[sr1200]

The dismantled one i was working on wasn't the u87 board... I got an Aurycle mic to use as the donor... then found the OPC body which had all 3 switches on it... so i built the Aurycle and that board just wouldn't work...

 the one that IS working however, has styrenes, TAB trafo and peluso capsule.

[geekmacdaddy]

Ordered the Cinemag CM2480 from Dave...he has to wind some next week so it will be 2 weeks before I see it....gaaah! Can't wait to bring this to fruition.

[kante1603]

Yes,I know how it feels to wait for transformers for ages

Heads up my brother,we're with you!

Udo

[geekmacdaddy]

 

[geekmacdaddy]

Of interest;

Emailed Matt from Microphone Parts, and he says that the new batch of RK-87's will be ready to ship in about 10 days.

[geekmacdaddy]

What to do while waiting on transformers to arrive in the mail (Brother Udo will certainly appreciate the sentiment);

[kante1603]

YES

[tskguy]

Hi all,

As many of you know I have been building capsules for a while, I haven’t really officially started production mainly because I  had a very limited number of gold coated targets. Until now!!! Check out these pics! So I now am officially selling these capsules in 2 styles, the HK87 and the HK67 both have identical back plates but have different back plate connections. Price is 250 plus shipping of your choice.   Anyway here some pics of the coating process..

Enjoy, Eric

[0dbfs]

What, Is that a spray bottle full of liquid gold in there?

Just kidding.... That looks like some awesome DIY!

Cheers,
jb

[wave]

Hello fellow cloners!
So if I wanted to drop a 2SK170 in place of the 2N3819, would the 25K trimmer still be the right range for biasing the different FET?

Dave

[poctop]

Hello fellow cloners!
So if I wanted to drop a 2SK170 in place of the 2N3819, would the 25K trimmer still be the right range for biasing the different FET?

Dave

Yep you should be all fine ,  my guess is it will bias around 5-6K,  Let me know
Dan,

[HellfireStudios]

I just got done testing my twenty five 2N3819's IDss last night. I had a reading of 3.8 mA on nineteen of them, 3.79 mA on five, and it fluctuated between the aforementioned values on just one of them. Does this sound right? I thought they were supposed to be all over the place in IDss values.

To test IDss, I set my DMM for mA. Then, I shorted Gate and Source of the FET, and attached the negative side of a 9V battery. The positive 9V side of the battery was hooked to one of my DMM probes. The other probe was hooked to Drain. Let me know if you see any problems with my testing method or results. Thanks.

[Hank Dussen]

I guess that's within the specs of the datasheet but it does seem low to me.
Mine were all around 10mA...
But maybe they're from from a low IDss batch?

[Hank Dussen]

with nothing connected to the  Front Capsule nor Rear Capsule ( important )For the sine Tone Method and THD Method : use the big RT and SW Pad on the board,  RT + ,SW -
feed it 0db (0.775VAC) at 1Khz and tweak the pot , if using your Daw as sine tone ,you should be able to hear what come from the mike with headphone or something you need to be able to ear separtely what come from the mike,  tweak the pot fully in 1 way and then fully the other way you should ear the sounds come louder and then get to almost dissapear, what you need to do is to tweak just after it becomes louder and there is no more Fizz in the sounds, you will clearly ear fizzing when the FET opens, just a bit past this point is the good approximative Bias, if you go too Far the Signal will becomes fizzy again but more subtile past this point.( easier to see on a scope) At this point the method is the same for using the Room capture demo software with the THD Meter you only use the THD Meter as an indicator , when the FET opens the Meter goes up at near 25-30% THD so you can clearly see what happening the more you tweak past the opening point the Less THD on the Meter until your reach about 0.4% thd that is fine. if you go too far the THD will slowly rise backup as the fizz in the sounds as the sine symmetry seen in a scope meter, doing it a couple of times is good pratice and you can actually verify your method with the ending drain Voltage if the ending voltage makes sense.

Hope this helps,
Dany,

I want to make some samples with different capacitors in the microphone to compare them.
Is it an idea to inject audio straight into the circuit as described above, and thus being able to objectively compare the sound of the caps, or is a stupid idea?

[Hank Dussen]

When I trim the transistor I hear the least distortion at about 3V at the drain.
When I set it to 10,5V it's a bit louder but also a bit more distorted.

When I push the input signal only the top of the sine wave clips. However I set trimmer I can't get any symmetrical clipping...

The IDss of that fet was about 11mA.

Any suggestions?

[Matador]

I just got done testing my twenty five 2N3819's IDss last night. I had a reading of 3.8 mA on nineteen of them, 3.79 mA on five, and it fluctuated between the aforementioned values on just one of them. Does this sound right? I thought they were supposed to be all over the place in IDss values.

To test IDss, I set my DMM for mA. Then, I shorted Gate and Source of the FET, and attached the negative side of a 9V battery. The positive 9V side of the battery was hooked to one of my DMM probes. The other probe was hooked to Drain. Let me know if you see any problems with my testing method or results. Thanks.

Transistors from similar lots will be close.  If you ordered them simultaneously from the same place it's likely that they broke up a tape and reel which would explain the similarities.

The datasheet is the the manufacturers "get out of jail card" so they tend to be overly conservative.

Your testing method was spot on.

[Matador]

When I trim the transistor I hear the least distortion at about 3V at the drain.
When I set it to 10,5V it's a bit louder but also a bit more distorted.

When I push the input signal only the top of the sine wave clips. However I set trimmer I can't get any symmetrical clipping...

The IDss of that fet was about 11mA.

Any suggestions?

What is the voltage at the top of the Zener diode and the top of the drain resistor?

[Hank Dussen]

Been doing some more measurements.

I've tried two fets, IDss of 11mA and 12ma.

When I set the fet drain to 10,5V I get these voltages:
fet source: around 3V
zener: 23,4V
top R14: around 21V

These voltages seem to be right according to the schematic but there's no symmetrical clipping, only the top, however I set the trimmer and it sound too distorted as well...

BTW: I've got the scope on the output of the mic.

[poctop]

Been doing some more measurements.

I've tried two fets, IDss of 11mA and 12ma.

When I set the fet drain to 10,5V I get these voltages:
fet source: around 3V
zener: 23,4V
top R14: around 21V

These voltages seem to be right according to the schematic but there's no symmetrical clipping, only the top, however I set the trimmer and it sound too distorted as well...

BTW: I've got the scope on the output of the mic.

you have to put the scope on the Drain Leg of the Fet ,  to do the adjustment ,  these voltage looks perfect to me ,  Have you actually set the capsule and try the mic ,  is your transormer connected properly ? maybe the signal is fizzing from the starting point ?
Hope this help,

Dany,

[wave]

Some more T-13s showed up today!



Also, for your listening pleasure I have an A/B test of my RK-87 and Eric's K-87.
The tracks are acoustic guitar and they were done by recording with one mic. First the RK-87 was used then I swapped out the capsule for Eric's
Check out the tonal difference.

https://www.dropbox.com/sh/04bghkvc9m22b9i/bYHNf_5RiF

Dave

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