Lowish Gain in Royer1 Mod?

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tk@halmi

Well-known member
Joined
Jun 3, 2004
Messages
999
Location
Oregon, USA
After several moths of work, in about 30 minute a week increments, I finished the Royer1 mod on a Nady SCM900. It sounds OK, very low noise and no hum at all. However, the gain is rather low. We recorded voice and acoustic guitar this afternoon. The preamp had to be cranked up to 50dB or more to get good level for recording. Usually, 35dB to 40dB is plenty for recording a guitar. The low end seems to be week. Perhaps it needs a good clean up after all the fondling it got during construction. Any other pointers from more experienced builders?
Thank You,
Tamas

 
I did this mod with a marshall v69 and then transplanted the guts into an older studio projects B1.  No level problems throughout the process, despite some gratuitous fondling of innards.... I used perfboard.  I do recall that the output level from the mod was somewhat lower than the stock version.  The thin low end comment raises an eyebrow though....

Did you use the (teflon?) standoffs for where the capsule wires join the circuit? 

Did the mic have good output (level and freq response) as stock?

Assuming your supply voltages are correct under load and are all getting to where they need to be, and we're not dealing with a wiring error (xfmr backwards? single ended?) , and you can rule out the capsule, then maybe you have a bad tube?  Verify the values of your plate and cathode resistors?

Something about working 30 minutes a week spread out over months makes me think that something very simple was missed, it could be a real forehead-slapper!  Can you post a picture?

Good luck,

Frank


 
Tamas

  Can you post the following with the microphone powered up?
B+ voltage
cathode voltage
Heater voltage
Voltage at the capsule divider node
plate voltage

The voltages will help with the gain stage troubleshooting. I guess you checked your G2 and other tube connections

Next what transformer did you use? check all the solder connections make sure it is not backwards like posted above.

Weak low end and low output can be an open on one of the primary wires.  Yes you can get an output with a broken primary wire.  I know this because I dropped a  transformer and one of the wires broke inside it and it had low output and no bass.
 
Hi Guys,

Thank You for the suggestions. I traced through the mic circuit one more time.
Here is what I measured on the mic after a five minute warm up.

B+ voltage = 104V
cathode voltage = 0.42V
Heater voltage = 6.4V (regulated, still burning in)
Voltage at the capsule divider node = 55.8V
plate voltage = 47.7V

I am using a JAN 6205 tube and I made sure G2 and G3 are hooked up right.
The output transformer is the Jensen JT-DB-EU.
The grid connection is made in the air with the red wire of the capsule to reduce paths to leakage.

As far as the original level of the mic before modifications I do not have a recording, but as I recall
the level was more "average" for a condenser mic.  I guess I will need to unhook the transformer,
measure the ratio, and check for DCR.

Thanks,
Tamas
 
the transformer has a 12:1 ratio, while many other circuits use transformers with lower step down ratios ( e.g. 6:1 )
step down is ~15dB for the 1:6 transformer and ~22dB for the 12:1
maybe that explains a part of what happens.

-max
 
This problem turned out to be a bad capsule after all. I took the mic apart and measured the tube stage which gives a gain of 26dB. Then I checked the transformer on the scope and the step down is 20dB unloaded (I am sure it shoots up when loaded.)
Luckily I have another unmodified SCM900 so I swapped the capsule and got about 6dB gain back. It is still about 3dB less sensitive than the original circuit, but it sounds very nice on vocals.

This was also a good opportunity to clean up and shorten wires for better fit.

I don't know what happened to the first capsule as I never even removed it from the head. I got these mics part of the SCM900/CM90 bundle for $60 from MF so I got two kits. I wanted to do the Royer2 mod in the CM90s, but the screws are such low quality that they all stripped before they even budged to come out.

By the way, the 5 pin DMX cables work great! They saved some time and money, but they look very durable and look high quality.


Cheers,
Tamas
 
Get a very clean artist brush and carefully clean the clear area between the gold and the edge of the diaphragm, no water at first just brush any dust off at first.
  I have bought microphones that did not work right out of the box and it turned out to be dust at the clear area of that style china capsule.

You screw the screws in on some SD microphones not out and slide the case off.
 
Just thought about sharing with you folks some experience I've learned from Royerizing a few Mics:

Hum Problem:

  • Follow Royer's PCB Layout and keep a good distance between components.
  • Make sure the tube 5840 you are buying is tested. The first tube I bought was a weak Tube and was one of the reasons for hum and Low Output. Buying a  NOS tube (New Old Stock) not necessarily means that the tube will be in excellent working condition, most of this tubes are 40+ years old and small tubes are more sensitive to damage than a tube of regular size. Buy it from a trusted source and make sure is TESTED. Its better to pay a couple of more dollars for a tested tube. I personally prefer Raytheon or Sonotone tubes.
  • Good grounding and shielding is a must.
  • If you have built the microphone right, ground loops within the circuit should not be an issue.

Low output Problem:
  • Make sure the tube is in very good condition.
  • The main reason for the lowish output (you have to crank the gain almost all the way up in some preamps) in the original design is the 12:1 Ratio of the Jensen Transformer. I used a BV8 (Neumann) clone from aamicrophones.com http://www.aamicrophones.com/parts.htm The Ratio is 6.5:1 and sounds better for voices to my ears than a Jensen JT-DB-E. The 1uF Plate Capacitor must be changed to 2.2uF in order to use the AA BV8.
  • Use a teflon isolated terminal to make the connection between the capsule and tube.

Better sound:

  • Use a Peluso Capsule. Trust me, It is woth it. Change the 1uF Plate Capacitor for a 2.2uF for better Low end

In this version I used the body of a dead M-Audio Nova and painted the cylinder in smooth red satin finish from RustOleum:

296320771_o.jpg

Royer's Mod
296321297_o.jpg

Royer Mod
296321335_o.jpg

 
Cool build, mrvision!

I second about using a 2.2uf and a different capsule. I did the mod and wasn't excited about it with the stock 2001 capsule, but a Peluso capsule made it a really nice mic. Still bright, but in a more musical way.

Who makes those AA transformers?

Burn in those sub-minis!
A week.
I'm serious.

-paul
 
2.2uf might not be "better" than 1uf it depends

Most low output is a dirty capsule or miswired circuit with a new(NOS)5840.  I have not used a weak 5840, I have a few noisy ones.

The stock circuit has good output even with the 12:1.

The posting that you need a 2.2uf for bass with the aabv8.  Anyone test the aabv8 inductance? (I have not tried that transformer).
 
Gus said:
Anyone test the aabv8 inductance? (I have not tried that transformer).

Gus, I think Rossi maybe tested this when he bought a BV8-ish transformer from Beezneez microphones (I'll be $10 they are the same Chinese transformer). I don't think anyone seemed particularly impressed under first impressions if I remember correctly. Sadly the Black Market posts get cleared out after a certain duration, so the post is no longer there. I think I'd go for one of Oliver Archut's budget models myself. Which reminds me, I'll bump that old thread....
 
Hi tk@halmi  and All,

Since you mentioned about the tiny screws that stripped out that secure the body tube, I thought I would copy my post with some things I found/figured out during my experiences with these mods:


Hi All,
Not sure if any of you saw my post from awhile ago, thought I might post it here as well.
unfortunately my brother-in-law got laid off so I had to go to another machine shop to have these made, so the price went up.
I have made an Adapter that screws on the MXL 603 mic body and allows you to use any AKG CK series capsule or any Bayonet style capsule, Blue,Red, Neumann etc.
They are made of Stainless steel, delrin insulator and a brass or beryllium/copper center contact.
Cost would be $68.00 each , shipping INCLUDED.
I have used all the capsules pictured with my Modded 603's and they sound AWESOME ! Very Happy

http://picasaweb.google.com/audiophreeek/MXL603CapsuleAdapter#

.... some other stuff .....

1.) MXL 603s have 3-- thats right THREE different bodys styles/styles....all the threads are the same)

One is the original body style with the three tiny little screws that secure the body tube to the connector base
I found a second style - the " MXL 603 Mogami " - and the Body tube on THIS one is BRASS and THREADS on to the connector base... MUCH BETTER!
And yet a third style that is like the second but it does not say "Mogami" and the label or badge is PRINTED on the body tube not embosed or engraved like the first two. :Smile


2.) I just finished a Gyraf G9, my first lage project and it KICKS ASS !!! - Thanks Jacob , I had these PCB mount screw connectors hanging around, and had an Idea.... the perf board I had was the right spacing and the connector fit, and it also fit inside the body tube, I just hot glued it to the perf board ..making a socket for the tube, I used a 5703wb tube and copied the C60, but with the pin combining of the 5840 in this and Royers circuit it makes for a nice tiddy Mod without having to fly the leads over, under and around, you just run the wire from the connector to the terminal pins. - sweet right? Wink

Here is a link to show the differences, I left the one w/ the printed label out because other than that, its no different than the brass threaded one

http://picasaweb.google.com/audiophreeek/603Mogami?feat=directlink

all the best,
Chip
 
rodabod said:
Gus said:
Anyone test the aabv8 inductance? (I have not tried that transformer).

Gus, I think Rossi maybe tested this when he bought a BV8-ish transformer from Beezneez microphones (I'll be $10 they are the same Chinese transformer). I don't think anyone seemed particularly impressed under first impressions if I remember correctly. Sadly the Black Market posts get cleared out after a certain duration, so the post is no longer there. I think I'd go for one of Oliver Archut's budget models myself. Which reminds me, I'll bump that old thread....

Yes, I did some tests on the Beezneez ones. I'm not absolutely sure they're the same, but they certainly come from the same source. The Beezneez one was actually a Peluso design. The ones I got were about 11:1, if I remember correctly. Dave Thomas claims his to be 6.5:1, but maybe he just took that number from the original BV8, I don't know. In his own circuits he uses his transformer with an additional cathode follower stage.

On the ones I tested, there was a slight LF roll off. They do work okay, but calling them BV8 may be missleading. I'll see if I can find my notes and graphs.
 
Does anyone know or have any help to help me modify my M-Audio Nova microphone, like the post above ?
Thanks
 

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