Two-Tube 'MILA-like' 6SN7 mic pre thoughts

[Matador]

Here is the updated schematic and ground topology:

http://www.musicalsparks.com/images/6sn7_preamp/6SN7_Mila3_V4.pdf

 

[Kingston]

What tipped me off is that I listened carefully through a set of headphones before I even activated power to the box (with the exception of the currently-external heater supply).  I could hear a faint hum even before I powered anything on.  A clip on the secondary of the output transformer leads shows a slight 60Hz hum signature (according to the frequency counter on the scope).  I do not see this pre-output transformer though.

Here is an MP3 file of the hum along with the "wind" noise that I'm getting (you can hear it come in even before I thump the mike a few times to make sure I'm "live").

Detach the output transformer from the case and try rotating it to all directions while you are looking at the FFT view. Then do the same by rotating the PSU transformer (even if it's toroidal it can help several dB).

Then there's the external PSU option as a last resort. I've had to do this once on a project that I thought I had already finished, case drilled and all. Edcors have no shields and distance is the best defense. Or a PSU transformer shield, but these are hard to find.

Any random wind noise is very hard to troubleshoot until significant cooking. There's a remote possibility of dirty contacts on a high impedance part of the circuit (read: a potentiometer). Rotary switches are suspect too. Try some clean up duty. That 500k pot for example is immediately suspect. Try 50-100k and scale C3 accordingly. That 1meg feedback pot is another suspect. Replace with a simple (20-100k) resistor for a quick test.

I suppose you have tried another set of tubes already, or at least switching them around? This can get expensive with 6sn7, but there's always the cheap soviet option (6N8S).

PS. I said this in chrispsound's preamp thread already, but there's no need to have a separate feedback cap (C9) unless you wanted to specifically tune the low frequency response. You can take feedback from output cap (C6) for a quick optimisation.

 

[Kingston]

Here is the updated schematic and ground topology:

http://www.musicalsparks.com/images/6sn7_preamp/6SN7_Mila3_V4.pdf

That ground topology is strange and a suspect too. Hierarchical ground is much easier to control, and as a concept "star ground is for electricians".

Instead of three separate connections to star (first filter/HT supply/preamp), you should have only one. HT supply (including first filter cap) and preamp ground buses connected directly. Now you can control the "star point" with much finer grain. You should really think the ground reference as one long bus from HT all the way to preamp input transformer. Any separate branches to star only increases overall ground resistance, which is a bad thing for noise.

http://www.groupdiy.com/index.php?topic=41833.msg528292#msg528292

 

[Matador]

I've tried many different experiments with no success:

1) Removed the first filter cap ground to star point and tied it in with the ground bus bar in the power supply
2) Extend the ground bus bar around to tie together the preamp torrent board and power supply supply
3) Removed output transformer, and tapped unbalanced signal from after 4uF coupling cap (transformer replaced with
47K resistor)
4) Jumpered across the power supply choke
5) Removed 48V torroid transformer and moved heater supplies to external DC power supply
6) Broke feedback path - hum/noise increased slightly
7) Exchanged tubes

I did however find something interesting:  when I placed the top cover plate on the chassis without mounting screws, it rattles!  When I placed my hand on the chassis, I can feel a faint 60Hz vibration.  I'm wondering now if there is a mechanical coupling between a transformer and the tube sockets.  I did a pencil test, where I strike the side of the tubes with a pencil while listening through headphones:  there is a loud PLINK sound when struck (indicating these EH tubes aren't stellar from a microphonics point of view).

Next I will try rubber isolating the transformers and tube sockets.  I'll also try wearing some welding goggles and physically holding each tube to dampen vibration.

All in all I think I'm planning on moving to an external power supply based on Gyraf's G7 design.  That will also solve the dropout problem I have in the heater regulator since my current Hammond power transformer only have 6.3V AC secondary windings.  If I move to a 120V:9V then to a 9V:230V then my heater regulator board has more voltage to work with.

Any suggestions for good quality multi-conductor cable?  I need:

1) 300V supply
2) 6.3V supply
3) 48V supply
4) Ground

I'm thinking a 5 conductor XLR jack / wire combo so I can use two wires to carry the heater current.

Nice thing about an external supply is that I can power many different circuit topologies / preamps with a single supply design.

 

[ruffrecords]

I did however find something interesting:  when I placed the top cover plate on the chassis without mounting screws, it rattles!  When I placed my hand on the chassis, I can feel a faint 60Hz vibration.  I'm wondering now if there is a mechanical coupling between a transformer and the tube sockets.  I did a pencil test, where I strike the side of the tubes with a pencil while listening through headphones:  there is a loud PLINK sound when struck (indicating these EH tubes aren't stellar from a microphonics point of view).

Next I will try rubber isolating the transformers and tube sockets.  I'll also try wearing some welding goggles and physically holding each tube to dampen vibration.

Steel enclosures are a PITA for conducting magnetic fields from mains transformers - its the induced magnetic field in the chassis that makes it vibrate. I avoid them whenever possible and/or use an external supply. If you are stuck with one it is a good idea to mount the transformer on non-magnetic stand offs held in with non-magnetic screws.

All tubes at the front end of a high gain amp will make a nice 'plink' when you tap them. There is a large variation in the amount of 'plink' and also the amount of noise generated even between tubes from the same manufacturer.

All in all I think I'm planning on moving to an external power supply based on Gyraf's G7 design.  That will also solve the dropout problem I have in the heater regulator since my current Hammond power transformer only have 6.3V AC secondary windings.  If I move to a 120V:9V then to a 9V:230V then my heater regulator board has more voltage to work with.

Any suggestions for good quality multi-conductor cable?  I need:

1) 300V supply
2) 6.3V supply
3) 48V supply
4) Ground

I'm thinking a 5 conductor XLR jack / wire combo so I can use two wires to carry the heater current.

Nice thing about an external supply is that I can power many different circuit topologies / preamps with a single supply design.

External power supplies are a great idea for all the reasons you have thought of. I use an 8 (so I can double up on the heaters if necessary) way cable and 8 way Speakon connectors. I use military grade screened cable but it is very expensive.  Might be better to make your own.

Cheers

Ian

 

[Matador]

My case is actually aluminum.  I was assuming the vibration was coming between the mains transformer and the choke, or perhaps the other torroid transformer I use for 48V phantom power.

 

[chrispsound]

What kind of mains transformer are you using?  I had a crusty old giant EI 350V guitar amp mains transformer I got from ebay that vibrated no mater where I put it and what I loaded it with.  Would it be too hard to eliminate phantom from the equation?  Eliminate the voltage regulator by trying balanced and elevated AC heaters if all else fails(with the phantom transformer disconnected).  If you are building only one channel try using the Bo Hanson phantom power schematic using a bridge rect., couple of resistors, 3 caps and a zener with this power transformer or similar:
http://www.mouser.com/Search/ProductDetail.aspx?R=3FS-448virtualkey55420000virtualkey838-3FS-448
It even has bolt holes for mounting if you don't want to make a board for it.  All just as hard as making an external power supply(maybe even cheaper).  ChrisP

 

[Matador]

I already stripped out all of the phantom and heater regulator circuits from the chassis just to experiment - no change in noise / hum levels.

The power transformer is indeed a guitar amp transformer - a Hammond 269GX to be exact.  It's about 50 VA overkill but it's what I had on hand that was within reason for the voltages I needed.

 

[letterbeacon]

External power supplies are a great idea for all the reasons you have thought of. I use an 8 (so I can double up on the heaters if necessary) way cable and 8 way Speakon connectors. I use military grade screened cable but it is very expensive.  Might be better to make your own.

Cheers

Ian

Do you have a link to the military grade cable at all?

 

[ruffrecords]

External power supplies are a great idea for all the reasons you have thought of. I use an 8 (so I can double up on the heaters if necessary) way cable and 8 way Speakon connectors. I use military grade screened cable but it is very expensive.  Might be better to make your own.

Cheers

Ian

Do you have a link to the military grade cable at all?

http://www.rapidonline.com/Cables-Connectors/Unistrand-100m-Defence-Standard-Signal-Cable-8-Core-16-2-8C-02-0293

Cheers

Ian

 

[letterbeacon]

Yikes, you weren't kidding about it being expensive!

 

[gemini86]

That's for 100 meter reels. which comes out to about 1.4GBP per meter. Or 70 cents per foot. That's a great deal.

 

[johnheath]

I already stripped out all of the phantom and heater regulator circuits from the chassis just to experiment - no change in noise / hum levels.

The power transformer is indeed a guitar amp transformer - a Hammond 269GX to be exact.  It's about 50 VA overkill but it's what I had on hand that was within reason for the voltages I needed.

Sorry for bringing this topic to life, but I have read it and I do not know the full answer to your problem but I have tried a Hammond 390JX in an attempt to use it in a LA2A but it hums like crazy… the solution in that case was to switch to a Sowter equivalent and the hum was gone.

Best regards

/John