ruffrecords
Well-known member
kambo said:
The two heaters are in series so the current is still 300mA.
Cheers
Ian
AC heaters ultimate hum limit
- Thread starter ruffrecords
- Start date
Help Support GroupDIY Audio Forum:
The two heaters are in series so the current is still 300mA.
Cheers
Ian
ruffrecords
Well-known member
Whoops said:
Yes, they were useful. They confirmed what I had read elsewhere and what others had said here.
Cheers
Ian
ding
Well-known member
ruffrecords said:
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
Well-known member
kambo said:
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
Well-known member
ding said:
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
Well-known member
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
Cheers
ian
Attachments
ruffrecords said:
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
Well-known member
kambo said:
Henke
Well-known member
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.
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
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
