'Phantom Power' PSU - UK Vintage Radio Repair and Restoration Discussion Forum

robjkmannering · 20th Aug 2011, 3:58 pm

I found this vintage power supply in a local seconhand shop.
It appears to be an HT unit for portable valve sets that has been modified (in the 70's?) as a 30v regulated 0.1A PSU.
Does anyone know about the 'Phantom Power' name? Could it have been a kit?
The transformer has 45-0-45V secondary that would tie in with 90V HT and the later modification of the regulator (which works well) is built on veroboard from 2 BC107s and a TIP31 transistor. The front panel has been modified with dyno tape labels and the quality of construction is quite good.
Also, what would 30V @ 0.1A be used for? too high for logic and not split for op-amps. The current rating is also quite low.

Rob

Dave Moll · 20th Aug 2011, 4:09 pm

My only experience of the term "phantom power" is when the voltage to operate a device (e.g. a condenser microphone) is fed as a DC component along the signal cable. The device is then designed to filter out the two components as appropriate.

robjkmannering · 20th Aug 2011, 4:41 pm

Thanks Dave,
I have just read up on this. The standards for phantom power appear to be 48,24 and 12V at 1-2mA.
Maybe this is a homebrew version along those lines?
As the case is a lovely enammelled green, I think I will find a smoothing choke and reuse it as a valve portable supply.
Regards,
Rob

jay_oldstuff · 20th Aug 2011, 9:50 pm

I have seen audio kit in the past that to use with condenser mics you had to supply the phantom power from an external source. I remember a mixing desk we had at secondary school probably from mid 70's when the school was built that used an external PSU for phantom power. phantom power has now been pretty much standardised at 48V but in the past it was anything from 12v up to 50v. and i can tell you 48v from a wet mic hurts.

Jay

richrussell · 22nd Aug 2011, 10:26 am

I remember when the singer in a band I was in at university got his phantom powered microphone a little too close to my guitar. As it touched the strings, one of the pickup coils fizzed, releasing smoke, shortly followed by my amplifier making a rather large bang. Then the fuses went in both my amp and the microphone preamp (which incorporated the phantom supply). One £20 pickup and a couple of fuses and we were going again

That was 48V dc and obviously not current limited.

Kat Manton · 22nd Aug 2011, 5:42 pm

That sounds much more like an earth fault. Whether it's phantom powered or not, the microphone casing should be at mains earth potential and so should the guitar strings; each earthed via the equipment they're connected to. Touching two items together which should be at the same potential shouldn't blow fuses and fry pickups.

Phantom power is current limited by the way it's superimposed on the audio input. On the mixer diagram I've just looked at, the +48V rail is connected to the 'hot' and 'cold' microphone socket pins via 6.8k resistors. So the resistors are in series across the audio input signal, totalling 13.6k. They're in parallel as far as the DC supply is concerned, the source resistance being 3.4k (limiting short-circuit current to 14 mA.)

The DC supply can be superimposed on the audio signal via an input transformer instead of resistors, thus allowing more current without loading the audio input. (Centre-tapped primary, +48V DC connected to centre tap.) That's not very common though. From portable "powered mixers" (P.A. and mixer in same case) to high-end studio consoles, everything I've worked on (bar one huge studio console) uses a couple of resistors, limiting current to a few mA. Decent transformers are expensive and only really turn up in very old or very high-end equipment. Besides, the DC resistance of the windings would also limit current.

The supply is also designed (or should be) to survive short-circuits indefinitely. Some microphones contain matching transformers; depending on how they're wired these can present a low DC resistance between hot/cold and ground. Unbalanced equipment and cables are often wired so the cold input is shorted to ground. Short circuits are expected, designed-for and shouldn't cause damage.

OTOH, the mains supply has a fairly low source impedance...

Kat

richrussell · 22nd Aug 2011, 7:15 pm

Given the standard of most of my mate's kit, I wouldn't rule out an earth fault. And as we were all studying electronics at the University of York, you'd have thought we'd know better!

Kat Manton · 22nd Aug 2011, 9:06 pm

Given the state of some of the kit I've repaired, it wouldn't surprise me either! You'd think some people are trying to emulate Keith Relf...

I suspect there was too much blood in my caffeine stream, too...

Quote:

Originally Posted by Kat Manton

So the resistors are in series across the audio input signal, totalling 13.6k.

What I meant to say was there's a 6.8k resistor from each side of the balanced audio input to the +48V supply, which is at ground as far as AC is concerned. Or something. I should've just posted a diagram.

Regardless, due to how it's connected, the supply doesn't load the audio input much but also can't supply much current. It doesn't need to, a FET pre-amplifier in a condenser microphone doesn't need much.

I'm still not going to plug any of my guitars into a mixer with phantom power turned on, even though I don't think there's enough current available to fry a pickup even if it's directly connected. OTOH, one of them has a lot of onboard electronics and eats PP3 batteries in pairs so I'm tempted to modify it for phantom power.

Kat

mhennessy · 24th Aug 2011, 9:57 am

Don't think you were too far off, Kat. A genuinely floating source would indeed "see" 6K8+6K8. But having said that, the phantom resistors are in parallel with the rest of the input stage (should be around 1.2K in total), so just a small part of the scheme, relatively speaking...

Also concur that 6K8 seems to be standard practice IME. We still show conceptual diagrams with centre-tapped transformers here, but I don't recall seeing one in real kit. Instrumentation amplifiers seem to be the way to go - usually built discretely from 3 op-amps, which is cheaper than an SSM2017 or the modern equivalents...

I haven't seen many microphone schematics over the years as manufacturers don't tend to publish them, but I'm always impressed with the complexity - mostly because of the need to power everything via those resistors. But Douglas Self published a design that involved a DC-DC converter to make 32V from the incoming phantom power, which powered an AD822 dual op-amp, the input FET and another transistor. Additionally, 63 volts is generated to energise the capsule via a 10G(!) resistor. Sadly, no details of the power supply circuitry were given as it was a discussion about the audio challenges...

Is there a chance this was used for something other than audio? There are countless other possibilities in scientific instrumentation, for example...

Cheers,

Mark

20th Aug 2011, 3:58 pm	#1
robjkmannering Retired Dormant Member Join Date: May 2009 Location: Norwich, Norfolk, UK. Posts: 717	'Phantom Power' PSU I found this vintage power supply in a local seconhand shop. It appears to be an HT unit for portable valve sets that has been modified (in the 70's?) as a 30v regulated 0.1A PSU. Does anyone know about the 'Phantom Power' name? Could it have been a kit? The transformer has 45-0-45V secondary that would tie in with 90V HT and the later modification of the regulator (which works well) is built on veroboard from 2 BC107s and a TIP31 transistor. The front panel has been modified with dyno tape labels and the quality of construction is quite good. Also, what would 30V @ 0.1A be used for? too high for logic and not split for op-amps. The current rating is also quite low. Rob Attached Thumbnails

20th Aug 2011, 4:09 pm	#2
Dave Moll Dekatron Join Date: Feb 2005 Location: West Cumbria (CA13), UK Posts: 6,127	Re: 'Phantom Power' PSU My only experience of the term "phantom power" is when the voltage to operate a device (e.g. a condenser microphone) is fed as a DC component along the signal cable. The device is then designed to filter out the two components as appropriate. __________________ Mending is better than Ending (cf Brave New World by Aldous Huxley)

20th Aug 2011, 4:41 pm	#3
robjkmannering Retired Dormant Member Join Date: May 2009 Location: Norwich, Norfolk, UK. Posts: 717	Re: 'Phantom Power' PSU Thanks Dave, I have just read up on this. The standards for phantom power appear to be 48,24 and 12V at 1-2mA. Maybe this is a homebrew version along those lines? As the case is a lovely enammelled green, I think I will find a smoothing choke and reuse it as a valve portable supply. Regards, Rob

20th Aug 2011, 9:50 pm	#4
jay_oldstuff Octode Join Date: Jan 2005 Location: Hyde, Greater Manchester, UK. Posts: 1,074	Re: 'Phantom Power' PSU I have seen audio kit in the past that to use with condenser mics you had to supply the phantom power from an external source. I remember a mixing desk we had at secondary school probably from mid 70's when the school was built that used an external PSU for phantom power. phantom power has now been pretty much standardised at 48V but in the past it was anything from 12v up to 50v. and i can tell you 48v from a wet mic hurts. Jay __________________ The light at the end of the tunnel is probably the headlight of an oncoming train

22nd Aug 2011, 10:26 am	#5
richrussell Heptode Join Date: Jul 2008 Location: Selby, North Yorkshire, UK. Posts: 979	Re: 'Phantom Power' PSU I remember when the singer in a band I was in at university got his phantom powered microphone a little too close to my guitar. As it touched the strings, one of the pickup coils fizzed, releasing smoke, shortly followed by my amplifier making a rather large bang. Then the fuses went in both my amp and the microphone preamp (which incorporated the phantom supply). One £20 pickup and a couple of fuses and we were going again That was 48V dc and obviously not current limited.

22nd Aug 2011, 5:42 pm	#6
Kat Manton Retired Dormant Member Join Date: Jul 2005 Location: West Yorkshire, UK. Posts: 1,700	Re: 'Phantom Power' PSU That sounds much more like an earth fault. Whether it's phantom powered or not, the microphone casing should be at mains earth potential and so should the guitar strings; each earthed via the equipment they're connected to. Touching two items together which should be at the same potential shouldn't blow fuses and fry pickups. Phantom power is current limited by the way it's superimposed on the audio input. On the mixer diagram I've just looked at, the +48V rail is connected to the 'hot' and 'cold' microphone socket pins via 6.8k resistors. So the resistors are in series across the audio input signal, totalling 13.6k. They're in parallel as far as the DC supply is concerned, the source resistance being 3.4k (limiting short-circuit current to 14 mA.) The DC supply can be superimposed on the audio signal via an input transformer instead of resistors, thus allowing more current without loading the audio input. (Centre-tapped primary, +48V DC connected to centre tap.) That's not very common though. From portable "powered mixers" (P.A. and mixer in same case) to high-end studio consoles, everything I've worked on (bar one huge studio console) uses a couple of resistors, limiting current to a few mA. Decent transformers are expensive and only really turn up in very old or very high-end equipment. Besides, the DC resistance of the windings would also limit current. The supply is also designed (or should be) to survive short-circuits indefinitely. Some microphones contain matching transformers; depending on how they're wired these can present a low DC resistance between hot/cold and ground. Unbalanced equipment and cables are often wired so the cold input is shorted to ground. Short circuits are expected, designed-for and shouldn't cause damage. OTOH, the mains supply has a fairly low source impedance... Kat

22nd Aug 2011, 7:15 pm	#7
richrussell Heptode Join Date: Jul 2008 Location: Selby, North Yorkshire, UK. Posts: 979	Re: 'Phantom Power' PSU Given the standard of most of my mate's kit, I wouldn't rule out an earth fault. And as we were all studying electronics at the University of York, you'd have thought we'd know better!

24th Aug 2011, 9:57 am	#9
mhennessy Dekatron Join Date: Jul 2007 Location: Evesham, Worcestershire, UK. Posts: 4,244	Re: 'Phantom Power' PSU Don't think you were too far off, Kat. A genuinely floating source would indeed "see" 6K8+6K8. But having said that, the phantom resistors are in parallel with the rest of the input stage (should be around 1.2K in total), so just a small part of the scheme, relatively speaking... Also concur that 6K8 seems to be standard practice IME. We still show conceptual diagrams with centre-tapped transformers here, but I don't recall seeing one in real kit. Instrumentation amplifiers seem to be the way to go - usually built discretely from 3 op-amps, which is cheaper than an SSM2017 or the modern equivalents... I haven't seen many microphone schematics over the years as manufacturers don't tend to publish them, but I'm always impressed with the complexity - mostly because of the need to power everything via those resistors. But Douglas Self published a design that involved a DC-DC converter to make 32V from the incoming phantom power, which powered an AD822 dual op-amp, the input FET and another transistor. Additionally, 63 volts is generated to energise the capsule via a 10G(!) resistor. Sadly, no details of the power supply circuitry were given as it was a discussion about the audio challenges... Is there a chance this was used for something other than audio? There are countless other possibilities in scientific instrumentation, for example... Cheers, Mark