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High Current vs High Voltage Amplifiers


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#1 of 21 OFFLINE   Jigesh Patel

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Posted October 06 2003 - 12:22 AM

Given the same power ratings, which type (high current output vs high voltage output) of multichannel amplifier should sound better with speakers of around 8-ohm nominal impedance and around 89 or 90dB sensitivity? The intended usage is 65% movies and 35% hi-rez music.

Thank you.

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#2 of 21 OFFLINE   Yogi

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Posted October 06 2003 - 01:53 AM

With speakers of higher impedance (easy to drive 8 Ohms), amps with higher voltage output can perform good, however speakers with low impedance (tougher loads, 4 ohms that drop down to 2 ohms like electrostats) one needs an amp that can deliver high current. Tube amps operate at higher voltages while SS amps operate at higher currents. Thats why tube amps perform well with easy speaker loads while you almost always need a good SS amp to drive large electrostats.

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#3 of 21 OFFLINE   Michael R Price

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Posted October 06 2003 - 02:13 AM

I don't think either of those terms really mean anything, when you think about it. Given the same power ratings, current is voltage divided by resistance, no matter how you slice it! Seriously, though, I don't think you should pay much attention to claims like that. It's a good thing if the amplifier is *capable* of delivering a lot of current, because that means might distort less under load. The only reasonable way to gauge that is by looking at how much the power rating increases with lower impedances (4 or 2 ohms).

Actually, what specific speakers do you have? Some of them aren't actually 8 ohms, especially in the bass.

#4 of 21 OFFLINE   Jigesh Patel

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Posted October 06 2003 - 02:19 AM

I have Paradigm Reference/40, CC and 20 (front, center, surround) with SVS 25-31PC+ and the receiver is Denon AVR-3802. The speakers are said "compatible with 8 ohms" on Paradigm website and I understand impedance dips occasionally with frequency. I was considering an outboard amplifier (like Parasound HCA-1205A, where HCA stands for high current amplifier despite its being a Solid State amplifier).

Thanks.

Jigesh

#5 of 21 OFFLINE   Shiu

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Posted October 06 2003 - 01:11 PM

I think Yogi's example (Tube vs Solid State)is an interesting one. While it is true that current = voltage divide by impedance, a high voltage, low current system can theorectically delivers the same power as a low voltage, high current system. A high impedance coil (lighter gauge wire and more turns) with a lower current rating in a speaker, should be able to produce as much force as a low impedance coil (heavier gauge wire and less turns) that has a higher current rating. I am not sure if I remember correctly, but in my vague memory, there used to be amplifiers that drove speakers with as high as 800 ohms.

#6 of 21 OFFLINE   Chu Gai

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Posted October 06 2003 - 09:39 PM

Ummmm, not sure what you mean by low current, but if it's low enough, there's no way those two units are equivalent. You've got to have some amps in order to deliver the power especially if the impedance is low.

#7 of 21 OFFLINE   Jason.Soko

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Posted October 07 2003 - 09:45 AM

Current drives speakers, that is all their is to it. Thereis a reason why they sell 1000watt amplifiers at walmart for $100 dollars while my 70w amplifier cost $2500.
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#8 of 21 OFFLINE   Jonathan M

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Posted October 07 2003 - 11:23 AM

An amplifier is simply something that takes a given voltage, and amplifies it by some set factor.

This is all fine and dandy until you actually hook a speaker up to your amplifier in order to get sound out.

The speaker represents a load on the amplifier which is generally some complex impedance that varies with frequency. Usually the impedance is high at any resonance (port or loudspeaker) and low in between the impedance peaks. It also tends to rise as frequency increases.

Given this impedance, the amplifier will try and amplify it's source voltage by it's constant factor, and present it across the load. This inturn produces a current in the load, equal to the voltage divided by the impedance.

Where does this current come from? The power supply of the amp (ie this current comes from the DC supply rails of the amplifier which comes from the mains via (generally) a transformer, rectifier, and capacitor filter bank. If these components, as well as the mains itself, are capable of supplying the required current, then everything is peachy, and the speaker will produce what is was asked for (subject to it's response variations). Problems occur when one or more of the links in the chain are unable to supply the required current. In this case, they'll supply as much current as they can, and thus force the voltage rails to droop, which then lowers the voltage at the output (so that it still satisfies ohms law V = IZ).

As for which is better, high current or high voltage, the answer is it depends on what the impedance is, and how much it varies, along with how sensitive the speaker is, and what sort of volume you require.

Hope this helps, or at least has been a little informative!

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#9 of 21 OFFLINE   Shiu

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Posted October 07 2003 - 11:37 AM

Chu, may be I should have said "lower current", instead of low current. It is just "relatively" speaking. Example, a coil with 10 turns passing 10 amps compare to a coil with 20 turns passing 5 amps. The product "ampere-turns" is the same.

#10 of 21 OFFLINE   Chu Gai

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Posted October 07 2003 - 12:18 PM

This may be a bit of confusion upon my part in possibly misunderstanding what you've written, but the vast majority of speakers are designed to give a flat FR when driven by a voltage source. Now that doesn't mean that current isn't needed. Perhaps we're saying the same thing.
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#11 of 21 OFFLINE   Shiu

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Posted October 07 2003 - 12:47 PM

What I wrote was based on electrical theory only, in response to a post about "current=voltage/impedance). I did not mean to ignore the practical design considerations in the real world where high current design seem to be the norm. I would delete my previous posts when I get a chance to figure out how......but before they get deleted let's be reminded that aside from V=IZ, power, P=Voltage sqaure/impedance, or P=current square X impedance, so if you double the voltage,quadruple the impedance, current will be halved but power will remain the same, assuming 100%impedance match between the source (amplifier) and the load (speaker)for maximum power transfer. Again, this is only theorectical, I do recognize the fact that speaker impedance typically lies within the range of 4 to 8 ohms nomial, some (just a few) may go as low as 2 ohms or as high as 16 ohms.

#12 of 21 OFFLINE   JohnnyG

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Posted October 08 2003 - 04:59 AM

Quote:
...but the vast majority of speakers are designed to give a flat FR when driven by a voltage source.

But the current needed to provide that voltage will vary with the frequency since the impedance is not constant.

#13 of 21 OFFLINE   Chu Gai

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Posted October 08 2003 - 07:14 AM

it better!

#14 of 21 OFFLINE   Michael R Price

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Posted October 08 2003 - 08:04 AM

Current drives speakers but how do you get a higher current into a fixed impedance speaker without a higher voltage? And what does it mean if an amplifier is designated "high current" or "high voltage" anyway? (How about both? Posted Image)

#15 of 21 OFFLINE   Mark Russ

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Posted October 08 2003 - 11:16 AM

I copied and pasted this from part of a review on a B&K receiver (just so you all wouldn't think that I actually knew what the hell I was talking about here):

The rail voltage is relatively high for the size for the transformer, considering that it supplies all five channels. Explain? Love to. The consequence of a high rail voltage is that for a given power supply step-down transformer rating (measured in VA, Volt Amperes, i.e., watts), a higher voltage output translates into a lower continuous current output capability.

In most circumstances, engineers only count on between 70% - 80% of the transformer capacity, both for safety margin, and because of the fact that transformers do “weird” things when they approach capacity, like heat up, supply far less voltage than required, or otherwise wreak havoc.

For instance, a transformer with a 600 VA rated limit, at maximum output, can theoretically supply current and voltage so long as the product of the two (expressed in VA) does not exceed 600. If the transformer will supply 60 volts rms, the total current cannot exceed 10 amps rms. However, if the transformer’s secondary only offered 20 volts rms, the current capability would triple to 30 amps rms.

In both cases, the transformer remains the same size. The only difference between the two is the ratio of turns between the primary and secondary. Which transformer offers more power capacity depends on the load. The first will perform better into higher impedance loads that would exhaust the voltage swing faster than the current capacity. The latter will better suit lower impedance loads that would bleed the first transformer dry of current before ever taking advantage of the higher voltage potential available. If you’d like to throw the numbers around:

Voltage (Volts) = Current (Amperes) x Impedance (Ohms), and so, one can derive that Current = Voltage/Impedance.

Power (watts) applied to a given load = (Voltage applied to the load) x (Current drawn by the load at that voltage) x the cosine of the phase shift between the current and voltage (in the case of reactive impedances). Got that? Film at 11.

Although a lot of people make a big hype about “high-current” amplifiers, usually without qualifying the term, sacrificing a bit of current capability for the sake of greater voltage swing is not necessarily a crippling feature, particularly taking into account a good deal of storage capacitance and the variable nature of both loudspeakers and audio.

Continuous current capability, while certainly nice to have on tap, is not absolutely necessary for short peaks if the storage capacitors in the power supply can stash away enough reserve energy. With very little current capability from the transformer, and very little storage capacitance, you’re very much in trouble. But, with a little saved in the bank, the capacitors can supply bursts of current for short-term demands, which a moderately robust transformer can make up between stressful moments. Equipped with a power supply boasting 54,000 µF (Micro-Farads) of filtering capacitance right after the rectifiers, and a power transformer that is pretty big to begin with, the AVR-202 can save enough juice for a rainy day or two, and perhaps even a minor flood once in awhile.

So, with a somewhat high rail voltage and a formidable set of storage capacitors, we can state that the power supply has been designed to exceed its rated 105 watts@ 8 ohms by a wide margin for short periods, especially if it should only have to stuff a channel or two at a time. In other words, the AVR-202 is optimized for transient-heavy, dynamic program material, coincidentally similar to action-oriented theatrical audio that often reaches barely sane, if not stupidly high levels of SPL between spans of intelligible dialogue. Stallone in other words.

#16 of 21 OFFLINE   Michael R Price

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Posted October 08 2003 - 11:38 AM

I think the general summation of this topic should be: More power, into your speaker load, is better.

I myself use 750VA at +/-75V (5 amps continuous), and 47,000uf per channel. Those power supply components only cost about $250, so it's probably a worthwhile investment. (Don't know how that compares to usual amplifiers, but it seems pretty strong to me.)

#17 of 21 OFFLINE   Chu Gai

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Posted October 08 2003 - 12:07 PM

ever see those 1 Farad capacitors for car stereos Mike?

#18 of 21 OFFLINE   Shiu

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Posted October 08 2003 - 12:12 PM

Mark, thank you for posting that article. It makes some of the points that I was trying hard to make. We should not keep quoting ohms law (V=IZ, or I=V/Z) only, without also considering the fact that power is proportional to both voltage and current. The example used in your article, 60 volt X 10 amps (high V, low I) = 20 volts X 30 amps (low V, high I) = 600 VA in both cases. Same power! It should be noted though, that the article seems to focus mostly on the power supply side of the amplifier whereas some of us were talking about the amplifier output side, where the amplifier's output impedance and speaker impedance matching also plays a major part on "power transfer" from the amplifier to the load.

Can anyone tell me how to delete my posts, is it allowed?
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#19 of 21 OFFLINE   Michael R Price

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Posted October 08 2003 - 12:43 PM

Chu,

Yes, but those are working at 12-16 volts. A twentieth of a farad stores as much energy at 75V...E = 1/2 CV^2, right?

#20 of 21 OFFLINE   Craig_Kg

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Posted October 08 2003 - 03:48 PM

Johnathon M already said it but maybe I can make what he said clearer.

RMS Power rating for an amp is normally defined by the voltage it will maintain across a resistive load through P = V*V/R.

Let's consider a monobloc to simplify things. If the amp has a rail voltage of 40V RMS and holds that for 8 ohms and it falls to 35V RMS for 4 ohms, then it has a power rating of 200W into 8 ohms and 306.5W into 4 ohms. A fall in rail voltage is normally caused by the power supply saturating so assume the power supply limit is about 300W.
In this case, the amplifier's output would vary across the range of a speaker that had impedance variations from 8 to 4 ohms (or less) IF the volume demand requires power > 300W to be delivered.

Now the voltage across the resistance dictates the current (I = V/R) so talking about a high voltage amplifier is pretty meaningless.

You might find an amplifier that has a very high power rating (assume into 8 ohms if not otherwise specified) but is not "high current" as it performs relatively poorly with low impendance speakers - in this case the power supply is the bare minimum for the specified rating. Meanwhile a "high current" amp of the same power rating would have a rating into 4 ohms that is about double its 8 ohm rating as it has a power supply that is over specified for its power rating into 8 ohms but is necessary to allow it to double down for linear operation into 4 ohms (and if REALLY high current, would double it output again for a 2 ohm load, etc).

Multichannel amps exacerbate this situation as manufacturers often make assumptions about channel demands not being simultaneous so they reduce the capacity of the power supply to only cope with some of the channels at full output rather than all.
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