Power Transformers for Different Circuits

captainate

Member
2014-05-26 3:48 am
Bit of a noob question here:

I'm racking some old tube preamps and installing a phantom power supply. There's a transformer for the preamp power supply, and a transformer for the phantom power supply, and I'm assuming it's ok to connect the primaries of both transformers to the same IEC socket. I can't imagine why this wouldn't be fine, but there are two things I wonder about with this configuration

(1) I've never seen this done, I imagine most gear with a preamp and a phantom supply are designed as such from the outset and use the same power transformer with the appropriate windings and circuitry to only need one transformer.

(2) There are different recommended fuse ratings, is it ok to put each fuse after the split but before each transformer? IEC socket -> Power Switch -> Split to each fuse -> Power transformers for preamp and phantom power supply?

Phantom Power supply - http://sound.westhost.com/p96-f1.gif

Power Supply for Tube Preamps - Imgur: The most awesome images on the Internet
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Transformer primaries are fused for safety in case of either a short on the secondary side or some interwinding drama and so each should have its own appropriately rated fuse fitted. So while its fine to run two or more transformers from a common source (the IEC socket) each must be protected in its own right.
 

captainate

Member
2014-05-26 3:48 am
So basically my plan is correct, IEC first, then power switch, split off to the correctly rated fuses for each circuit, then to the transformers?

And is there an answer to why this particular configuration isn't common? Where are the 48v supplies in the average preamp coming from if not from a second transformer? Are they designing power supplies that create multiple power rails from a common power transformer?
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Yes, that's basically it. You always need to consider worst case scenarios... so both trx's are fused but what about any possible safety risk between the IEC inlet (is that fused to ?) and the switch. Suppose a wire came off the switch and touched the chassis or anything internal. So you really need a fuse on the input side as well.

I don't know if the configuration is common or not tbh, partly because I don't delve much into valve gear :)

You can derive a stable 48 volts via several methods using existing supplies, although which would be a best method depends on 'what is in there' to begin with. You can derive it from an already present higher DC rail or you could perhaps generate it independently from the existing windings on the transformer. As phantom supplies are generally low current there is also an option of using voltage doublers and triplers to get up to the required voltage... although that wont be an issue with valve gear.

Always ways and means though.
 
Most OEMs when designing a mixer or preamp will specify a power transformer that supplies all the needed voltages. They have zero incentive to go to the extra expense of installing two transformers, one for the main rails and one for 48v. If they have say a 30v ct main winding, they would also have a second winding for the 48v supply. And as Mooly siggested, sometimes they add it to a circuit by using a voltage doubler or voltage multiplier to raise the voltage from a lower voltage source.

Some tube power amps derive their bias supplies from a dedicated bias winding or tap, others derive it through a cap off the HV winding. No reason your tube circuit power supply could not be similarly adapted. Any bias tap used for making a negative supply can also be used to make a positive supply with a rectifier pointing the opposite way.

There is nothing "wrong" with using separate transformers, it is just inefficient and less cost effective. That is why you never see it. Like you never see an automobile with a motor for each wheel.
 
OK OK but main stream autos will never have two engines and transmissions. That was the point of the analogy. yes, the coming generations of electric cars may embrace golf cart technology and take it to new levels with an electric motor on each wheel. But I bet all those motors don;t each have their own battery and charger system.
 
So what would a second 48VDC power rail coming off a rectified and filtered 310 VDC supply look like? I already built the power supply in the original post before even considering the fact that I'd want phantom power as well.

Supposedly current draw for phantom powered microphones are around 10mA, so it's not like the power supply I've already built will even notice a difference. Most of the problems mentioned in Rod Elliott's introduction to Phantom Power are issues related to stepping up from a low voltage source, not down from a high voltage source.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
310 down to 48 means you need to 'lose' around 262 volts. The big issue with regulators is heat dissipation. 262 volts at 10ma is 2.62 watts. OK, so in a valve amp that's nothing :)

The simplest solution is a resistor and zener stabiliser.

Better is the same again but twice over, so resistor plus zener, and again resistor plus zener. The first pair could stabilise down to say 100 volts and the second down to 48.

I love designing efficient circuits and one possibility might be to use a 'wattless dropper' (a capacitor) off the transformer secondary feeding into a linear high voltage 3 terminal regulator such as the TL783. That could give a super clean and stable supply with lowish dissipation.
 
The proper sequence for safety is:

IEC - Fuse - Switch. You do not need to switch and fuse each transformer separately, but if you do, then you would use a multi-pole switch to switch the hot (and cold, if you like) to each transformer. Might be hard to find more than a 4-pole, 2-position power-rated switch however.