Bridged amp wattage?

[Richardjhy]

Got a Rotel RX-855 receiver recently. It's 50 wpc/8 ohm and bridgeable, but spec didn't mention wattage when in bridge mode.

BTW, this receiver use a lot of cheap Jamicon electrolytic caps. Quite a few of them get bulged so I replaced all Jamicons on the main board.

 

[BillH]

I don’t see any mention of bridging in the owners manual. Could be.

 

[Richardjhy]

I don’t see any mention of bridging in the owners manual. Could be.

There is a switch on the back with normal and bridge modes.

 

[Richardjhy]

This receiver uses 2 pairs of 100W power transistors each channel which I think is overkill for normal mode.

 

[Dave_Reite]

Power is equal to Voltage-squared divided by R. (You're doubling the voltage with a single load connected across the A/B terminals.)
So, best case, you would get quadruple the power.
But, in the real world, probably not.

Dave.

 

[Richardjhy]

Power is equal to Voltage-squared divided by R. (You're doubling the voltage with a single load connected across the A/B terminals.)
So, best case, you would get quadruple the power.
But, in the real world, probably not.

Dave.

So therotically bridge mode can be 200W for this receiver? That makes it a good mono!

 

[unpluggged]

So therotically bridge mode can be 200W for this receiver? That makes it a good mono!

But in practice it will be limited by its power supply. And it's not uncommon for such amps to require an increased load impedance in bridged mode, so I suggest consulting the manual on which restrictions apply before trying to bridge it.

 

[Richardjhy]

Too bad - it can be hooked only with minimum 8 ohm speakers.

 

[BillH]

Here is the text from the manual

Use speaker systems with impedance ratings of 4-16 ohms,
when you hook up 2 sets of speaker systems, be sure to
speakers with ratings of 8-16 ohms. Before hooking up your
speaker system, check its impedance, which should be in-
dicated either on the back of the speaker, or in the speaker
instruction manual.

 

[sam_adams]

The switch on the back sets the device to bridged MONO.

 

[Richardjhy]

Here is the text from the manual

Use speaker systems with impedance ratings of 4-16 ohms,
when you hook up 2 sets of speaker systems, be sure to
speakers with ratings of 8-16 ohms. Before hooking up your
speaker system, check its impedance, which should be in-
dicated either on the back of the speaker, or in the speaker
instruction manual.

On the back of my receiver it says minimum 8 ohm speakers. Difference per region?

 

[wwenze]

This receiver uses 2 pairs of 100W power transistors each channel which I think is overkill for normal mode.

Transistors are rated for DC, but audio is AC. 10V DC has more power than a sine wave with 10V peak.

 

[solderdude]

This receiver uses 2 pairs of 100W power transistors each channel which I think is overkill for normal mode.

A 100 W transistor can dissipate 100 W of heat when cooled perfectly. This does not say much about how much power a device can deliver in a certain load. That also depends on the power supply voltage, load impedance and output voltage relative to the power supply voltage.
The highest dissipated power happens when the output voltage is half that of the power supply voltage.
Fortunately the output voltage varies dynamically in music. Also 8 ohm speakers can dip well below 8 ohm so can have higher than expected currents flowing. Furthermore there is thermal resistance so chip temperatures can be higher than acceptable for them than in lab conditions. You want output devices to operate within their SOA (safe operating area). That is why output devices need to be of certain wattage. They need to be able to supply higher than expected currents at all voltages without the die inside that device overheating.

 

[solderdude]

Got a Rotel RX-855 receiver recently. It's 50 wpc/8 ohm and bridgeable, but spec didn't mention wattage when in bridge mode.

BTW, this receiver use a lot of cheap Jamicon electrolytic caps. Quite a few of them get bulged so I replaced all Jamicons on the main board.

Bridging can double the output voltage in principle if the powersupply can provide the required current.
Usually that power supply is the limiter which causes the output voltage to not reach double the voltage.
50W in 8ohm = 20Vrms and 2.5Arms so double that is 40V rms and 5Arms in 8ohm so 4x the power. The power supplies usually can't reach that so in practice this will be limited.
For bridged operation most amps ( because of their current limits) allow bridged operation but only at double the impedance so a 4ohm rated amp can be used bridged in 8ohm only (doubling the output power and thus not exceed the max. allowed current.