Mic Input Transformer
- Thread starter Gearsix
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CJ
Well-known member
try it both ways while listening for hum, if you can get away with it ungrounded, then do that,
this will keep DC current off the primary to ground,
usually they are designed well balanced so that the DC is not a problem,
but if they are off a turn or two, you can get a slight imbalance which might hurt the low end,
this will keep DC current off the primary to ground,
usually they are designed well balanced so that the DC is not a problem,
but if they are off a turn or two, you can get a slight imbalance which might hurt the low end,
ruffrecords
Well-known member
Gearsix said:
As a rule, a floating input ( centre tap not used) is better for hum. The centre tap on mic transformers is often used/intended for supplying phantom power.
Cheers
Ian
SSLtech
Well-known member
Ground that center tap, and you'll have a HECK of a fun time when you connect 48V phantom power to both ends...
ruffrecords
Well-known member
Gearsix said:
The X187B is OK but it is a little lacking in primary inductance and hence bass response - I calculate it will be 3dB down at 16Hz with a 150 ohm source. It will be fine with a condenser mic with a source impedance of about 50ohms where it will be 3dB down at just over 5Hz.
Cheers
Ian
CJ
Well-known member
this has be discussed before,
http://www.groupdiy.com/index.php?topic=747.0
i see the problem now, the transformer will not be harmed, unless you leave out the phantom resistors,
but there will not be enough voltage to power the mic, as the voltage divider formed by the mic pri and phantom resistors will mean low voltage going to the mic,
in this example, we pic an arbitrary pri resistance of 5 ohms in each leg,
with 48 vdc applied and using 5 k phantom resistors, you will only have 48 millivolts to power the mic,
now if you want a balanced AC input, you could use a cap from the CT to ground,
i do not know if this would affect hum or not, might be worth a try,
http://www.groupdiy.com/index.php?topic=747.0
i see the problem now, the transformer will not be harmed, unless you leave out the phantom resistors,
but there will not be enough voltage to power the mic, as the voltage divider formed by the mic pri and phantom resistors will mean low voltage going to the mic,
in this example, we pic an arbitrary pri resistance of 5 ohms in each leg,
with 48 vdc applied and using 5 k phantom resistors, you will only have 48 millivolts to power the mic,
now if you want a balanced AC input, you could use a cap from the CT to ground,
i do not know if this would affect hum or not, might be worth a try,
Attachments
Usually, I don't use phantom powered mics because I have only two tube condenser mics (with their own PSU) and my other mics are dynamics (SM57, D112, ecc), but may be a problem if using dynamic mics I have a loss in bass freq.
Consider that I want to use these preamplifiers for recording drums and vocals
Consider that I want to use these preamplifiers for recording drums and vocals
Brian Roth
Well-known member
I would leave the CT disconnected....keep in mind that most preamps don't even HAVE a CT on the input transformer.
Bri
Bri
CJ
Well-known member
some of the Telefunken modules had CT's, Langevin AM16 has the potential for a CT, but yeah, you are right, it is old technology, WeCo, RCA used it, needed when they ran long cables near AM transmitters and stuff like that,
telephone gear used it a lot, but now everybody has cell phones,
i miss the old rotary phones, do they still work on today's phone lines?
telephone gear used it a lot, but now everybody has cell phones,
i miss the old rotary phones, do they still work on today's phone lines?
Brian Roth
Well-known member
CJ said:
I have an Olde Rat Shack phone on my nightstand, which is wired into my Olde copper Ma Bell POTS line. The RS phone has a switch on the side for "tone" or rotary pulses, and if accidentally switched to pulse mode, it still actually dials a call correctly!
click click click click click.....
But, dunno if that works with cable company or other "non POTS" circuits/VOIP/etc.
Bri
PS, I'll start new POTS thread in the brewery.....
> i miss the old rotary phones, do they still work on today's phone lines?
I recently saw somebody say the phone company threatened to charge MORE if he kept using his rotary phone.
Back in the last century, rotary pulse decoding was a big expense for the telco. They even optimized the dial so people might dial quicker.
Touch-tone was in part a cheaper system (though for many years they charged more).
Back 30+ years we got ESS, a computer did all the decoding. One computer for thousands of lines. Obviously this can decode either tone or pulse and the user-time does not matter (since one dedicated computer serves thousands of lines).
Ah.... There's no central computer anymore. COs have split into pole-top concentrator boxes. Even so, such boxes should have code to read pulse-dial. If nothing else, so that in an emergency (such as storm/flood) where POTS tone-phones might be soaked (and cellphones out), people might find a rotary phone in the attic and be able to use it to call for help.
I recently saw somebody say the phone company threatened to charge MORE if he kept using his rotary phone.
Back in the last century, rotary pulse decoding was a big expense for the telco. They even optimized the dial so people might dial quicker.
Touch-tone was in part a cheaper system (though for many years they charged more).
Back 30+ years we got ESS, a computer did all the decoding. One computer for thousands of lines. Obviously this can decode either tone or pulse and the user-time does not matter (since one dedicated computer serves thousands of lines).
Ah.... There's no central computer anymore. COs have split into pole-top concentrator boxes. Even so, such boxes should have code to read pulse-dial. If nothing else, so that in an emergency (such as storm/flood) where POTS tone-phones might be soaked (and cellphones out), people might find a rotary phone in the attic and be able to use it to call for help.
