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Old 29th October 2017, 03:23 PM   #1
lawbadman is offline lawbadman  Jamaica
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Default Amplifier Ratings

Hello guys,

Just out of curiosity. I have seen in the schematic for the QSC CX302 and only 3 pairs of output transistors are used...yet the are able to achieve 600 watts at 2ohm. I know QSC to be a reputable brand so I don't think they would lie.... But how is that possible while maintaining reliability? In the diy world it's the norm to have 1 pair of output transistors for every 100watts or so for reliability sake. Why is it not the same for pro audio?

Just a thought..

Thanks all
Lawrence






QSC QSC CX302 Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics
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Old 29th October 2017, 03:37 PM   #2
AndrewT is offline AndrewT  Scotland
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it comes down to reading the way the specification is written.

A 100W into 8ohms amplifier specification usually means to can drive 40Vpk & 5Apk into an 8r0 dummy resistor at the specified distortion.

But that is not a particularly onerous task. Driving a reactive speaker is much more onerous and reputable amplifier manufacturers while still specifying the same 100W into 8r0 @ 0.1% maximum distortion from 100Hz to 10kHz will also have designed it to survive normal and loud domestic duty where the peak outputs just hit the peak currents expected into a real speaker load. These peaks can easily and often exceed 200% of the maximum resistor current and can sometimes exceed 300% of the rated resistor current.

You will find that such an amplifier can easily drive a 4r0 dummy load to nearly double the 8r0 rating, for a short while as the heatsinks and output devices (including drivers) rapidly heat up.
If the 4r0 output is only 50% higher than the 8r0 output, then leave that amplifier in the dealer's showroom.
If it gets to 170W, then you probably have a good amplifier.
If it gets to 180W you have an excellent current delivery power amplifier that will perform with all varieties of 8ohms speaker.

But beware, many commercial speakers are now actually a combination of 4ohms drivers and 8ohms drivers. They should be rated as 4 to 8ohms speakers, but usually the speaker manufacturer specifies them as 6ohms and might not even show an impedance plot to forewarn the buyer.
A 4ohms to 8ohms speaker really needs a 4ohms rated amplifier. That amplifier should be completely happy driving a 2r0 dummy load to maximum power (around the 170% to 180% of 4r0 rating) for a minute, or two without triggering protection circuits and without blowing up.

A 100W into 8ohms amplifier that is rated for 4ohms duty should be able to get at least 180W into 4r0 and more than 320W into 2r0 while still managing to introduce no more than 0.1% distortion at the maximum output level.
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Last edited by AndrewT; 29th October 2017 at 03:40 PM.
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Old 30th October 2017, 05:23 AM   #3
jvhb is offline jvhb  Europe
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Also note that the stated test conditions change: (from CX302 Power Amplifier - QSC)

8Ω, 20 Hz–20 kHz, 0.05% THD
2Ω, 1 kHz, 1% THD

So they allow for much higher distortion, and only test at a single frequency.

In addition, since PA amps has forced air cooling, this does allow the transistors to (safely) put out more power than when passively cooled (as a general rule, and all other things being equal etc.)
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Old 30th October 2017, 11:07 PM   #4
djk is offline djk
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"Why would it make a difference?"
It's a matter of M_ean T_ime B_efore F_ailure, among other issues.

MTBF at 2R is much, much less.

The transformer is much smaller on the EP2500, as are the filter caps, and it only has one fan.
----------------------------------------------------------------------------------------------------------------------

As far as I am concerned, 'rated to 2 ohms' means that it will sound good and have a long trouble free life at 4 ohms. Large sound sompanies in the US sometimes run 2 ohms for mids and highs, but there is a dirty little secret you don't know. They sell the amps after one year and buy new. They get such a deal on the amplifiers that their cost seems like a deal to you on the 'tired' amplifiers, and you get the balance of the factory warranty anyway. The point is they always have new amps and it never costs them anything to own them (but they still won't run them at 2 ohms in the bass).

Years ago I did a M_ean T_ime B_efore F_ailure (MTBF) on an amplifier that claimed to double its power going from 8 to 4 and again from 4 to 2 ohms (it did). Based on the junction temperatures of the transistors the MTBF at 8 ohms was 10,000 hours, at 4 ohms it was 625 hours, and at 2 ohms it was 39 hours.

How long do you want your amp and speakers to last?

Will you get paid if your only bass amp goes thermal and takes a half hour to come back on?

How much will it cost to re-cone the bass drivers if the amp goes up in flames?

How much revenue will you loose from jobs you can't do while down?

Don't put all your eggs in the same basket!

I always do whatever I can to make sure I get paid at the end of the night, and I don't like to have to pay for repairs.

I did audio for about 30 years (doing L-band RF now), but as the sticker on a new car says:

Your Mileage May Vary!
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Old 31st October 2017, 12:24 AM   #5
wg_ski is offline wg_ski  United States
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Translation: even the reputable PA amp manufacturers run the output transistors (and the IGBTs in the power supply) WAY the **** outside their rated SOA.

Why? Modern transistors will usually take it... for 39 hours of course. Back in the 70s and 80s that sort of abuse might mean instant death. Mostly because the failure mechanisms were not as well understood and it was more typical to oversize to err on the side of caution.
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Old 2nd November 2017, 12:33 PM   #6
east electronics is offline east electronics  Greece
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Quite real arguments add to them that both in QSC and Behringer products may actually produce that amount of power 2 1KHZ and resistive load but beyond others all shorts of limiters will kick in with a reactive load ....
I could say that these amps are designed to measure that on bench but never produce this in real life .....
That's about it

Running outputs way out of their SOA actually will have a small benefit only, like when that amplifier operates in relative low power will have plenty of headroom and nice dynamic peaks before limiters kick in ... In some cases that could be a nice addon but when the discussion is about continuous available power in specific load is obviously useless ...

Kind regards
Sakis
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Last edited by east electronics; 2nd November 2017 at 12:38 PM.
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Old 2nd November 2017, 04:21 PM   #7
lawbadman is offline lawbadman  Jamaica
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Quote:
Originally Posted by djk View Post
"Why would it make a difference?"
It's a matter of M_ean T_ime B_efore F_ailure, among other issues.

MTBF at 2R is much, much less.

The transformer is much smaller on the EP2500, as are the filter caps, and it only has one fan.
----------------------------------------------------------------------------------------------------------------------

As far as I am concerned, 'rated to 2 ohms' means that it will sound good and have a long trouble free life at 4 ohms. Large sound sompanies in the US sometimes run 2 ohms for mids and highs, but there is a dirty little secret you don't know. They sell the amps after one year and buy new. They get such a deal on the amplifiers that their cost seems like a deal to you on the 'tired' amplifiers, and you get the balance of the factory warranty anyway. The point is they always have new amps and it never costs them anything to own them (but they still won't run them at 2 ohms in the bass).

Years ago I did a M_ean T_ime B_efore F_ailure (MTBF) on an amplifier that claimed to double its power going from 8 to 4 and again from 4 to 2 ohms (it did). Based on the junction temperatures of the transistors the MTBF at 8 ohms was 10,000 hours, at 4 ohms it was 625 hours, and at 2 ohms it was 39 hours.

How long do you want your amp and speakers to last?

Will you get paid if your only bass amp goes thermal and takes a half hour to come back on?

How much will it cost to re-cone the bass drivers if the amp goes up in flames?

How much revenue will you loose from jobs you can't do while down?

Don't put all your eggs in the same basket!

I always do whatever I can to make sure I get paid at the end of the night, and I don't like to have to pay for repairs.

I did audio for about 30 years (doing L-band RF now), but as the sticker on a new car says:

Your Mileage May Vary!

Eye opening stuff djk!
I will have to reevaluate how I wire amps now...
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Old 2nd November 2017, 04:23 PM   #8
lawbadman is offline lawbadman  Jamaica
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Quote:
Originally Posted by jvhb View Post
Also note that the stated test conditions change: (from CX302 Power Amplifier - QSC)

8Ω, 20 Hz–20 kHz, 0.05% THD
2Ω, 1 kHz, 1% THD

So they allow for much higher distortion, and only test at a single frequency.

In addition, since PA amps has forced air cooling, this does allow the transistors to (safely) put out more power than when passively cooled (as a general rule, and all other things being equal etc.)

jvhb good point!
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Old 2nd November 2017, 11:08 PM   #9
motronix is offline motronix  Israel
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most of my work today is car power amplifiers. On the car power amplifier market i can see that they drive 2 ohm with 1 NPN & 1 PNP so?
and the owner of this amplifiers load the amplifiers with 0.5 ohm
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Old 5th November 2017, 12:22 PM   #10
Burnedfingers is offline Burnedfingers  United States
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As one that has been employed in the commercial field for over 25years now I can attest to the quality of the QSC brand and particularly this model. I have used hundreds of the CX302's,CX602's, CX902's, and CX1202's.

Most generally the load is not below 4 ohms and none have been lost. Would I run a 2 ohm load with one? Probably not. I generally try to obtain a load of 4 ohms for one of these dispite the 2 ohm rating. To me a 2 ohm rating still says don't go below 4 ohms.

To be blunt I would gladly use a QSC product over say a Crown of the same or more power simply because 5 or 10 years later I still don't have a problem with the QSC power amps and I can't say that when Crown is used.
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