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Topic: Using DC rectified power to power an AC preamp (Read 333 times) previous topic - next topic

legham

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?
Thanks!

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

DVDdoug

Quote
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.)


Paul_KD7HB

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.

Paul

TomGeorge

#3
Dec 12, 2018, 02:06 am Last Edit: Dec 12, 2018, 02:07 am by TomGeorge
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?

Something like this;


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

legham

#4
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/

DVDdoug

Quote
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.

Quote
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.

MarkT

#6
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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