Phantom PCB

[baadc0de]

Hi all,

I received the PCBs today with thanks - nice work

I'd like to hook them up to some drip electronics fourseven preamps. Those guys are powered by a 370bx hammond transformer and the violet wire is unused by the PCB and specified as 50V. Can I get my power from there? The wire is declared as "Bias Tap with High Voltage C.T. Grounded" which I have no idea about.

The transformer is defined as:

550V C.T. @ 58ma.
5V C.T. @ 2A
6.3V C.T. @ 2A

And this is the schem:

can I get my power between violet and ... red/yellow?

Cheers,
B.

 

[ruffrecords]

can I get my power between violet and ... red/yellow?

Cheers,
B.

Possibly. Do you have a schemo for the PSU you can post? With that I will be able to better give an answer.

Cheers

Ian

 

[ruffrecords]

Here's the current state of play with the first batch of 30 phantom PCBs:

okgb            2-off        paid/posted
helterbelter    2-off        paid/posted
kazper          4-off        paid/posted
Bill Wilson     2-off        PM'd
kooma           2-off        paid/posted
peps            5-off        paid/posted
baadcose        2-off        paid/posted
neilsk          ??-off       PM'd
guy_4           2-off        paid/posted
mamiti          2-off        paid/posted
lyonsdk         3-off        paid/posted
neil            1-off        paid/posted

That accounts for 25. Because my PCB supplier over delivered I have 6 spare so if you want any pleas PM me ASAP. I'll update this post as things change.

Cheers

Ian

 

[letterbeacon]

can I get my power between violet and ... red/yellow?

Cheers,
B.

Possibly. Do you have a schemo for the PSU you can post? With that I will be able to better give an answer.

Cheers

Ian

Hi Ian,

I'm also planning to do a similar thing - using your circuit to derivive 48v from that Hammond 370bx transformer in a P2P REDD 47 build.  I'll be using the power supply in the schematic attached but with a couple of 1N1007 diodes to rectify the AC.

On the Hammond website it says this about the violet wire:

Bias Tap with High Voltage C.T. Grounded (VAC) - 50.

So do you think I'll be able to hook your circuit up to violet and red/yel to get 50VAC, whilst also using the 550V C.T. and the 6.3V?

Thanks a lot!

 

[gemini86]

I've seen the 50v tap used for phantom before... should be no problem.

 

[letterbeacon]

Great, thanks a lot!

 

[letterbeacon]

Just to confirm (and sorry if this is going OT a bit!), this is how I would connect up the secondary windings of the transformer:

Red + Red = go to the rectifiers of my power supply
Blk/ Yellow + Blk/ Yellow (5VAC) = go nowhere
Yellow + Yellow (6.3VAC) = go to my tubes' heaters
Violet (50VAC) + ? = go to the phantom power circuit

I'm slightly confused about what happens to the CTs on the secondaries.  Should I ground the High Voltage CT and the Low Voltage CT to star ground on my chassis?

 

[ruffrecords]

You need to be careful deriving phantom from the 50V tap on the Hammond transformer. As it is a single tap you can only use a half wave rectifier so you cannot use the bridge on the phantom power supply PCB.

Regarding the centre taps on the Hammond, these do form the 0V reference. So the CT of the HT winding is the HT zero volts. Since the 50V tap is also referenced to this CT then the phantom power zero volts is automatically the HT zero volts. This may or may not be what you require - normally you don't want to do this but in this case it is unavoidable.

The CT of the heater winding can also be connected top the HT zero volts assuming you do not want elevated heaters.

Cheers

Ian

 

[letterbeacon]

You need to be careful deriving phantom from the 50V tap on the Hammond transformer. As it is a single tap you can only use a half wave rectifier so you cannot use the bridge on the phantom power supply PCB.

I've just mangled your circuit to make it half wave (hope you don't mind!), how does the schematic attached look to you?  One of the problems with half wave is that it's hard to smooth, but hopefully because the current draw of phantom power is pretty small, the smoothing caps shouldn't discharge significantly.

Regarding the centre taps on the Hammond, these do form the 0V reference. So the CT of the HT winding is the HT zero volts. Since the 50V tap is also referenced to this CT then the phantom power zero volts is automatically the HT zero volts. This may or may not be what you require - normally you don't want to do this but in this case it is unavoidable.

The CT of the heater winding can also be connected top the HT zero volts assuming you do not want elevated heaters.

Cheers

Ian

Why would I normally not want to have the phantom power 0V be the same as the HT 0V?  Shouldn't everything in the case be referenced to the same ground?

Thanks for your help!

 

[ruffrecords]

I've just mangled your circuit to make it half wave (hope you don't mind!), how does the schematic attached look to you?  One of the problems with half wave is that it's hard to smooth, but hopefully because the current draw of phantom power is pretty small, the smoothing caps shouldn't discharge significantly.

That looks fine - just what I expected. Smoothing should not be a problem as you say for reasonable currents. Even with half wave rectification, a 100mA load should only cause about 500mV of ripple at the 4700uF

Why would I normally not want to have the phantom power 0V be the same as the HT 0V?  Shouldn't everything in the case be referenced to the same ground?

Ultimately yes, but a lot of noise performance depends on implementation and phantom power is particularly awkward to implement.

As a rule, you want the metalwork (chassis) to act as a screen and not carry signal currents. Then you connect your HT, and possibly phantom, zero volts to the chassis at one point, usually close to where the mains or power enters. That's all fine and easy enough to do with the HT zero volts but the phantom zero volts is different. Phantom power goes down the mic cable hot and cold wires and returns via the mic cable screen which is connected to Pin1 of your XLR - and we all know what we do with XLR Pin 1, we connected it to chassis right next to the connector. But you already connected phantom zero volts to the chassis some place else -  at the same place as the HT zero volts. This means that phantom zero volts signal current passes through the chassis to the XLR Pin1. This is bad news because any noise currents in the chassis now get superimposed on the phantom power 0V. The real answer is to connect the phantom power 0V to the chassis ONLY at the XLR inputs - that way the phantom power is not affected by noise currents in the chassis. Unfortunately in your case, the phantom 0V is already commoned with the HT 0V at the power supply.

Cheers

Ian

 

[letterbeacon]

Ah I see, thanks for the explanation -it's all a lot clearer now.

So what do you suggest I do?  Should I just get another transformer in the case to power the phantom board?  I was hoping to keep the one transformer in there but do you think a second one would be better?

Thanks!

 

[ruffrecords]

Ah I see, thanks for the explanation -it's all a lot clearer now.

So what do you suggest I do?  Should I just get another transformer in the case to power the phantom board?  I was hoping to keep the one transformer in there but do you think a second one would be better?

Thanks!

Well, this is a stand alone REDD 47 is it not? So, if it is all going to be in one single enclosure, I would recommend you connect the (common) HT and Phantom zero volts to the chassis right near the the mic input XLR. From this same (star) point, take a lead and connect it direct to the mains earth pin on your mains input connector.

Cheers

Ian

 

[letterbeacon]

Thanks for the reply!

So the thinking is that despite the phantom 0v and the common HT still being connected at the same point, the phantom 0V has a shorter distance to travel to the XLR pin, hopefully not picking up as much noise?

 

[ruffrecords]

Thanks for the reply!

So the thinking is that despite the phantom 0v and the common HT still being connected at the same point, the phantom 0V has a shorter distance to travel to the XLR pin, hopefully not picking up as much noise?

What you want to avoid is phantom current (power and signal) flowing in the chassis or if that is unavoidable, flowing over as short a distance as possible. You can do that by connecting phantom 0V really close to the XLR.  My mixers have external power supplies connected to the mixer by a screened cable (the screen is connected to the chassis at each end). The phantom 0V is completely separate from the HT 0V, it runs up the power cable and is connected to the chassis by the mic input XLRs. The HT V on the other hand is connected to chassis by the mains inlet in the power supply.

As I said, implementation can be complex!

Cheers

Ian

 

[baadc0de]

I can't find 1n5007, can I use 1n4007?

Shopping at farnell..

 

[ruffrecords]

I can't find 1n5007, can I use 1n4007?

Shopping at farnell..

Yes, 1N4007 will be fine.

Cheers

Ian

 

[zebra50]

Hi Ian,

Are there still some left - I'd be happy to take a pair off your hands.

Stewart

 

[ruffrecords]

Hi Ian,

Are there still some left - I'd be happy to take a pair off your hands.

Stewart

Yes, I have some left. 8GBP each including shipping. I am on holiday for a week starting Monday so don't send any money just yet.

Cheers

Ian

 

[morbleu]

I just red about this - since I'm building a drip 47 too I would like to join the club ... if there's one left for me 

andreas

 

[ruffrecords]

I currently have 6 phantom PCBs left from the last batch. They are 8GBP each including shipping.

PM me with your shipping details and quantity required and I'll send payment details by return.

Cheers

Ian