Valve mic power supply

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NOON

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I have to build a power supply for a Neumann KM56, 120V at ~1mA and 4V at ~100mA.
First thoughts are bog standard suitable TX, rectifiers, filters, TL783 for the 120V and LM317 for the 4V with trimpots for fine adjustment. But second thoughts are 'can this be improved?'

Any ideas on whether it's worth going to more effort/expense for this power supply?

Any ideas for fancy features? One idea that came to mind is a delay circuit for the high voltage that detects current on the filament rail and then switches a relay on after about 30 seconds. Gives the valve some time to warm up before HT hits and keeps the HT safely isolated from the outside world when nothing is connected.
 

abbey road d enfer

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You should check the NKM schematic. You can se that hey start with a very clean 4.2V (NiCad batteries regulation) and add two 2.25 ohm resistors in series. One of these resistors can be bypassed when using very long cables. This is what provides surge protection. Tube filaments are quite resilient in that respect, no need to make it too complicated. You must make sure the regulator is as clean as possible, since it's also what provides the tube's bias.
HT is less a problem since you can add as many RC's after the 783.
 

NOON

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Interesting scheme. I have a distrust of batteries though, recently reinforced by pulling apart a Mac SE/30 I found where the backup battery had s%$t itself all over the insides, eating it's way through the motherboard and some of the chassis metalwork. :)

It looks like the Neumann NN48a PSU is nearly identical to the NKM but in the NN48b they've changed to a zener regulated design. I'm struggling to get my overtired brain around the regulation scheme right now, a quick glance said 'capacitance multiplier' because I saw a cap on the base of a transistor, but then I realised it was on the low side. I still think it's doing something like that but it's too clever for my current state of mind, I'll try again later.
 

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NOON

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Or should I just make my life easier by using a modern ridiculously low noise regulator like the LT3045?
 

Tubetec

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One version of the Sony C37 psu uses a 6x4 tube rectifier which would gradually ramp up ht ,extra heater winding on the mains tx required of course , people will argue all day and all night it makes no difference and in the shorter term maybe thats true , Ive come to conclude though that a gradual ramp up of ht means less stress on the caps and makes sure the cathode is warming before the anode starts pulling electrons off it makes life easier for tube at switch on. With modern big high voltage caps and  the minuscule current involved in tube mics the noise levels you want are very do-able passively with RC smoothing .

Ive also seen the low ohm series resistors in each leg of the heater supplies like Abbey speaks of  , while your gear is operating ,yes a small loss in heat and volts , its at the moment of switch on these guys do their work , when the cold heater is a fraction of its operating resistance, not only as the heater warms does its resistance rise ,physically it needs to expand also, limiting the inrush current is kinder to your heater/cathode insulation . Its not something your going to notice over the course of weeks months or even years  ,more on a geological time frame with tube mics  :D
that being said Ive also seen series chained heaters coupled with no standby on guitar amps that eats out heater cathode insulation very very quickly .
 

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NOON

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The cathode is effectively directly heated in the original circuit of the KM56 (which mine is) and therefore requires a very low noise heater supply. I'm thinking of making up a board that has both LM317 and LT3045 heater regulating circuits so I can experiment with both. Will be an interesting learning experience.
 

Tubetec

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I did try the directly heated approach on an EL84 based preamp before  tying the + end of the heater to cathode, as it happened my heater and grid/cathode voltage co-incided .
Loss of the cathode resistor and associated bypass cap did something I liked very much to the tone ,but as you mentioned lost me a lot of ground in S/N terms . I figured i needed around a 30db quieter heater psu for direct connection between cathode and heater.
 

NOON

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The KM56c version changed the biasing scheme to a more conventional one with less demands on the power supply, but it seems the general consensus agrees with you that the original sounds better.
 

NOON

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I've sketched out this schematic. A bit of overkill here and there but why not for a one off?

Heater rail has a capacitance multiplier pre-reg and then a choice of LM317 or LT3045 regulator. I've taken them to separate outputs so I can fit a switch to quickly flick between them to see if I can hear any difference.

Still considering a delay and relay circuit off the heater to switch the HT after it's had time to warm up.
 

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