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Old 5th July 2012, 09:42 AM   #71
DF96 is offline DF96  England
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Quote:
Originally Posted by themagicmanmdt
power supply related observation: the noise on the B+ rail is higher after my 4th filter stage (which supplies the 1st gain stage) than the 3rd filter stage (which supplies the 2nd gain stage and CF output). i'll re-look at the grounding more critically per your post; thoughts on this, though?
Seems to confirm that you have a grounding problem.
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Old 5th July 2012, 11:59 PM   #72
gootee is offline gootee  United States
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Old 7th July 2012, 06:05 AM   #73
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just solved most of it...

first, changed the grounding scheme to what DF96/gootee had mentioned. this was the quietest solution I've tried! this helped the ground noise seen on the last cap, too. to further mention: i soldered the secondary CT right at the negative terminal at the cap (I mean...RIGHT at it!) along with the heater CT; this was also the quietest config i've tried.

further, i found the hum was also reduced by NOT terminating the secondary of the transformer with a 220k resistor to 'test'. i decided to hook up the mic leads to the XLR, and in doing so, unterminated the 22k resistor. the hum reduced equally as much as the power supply update mentioned above. terminating the secondary with a fixed load only creates more hum. the reflected impedance through the transformer seems to be fine, as well as amazingly quiet.

third; read the sticky regarding heater wiring, and decided to get anal with it and give them all a few more twists.

fourth; having the lid on and the mic at least 1' away from the preamp was also significant.




so, now, with the full ~80db of gain, i'm getting a S/N of at least 80db; and even better when running the EF86 as triode (although less gain; i have the ef86 on a pentode/triode switch); it's more than quiet enough for my ribbon mics for vocals in a room, ala 'smiley smile'

thanks for the help, everyone. i'll follow up as I continue to further chase down s/n reduction and that 'teeny' bit of hum left when it's running wide open. yet, i'll call it 'solved':

so, here, to recap, were all the things that ended up helping:

*power supply in a separate star than signal ground
*power supply star is connected with the sec. center tap and heater center tap very very close to the first filter cap pair twist (as close to the neg terminal as possible); then all filter caps, one at a time, with the connection to chassis ground after the last filter cap
*signal star connected with lowest current draw sections furthest from chassis ground on a small 'buss', with the highest current drawing sections closest to the star chassis ground connection
*grouping each stage in a 'sub-star' connection B/C the less long leads and 'loop' area between grounds, the better;
*twist ground leads going back to signal star connection if possible, just like a heater lead
*be careful when 'test-terminating' a transformer; it seem to create a small loop by terminating it on the transformer pins (solder-type, that is) - I used UTC O-8 inputs wired 1:10.
*use shielded signal cables, and terminate the shield on the side where the signal is originating, not terminating (ex: from plate blocking cap to volume pot; terminate the shield at the cathode of the tube it 'came from')
*isolating the power transformer with rubber washers helps remove mechanical hum.


that's all off the top of my head

anyone have further thoughts?

thanks; hopefully this is a good thread for the vault


oh look, here's a picture (hahahahahaha)

now, to have a variable NFB control and make it pretty.


now, time for me to help a bit, if i can, for everyone else...

much love.
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Old 7th July 2012, 01:15 PM   #74
DF96 is offline DF96  England
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Quote:
first, changed the grounding scheme to what DF96/gootee had mentioned. this was the quietest solution I've tried!
Strange, lots of people find this! Maybe because it is actually correct, but people seem reluctant at first to believe that such a simple change can make so much difference. Unlike some tweaks, this one is based on sound science like Ohm's Law.

Keep charging pulses in a tight loop well away from the signal ground.
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Old 17th March 2013, 02:22 PM   #75
AndrewT is offline AndrewT  Scotland
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similar to my warning shot fired off in December 2004 where I clearly state that keeping the pulse of the charge currents away from the audio ground.
Quote:
I see that users are still basing their grounding scheme on the post 143 diagram.
I strongly suggest you DO NOT use multiple grounding as shown on this diagram!!!!
The peak / pulse currents flowing between the capacitors through this plate will contaminate your audio signal.
If you insist on using the cap common as your clean and dirty ground then use the Sugden trick:- put a brass ( or better still a copper) bolt through the plate and nut it up firmly. Connect all your clean and dirty grounds to the other side of the nut, then they are isolated from the pulse currents between the caps. or connect a single wire from the cap common to a remote audio star ground.
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Old 17th March 2013, 03:08 PM   #76
DF96 is offline DF96  England
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I suspect that when people read 'copper is a good conductor' what they think is 'copper is an infinitely good conductor' so they then believe that any points connected by copper must be at the same potential. Spice reinforces this misunderstanding, so it confuses newbies.

When designing power supples and output stages it would be better for people to understand that copper wires (however 'oxygen-free') are resistors, albeit fairly low value resistors.
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Old 17th March 2013, 05:29 PM   #77
12E1 is offline 12E1  United Kingdom
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Quote:
Originally Posted by DF96 View Post
When designing power supplies and output stages it would be better for people to understand that copper wires (however 'oxygen-free') are resistors, albeit fairly low value resistors.
...And (although mostly off-topic for this forum) for switching power supplies and for any digital electronics, even short lengths of copper can have enough inductance to have a big effect on signal integrity.
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Old 17th March 2013, 05:38 PM   #78
DF96 is offline DF96  England
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Yes, although capacitance can sometimes help by turning copper into a short transmission line.
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