has a touch bit more color than the Desheng generic T-13 from Aliexpress so they are indeed different. The T-13 feels very clean like the Cinemag CM-13113, but with a little less heft, but the GZT is more balanced overall. Looking forward to expanding my search to other transformers during this microphone expedition.

Hi!
*So far, of all the ones you have tried, which transformer for clones: U87i (Dany B) and for U87ai (Michael Fuchs) would you choose, according to your taste, to compliment your voice, to induce your pleasure to use, to excite just the way you like it, to attract attention, to convey the desired vibe?
*Which one do you think is closer to the Neumann U87ai you got from GuitarCenter?
*I have been insisting on these questions for some time and I keep bothering you because I set your voice in my audio memory as a VO reference. When I recognize her in a commercial it will be cool!

 

[Drosselmeier]

Thank you! That's good to know. I've bought a few of Guosheng's mics and a bunch of his capsules and transformers over the years. The GZT-87 is one of my faves and I've come to the conclusion that it has a touch bit more color than the Desheng generic T-13 from Aliexpress so they are indeed different. The T-13 feels very clean like the Cinemag CM-13113, but with a little less heft, but the GZT is more balanced overall. Looking forward to expanding my search to other transformers during this microphone expedition.

t13 transformer from ali is not the best choice

 

[Wordsushi]

Hi!
*So far, of all the ones you have tried, which transformer for clones: U87i (Dany B) and for U87ai (Michael Fuchs) would you choose, according to your taste, to compliment your voice, to induce your pleasure to use, to excite just the way you like it, to attract attention, to convey the desired vibe?
*Which one do you think is closer to the Neumann U87ai you got from GuitarCenter?
*I have been insisting on these questions for some time and I keep bothering you because I set your voice in my audio memory as a VO reference. When I recognize her in a commercial it will be cool!

Okay, so far... when it comes to stuff I've built, I have found that once I switched to the superior HL-95 body, the UTM 5087 with the Fuchs and Arienne k87 v1 is the best match for the u87ai on me. Now that I've had time to listen to a number of different transformers in the ORS 87, just not the UTM, I'm probably calling the GZT-87 my next favorite choice as it also sounds close in my Holy Fuchs mic, but the UTM is closer.