Phantom PCB

[ruffrecords]

I'm hoping to connect up the phantom PCB this weekend, but I wondered if I could run this by you:

I'm using a toroid transformer with dual 0-25VAC secondaries.  I connect the two 25VAC leads to the two VAC inputs on the phantom PCB.  I tie the two 0V leads together, but where should I connect those?  At the star ground point in my pre amp that's connected to the chassis?

You need to connect the two 25VAC windings in series to give a single 50VAC. So connect the 25VAC of the first to the 0V of the second and take the 50VAC between the 0V of the first and the 50VAC of the second. Check with a meter on its AC range after you have done this that you do get 50VAC.

I'm using the 48VDC to connect to a JLM go between kit.  Where shall I connect the 0VDC?  Pin 1 of the XLR input?

Thanks for your help.

The 0VDC should go to pin 1 of the XLR input. This same pin 1 should also be connected directly to the chassis as close to the XLR as possible. The system 0V should be connected to the chassis at the power supply.

Cheers

Ian

 

[earthsled]

Hi there -- I have a few questions, if you don't mind...

If I were to spec a custom power transformer winding for this circuit, would 50VAC be the ideal voltage? Would a slightly lower voltage like 40VAC be better to use for less heat/power dissipation?

Also, if my current draw is around 100mA (max) what would the current spec on the secondary winding need to be? I calculate about 350mA -- is this correct?

Finally, is there any harm in substituting 4x 1000uF caps in parallel for the 4700uF cap (considering 100mA of current draw)?

Thanks!

 

[ruffrecords]

Hi there -- I have a few questions, if you don't mind...

If I were to spec a custom power transformer winding for this circuit, would 50VAC be the ideal voltage? Would a slightly lower voltage like 40VAC be better to use for less heat/power dissipation?

You need 50VAC to get about 70V raw unsmoothed at the input to the regulator chip. It likes to see about 20V across it in order to reduce ripple to very low levels.

Also, if my current draw is around 100mA (max) what would the current spec on the secondary winding need to be? I calculate about 350mA -- is this correct?

Depends. if you are using a full wave bridge then the transformer current rating should be 1.61 times the dc load (161mA). If you are using a centre tapped secondary it should be the same as the dc load. See Sowter's how to specify ht windings for detials:

http://www.sowter.co.uk/rectifier-transformer-calculation.htm

Finally, is there any harm in substituting 4x 1000uF caps in parallel for the 4700uF cap (considering 100mA of current draw)?

That should be fine.

Cheers

Ian

 

[earthsled]

You need 50VAC to get about 70V raw unsmoothed at the input to the regulator chip. It likes to see about 20V across it in order to reduce ripple to very low levels.

Ah. Okay. I gather this is a bit different from the typical bridge supply which would need Vdc x 0.71 -- is this correct?

Thanks!

 

[ruffrecords]

You need 50VAC to get about 70V raw unsmoothed at the input to the regulator chip. It likes to see about 20V across it in order to reduce ripple to very low levels.

Ah. Okay. I gather this is a bit different from the typical bridge supply which would need Vdc x 0.71 -- is this correct?

Thanks!

I think it is the same. The required dc is 70V which times 0.71 is just over 49VAC.

Cheers

Ian

 

[earthsled]

I think it is the same. The required dc is 70V which times 0.71 is just over 49VAC.

My mistake. I was thinking about the DC post regulation - but of course, that doesn't make sense -  sorry.

So, with 20V across the reg at 100mA, I should expect about 2 watts of dissipation - correct? I may choose a slightly larger heatsink if this is the case.

Thanks for your help!

 

[earthsled]

One more question --

Depends. if you are using a full wave bridge then the transformer current rating should be 1.61 times the dc load (161mA). If you are using a centre tapped secondary it should be the same as the dc load.

-- is there any need to add a safety margin to the transformer's current rating, or is it safe to use one rated for exactly 161mA?

Thanks!

 

[ruffrecords]

One more question --

Depends. if you are using a full wave bridge then the transformer current rating should be 1.61 times the dc load (161mA). If you are using a centre tapped secondary it should be the same as the dc load.

-- is there any need to add a safety margin to the transformer's current rating, or is it safe to use one rated for exactly 161mA?

Thanks!

I think it is always wise to add a safety margin if for no other reason than to reduce the effects of mains voltage variations. I would have thought specifying 200mA would be sufficient for your application.

Cheers

Ian

 

[letterbeacon]

I'm hoping to connect up the phantom PCB this weekend, but I wondered if I could run this by you:

I'm using a toroid transformer with dual 0-25VAC secondaries.  I connect the two 25VAC leads to the two VAC inputs on the phantom PCB.  I tie the two 0V leads together, but where should I connect those?  At the star ground point in my pre amp that's connected to the chassis?

You need to connect the two 25VAC windings in series to give a single 50VAC. So connect the 25VAC of the first to the 0V of the second and take the 50VAC between the 0V of the first and the 50VAC of the second. Check with a meter on its AC range after you have done this that you do get 50VAC.

Cheers

Ian

Hi Ian,

I've just hooked up your PCB and it all works perfectly, thank you very much!

One question though - I hooked up the power transformer as you described above, but what shall I do with the 25Vac of the first winding and the 0Vac of the second once I've connected them together?  I thought I should connect them to the chassis but my transformer wasn't very happy with me when I did that!  Should I just leave them connected and isolate them from everything else?

Thanks,

Matt

 

[s2udio]

The link is fine .....the transformer becomes a series linked 25-0-25v transformer ...=0-LINK-50v
Do not put the centre tap[LINK] to GND ..
As the grounded end of the single winding will be a dead short....

 

[letterbeacon]

Great, thank you!

 

[ruffrecords]

I think it is the same. The required dc is 70V which times 0.71 is just over 49VAC.

My mistake. I was thinking about the DC post regulation - but of course, that doesn't make sense -  sorry.

So, with 20V across the reg at 100mA, I should expect about 2 watts of dissipation - correct? I may choose a slightly larger heatsink if this is the case.

Thanks for your help!

That is correct. I forget the exact spec.of the heat sink but it should be quite adequate for 2W of dissipation. At worst it will have a thermal resistance of around 10 degress C per watt which means effectively it will rise little more than 20 degrees C above ambient at full load.

Cheers

Ian

 

[earthsled]

Hi Ian,

I have a +24V off-the-shelf linear power supply. The power transformer in this particular supply outputs about 30V-0-30V AC. Your circuit requires 50V AC, but 60V seems fairly close. So, would it be possible to use the power transformer in this supply to feed both the +24V circuit and your +48V PCB?

Thanks!

 

[ruffrecords]

Hi Ian,

I have a +24V off-the-shelf linear power supply. The power transformer in this particular supply outputs about 30V-0-30V AC. Your circuit requires 50V AC, but 60V seems fairly close. So, would it be possible to use the power transformer in this supply to feed both the +24V circuit and your +48V PCB?

Thanks!

60V is fine for the phantom PCB, you just need to make sure the reservoir capacitor is rated at 100V.

Using the 30-0-30 transformer might be difficult. The problem arises because at some point the 0V of the +24V supply and the 0V of the phantom supply will need to be connected together. This means, if they use the same transformer winding, they must have one terminal of that winding as their common 0V. That in turn means you cannot use full wave bridge rectifiers.

So, the answer to your question is yes it is possible but you would have to use half wave rectification on both supplies. WHile this is perfectly possible, you do have to make extra effort to reduce ripple.

Cheers

Ian

 

[letterbeacon]

I've always had a bit of hum in my BA-2 pre amp, and always assumed it was the poor layout of the heater wires.  It turned out that this was the source of a fair bit of the hum which I have now got down to -38dBfs, but I noticed that when I touched the metal case of my pre amp, the hum would drop slightly in level, leading me to think it could be a grounding issue.

I just tried disconnecting the 48VDC and the 0VDC that goes from my Phantom PCB to my JLM Go Between and XLR pin 1 respectively, and have discovered that this lowers the hum to -68dBfs.  That's quite a difference!

Have I got my grounding correct?  I followed your advice earlier in the thread where you said:

The 0VDC should go to pin 1 of the XLR input. This same pin 1 should also be connected directly to the chassis as close to the XLR as possible. The system 0V should be connected to the chassis at the power supply.

Cheers

Ian

I have the 0VDC going from the Phantom PCB to pin 1 of the XLR.  Pin 1 is connected to the ground tab on the Neutrik XLR connector, which is in turn connected to the chassis.  The power supply's ground is connected to a point on the chassis just next to the IEC socket.  So what do you think I'm doing wrong?

Is there a ground loop because the pin 1 of the XLR input is connected to the chassis AND the 0VDC?

Any help would me much appreciated!

 

[ruffrecords]

I have the 0VDC going from the Phantom PCB to pin 1 of the XLR.  Pin 1 is connected to the ground tab on the Neutrik XLR connector, which is in turn connected to the chassis.  The power supply's ground is connected to a point on the chassis just next to the IEC socket.  So what do you think I'm doing wrong?

Is there a ground loop because the pin 1 of the XLR input is connected to the chassis AND the 0VDC?

Any help would me much appreciated!

It may be something to do with the JLM. The cable from the JLM to the mic pre - what do you do with the screen at the mic pre end??

Cheers

Ian

 

[letterbeacon]

Hi Ian,

I currently have the JLM set up like this:

The screen of the cable marked 'To Mic Pre Amp Input' is then connected to pin 1 of the output XLR (as are all the screen connections in the pre amp).

 

[ruffrecords]

OK, the screen of the cable marked 'to mic pre' should not be connected to anything at the mic pre end because it is already connected to chassis via pin 1 of the mic input XLR.

Cheers

Ian

 

[letterbeacon]

Perfect -that did the trick nicely, thanks Ian!

 

[ruffrecords]

Perfect -that did the trick nicely, thanks Ian!

Has that fixed it then? What hum level do you get now?

Cheers

Ian