Post by Graham on Apr 14, 2020 12:28:14 GMT 12
Question for circuit gurus. ! My understanding is that most electronic circuits operate on DC. Why then do so many circuits have have + n - as their input or even ac n ac from a center tapped transformer. The ones with ac input then have a diode bridge to convert it to dc anyway. I'm confused. please explain . 
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Post by Owen Y on Apr 14, 2020 13:32:51 GMT 12
Graham, - What do you mean by + n - and ac n ac ? - When you say 'input', you mean mains power input I think, not signal input ? - Do you mean, if our ccts are DC powered ccts, then why don't we use DC supplies to run them, eg a battery?
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Post by Graham on Apr 14, 2020 13:57:22 GMT 12
I'm not talking about signal input but power input. For example a phono preamp circuit I was looking at the other day has 3 pins for ac power listed as ac15v / 0 /ac15v presumably from a center tapped transformer. These are followed by a 4 diode rectifier block. Why not just supply dc in the first place ? Another example also has the 3 pin but for dc listed as +15 / 0 / -15. For the second example why not just supply 15v +/- ?
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Post by Owen Y on Apr 14, 2020 14:52:53 GMT 12
Yes can do. How would you do that? Using a battery, batteries? Can do such 'dual symmetic' +/- supplies using batteries, I believe. Battery voltage drops in use & needs a charging cct. No free lunch 
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Post by Graham on Apr 14, 2020 15:19:08 GMT 12
Not batteries necessarily , I was thinking good quality wallwart or regulated PSU providing simple dc with + and - polarity.
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Post by Owen Y on Apr 14, 2020 17:11:01 GMT 12
Ah OK, an outboard PSU. As opposed to an onboard PSU. Either traditional transformer + rectification, or Switch Mode.
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Post by colinf on Apr 14, 2020 18:41:23 GMT 12
Hi Graham, short explanation is yes you can feed it with dc instead of ac but....you need + and - dc. Here’s the long explanation... Operational amplifiers (opamps) are small amplifier circuits built into an integrated circuit. One of the oldest ones was the 741 of the 1970s, (and later the NE5534, the most produced and used opamp in the world, still being made). They were convenient to use and became very popular for their good performance compared to most discrete circuits of the time, and so the opamp standard was born, with standardised pin connections. An example of today’s opamps would be opa134, opa627, opa1641, LM833 (a dual opamp in one 8 pin ic package) etc, all with the same connections as the 741. Opamps have two power supply pins, a plus and a minus. Almost all work from +5v and -5v up to +18v and -18v. They can work on single power supplies as well of +10 to +36v but need capacitor coupling in order to do that. Some designers build discrete versions of opamps with separate transistors, diodes and resistors but the + and - power supply standard still remains. So to make a + and - power supply you’d typically use a centre-tapped transformer with a full wave rectifier bridge, say 12-0-12 ac. You could also use a single transformer winding of 12v ac with two half wave rectifiers to make the + and - power supplies. Most opamp circuits use regulators to smooth the supply. Some devices that use opamps, like phono preamps, have 3 pins for their power supply with 2 ac lines and 0v. Ie say 12-0-12v transformer. It goes straight to a rectifier bridge to make the + and - power supplies. If you wanted to use a dc supply (like batteries or a separate psu) in place of a transformer you’d just use a + and - dc power supply straight into the 3 pin ac socket on the unit. If you have access to the schematic you can work out appropriate dc voltages and even bypass the rectifier bridge. Hope this helps!
AMR-iFi R&D
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