AC heaters ultimate hum limit

[ruffrecords]

you need 4700uF for per  300mA / heater... looks like u have two tubes pulling 300mA each!
i would at least triple that amount .... dont forget to ground it either !

The two heaters are in series so the current is still 300mA.

Cheers

Ian

 

[ruffrecords]

I see,
so let us posted on your experiments, this is a quite interesting thread.

The documents I emailed you about Heater supplies were of any use?

Yes, they were useful. They confirmed what I had read elsewhere and what others had said here.

Cheers

Ian

 

[kambo]

is this your current layout:
http://groupdiy.com/index.php?topic=62094.msg787753#msg787753

edit:  got confused with revisions on ur layout...  i c ur 12VAC layout

 

[kambo]

this design is what i use all the time.
http://groupdiy.com/index.php?topic=57166.msg728679#msg728679
without the RC stage, i was always getting hum.

 

[ding]

I then decided to try a simple dc heater supply just using a bridge rectifier and a 4700uF smoothing capacitor, which should reduce the hum amplitude applies to the heaters by about 20dB. To my surprise the overall hum was higher.

Can someone explain this to me? My erroneous understanding is that if you have DC HT and DC heaters there should be no/low hum due to the lack of AC inductance on the audio signal. Obviously if the DC is not a straight line and has some ripple you would get some inductance there but way less than with straight AC. What am I not understanding here?

 

[ruffrecords]

is this your current layout:
http://groupdiy.com/index.php?topic=62094.msg787753#msg787753

edit:  got confused with revisions on ur layout...  i c ur 12VAC layout

That is the new layout  which does have 6.3VAC heaters. There are 3 layouts in existence. The first was 12VAC only and that is the one I am currently experimenting with. The second layout is the one available in the PCB Emporium and this can be configured for 12V or 6V heaters. Lastly is the new layout, that you referred to - this is currently with the PCB manufacturer.

Cheers

Ian

 

[ruffrecords]

I then decided to try a simple dc heater supply just using a bridge rectifier and a 4700uF smoothing capacitor, which should reduce the hum amplitude applies to the heaters by about 20dB. To my surprise the overall hum was higher.

Can someone explain this to me? My erroneous understanding is that if you have DC HT and DC heaters there should be no/low hum due to the lack of AC inductance on the audio signal. Obviously if the DC is not a straight line and has some ripple you would get some inductance there but way less than with straight AC. What am I not understanding here?

You have it right.  You have to remember that the levels of hum I am measuring are very low. Typically 1mV at the output of an amplifier with 70dB of gain so referred to the input the hum level is about 0.3uV. and that is with AC heaters. In all my previous designs, the power supply PCB has been separate from the preamp PCB. In this case they are on the same board so as well as the heater ac voltage of 12V there is the HT AC of 240V. It just so happens that the HT AC is quite close to the grid of the first stage so even if the heaters were pure dc there would be a possibility of  coupling the HT ac to the grid of the first tube.

Cheers

Ian

 

[ruffrecords]

I have made some further progress. I have added a low drop out 12V regulator to the dc heaters circuit. I just tested it to be sure it works and initially it gives results very similar to those I got with the simple bridge/capacitor rough dc supply. However, because there is now no 100Hz component (and harmonics) in the heater supply, the 100Hz component in the output is no longer completely cancelled (although it is much lower). However, I now discover that there is an optimum polarity for the HT ac polarity that now reduces the 50Hz level by about 4dB and the overall noise level by about 3dB (see attached spectrum). SO it looks like hum is getting into the input circuit from several nearby sources, one of which I have now eliminated by using dc heaters. It will be interesting to see if the new PCB layout improves matters further.

Cheers

ian

 

[kambo]

did you use your DC heater supply that u used on ur first post on this topic ?
(dc supply with no hum on graph)

 

[kambo]

I understand you point about moving the transformer far away but I do not think magnetic induction is the issue as the hum does no vary with the orientation of the mains transformer.

Cheers

ian

hey Ian, i have been trying to introduce some hum to my existing pres...
PCB and P2P mic pres...
my power transformers are away from the boards/tubes... so i unscrewed them and started to move them close to the boards, couple of inches was closest i could get; due wires....
hum was getting higher...  20db+ more...
changing the axis + orientation of the transformers helped! but i wasnt able to achieve same results...
but not too far off as in ur graphs!

 

[ruffrecords]

I understand you point about moving the transformer far away but I do not think magnetic induction is the issue as the hum does no vary with the orientation of the mains transformer.

Cheers

ian

Can you post spectra showing transformer far away and close?

Cheers

ian


hey Ian, i have been trying to introduce some hum to my existing pres...
PCB and P2P mic pres...
my power transformers are away from the boards/tubes... so i unscrewed them and started to move them close to the boards, couple of inches was closest i could get; due wires....
hum was getting higher...  20db+ more...
changing the axis + orientation of the transformers helped! but i wasnt able to achieve same results...
but not too far off as in ur graphs!

 

[kambo]

here u go :

original position

 

[kambo]

very close : 2 inch

 

[kambo]

very close but adjusted for best hum in 3D....

 

[Henke]

What's the exact configuration of your DC heaters? I've been experimenting a lot with this over the years and usually have the best results with a "balanced" approach. That is, make sure to reference the midpoint (through either humdinger/CT on transformer end or at the heater CT if available) instead of one of the ends, so instead of eg 12.6V and 0V , you'll have +6.3 and -6.3V (or +/-3.15V with 6.3V heaters). The idea is to keep the residual AC ripple equal in both leads, and it will cancel more effectively than with one side "AC grounded".  This together with some 50 volts of elevation, to further reduce the cathode coupling. The approach can be used with or without regulators - i won't be bothered with them any more as i can get equally good results with one or two balanced RC sections with generous capacitance, which is much less prone to failure than a reg.

 

[kambo]

12.6 VDC, for dual 6sn7+5687 micpre
diode bridge+22000uf+1k(ish)+22000uf

 

[kambo]

more real life example :
output of DI connected to mic input : max gain on 6sn7+5687 micpre

(when i connect 200ohm resistor to mic input its picking my movements too...
proper pad etc connections are on the other side of the studio, its hassle to reconnect everything!

this should give you an idea!

org position

edit : analog synth is connected to DI in too... DI is Radial -ProDi

 

[kambo]

very close

 

[kambo]

note :

on first 3 test :
trx was close to right channel, but i was testing left channel
that is 4 inch away...

on second set of test, (with DI), i was close to right channel, and testing right channel
this is 2 inch away

 

[kambo]

ok, this is most proper test, but pretty much the same results on hum...

200ohm in to mic in ,. max gain on 6sn7-5687 micpre
mic out to Lynx aurora in....
fabfilter proQ is pretty trustful.