Amp bridge adapter - Inverting opamp hum

Thread Starter

IvarsOzolsDB

Joined Aug 24, 2012
40
I'm trying to bridge an old 2ch Rotel RA-06 amp that natively does not have that kind of function, to drive a large sub at home AV.

I'm using Figure 2 from this project https://sound-au.com/project14.htm to get inverted signal. Here is direct link to schematic:
https://sound-au.com/p14_fig2.gif

Attached is service manual with schematic on Rotel amp.

Bridging adapter pcb is put inside amp, power for adapters pcb is tapped from +/-18V rails of amp. Single ground connection from adapter PCB to amp PCB. Outputs from bridge adapter is soldered directly to amps RCA inputs. No extra grounds on signal wires. Adapter opamp is bypased with 100nF to power rails and 2x100uF to ground from each of power rails.

Signal is going fine on both channels. The problem I'm having is on inverted channel - 50Hz hum on amp output, but no hum on non-inverted channel. If I connect signal without adapter, both channels have no hum. What am I missing? Signal source is Yamaha AVR subwoofer line output.
 

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Ian0

Joined Aug 7, 2020
9,792
R635/R613 and R636/R614 are the feedback networks that set the gain of the amplifier.
All you have to do to make it into a bridge-tied-load amplifier is connect a resistor of the same value of R635 between the output of the top channel and the junction of R636/R614.
Don't forget that the amplifier will only drive twice the rated load impedance when it is converted to bridge-tied-load.
 
Last edited:

crutschow

Joined Mar 14, 2008
34,392
I expect the problem is with the ground connection for the inverting amp, since any ground noise will be amplified by the inverting amp, but not the non-inverting amp.
You might try connecting that ground to different locations and see if that makes a difference.
 

Thread Starter

IvarsOzolsDB

Joined Aug 24, 2012
40
R635/R613 and R636/R614 are the feedback networks that set the gain of the amplifier.
All you have to do to make it into a bridge-tied-load amplifier is connect a resistor of the same value of R635 between the output of the top channel and the junction of R636/R614.
Don't forget that the amplifier will only drive twice the rated load impedance when it is converted to bridge-tied-load.
Thank you very much. This worked like a charm.

Impedance is not an issue as amp is capable driving 4ohm load, but the subwoofer is 8ohm.
 

LowQCab

Joined Nov 6, 2012
4,053
The "Effective" Impedance that each of the Amplifiers Outputs will "see"
is half of the actual Load-Impedance of the Speaker when connected in a Bridged configuration,
so both Amplifier Channels will see a 4-Ohm-Load,
and are likely to run quite hot.
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Ian0
Simply placing the Sub in a particular corner of a particular room
can easily increase Speaker efficiency by ~4X over placing the Sub in the middle of the room.

Speakers are "Impedance-Transformers", and most of them have really-really poor efficiency.

Horn-Loaded-Speakers do have massive advantages,
but modern designs don't look anything like your ancient comical picture.

Here's a modern design ........
.
Tuba Home Theater .jpg.
36Corner.gif
This is the Output with 1-Watt of Power applied.
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