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Old 3rd May 2009, 08:46 PM   #1
OMNIFEX is offline OMNIFEX  Jamaica
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Question Voltage Drop Based On The Load

Hi

How much voltage drop is acceptable based on the load?

It is quite common to find an amplifier rated 200 watts per channel @ 8 ohms and, 300 watts per channel in a 4-ohm load.

That equates to a 6-volt drop when comparing the loads.

Is there a standard how low the voltage can deteriorate before it is classed as unacceptable for a lower impedance?



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Old 3rd May 2009, 09:10 PM   #2
AndrewT is offline AndrewT  Scotland
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A very good amp will be ~-0.4dBV into a half impedance load.
a good amp ~-0.8dBV.
a bad amp ~-1.5dBV.

300W into 4r0 is about -1.24dBV relative to 200W into 8r0. It's bordering on bad.

An example of a good amp using just 1pair of 2n3771 in the output stage gives 110W into 8r0 and 200W into 4r0. That crimson 1704 loses just -0.4dBV into half impedance.
Cordell reckons that a good amp should deliver at least 80% more power into half impedance. This is equivalent to -0.46dBV
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Old 4th May 2009, 04:19 PM   #3
OMNIFEX is offline OMNIFEX  Jamaica
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Thank you AndrewT!

Based on those requirements 90% of the amplifiers would be classed as a “bad amp” for I’ve calculated voltage drops as much 12 volts moving from an 8 to a 4-ohm load.

What I find odd are amplifiers rated 2-ohm minimum offering a larger drop in voltage in 4 ohms than, amplifiers rated 4-ohm minimum.

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Old 4th May 2009, 04:22 PM   #4
AndrewT is offline AndrewT  Scotland
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and then we hear complaints about lack of bass when the purchaser has been assured that the frequency response goes all the way down to 4Hz @ -1dB.

The ability to deliver current when the speaker demands it is crucial to the way the speakers sound.

Your conclusion is right. Most cheap amplifiers cannot deliver sufficient current. And some expensive ones are almost as bad.

BTW,
that 2ohm rating is almost certainly a 2r0 rating.
A 4ohm reactive speaker is far more demanding than a 2r0 resistive load.
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Old 4th May 2009, 05:57 PM   #5
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Quote:
Originally posted by AndrewT

A 4ohm reactive speaker is far more demanding than a 2r0 resistive load.
Definitely not more demanding for the power supply. The output stage though (or most likely an over-conservative protection circuit) might cause problems of course.

If an amplifier can drive the speaker load without distorting or being damaged it is good. How much the power increases when the impedance is decreased doesn't tell anything. It's perfectly possible to make an amp that will double power from 4 to 2 ohms resistive, but it might still be unsuitable even for driving a 16 ohm speaker load.

When the limit comes from a certain mechanism (fuse, overheating, second breakdown, current limit, certain kind of VI limiter, voltage clipping, etc.) it might be possible to have a rule of thumb for that specific mechanism - but not generally.

An amp with enough power supply capacitance will sound better than one with little if driven into clipping. How much the power increases with decreasing impedance doesn't tell which mechanism it is. If it's the transformer regulation that causes the voltage to drop (but there still is enough capacitance in the supply) it will sound much better than if the transformer has good regulation but there is too little capacitance.

But on the other hand, if your amp doesn't clip as heavily it will sound even better.

In other words, if you want to play music at 90Vpk into 4 ohms, the amp that can do 70Vpk at clipping into 8 ohm and 65V into 4 ohm loads will sound worse than the one that can do 120Vpk with 8 ohm loads or 90Vpk with 4 ohms (assuming the amplifiers are designed to not distort in another way, like VI limiters activating).

In this case the amplifier with stiffer supply is worse.
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Old 4th May 2009, 11:33 PM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by megajocke
Definitely not more demanding for the power supply. ..............

How much the power increases when the impedance is decreased doesn't tell anything.

It's perfectly possible to make an amp that will double power from 4 to 2 ohms resistive,......................

An amp with enough power supply capacitance will sound better than one with little if driven into clipping.



In other words, if you want to play music at 90Vpk into 4 ohms, the amp that can do 70Vpk at clipping into 8 ohm and 65V into 4 ohm loads will sound worse than the one that can do 120Vpk with 8 ohm loads or 90Vpk with 4 ohms (assuming the amplifiers are designed to not distort in another way, like VI limiters activating).

In this case the amplifier with stiffer supply is worse.
we are going to have to disagree on all these points.
Quote:
But on the other hand, if your amp doesn't clip as heavily it will sound even better.
we at least agree on one point.
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Old 5th May 2009, 04:08 AM   #7
OMNIFEX is offline OMNIFEX  Jamaica
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Quote:
Originally posted by AndrewT
and then we hear complaints about lack of bass when the purchaser has been assured that the frequency response goes all the way down to 4Hz @ -1dB.

The ability to deliver current when the speaker demands it is crucial to the way the speakers sound.

Your conclusion is right. Most cheap amplifiers cannot deliver sufficient current. And some expensive ones are almost as bad.

BTW,
that 2ohm rating is almost certainly a 2r0 rating.
A 4ohm reactive speaker is far more demanding than a 2r0 resistive load.
I am realising how important current is every single day. Hopefully by 2010 I will have enough knowledge behind basic amplifier topology to build my own.

After comparing the numbers I feel like I was ripped off with some of the amplifiers I own. It is becoming very clear why many design their own amplifiers.


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Old 5th May 2009, 04:50 AM   #8
wg_ski is offline wg_ski  United States
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Quote:
Originally posted by OMNIFEX
.

After comparing the numbers I feel like I was ripped off with some of the amplifiers I own.


Cheers!
I wouldn't go so far as to say "ripped off". If they designed the MA5000 with a supply stiff enough to satisfy the purists, it would put out 12,000 watts per channel at 2R, weigh six hundred pounds, require 50A/240V service to get safety agency certs, and be priced accordingly. And the ODEP would kick in a whole lot sooner unless you cooled it with a 5000 BTU air conditioner.

But modest improvements could be made over most of what is out there. Like 50% more copper and iron, double the output stage SOA, and a full 20kuF per rail. That sort of thing won't improve 2 ohm power by much, but will get you more runtime at 1/4 or 1/8 power at that low a Z with better bass when driven to clip. And you can refine the front end circuitry easily and cheaply enough and get better sound across the board.
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Old 5th May 2009, 05:04 AM   #9
OMNIFEX is offline OMNIFEX  Jamaica
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Thanks for sharing your input megajocke! There are a few things I am having difficulty understanding here. Hopefully you can share your thoughts based on my reply.


Quote:
Originally posted by megajocke



If an amplifier can drive the speaker load without distorting or being damaged it is good. How much the power increases when the impedance is decreased doesn't tell anything. It's perfectly possible to make an amp that will double power from 4 to 2 ohms resistive, but it might still be unsuitable even for driving a 16 ohm speaker load.
I always assumed the user is the one who has the power to destroy speakers. An amplifier cannot distort or destroy speakers unless the user drives the amplifier into clipping.

All of my speakers are 8-ohm nominal in which will only see an increase in its impedance based on how it reacts in the enclosure. How can an impedance rise at a particular frequency that is not a steady tone create havoc to the amplifier when it is still seeing a load?


Quote:
When the limit comes from a certain mechanism (fuse, overheating, second breakdown, current limit, certain kind of VI limiter, voltage clipping, etc.) it might be possible to have a rule of thumb for that specific mechanism - but not generally.

An amp with enough power supply capacitance will sound better than one with little if driven into clipping. How much the power increases with decreasing impedance doesn't tell which mechanism it is. If it's the transformer regulation that causes the voltage to drop (but there still is enough capacitance in the supply) it will sound much better than if the transformer has good regulation but there is too little capacitance.

But on the other hand, if your amp doesn't clip as heavily it will sound even better.
I am not one of those blokes who are trying to extract every ounce of wattage from an amplifier. I am merely comparing the average voltage based on continuous pink or sine wave power at the given load.

Why can I not have a strong power transformer to give me a 2 – 3 volt drop comparing 8-ohm versus 4-ohm and, 4-ohm versus 2-ohm loads with enough capacitance to double the strength?

Quote:
In other words, if you want to play music at 90Vpk into 4 ohms, the amp that can do 70Vpk at clipping into 8 ohm and 65V into 4 ohm loads will sound worse than the one that can do 120Vpk with 8 ohm loads or 90Vpk with 4 ohms (assuming the amplifiers are designed to not distort in another way, like VI limiters activating).
If I want 90 volts in a 4-ohm load, I would not bother hunting down an amplifier that offers 90 volts peak in a 4-ohm load. I would be short-changing myself. I am a firm believer of continuous sine wave power. I don’t fancy burst ratings to make the amplifier look more desirable on the specifications sheet. More times when I encounter such an amplifier, I divide the volts by 1.414 to find the continuous average power.

If the amperage is listed based on various impedance loads with duty cycles, I will use that instead of the advertised wattage chart.



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Old 5th May 2009, 05:28 AM   #10
OMNIFEX is offline OMNIFEX  Jamaica
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Cheers wg_ski!


I am not concerned with 4-ohm to 2-ohm ratio as I am with the 8-ohm to 4-ohm ratio.

http://www.mc2-audio.co.uk/t3500/spec.asp

A mere 1-volt drop from 8 to 4-ohms

http://www.qscaudio.com/products/amp...owerlight3.htm

This amplifier offers a 9.5-volt drop from 8 to 4-ohms.

It is voltage drops like the ones shown above which brought forth my question.
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