6.3VAC heaters to filtered/regulated 6.3VDC. best solution?

Hello,

I'm trying to sort out some 50hz hum problems on my two channel NYD two bottle preamp. The one solution that gets most commonly suggested is to convert the raw 6.3VAC heater, grounded with 100ohm resistors, to 6.3VDC. I couldn't find one single answer here on prodigy pro everyone would agree with, as there seems to be different schools of thought on the issue.

First of all, should it be rectified and filtered only, or should I also regulate it?

What about grounding of DC heaters? Do I just use the ground rail common to all of the preamp?


The PSU is specified as 6,3VAC 3A, of which the tubes use just under 2A in the 2 channel two bottle. Would this prove a problem in converting to DC?

Cheers,

Michael

I don't know this circuit but heaters are generaly unregulated AC and come directly from power xfmr windings

With a 6.3VAC transformer, you will probably need to stick with an unregulated supply. Regulators need several volts higher input VDC than the desired regulated output voltage.

In *theory*, a 6.3 VAC secondary will produce 6.3 * 1.414 = 8.9 VDC. But, that assumes NO voltage drops in the diodes in the rectifier, and an infinitely large filter capacitor.

Per this article from an old National Semiconductor manual (see page 4):

http://www.brianroth.com/library/national-ps-design.pdf

2000 uFd with a 1 Amp load will result in 3V p-p ripple. That means the "ideal" (and that ignores the diode drops!) rail will sag from 8.9 VDC to 5.9 VDC during parts of the AC mains swing.

I would suggest at least 5000 uF for a 2 Amp supply. Select diodes with the lowest possible voltage drop at 2 Amps. If needed (most likely) to "tune" the final voltage, install a dropping R between the output of the bridge and the positive side of the filter cap.

Bri

You could also do a little experiment to see if d.c. heater voltage is going to help. Set things up to be able to rapidly switch off the primary of a separate transfomer for the heaters while listening closely to the hum. The heaters will cool rapidly but not so quickly that you can't hear a second or two of hum reduction, if indeed the a.c. voltage is a significant contribution.

I don't have a schemo of this one in front of me right now, but some circuits don't like to have sustained voltages on other tube electrodes with the cathode cold, so make the experiment brief.

michael

Do you have the heater circuit grounded at all? If your heater circuit is not grounded at all expect hum. When your talkng about the 100 ohm resistors that is a thing that is normally done with ac heaters so that part of the circuit is grounded. A single 100ohm resistor from both sides of the 6.3v to ground.

If you go with a rectified and smoothed option, you have to be careful you don't substitute one problem for another. There could be several reasons why hum is coupling from your heater circuit into the signal path, and one route is likely to be capacitative. FW rectified and partially smoothed AC has many harmonics of 100 (120) Hz, and can be more invasive through capacitative coupling than the original AC. Add this to the usual hum suppression measures of original heater circuits such as centre-tapping the heater winding to ground, and you are building up for a disappointment.

DC firing of heaters was, of course, the traditional solution to hum coupling in the highest-quality gear of old, although generating low-voltage, high-current regulated DC was not easy in the pre-semiconductor era. It was not uncommon to have an external power unit with a huge multi-voltage hawser connecting it to the audio equipment.

Nowadays, a fully-regulated 2A DC supply set to 6.3V output is a cheap and reliable item. If you were to go that route, make sure you include a relay that connects the mains to the transformer in the audio equipment only when the DC is present on the heaters. Some valve (tube) designs rely on currents flowing in anode circuits to avoid over-volting other components, and loss of heater power can be a disaster.

You don't need to be too particular about where you ground a DC heater circuit, as there should be no AC component to balance out. If I understand your post correctly, the original cicuit has a series pair of 100 Ohm resistors across the heaters with their centre point taken to ground. I can't see why on earth the resistors are as low as 100 Ohm, but doing it this way is the next best thing to having a centre-tapped heater winding. For DC work, there is no point in changing the configuration, but I would up the values to at least 1KOhm to reduce unnecessary power dissipation.

Brian Roth and Boswell,

thank you for that. I suspected it would have to be big caps. dang!

bcarso,

sounds simple enough. I'll try. I have a fast spectrum analyser so I can see the result within less than half a sec, hopefully not killing any parts while at it.


Rob Flinn,

yes the heater is grounded with this virtual center tap, using 100ohm resistors. It's a remarkable difference to no ground at all.


Thanks everybody.


Oh speaking of other sources of hum, I was jiggling the PSU transformer around to find an optimal position. I was able to shave off about 5-10dB of hum just by doing that. The channel closest to PSU seems to automatically have about 10dB of more hum, no matter what I do. I also have an aluminum compartment in the 2U rack just for the PSU. That only shaves off about 1-2dB of hum.

Aluminium or other light alloys and stainless steel are no good at containing magnetic fields, as they are not ferro-magnetic. You need to use tinplate (sheet mild steel plated with tin to prevent rusting) or, if you can afford it, mumetal to contain hum fields (in or out).

I think your best bet for the heater circuit would be an external 6.3V DC supply, but a linear, not a switched-mode one.

However, since you are getting field effects from the main transformer, you may have to consider re-building the whole power supply section on a separate chassis that you can put at a short distance from the audio circuit, if that is not what you have already done. You could then try a tinplate screen between the two.

Since I can't get to the electronics store on sunday to get parts fot the DC converter, I had already in mind to strip away the PSU and move it out of the rack.

Too bad it's such a sunny day out today.

I think your best bet for the heater circuit would be an external 6.3V DC supply, but a linear, not a switched-mode one.

Click to expand...

Excuse the beginners question, but how would this differ from the rectified/filtered VAC to VDC I had already planned?

The mains transformer I have is of very high quality (A German Welter) so I'm hesitant to *not* use its 6.3 out as well, since it was designed for heaters after all.

Boswell speaks sooth, particularly the perils of higher harmonics arising in the rectification process and their audibility.

When saying "linear" typically this means a linear regulator as opposed to a switching one. But since as Brian has pointed out you really don't have enough headroom out of your existing 6.3VAC winding to reliably rectify, filter, and regulate, you should either use another trafo or go with passive filtering and adjust a series R to give proper voltage on the heater---if it looks like d.c. heating is going to help that much. EDIT: In fact, put the series R between an output leg of the bridge and the big cap and you will cut down on the peak currents out of the trafo and reduce high frequency content in the magnetic field emissions.

But---the fact that you are getting large changes from trafo orientation indicates your problems are, initially at least, dominated by pickup loops in the circuitry. If possible identify the loops and make them smaller in area.

A final trick: bias the heaters slightly positive (~10V) relative to the cathode. This curbs the tendency of the heater itself to act like a little cathode. If the real cathode is not tied hard to ground (a.c. ground througn a big cap, or some other low Z) noise and hum can couple in this way as well. The schematic should be inspected to assess this mechanism.

[quote author="Kingston"]

The mains transformer I have is of very high quality (A German Welter) so I'm hesitant to *not* use its 6.3 out as well, since it was designed for heaters after all.[/quote]

Bear in mind that regardless of quality power transformers are challenged driving high pulsed-current loads like diodes feeding capacitor input filters. Resistive loads are far more benign as far as their effect on the spectrum of trafo magnetic field emissions.

Sure, you can do the ac-dc rectified thing. It works fine. You don't need to regulate it, unless you ac mains is all over the place. Just fine tune the resistor voltage drop.

Keep in mind, you'll only get about 1/2 the rated current if you do this!

The conversion to dc has a lot of losses, especially in the self-resistance of the secondary windings. It limits peak current, and no matter how many uF you add, the average dc voltage will not rise. So first make sure the ac current rating of the tranny is twice as much as you need for output. Otherwise forget it. Use an external supply.

If you're ok, use schottky rectifiers. Overrate them to minimize loss. Check out the Cornet phono stage schematics at my website for a circuit to look at.

jh

You don't need a ton of capacitance if you use a reg. You need the DC pulse to have a min above reg dropout, of course.

I really doubt that AC heaters are the root of the problem.

[quote author="NewYorkDave"]I really doubt that AC heaters are the root of the problem.[/quote]

Hear, hear!

Peace,
Al.

[quote author="NewYorkDave"]I really doubt that AC heaters are the root of the problem.[/quote]

Could be they're not, but with my skills I'm sort of shooting in the dark. I don't know how to locate or measure the ground loops within the actual signal path.

Right now I'm moving the PSU out of the rack, but the problem could be layout related just as well.

I'll also try rotating and moving the audio transformers as well.

Don't forget the ol "turn the pwr sw off and listen" trick.
If the hum disappaears as soon as you kill power, it is from the circuit.
If the slowly fades with the pwr, most likely an external source of noise.

Click on fil 1 3 4 5 and 7 for some heater circuits from the freaks who know better than to use regs in a heater supply.

http://vacuumbrain.com/The_Lab/TA/Power_Supplies

PSU is out of the box now, although not an awfully long way away from the rack. Seemed to make a 10dB or so difference on the 50hz peak. rotating it around affects about 2dB. Same with the audio transformers.

CJ,

the hum fades slowly with the power. Oh and thanks for those links.



I think I'll call it a day and admit this hum troubleshoot goes way over my head.. at least today. Spectrum shows otherwise perfect noise floor, but both channels have approximately +20dB 50hz hum above the general noise floor, despite the used gain setting.

It's negligible in practise unless I use a ribbon mic.

If the hum fades slowly (this is essentially the same experiment I suggested earlier, although I proposed that you switch the heaters off independently) then forget the heater supply for the moment. And for that matter, note that your hum from inperfectly filtered power supply rails is also ruled out. You may return to these when you've found your primary source, but for the moment if there is no dramatic difference on power-down don't spin your wheels worrying about the power supplies.

What does this leave?

Pickup. Pickup from the outside world, or at least the world just inside the box (the mains cabling to the power switch and trafo for example).

If it is magnetic field pickup then look for a source nearby, like some other piece of equipment's power transformer. Reorienting the unit relative to this interference should make a substantial difference.

If it is electrical, then better shielding with simple electrically conductive material should do the job. Where are the sensitive circuit nodes relative to the mains input and switch? Put a shield tied to circuit common between them. Are those same nodes looking out into the world somehow? Prevent this with more complete shielding. Is the hum getting in through imperfectly shielded cable? Less likely if it is a balanced input, but possible. Electric field shielding is easy compared to magnetic.

Is it conducted noise? There is a capacitance between your power transformer primary and secondary windings. If you pull out the plug to switch off rather than use the power switch and the hum immediately goes away then it's this mechanism. If this is present when you have other things connected to the preamp (and you would have to have something connected that's allowing you to hear and/or measure the hum) then you may have currents flowing in "grounds" that are getting into the signal path somehow. This is much less of a problem with balanced systems using transformers, but if you work hard enough at it you can manage to get trouble. In some circles this is known as the "Pin 1" problem, as it arises from a poor choice of where to return the no. 1 pin(s) on the XLR connector(s).

Good analysis, bcarso.

I don't think we've been told whether the hum is there when the inputs to the preamp have been disconnected and replaced by short circuits. This would diagnose effects due to conducted hum and ground loops at the input.

By the same token, what is the pre-amp output connected to? Is conducted hum coming back in via the output leads? Not so easy to do a simple check here, but sometimes you learn something about the problem by shorting the preamp outputs in turn with the output cables still connected and the preamp inputs shorted.