Amps With Two Output Question....

[LukeFRC]

stereo amps.....

you know the ones with 2 outputs, or one bridged output.

well can you run only half of one, or will that make it go pop?

For instance i have a 4 ohm amp..... but the bridged output was minimum resitance of 8ohms.... so you would run it through a output.... but would you need to have another speaker for the other output?

[alexclaber]

You can use either side on its own or both at once, or you can use the bridged output. You can't used bridged at the same time as using each side separately but you can certainly use just one side of the amp singly with zero problems - in fact running one side of the amp into its minimum impedance you'll usually get a bit more power per side than using both sides at once, which is nice.

[LukeFRC]

[quote name='alexclaber' post='1094592' date='Jan 18 2011, 10:47 PM']You can use either side on its own or both at once, or you can use the bridged output. You can't used bridged at the same time as using each side separately but you can certainly use just one side of the amp singly with zero problems - in fact running one side of the amp into its minimum impedance you'll usually get a bit more power per side than using both sides at once, which is nice.[/quote]

awesome, thanks for the reply.
I think when i started playing i was lent an amp and told it will blow up if i turned it on with no load so thats where my questions came from

[alexclaber]

[quote name='LukeFRC' post='1094597' date='Jan 18 2011, 10:50 PM']I think when i started playing i was lent an amp and told it will blow up if i turned it on with no load so thats where my questions came from[/quote]

That's only true for valve amps (i.e. the really heavy ones!)

If an amp says "minimum load 'x' ohms" on the speaker outputs then it's safe to use without anything connected because having nothing connected is an infinite load. Valve amps don't have a minimum load, they have a maximum load,but usually just specify 2 and/or 4 and/or 8 and/or 16 ohm outputs (because they are transformer coupled) and having nothing connected is an infinite load, which makes them sad.

[obbm]

I assume your talking about a solid-state stereo amp in which case it's fine to run without a load.

Valve amps on the other hand must have a load matched to the output transformer setting.

[tauzero]

[quote name='alexclaber' post='1094592' date='Jan 18 2011, 10:47 PM']in fact running one side of the amp into its minimum impedance you'll usually get a bit more power per side than using both sides at once, which is nice.[/quote]
Depends what you're running into. If you have two 8-ohm cabs, you're probably better off running one off each output than both (ie a 4 ohm load) off one.

[alexclaber]

[quote name='tauzero' post='1094846' date='Jan 19 2011, 09:49 AM']Depends what you're running into. If you have two 8-ohm cabs, you're probably better off running one off each output than both (ie a 4 ohm load) off one.[/quote]

Sorry, I realise that could have been misinterpreted, as indeed it was!

If your amp can drive down to 4 ohms on each side, or 8 ohms bridged, and is 500W/ch into 4 ohms, 300W/ch into 8 ohms and 1000W/bridged into 8 ohms, the xxxW/ch ratings should be based on both channels being driven. If you're only driving one channel and the other channel is silent then I'd expect to get closer to the 600W into 4 ohms on that side as the power supply has lots of spare current capacity. My point is merely that if you have an amp which can't bridge into 4 ohms but you have a single 4 ohm cab, don't feel you're wasting the ability of the amp because the redundant power amp channel is actually contributing in that your amp has a power supply section twice as large as if it was a single channel 500W @ 4 ohms amp, hence it can get closer to doubling its 8 ohm power output when driving 4 ohms.

If you have two 8 ohm cabs then use one of each side of amp, which will get you a bit more output than with both on one side because the power transistors aren't working so hard.

[tauzero]

[quote name='alexclaber' post='1094887' date='Jan 19 2011, 10:32 AM']Sorry, I realise that could have been misinterpreted, as indeed it was!

If your amp can drive down to 4 ohms on each side, or 8 ohms bridged, and is 500W/ch into 4 ohms, 300W/ch into 8 ohms and 1000W/bridged into 8 ohms, the xxxW/ch ratings should be based on both channels being driven. If you're only driving one channel and the other channel is silent then I'd expect to get closer to the 600W into 4 ohms on that side as the power supply has lots of spare current capacity. My point is merely that if you have an amp which can't bridge into 4 ohms but you have a single 4 ohm cab, don't feel you're wasting the ability of the amp because the redundant power amp channel is actually contributing in that your amp has a power supply section twice as large as if it was a single channel 500W @ 4 ohms amp, hence it can get closer to doubling its 8 ohm power output when driving 4 ohms.[/quote]
Surely the output power will be limited by the power supply rail voltage rather than the current?

[alexclaber]

[quote name='tauzero' post='1095522' date='Jan 19 2011, 07:17 PM']Surely the output power will be limited by the power supply rail voltage rather than the current?[/quote]

If that was the case then all amps would continue to deliver more and more power as the load impedance drops, according to P=(V^2)/R

[dincz]

[quote name='tauzero' post='1095522' date='Jan 19 2011, 08:17 PM']Surely the output power will be limited by the power supply rail voltage rather than the current?[/quote]

Amounts to much the same thing. As you approach the power supply's maximum current capacity, the rail voltage will drop. So if you run only one side, the power supply will be loafing and giving full rail voltage.

[tauzero]

[quote name='alexclaber' post='1095646' date='Jan 19 2011, 08:42 PM']If that was the case then all amps would continue to deliver more and more power as the load impedance drops, according to P=(V^2)/R[/quote]
No, the voltage is split between the load resistance and the amp's internal resistance in the output stage. As the load drops, the potential divider between the internal and external resistance puts proportionally less voltage across the load.

Plus the amp will blow up in the limiting case.

[tauzero]

[quote name='dincz' post='1095720' date='Jan 19 2011, 10:02 PM']Amounts to much the same thing. As you approach the power supply's maximum current capacity, the rail voltage will drop. So if you run only one side, the power supply will be loafing and giving full rail voltage.[/quote]
If the load is the same overall, then the current draw will be the same whether it's one channel or two.

[alexclaber]

[quote name='tauzero' post='1096546' date='Jan 20 2011, 04:49 PM']No, the voltage is split between the load resistance and the amp's internal resistance in the output stage. As the load drops, the potential divider between the internal and external resistance puts proportionally less voltage across the load.

Plus the amp will blow up in the limiting case.[/quote]

The internal resistance of modern s/s output stages is not large enough to explain the discrepancy between maximum output voltage into an 8 ohm and a 4 ohm resistive load.

[quote name='tauzero' post='1096552' date='Jan 20 2011, 04:52 PM']If the load is the same overall, then the current draw will be the same whether it's one channel or two.[/quote]

Yes but it isn't the same - if you are using one 4 ohm cab then your amp will be able to deliver a greater voltage on that side than if driving two 4 ohm cabs at once, one on each output stage.

[dincz]

[quote name='tauzero' post='1096546' date='Jan 20 2011, 05:49 PM']No, the voltage is split between the load resistance and the amp's internal resistance in the output stage. As the load drops, the potential divider between the internal and external resistance puts proportionally less voltage across the load.

Plus the amp will blow up in the limiting case.[/quote]

The voltage divider idea is correct, but in practice the output impedance of a solid state class A/B amp is in the order of milliohms - not enough to make a scrap of difference. The power supply is where the serious drop happens.

Note I said power "supply" not "supplies" as a common supply for both channels is the norm.

Edited January 20, 2011 by dincz

[alexclaber]

[quote name='dincz' post='1096586' date='Jan 20 2011, 05:20 PM']Note I said power "supply" not "supplies" as a common supply for both channels is the norm.[/quote]

Indeed it is! When I was getting a lightweight power amp to drive my Acmes I considered the Stewart World 2.1 which is a true dual mono amp, with the left and right side completely separated back to the mains input, which is good for when things blow up because it's highly unlikely that both sides would die at once. However, I ended up with a QSC PLX3002 which has the more normal common supply for both halves of the power amp, so you don't have the entire spare amp with you if you're just using one side but does have an effectively overspecced power supply.

If you look inside a Hartke LH500 and LH1000, the LH1000 has two sets of output transistors, so a doubling up of the LH500 because it's a dual power amp version, but it doesn't have two power supplies, it has one power supply which is twice as big (absolutely literally in the case of the massive toroidal transformer).

[dincz]

Not so massive in this case but it illustrates the point.
[attachment=69486:01.JPG]

[fatback]

[quote name='obbm' post='1094630' date='Jan 18 2011, 11:16 PM']I assume your talking about a solid-state stereo amp in which case it's fine to run without a load.

Valve amps on the other hand must have a load matched to the output transformer setting.[/quote]

Useful info.

I was told by somebody not to use my amp as a DI with no load. nice to know I can.