Hi all,
I recently bought a Preamp from ebay to convert a mic-level input to a line-level one for use with my PC. It says to use AC power at the input, at 15v, however getting a 240-15v transformer is fairly expensive and somewhat dangerous. As such, I feel I have 2 options:
Buying a cheap, small transformer, such as this (https://au.rs-online.com/web/p/pcb-transformers/1739686/). I'm not sure it it will provide sufficient power, and I would prefer not to interface directly with 240v mains electricity if i can help it.
I don't know what that's supposed to mean. A 30V center-tapped transformer??? Or, maybe + and - 15VDC???
It's common to build a dual (positive & negative) power supply from a center-tapped transformer, and it's common for op-amps to require dual power supplies.
And typically, you could feed-in DC instead of AC but with the information given I just don't know.
And, what kind of microphone do you have? Electret condenser computer mics expect 5V power from the soundcard. Stage & studio mics are balanced (3-wire connection) and studio condenser mics require 48V phantom power. (Dynamic mics don't need power and some stage/performance electret mics have a battery built-in.)
Looking at all the Ebay pictures, I can see one 7815 regulator and one 7915 regulator. So, a 30 volt center tap transformer will give the correct input. But the more common 24 VAC center tapped transformer will also work. The 15 volt is peak to peak.
Hi,
I think you will find the pre-amp is designed like that so you use a transformer input and the on board linear regualtors to minimize the noise component.
A 15-0-15V output transformer , the ebay item does not say what its current consuption is?
Thanks for all your help guys, just FYI planning on using a dynamic mic.
Figure I'll just go with a centre tap 15-0-15 transformer from RS, hopefully 12VA should be enough (doubt the linear regulators could sustain much more without a heatsink). I tried it with +12_GND_-12 from a computer power supply, and it worked, however there was a significant hum - whether its from the ripple of the DC, interference somewhere, or the lower voltage, I'm not sure yet!
however there was a significant hum - whether its from the ripple of the DC, interference somewhere, or the lower voltage, I'm not sure yet!
If it's 50/60Hz power line hum it's probably not from the power supply because computer power supplies are usually high-frequency switching power supplies.* And, if it's designed to run-off AC, it should be better/quieter with "double filtered" DC.
The thing will probably "run" from a pair of 9V batteries. If you get hum when operating with batteries you know it's not power supply hum.
Mic preamps are high-gain and they can be prone to hum pickup. If the board is picking-up "stray" power line hum, the hum should change as you move it around, re-orient it, or put you hand next to it, etc. Connecting a mic might lower the hum because of the microphone's low impedance. But on the other hand, the mic itself, or cable, can pick-up hum.
planning on using a dynamic mic
If it has an XLR connector, that's a [u]balanced connection[/u] and you'll get better results with a proper microphone preamp. The balanced low-impedance connection cancels hum-pickup from the mic and cable.
The cheapest way to get a proper microphone preamp is to get a [u]small mixer[/u] that has a mic input. (You don't have to mix with it.)
I had some little computer speakers with a missing wall-wart power supply once. So, I had the "bright idea" of tapping-into the computer's power supply and I mounted a power-outlet jack in a blank PCB bracket on the back of the computer.... The noise was terrible and I had to just buy a power supply.
To power this I'd find a 5V in, +/-18V out DC-DC converter. Then it feeds 7815 and 7915 linear
regulators for a nice low-noise +/-15V rail. Easy to power from any system with 5V and enough
current. You'll have to calculate the currents/powers needed from the specs or measurements.
You'll get away with using a lower set of voltages, +/-12V for instance.
Given microphone level signals I'd advise against directly using switch-mode converters
without linear regulation to suppress noise.
These days though I'd have thought its perfectly possible to find a good preamp using 5V
supply only, though not with NE5532's, you'd need good low voltage rail-to-rail opamps.
