Problem with Phantom on Green Pre's

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matta

Well-known member
Joined
Jan 11, 2005
Messages
1,640
Location
Cape Town, South Africa
Hi Guys,

I fired up the pair of greens (V14) I am building this weekend and they work, within reason... they are really clean and quite, the Phase and High Pass work but the Phantom is freaking out.

I suspect on one channel the switch might be faulty as it is not throwing from one pole to the other, so it doesn't matter if you switch in our out the voltage stays on one pin, through it does effect the audio circuity with a VERY loud hum when switched OUT, but fine when switched in...

On the second pre I get they same hum when the Phantom is engaged. I am too afraid to connect it to a condenser for fear I might blow it up, right now I am using a low cost dynamic, which works fine except when the phantom is engaged, it still works, but the hum is induced.

I measures the phantom rail and it is 46.7V when switched out and 41.2V when switched in.

Any idea on what is causing the hum or how to fix it? I am stumped, being a newbie and would appreciate any advise.

Thanks

Matt
 
I measures the phantom rail and it is 46.7V when switched out and 41.2V when switched in.

What does the voltage read before the 48v regulator? If you transformer is 15-0-15 it should be about 54v.

Can you measure the ripple on the Phantom supply? Best on a scope..

Regards
Peter
 
Thanks, I suspect that might be a problem.

I am not sure where to test the voltage into the regulator? I assume I put the common lead (black) on my multimeter
on the ground in (0) on the PSU and the Voltage (red) lead on the positive leg of the 1000uF Cap before the LM317
regulator. If this is the case I am only getting 21.2V, which is the same voltage I get out of the Diode Bridge!
Half of what you said I should get.

On the output terminals on the PSU I get -14.8, +14.8, 0 and 47.2V.

Anyone else have ideas or would be willing to check theres and compare figures?

Thanks

Matt
 
what power supply circuit schematic are you using?

If it is the SSL 9K one you need a 20-0-20 transformer. If you don't have enought voltage the 48 will have ripple under load and you hear it in the signal.
 
Hey Gus,

It is Peter C PSU. It requires 15-0-15V. I have a 15-0-15V 2A CT type Transformer, but that is weird is that the output on the secondaries is 15.3V, which is bizare as there is normaly a 10% difference.

Thanks

Matt
 
Okay...

So here is the deal... turns out the PCB switch I was using for Phantom was bust, causing the irregularities in voltage and the hum!

Tonight for the heck of it I desoldered the switch and used hook-up wire to 'hardwire' the Phantom into the circuit... PRESTO, no hum and phantom works like charm... damb switch....

The other Pre is working fine as well. I think the hum I was getting was also partly due to the power tranfo being a little too close to the audio circuitry, still waiting on my nice custom wound Toroidal ;-)

My only concern, though this might be normal, is on power down there is a momentary pause and then you can hear what sounds like the cap's discharging briefly, making a 'sucking/zapping' sound.

Anyone else get this? This might be totally normal and usual for the caps to discharge, but I wanted to check.

Thanks again

Matt
 
Yeah, I'm getting that too. Kind of a "zip, zing, zoom, zam" thing? I thought it was the phantom power too.
 
You guys have obviously never turned off a B3 while playing...... :shock:

That kind of stuff is pretty normal if you are powering off while listening. Watch out for your speakers though, those woofers can experience a HUGE kick.

Peter
 
not totally on topic, but for saftey, I ALWAYS mute before turning anything on or off or unplugging or plugging anything in... I don't like surprises. One time I was assisting on a session and I unplugged a mic on an open channel, the engineer happened to have the monitors up to a relaively loud level and I could hear the KABOOM from the well isolated controll room (I was in the recording space). Never again...
 
Peter,

You mean a REAL B3!!!! I thought those things only came inside sound modules ;-) Just kidding... Yeah I thought it was pretty normal for the caps to discharge. I was monitoring through a console and forgot to mute the channel, so know EXACTLY what the 'Zip, zap, zoom' sound is all about, thank fully it was with headphone not my monitors. JC that sounds painful just reading your your post, damb that engineer must have been deaf for at least a week! Anyways, I am so stoked to have my PRE's and they work! Hopefully going to get the knobs and switch and a couple other bits and have them racked up in a week or 2, will post pics.

Cheers

Matt
 
B3???
These monsters do all kind of strange noises anyway... :grin:
But really FUN. We had one of these in the studios where we had our bandroom in. Good sound but awful heavy for going on tour with it together with a leslie box...and you have to be careful not to break it if it's a little bit older :grin:
 
Hello All,
I have recently completed 4 channels and all sound great. The one issue I get is when I switch off phantom power. The DC volts drain extremely slowly from the 5 blocking caps even when all the power is turned off. Is this to be expected? Is there an issue with my channels? Is there a simple solution? It is not a big issue at the moment as I own no ribbon mics, but it would be nice to know when the switch says 'no phantom power' there is no phantom power.

thanks for any suggestions... and for the wealth of information already available.
 
[quote author="mburch"]Hello All,
I have recently completed 4 channels and all sound great. The one issue I get is when I switch off phantom power. The DC volts drain extremely slowly from the 5 blocking caps even when all the power is turned off. Is this to be expected? Is there an issue with my channels? Is there a simple solution? It is not a big issue at the moment as I own no ribbon mics, but it would be nice to know when the switch says 'no phantom power' there is no phantom power.

thanks for any suggestions... and for the wealth of information already available.[/quote]

This is an old post, but I'm getting the same thing. It drops agonizingly slow when phantom is switched off. I'm measuring the phantom with my negative probe on the 0 on the PSU and hot and cold pins on the XLR. Is this a known issue? Can anybody else test their properly working units to let me know?

Thanks

Matt
 
Ok, I tried it under load and it goes down much faster but now my voltage is low with a microphone plugged in. It started as 49.1V(with no mic) and it's down to 36V at the XLR with it plugged in. Is this normal? It still has the 49.1V at the switch and at the PSU.

Thanks,

Matt
 
[quote author="fucanay"]Ok, I tried it under load and it goes down much faster but now my voltage is low with a microphone plugged in. It started as 49.1V(with no mic) and it's down to 36V at the XLR with it plugged in. Is this normal? It still has the 49.1V at the switch and at the PSU.

Thanks,

Matt[/quote]

Hi Matt,

if you're using the version with LM317 'new psu' (not the 4x 12V zener), take out C14 (10uF/63V elco between reg.pin 1 and ground).
I got the same problem as I was fooled by the datasheet for 'improved ripple rejection' of the datasheets. (rejection is fine but load regulation is poor). Without this cap, 48V was constant and not affected by load of led or mic.
 
Matt wrote:

It started as 49.1V(with no mic) and it's down to 36V at the XLR with it plugged in. Is this normal?

You have a 13v drop across 3400 ohms (the 2 x 6k8 in parallel) so maths wise, 13v/3400ohms = 3.8mA which is fairly typical for draw on a phantom supply.

Peter
 
[quote author="peterc"]Matt wrote:

It started as 49.1V(with no mic) and it's down to 36V at the XLR with it plugged in. Is this normal?

You have a 13v drop across 3400 ohms (the 2 x 6k8 in parallel) so maths wise, 13v/3400ohms = 3.8mA which is fairly typical for draw on a phantom supply.

Peter[/quote]

As long as it's normal, I won't worry about it. This is my first mic pre and I've never tested these voltages before, so i was unsure.


Thanks again Peter

matt
 
I build a SSL9k. How i found in the green pcb the exact routing for the phantom power. I understand now how i set it up. Great thx for running this forum and post replys.

Maybe someone can tell me why "the shorty" that the two 6k8s produce, have no effect to the mic in signal?

IN pin 2 and pin 3 are linked over the resistors!
 

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