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

[pyjaman]

try reversing the anode lead with the B+ lead, this might change the stray C in a good way, maybe worse, but give it a try,

On the output tube?

yes, flip the transformer primary leads (nodes 5 and 1 on your schem) ... I guess. You'll have to flip sec leads too...
Laurent.

 

[chrispsound]

I am adhering to the NYD MILA-1 topology more than Matador is, just changed some resistors and the heaters to accommodate a 6CG7.  But I get what Laurent is saying.  On Dave's original diagram it would be the pin 1(+) and pin 4(-) on the primary of the Edcor output transformer.  I did reverse polarity on the primary of the Edcor  XSM 15k:600 output transformer and the boost flipped with it to the low end @ around 20Hz and the rest is dead flat.  I also tried moving the output cap lead to the center tap and it had the same effect as switching the polarity.

 

[chrispsound]

Edcors output transformers state polarity and pin # on the top and pin #'s at each  pin.  One thing I noticed on NewYorkDave's MILA-1 original schematic is that pin 8(-) on the secondary of the Edcor output transformer is connected to pin 2(+) on the XLR.  In my pre (MILA-1 with 6CG7's),  with the primary is wired as in the schematic ( pin 1(+) and pin 4(-)) and  pin 5(+) on the secondary of the Edcor output transformer is connected to pin 2(+) on the XLR, I get a boost in the high's(wich I assume would be more than 1db if RMAA went past 20kHz).  If I switch it to the way NewYork Dave has it and keep the primary the same I still get the boost in the high's.  It's not until I switch polarity at the primary that I get a boost (more than 3db since RMAA sweeps lower frequencies than it does the highs) in the low end at around 10Hz and is flat until RMAA's limit at 20kHz.  I am going to make a seperate thread for this build when I get all my results in order, sorry Matador to have hijacked it this far.  ChrisP

 

[letterbeacon]

Is that copper tape over the heater wires?  That seems like a good way of shunting any noise from the heaters to ground -belt and braces with the twisted wires!  Do you need to solder a connection from the tape to the chassis?

 

[usekgb]

I must have missed this thread.  Very cool stuff!

 

[emrr]

Lassoharp and I swapped every possible orientation on a push-pull circuit with a grounded ladder attenuator on the 600 ohm secondary side, and every single change made a difference in the response plot, including the balanced primary connections. 

 

[Matador]

Is that copper tape over the heater wires?  That seems like a good way of shunting any noise from the heaters to ground -belt and braces with the twisted wires!  Do you need to solder a connection from the tape to the chassis?

This design uses DC heaters, so the tape doesn't do much.  However it does keep the wiring tidy and that's why I used it.  It holds much better than kapton or electrical tape.

If AC heaters were used, it might be of a benefit.  This tape is used to shield instrument cavities, and has a conductive adhesive so it doesn't need soldering either to itself or to the bare metal.  You can get it from Stewart MacDonald.

 

[chrispsound]

Lassoharp and I swapped every possible orientation on a push-pull circuit with a grounded ladder attenuator on the 600 ohm secondary side, and every single change made a difference in the response plot, including the balanced primary connections. 

Found that out,  Doug I have a thread started in the lab with the schematic and sweep of my pre, any tips woyld be greatly appreciated.  Thanks ChrisP

 

[Matador]

Well, it took about 6 hours but I have everything stuffed and mounted, and it passes signal.

Before mounting the transformers, I attached my signal generator right to the 1M grid resistor of the first stage, and placed a 10K resistive load on the output cap.  With everything dimed, feedback loop disconnected, and a 25mVpp signal going in to the input, I get about a 100Vpp clipped output swing before the output transformer.  I must have something wired incorrectly as I should have only seen about a 40V swing (or about 65dB of gain).  Perhaps I wired the input transformer as a 1:20 instead of a 1:10:  I'll check that first (I'm using a CMMI-10C).  With the output transformer connected, I must put the interstage volume pot down near the bottom before I can hear a clear undistorted signal.

Since I used a linear taper 1M feedback pot, the gain changes drastically with a slight twist of the control, so I think I'll yank it out and put in a 12-step Lorlin instead.

The other problem is a 120Hz hum that appears even with the volume controls all the way down.  I suspect the Edcor is picking up my torroid phantom transformer as I don't see this hum on the scope if I check the signal "pre" the output transformer.  I may install an unbalanced jack to confirm this.

Lastly, I hear a lot of tube "blow" noise:  I'm using EH6SN7 tubes so they may need 3 or 4 days of burn in time.

 

[Matador]

Another thought:  currently, the secondary center tap of the Edcor is left floating, and like in the MILA design the feedback comes from the output transformer coupling cap.  If I ground reference the secondary center tap, and move the feedback from the primary to the secondary, wouldn't I improve the CMRR around the secondary by incorporating it into the feedback loop (and thus reduce the audible hum in the output)?

 

[ruffrecords]

Lastly, I hear a lot of tube "blow" noise:  I'm using EH6SN7 tubes so they may need 3 or 4 days of burn in time.

I use a lot of EH6CG7 tubes (the B9A equivalent of the 6SN7) and they are always quiet. What exactly is 'blow' noise??

Cheers

Ian

 

[ruffrecords]

The other problem is a 120Hz hum that appears even with the volume controls all the way down.  I suspect the Edcor is picking up my torroid phantom transformer as I don't see this hum on the scope if I check the signal "pre" the output transformer.  I may install an unbalanced jack to confirm this.

Since this is 120Hz it is almost certainly on the HT supply. The PSRR of a single ended output stage is next to nothing. Can you post a PSU schematic?

Cheers

Ian

 

[ruffrecords]

I just looked at the schematic. C8 and C9 are too small. They should be more like 22uF electrolytics not 0.22uF.

Cheers

Ian

 

[Kingston]

I just looked at the schematic. C8 and C9 are too small. They should be more like 22uF electrolytics not 0.22uF.

What schematic are you looking at exactly? I can't seem to find any 0.22uF values on the latest revision, http://www.musicalsparks.com/images/6sn7_preamp/6SN7-Preamp-v3.pdf

[edit]

looking at the PSU there are some errors and unnecessary things. Firstly the 1N4007 diodes are not enough for even two 6SN7 tubes. Heater draw is more than 1 amp and they will fry. I've done it too. It can happen some years after perfect operation. I've started using way over-specified bridge rectifier chips for this task, but if you already made a PCB or layout, any +3A standard diode will do. Secondly, why have you biased the heaters up 80V? There are no cathodes biased above even 10V and it's perfectly safe to ground-reference the heaters, and actually more failure proof. The heaters are regulated DC so the biasing will have no effect on any kind of hum either.

 

[ruffrecords]

I just looked at the schematic. C8 and C9 are too small. They should be more like 22uF electrolytics not 0.22uF.

What schematic are you looking at exactly? I can't seem to find any 0.22uF values on the latest revision, http://www.musicalsparks.com/images/6sn7_preamp/6SN7-Preamp-v3.pdf

My apologies, I am confusing two different threads on the same topic. I was referring to the schematic in this thread:

(Edit: fixed link)

http://www.groupdiy.com/index.php?topic=50832.msg645242#msg645242

Cheers

Ian

 

[Kingston]

The other problem is a 120Hz hum that appears even with the volume controls all the way down.  I suspect the Edcor is picking up my torroid phantom transformer as I don't see this hum on the scope if I check the signal "pre" the output transformer.  I may install an unbalanced jack to confirm this.

120hz is most certainly not caused by any transformer proximity effect, or at least that never happened on any projects I've done. Transformers will only pick the fundamental 50/60hz AFAIK.

The harmonics have practically always been caused by rectifier crud, and that means you have a ground reference problem somewhere. Rectified AC leaks into your audio circuit that should be perfectly clean. Where is your PSU "star" and how exactly have you wired it?

You might not see it on the scope if you are referencing the scope on some other point than "B-" or "audio ground" (what the preamp circuit sees as its reference).

 

[Matador]

Here is the simulation schematic I based my layout on:

I pretty much followed Rane's advice for star grounding:  I have the star point directly on the chassis, and I run my first rectifier cap ground directly to this point.  The rest of the power supply bus bar ground ties to this point as well.  Then I have a third wire connecting my heater regulator board to this same point, and lastly I have the ground bus bar on the turret board tied to this point as well (this is the ground reference for all of the amplifier stages).  The transformer secondary currently has no ground reference.

I'll have to look more closely at what frequency the hum is at:  I do notice however that the magnitude and nature of the sound is changed by twiddling the feedback pot.  The hum is loudest when the pot is at maximum value, but it's only a slight change.  My power supply is essentially a CLCRCRCRC filter chain (pretty much identical to the V3 schematic referenced above).

I have referenced my scope right on the star point.

Lastly, the "blow" noise sounds like tape hiss.  It changes slightly in character as I fiddle around with the heater voltage.  In order to eliminate heater noise I've connected the heater to a dedicated external supply for testing.

 

[Kingston]

I'll have to look more closely at what frequency the hum is at.

There's a huge difference between the causes for harmonics or pure 50/60hz. FFT is the best tool for it and any soundcard input is good enough for this test.

 

[Matador]

It appears I was incorrect:  the hum is indeed 60Hz!

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").

http://www.musicalsparks.com/images/6sn7_preamp/6sn7_noise.mp3

 

[chrispsound]

I used the actual MILA power supply with two 47uF caps instead of 22uF.  It elevates the heaters a bit and balances and references them to ground and is extremely easy to build(I done did it).  With 2 6CG7's and AC heaters I get a -93db noise floor(according to RMAA) at decent recording levels.  I don't hear any hum when I record vocals.  Off topic:The link that Ian posted is actually my preamp and C8 and C9 are 22uF as per the schematic and the picture.  Thanks ChrisP