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Rail voltage under load questions.

S

SansuiHafler

Active Member
I have two identical Hafler power amps except that one has larger power supply caps (64,000mfd vs 40,000). Should the supply rail voltage be different assuming it’s tested using the same audio input and output load conditions? Should one amp voltage dip down more than the other? Trying to understand my VOM readings. I do not have a scope.
 
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Rail voltages shouldn't "DIP". That shows the Power Supply may be running out of gas. a scope is really indispensable.
 
Honestly you probably are not going to measure the difference. Bigger caps allow for more instantaneous current to be pulled but we're talking tiny fractions of a second. If you try to measure power supply sag under sustained output the caps frankly don't matter for that, its all about the transformer. In a perfect world the supply rails don't drop, however we live in reality and there is a source impedance involved so they do drop by some amount in an unregulated supply.
 
spark1 said:
Voltage sag under peaks in load is normal.
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Some sag perhaps but not much.

One spec you often see for power supplies is "load regulation" meaning how much the output voltage varies while going from no-load to full-load. And good power supply will have very tight load regulation and so should the power supply section of your amp.

But in answering the OP's question...both amps should measure the same rail voltages under identical conditions.
 
RVT1K said:
Some sag perhaps but not much.

One spec you often see for power supplies is "load regulation" meaning how much the output voltage varies while going from no-load to full-load. And good power supply will have very tight load regulation and so should the power supply section of your amp.

But in answering the OP's question...both amps should measure the same rail voltages under identical conditions.
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Hmm. I've never owned an amplifier with a fully regulated power supply.
 
Thanks guys, I’m going to ignore the rails for determining any difference and instead focus on measuring the outputs to the best of my no-O scope ability under the same conditions.
 
Both should measure the same rail voltage, until you start to "tax" the rail in which case the voltage will sag. In theory, all other things being equal (are they ever?), the system with less capacitance will sag sooner and worse, but you may not be able to measure this difference in practice.
 
bonzoro said:
WOPL's don't sag
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What's a WOPL? :dunno:
Of course unregulated power supplies sag. They would have to have zero output impedance to have zero sag, or put another way, they would have to have an infinite power output capacity to have zero sag. Clearly, that doesn't happen in the real world.

Btw, the reason why power amps usually don't have a regulated power supply is because if they did, the regulator would dissipate power that could be used to drive the speakers instead. An unregulated power supply is simply more efficient, despite the inevitable voltage sag.
 
White Oak Phase Linear. basically a Phase Linear 400 or 700 case and power supply with completely different guts inside. The one based around the 700 makes closer to 500 per channel vs the stock 350 per channel. I think the other one is more like 250w vs 200? something like that. More, whatever the exact number happens to be.

100% all of that about the unreg supply. You'll see it for low current stuff only, phono stage, tuner, whatever other stuff that wants stable voltage but not buckets of current.
 
That's why you will see, in almost all solid state amplifiers, AC input, transformer, some diodes to rectify the current, the large filter capacitors, and then, right to the power amplifiers. No regulation parts.

Doug
 
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When purchasing parts in bulk it is hard for an OEM to say no to a fantastic deal even if it is marginal to the specifications as in bare minimum or even under what is ideal. The same thing is true in reverse it never hurts to exceed specifications with power filter caps assuming they fit in the space allowed and it is even better if they get a great deal on them!

i say this all the time this is why my fantastic and I love to death Carver brand amp ended up with power filter caps that are are rated 7 to 10 volts under under what is actual on the rails ended up in the amp! Just like when you pop the hood on a a big V8 engined car or truck and find a battery that is far smaller than ideal because the OEM got a fantastic deal! It is going to make it out of warranty no problem with no failures and will work just fine meeting the specifications of the unit which normaly have plenty of wiggle room built in well in the past at least.

Every company does this especially if the end consumer can not easily see that corners have been cut. Traditionaly capacitors, switches, pot's, power cords, bulbs where the places cost cutting showed up first.
 
spark1 said:
Hmm. I've never owned an amplifier with a fully regulated power supply.
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What do you mean? No way your amps operate at mains voltage! Even if they do not have transistors they have to have voltage regulation even if they used just tubes and resistors, diode and capacitors in strongs. When car audio amplifiers became fully regulated with mosfet power supply it was one of the best things that ever happened to car audio not counting those people doing DB Drag's and such. It made sound quality much more precise since minor voltage flucuations where no longer a big deal.

Every part of home audio benifits from voltage regulation. Even if a design does not have active voltage regulation rest assured it would benifit from it.

That said voltage fluctuations are not quite the same as sag since you have to know the load and output to calculate sag in a precise way since voltage flucuation alone is not a liable indicator alone. You will always see flucuation depending on the tool being used and what the beuffering looks like and the dwell on the trigger is and the buffering on the read out or meter if analog what the refresh rate is etc....

A lot of gear can take up to 5 minutes to stabilize as a norm.


You know each digit on a cheap VOM is not a significant digit and watching the last digit move up and down a bit is no big deal so how much flucuation are you seeing? Nuance!

If you think a cap is getting old test it. Just make sure you discharge the cap before testing and discharge it again before putting it back into the circuit! Electrolytic caps age and slowly go bad even with out use. Extreme heat, vibration, lack of use, constant use, use at the limit or ratings all prematurely age electrolytic capacitors. Even silver dipped mica or metal clad mica have their limits when heat is concerned!
 
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