80hinhiding
Well-known member
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- Sep 27, 2016
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Single rail vs Dual rail - Pros and Cons
- Thread starter 80hinhiding
- Start date
Help Support GroupDIY Audio Forum:
Single rail is cheaper, dual rail allows DC coupling audio path,
JR
JR
^^ good summary.
All amplifying devices (tube, BJT, FET) are naturally uni-polar.
Audio is bi-polar.
The obvious way to fake it is to run a single supply, bias-up halfway, then couple-out with cap or transformer to get audio both ways around zero.
But there are other problems. Simulate an airplane climbing and descending. Flight Trainers did this. Positive voltage from "throttle" charged-up a capacitor, negative voltage discharged it. In this simple case, if cap went below zero, a bell dinged, you crashed. More elaborate trainers would track load, downdraft, different landing elevations, so might need full resolution below zero. Another use was megaWatts of electric power sloshing around a large power grid, where "negative energy" has a vital meaning (generator asked to absorb precious power).
For these jobs a system was developed using both + and - supplies. As the theory was mathematical operations, the amplifiers used to force the resistors and caps were Operational Amplifiers. Real hot stuff in the 1950s. As that field was lost to digital computers (which also have a below-zero problem handled different ways) transistor op-amps pushed the price down so opampery (and +/- supplies) moved out to real-world applications. The opamp makers published excellent books to promote opamp sales. While these opamps were a bit slow, some were used in audio and several designers traded-off DC precision for audio smoothness.
The concept of high gain to get precision response aided complex audio circuits. Using judo on caps to get choke-like response was a big deal. The response-to-DC is a non-benefit, and stray DC is always a problem, generally forcing some caps here and there.
At your point on the path: the benefits of bipolar are lost if your amplifier does not have a GOOD input with zero DC offset. Single-device input stages are no good. (The few exceptions tend to prove the rule.) Need an input pair with excellent balance so you can force the output to zero DC.
So I would say a beginning designer could work all single-supply. Except the fad for opamps is SO big that most of the chips you may add-in assume +/- supply. Mixed power systems sure are possible but may be inelegant.
All amplifying devices (tube, BJT, FET) are naturally uni-polar.
Audio is bi-polar.
The obvious way to fake it is to run a single supply, bias-up halfway, then couple-out with cap or transformer to get audio both ways around zero.
But there are other problems. Simulate an airplane climbing and descending. Flight Trainers did this. Positive voltage from "throttle" charged-up a capacitor, negative voltage discharged it. In this simple case, if cap went below zero, a bell dinged, you crashed. More elaborate trainers would track load, downdraft, different landing elevations, so might need full resolution below zero. Another use was megaWatts of electric power sloshing around a large power grid, where "negative energy" has a vital meaning (generator asked to absorb precious power).
For these jobs a system was developed using both + and - supplies. As the theory was mathematical operations, the amplifiers used to force the resistors and caps were Operational Amplifiers. Real hot stuff in the 1950s. As that field was lost to digital computers (which also have a below-zero problem handled different ways) transistor op-amps pushed the price down so opampery (and +/- supplies) moved out to real-world applications. The opamp makers published excellent books to promote opamp sales. While these opamps were a bit slow, some were used in audio and several designers traded-off DC precision for audio smoothness.
The concept of high gain to get precision response aided complex audio circuits. Using judo on caps to get choke-like response was a big deal. The response-to-DC is a non-benefit, and stray DC is always a problem, generally forcing some caps here and there.
At your point on the path: the benefits of bipolar are lost if your amplifier does not have a GOOD input with zero DC offset. Single-device input stages are no good. (The few exceptions tend to prove the rule.) Need an input pair with excellent balance so you can force the output to zero DC.
So I would say a beginning designer could work all single-supply. Except the fad for opamps is SO big that most of the chips you may add-in assume +/- supply. Mixed power systems sure are possible but may be inelegant.
midwayfair
Well-known member
JohnRoberts said:
Allows, but sometimes there are input and output caps anyway to prevent DC offset ... it's hard to get an op amp absolutely perfectly at 0V. There aren't too many discrete designs that could avoid them.
Ground is certainly the quietest possible Vref for an op amp. Mimicking its total absence of noise and stiffness requires additional filter capacitors, plus the voltage divider or trim pot to make the Vref.
But of course, if I'm going to bring cost into it -- what's the cost of a few components compared to the cost of a power transformer, rectifier parts, and the really big caps needed to filter 60Hz/120Hz compared with a $15 (shipped) SMPS wall lump you can filter relatively easily?
I personally ended up going single-supply (48V) for preamps. They're for me in my house, not for other people, so the inconvenience of "Where did I leave the 48V wall lump" and slightly more noise weighed less than "risk getting electrocuted because I spaced out for a second building the power supply a month ago." I know I can also get AC wall lumps and avoid the safety issue, but I end up behind on parts. (I'm occasionally lazy and often cheap.)
