Phaedrus Audio: Spirit Tube U47 Circuit // Need help, please!

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grobaesthet

Member
Joined
Jan 20, 2020
Messages
20
Location
Berlin, Germany
Dear all,

I started with the U47 featuring the spirit tube from phaedrus audio and using their circuit.
But I have difficulties with the circuit, which might explain, why my construction does not work.
There is no voltage anywhere in my circuit.


Attached you will find the schematic with the two areas I can't get my mind around.

1.
The resistor with the "NF". I thought this would mean "non fitted" and should be left out. so there is no connection between the 100M resistor and the ground.

2.
The marked connection from the transformer on the XLR-side. This looks like the "2" and the "3" from XLR go into the transformer and from there out of the transformer giving 45 V to the circuit.
I have a FAB V8 which has the two ins and two outs and two brown cables for ground. I can't see how I can get the marked lead out of it.

Thank you all and best regards
Simon
 

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Hi Simon, I don't think this will work with your output transformer without a center tap.

You have to decouple the operating voltage of your microphone from the phantom power with two resistors.

Here the Neumann KM84 as an example. R9 and R10 doing this here.

24743-e84d393c822fb25aef98767d9fcf272a (1).jpeg
 
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Thank you rock soderstrom and Khron!

Seems I was lucky with my interpretation of "NF"!

When finishing the project last night, I was already thinking, that my transformer was missing something. Didn't know about the center tap, when ordering from FAB.
About the two resistors:
Is it enough to just solder the two resistors in the circuit as seen in the KM84 schematics?
And: 2,2k or 6,81k? I have 2 2,2k (from my last project) but also two 6.8 k resistors. When measuring them they have 6.78 and 6.79 Ohm. Is this balanced enough or should i get other ones?
Thank you all!
Simon
 
Thanks Khron and Gus,
I really appreciate the time and expertise you are giving to me!

I just finished the U47 project and am really happy, as this is my first working mic-project I built from ground up. (Not just modding existing SP C1-circuits)

I used the 6.81 resistors, as this was also what "phaedrus audio" were advising. But I fitted the resistors on cables, coming directly from the XLR socket. So it will be an easy task to swap to the 2.2 k resistors if needed.

All in all I really like the sound of the "spirit" U47. Much more details and body as my TLM103 and my Peluso-modded studio projects C1.
But I have much more noisefloor!
I will try to check and resolder the mic to see, where the noise may come from.
Maybe I was shooting to high with my extra-large caps (Jupiter red, Vishay MKT 1813,etc) or its the long cables I left inside (from and to transformer, XLR to phantom resistors, ..)and also the solderjoints are quite bubbly...
So I bet, there is something I can do, to upgrade this mic...

As soon as I have recorded samples and have gathered new input for the noisefloor, I will post it here!

Thanks and all the best!
Simon
 
Thank you all for your input and all your effort!
I did not have the time to retest the mic today. Hopefully tomorrow I can check it again and give some more detailled feedback.
Thanks and all the best!
Simon
 
Thank you all for your input and all your effort!
I did not have the time to retest the mic today. Hopefully tomorrow I can check it again and give some more detailled feedback.
Thanks and all the best!
Simon
Hello again and sorry for the long silence in this thread!

After a few weeks of work and family business I was able to test my Phaedrus spirit-tubed "U47" against a studio projects C1, a Neumann TLM103 and our old Neumann M269.
The C1 had the lowest self noise. The "U47"s self noise was quite on par with the self noise of the TLM 103 but a little bit more on the whithe-noise-side and lower on the bass.
The M269 had the highest self noise which sounded quite like the "U47" self noise.
So in comparison to the other mics, my "U47" seems to work quite okay and better than it did on its first test drive.

After the first test drive I did the following:
1. Cleaned the PCB with 99,9% isopropyl alcohol
2. Changed the two power-decoupling resistors from 6.81 to 2.0 kOhm.

The power is still not as high as in the circuit diagram provided by phaedrus. Most voltages are 2 Volts under the recommondation.

But most important:
The sound of the mic is very good. I have no U47 to test against mine but I have experience with U87 and U67 and think, the phaedrus "U47" is playing just a bit below this league.
Hopefully I will have the time and opportunity, to get one of our voice artists to test my "U47" and allow me to share the recordings.

Following are pictures of my "U47"-build.
I am quite sure that you will have a hard time to look at my poor soldering job and the totally amateurish build, which does not stand to the very expensive and great parts used (Thiersch blue M7, Fab V8 transformer, Phaedrus spirit tube "gold").
Do you think the mic could sound better, if it had a better built quality?
For example: The tube socket is soldered to the PCB via 3-5cm long cables and not "hardwired"
The capacitors are very big and perhabs not qualified for mics but better for tube amps? ("aimed to high?")
The "earth" is soldered very bulky as I thought, this would not matter.

As long as it's just about cosmetics and quality craftmanship i would not have any proplem with this, but if the self noise or the sound of the mic would benefit by a redo, I would give it to someone, who has better skills.

Thank you and all the best!
Simon
 

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First, I would intensively clean the high impedance areas around the "tube" with 99,5% isopropyl alcohol. Even the smallest amount of contamination can lead to unwanted noise.

How does the noise sound? Hiss, hum, ocean beach or frying eggs?
Hi rock soderstrom,
just updated this thread and wanted to thank you for your tip with the 99,9 isopropyl alcohol. I used the 70% stuff before and now re-cleaned the PCB with 99,9 and this reallly helped!
The noise is still there. It sounds static and is more on the hiss side. It is continuos and quite broad on the upper frequency. It was indeed a few dBs louder before the usage of the 99,9% isopropyl alcohol. So thank you very much for this hint!
Best
Simon
 
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I have low S/N. Back ground hiss is there. I add a DC-DC on it. Hiss is gone.
Hi,
thanks for your suggestion! I really hoped that I would not have to use another PCB in this build as there is no more "real estate" in the door body.
I hope to get my issues fixed without the dc-dc-booster. But if not, I will gratefully come back to you!
Best!
Simon
 
Thank you Tubetec!
I was not aware of the different tube-sockets. And the PTFE-version makes total sense! Just ordered one and when rewiring it, I will also turn the valve (or rewire the circuit ) to have shorter grid-cable.
thank you very much for your input!
Best,
Simon
 
Dear all,

I started with the U47 featuring the spirit tube from phaedrus audio and using their circuit.
But I have difficulties with the circuit, which might explain, why my construction does not work.
There is no voltage anywhere in my circuit.


Attached you will find the schematic with the two areas I can't get my mind around.

1.
The resistor with the "NF". I thought this would mean "non fitted" and should be left out. so there is no connection between the 100M resistor and the ground.

2.
The marked connection from the transformer on the XLR-side. This looks like the "2" and the "3" from XLR go into the transformer and from there out of the transformer giving 45 V to the circuit.
I have a FAB V8 which has the two ins and two outs and two brown cables for ground. I can't see how I can get the marked lead out of it.

Thank you all and best regards
Simon
I just saw this thread. Bv.08 has all the leads coming out so achieving center tap is not a big deal. But, as we can see this variation of the transformer has internal junction of the coils. You should contact manufacturer and ask is it possible to remove the top insulating tape and find that junction. I'm sure it's possible. That way you can add the CT lead and make it as the author of the circuit recommended.
 
I built many pc with this Phaedrus tube and I remember the schematics values are not really correct,some recalculations were needed. But the mics sounded good without the 2.2k phantom resistors or DC-DC converter.
 

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I built many pc with this Phaedrus tube and I remember the schematics values are not really correct,some recalculations were needed. But the mics sounded good without the 2.2k phantom resistors or DC-DC converter.
Wow, your build looks fantastic!
When rewiring the tube next weekend, I will also install a new pair of 2,2k decoupling-resistors. I have ordered 10 pieces and will match a perfect couple with a "go between kit".
Thank you!
Best
Simon
 
I just saw this thread. Bv.08 has all the leads coming out so achieving center tap is not a big deal. But, as we can see this variation of the transformer has internal junction of the coils. You should contact manufacturer and ask is it possible to remove the top insulating tape and find that junction. I'm sure it's possible. That way you can add the CT lead and make it as the author of the circuit recommended.
Hi Moby,
thank you for your good expanation and suggestion!
I will write to FAB about this. If it is an easy hack I iwll try to modify my V8 transformer. If not I will have to live with the decoupling resistors.
Thank you and all the best!
Simon
 
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