One experiment to see if noise is coming from heater or high voltage is to get 4 AA size batteries and run the heaters on battery power. Disconnect the heater power at the transformer. The battery power is not a solution just a test. if the buzz is still there when on battery powered heaters you know it is not coming in via heaters and must be a B+ power thing or even florescent lighting in the house. If the battery fixes the buzz then you know you still have a problem with the heater supply. AA size batteries will last long enough for several tests.
Also is the microphone plugged in and how much cable is there. Or is this a test with no mic cable attached?
No mic attached. Just R50 is plugged into XLR.
Made experiments with disconnecting power:
a) disconnected anode AC for couple of seconds - preamp run on caps, no changes in buzz.
b) disconnected heater AC for couple of seconds - buzz disappeared. With reconnection buzz reappeared.
So, it needs more smoothing caps or a choke on heater power, or it needs a regulator.
There is already 4 RC chains (10000u and 10R) that went with no ripple in simulations. Or should I increase resistors, let say to 100R?
Made experiments with disconnecting power:
a) disconnected anode AC for couple of seconds - preamp run on caps, no changes in buzz.
b) disconnected heater AC for couple of seconds - buzz disappeared. With reconnection buzz reappeared.
So, it needs more smoothing caps or a choke on heater power, or it needs a regulator.
There is already 4 RC chains (10000u and 10R) that went with no ripple in simulations. Or should I increase resistors, let say to 100R?
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Common-mode chokes in raw supplies, especially the heater supply. Bypass each side of the heater to ground separately with identical caps. Take a look at the preamp supplies in "Valve Amplifiers," or if you're using regulators, the differential regulation and bypassing I showed in "His Masters Noise."
I've made following experiment.
Disconnected heaters from PSU.
Connected heaters to car booster, it produces around 12+ DC.
Started the preamp - works, no buzz.
Connected R 20 Ohm to the heater circuit of PSU that should simulate heaters for the PSU. Buzz appeared. Disconnected the 20R - no buzz.
So, I suppose, heater's voltage is clean enough, it is the mains transformer under heater's load radiates EMI that is picked up by the preamp. Am I correct?
What to do? Thin aluminium shielding does not help much.
I am thinking about a separate external PSU (a wall wart) just for heaters.
Any other ideas?
Disconnected heaters from PSU.
Connected heaters to car booster, it produces around 12+ DC.
Started the preamp - works, no buzz.
Connected R 20 Ohm to the heater circuit of PSU that should simulate heaters for the PSU. Buzz appeared. Disconnected the 20R - no buzz.
So, I suppose, heater's voltage is clean enough, it is the mains transformer under heater's load radiates EMI that is picked up by the preamp. Am I correct?
What to do? Thin aluminium shielding does not help much.
I am thinking about a separate external PSU (a wall wart) just for heaters.
Any other ideas?
This is a point on which Morgan and I have disagreed. I use resistor strings. His rationale for the THINGY is that there could be heater-cathode leakage currents which would upset the voltages in the string. My riposte is that if there's that much leakage current, replace the tubes!
Hi Ian. There is no caps across each diode. But I've placed 0.1uF and serially 500R across heater secondary.
But actually most crucial thing was R before the first reservoir C.
Without this R the C sucks too much current that in pikes overloads transformer, and being toroid it, I think, got saturated and buzzed.
Buzz become less than hiss after I've placed 4.7R between first C and diode bridge. Other three Rs I've decreased from 10R to 3.3R to avoid excessive voltage drop.
Currently there is four RC cascades after rectifier: (4.7R, 10000uF)||(3.3R, 10000uF)||(3.3R, 10000uF)||(3.3R, 10000uF). Voltage with heaters load is 10.2V. It is less than required 12.6V (heaters a wired parallel in each envelope, and sequentially between envelopes).
I read different opinions regarding decreased heaters voltage, from "less noise", "longer life", to "dull sound" and "shorter life due to cathode poisoning".
I do not know, but here is little I can do. Without load it shows 13.2V after rectifier. Even with a regulator it will be dropped by at least 2.5V to be smooth.
My transformer is rated 2x6.3V 1.5A. May be it is too weak for 2 envelopes, may be it should be 2x2A?
What do you think about heaters voltage decreased to 5V?
BTW, I tried with regulator, it buzzed whatever voltage I set. For very same reason - no R between rectifier and first C. (here is regulator schematic http://analogmetric.com/download/HV400 variable high voltage regulator New Schematic.pdf )
Yes, I'd agree. I use just resistor divider.
One big disadvantage of mu followers is the need for separate heater supplies for top and bottom if you want to do things optimally. If you can't do that, then indeed you want to make the heater supply extremely quiet, both differential mode and common mode, and you may have to do some tube selection for minimum heater-cathode leakage. See the power supply design for Morgan Jones's mu follower RIAA stage for a good example of what's needed.
There is another secondary winding in the transformer - 18V AC, 1A.
I rectified it through a separate bridge to DC and paralleled with 12.6V DC, 1.5A.
So to help the latter to cope with current.
Yes it decreased current through secondaries while increased voltage for heaters to required 12.6V DC.
But it did not decrease buzz.
Whatever I do I cannot decrease buzz lower than -65dB.
Buzz decreases when current through transformer decreases. There is no buzz if heaters are feed from battery. The buzz appears as soon as I put some comparable resistance across heater's PSU output. The less resistance the less buzz. And vice versa.
Does it mean that 60W transformer is not enough?
Should I resort to a separate external heaters PSU, probably switch mode?
I rectified it through a separate bridge to DC and paralleled with 12.6V DC, 1.5A.
So to help the latter to cope with current.
Yes it decreased current through secondaries while increased voltage for heaters to required 12.6V DC.
But it did not decrease buzz.
Whatever I do I cannot decrease buzz lower than -65dB.
Buzz decreases when current through transformer decreases. There is no buzz if heaters are feed from battery. The buzz appears as soon as I put some comparable resistance across heater's PSU output. The less resistance the less buzz. And vice versa.
Does it mean that 60W transformer is not enough?
Should I resort to a separate external heaters PSU, probably switch mode?
I don't understand. You can't parallel two voltage supplies unless they are absolutely identical in all respects.mm7 said:
If buzz occurs as soon as the DC heater supply has a load attached then you have coupling between the heater supply and the main circuit. This could be via wiring or via the mains transformer. You could try a choke input heater supply, as this will reduce charging pulses. The choke may be big, though!
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