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Topic: Using DC rectified power to power an AC preamp (Read 177 times) previous topic - next topic


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:

1) 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.

2) Buying a cheap AC-DC 15v switching power supply from ebay ( https://www.ebay.com.au/itm/AC-100V-240V-Adapter-DC-15V-1A-Switching-Power-Supply-1000mA-15W-AU-plug-5-5mm/221526192322?hash=item3393fd70c2:g:2E8AAOSwwbdWLZnm:rk:21:pf:0 )
The issue with this is that I am not sure the filter capacitors will be suitable to filter the ripple produced by any switching PSU. If they are built to smooth half-wave rectified AC, will the do a good job at DC with what I would guess is a evry significant amount of ripple?

Additionally, the listing says it requires 2 15v sources - how would I achieve this?

Are there any other options I could use?

Ebay listing: https://www.ebay.com.au/itm/NE5532-OP-AMP-HIFI-Preamplifier-Signal-Amplification-Board-for-Bluetooth-Pre-amp/232987496280?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2648


Power supply:  Dual 15V  AC
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.



Dec 12, 2018, 02:06 am Last Edit: Dec 12, 2018, 02:07 am by TomGeorge
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?

Something like this;

Tom... :)
Everything runs on smoke, let the smoke out, it stops running....


Dec 12, 2018, 02:29 pm Last Edit: Dec 12, 2018, 02:30 pm by legham
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!

Thanks for all your help :)

Transformer: https://au.rs-online.com/web/p/pcb-transformers/0504442/


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 balanced connection 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 small mixer 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.


Dec 13, 2018, 03:36 am Last Edit: Dec 13, 2018, 03:37 am by MarkT
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.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

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