Some help with understanding how to bridge load an amplifier

[Trdat]

Can someone help with a laymen explanation on what the diagram means? I understand the speaker connection part but the source connection lets go back to basics what is source? What am I connecting to what firstly? What is the balance side and what is the input?

Lastly, it recommends bridge load on 8ohm and up but also mentions that minimum load is 4 ohm so if the speaker is 8ohm and reaches minimum of 5 ohm how would that play out?

 

[sergeauckland]

Starting at the beginning:-
A BTL, (Bridge Tied Load) works by having two identical amplifiers in which one goes positive with a positive input, and the other goes negative with a positive input (and therefore positive with a negative input!) That means that ignoring the -ve outputs, the two +ve outputs go in opposite directions, and therefore putting the loudspeaker load between them, means they provide twice the voltage (and therefore four times the power) of a single amplifier. Any stereo amplifier with balanced inputs (that isn't already a BTL amplifier) can be used bridged just by connecting the inputs in inverse polarity and putting the loudspeaker between the +ve terminals.

Regarding the load, each half of the bridge doesn't see the loudspeaker between its output and earth, it sees the loudspeaker between its output and its opposite, so drawing twice the current. Twice the current means each half of the bridge sees half the load, so with an 8 ohm loudspeaker, each half sees 4 ohms. If your loudspeaker has an impedance that drops to 5 ohms at some frequency, each half of the bridge will see 2.5 ohms. If each half is rated at 4 ohms, that could well overload the amplifier. Whether it does, depends to a great extent at what frequency the 5 ohm minimum is at. If at a few hundred Hz or below, then that's where the most energy in music is and is likely to be problematic. If the minimum is at, say 8kHz or above, then it's unlikely to stress the amplifier as there's much less energy in music at those frequencies.

The Hypex amp above is rated at 4 ohms minimum for BTL use, so should be OK with your loudspeaker. This means that in single-ended use, it should be OK down to 2 ohms which is impressive.

Hope this all makes sense.

S.

 

[Trdat]

A BTL, (Bridge Tied Load) works by having two identical amplifiers in which one goes positive with a positive input, and the other goes negative with a positive input (and therefore positive with a negative input!) That means that ignoring the -ve outputs, the two +ve outputs go in opposite directions, and therefore putting the loudspeaker load between them, means they provide twice the voltage (and therefore four times the power) of a single amplifier. Any stereo amplifier with balanced inputs (that isn't already a BTL amplifier) can be used bridged just by connecting the inputs in inverse polarity and putting the loudspeaker between the +ve terminals.

Appreciate the response.

I did put the amp together myself so I should be able to figure this out. When we say source are we essentially talking about the wires coming out from the amp board and output is the binding post sockets?

So all i am doing is keeping the left binding socket plus minus the same and then the right binding socket internally I switch the plus with the minus? Did I understand correctly?

And then of course I just need to match the external wiring with the correct layout to the inside.[/QUOTE]

 

[Trdat]

The Hypex amp above is rated at 4 ohms minimum for BTL use, so should be OK with your loudspeaker. This means that in single-ended use, it should be OK down to 2 ohms which is impressive.

I am trying to make sense of it all, when you say single ended use what does that mean?

 

[sergeauckland]

I am trying to make sense of it all, when you say single ended use what does that mean?

SIngle-ended as opposed to bridged. In other words using the amplifier as a two-channel stereo amp rather than a single channel mono amp of higher power.

S.

 

[sergeauckland]

Appreciate the response.

I did put the amp together myself so I should be able to figure this out. When we say source are we essentially talking about the wires coming out from the amp board and output is the binding post sockets?

So all i am doing is keeping the left binding socket plus minus the same and then the right binding socket internally I switch the plus with the minus? Did I understand correctly?

And then of course I just need to match the external wiring with the correct layout to the inside.

[/QUOTE]
Keep the output sockets exactly as normal, and just use the two +ve outputs when bridged. On the inputs, connect the +ve input of channel A to the -ve input of channel B, and the -ve input of channel A to the -ve input of channel B. That will give you one set of balanced inputs going to one set out outputs.

For stereo, you'll need to do this to another pair of amps in exactly the same way, making sure you use the same +ve and -ve inputs as the first pair.

S.

 

[ironhorse128]

I was thinking about buying two NAD THX 216s (https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Nad-216-THX-Brochure.pdf) amps and run them in bridge mode to power Revel F208s which seem to be pretty hard to drive as they dip down to 3.4 ohms multiple times.

Would the NAD THX 216s in bridge mode be able to drive the F208s?

 

[Trdat]

Keep the output sockets exactly as normal, and just use the two +ve outputs when bridged. On the inputs, connect the +ve input of channel A to the -ve input of channel B, and the -ve input of channel A to the -ve input of channel B. That will give you one set of balanced inputs going to one set out outputs.

For stereo, you'll need to do this to another pair of amps in exactly the same way, making sure you use the same +ve and -ve inputs as the first pair.

I am ready to bridge load my hypex amp and just want to confirm the above. The instruction given in the manual seem a little different than what you have mentioned, unless its exactly the same and I have understood incorrectly. Because your saying connect the non inverting of Channel A to the inverting of Channel B and the inverting of Channel A to the non inverting of Channel B. Do you mean connecting these to each other and not to the source as mentioned in the manual? This seems logical cause it gives me one set of balanced inputs.

The manual describes that between the source and input one of the channels should be inverted. To me that means, when connecting inverting and non inverting audio from the module(J4.1 J4.2) to the XLR socket I set the one channel accordingly remaining as per normal while the second channel I switch the plus to minus and minus to plus. Is this correct?

And if it is then which XLR socket do I use to connect from my pre amp?

If your correct than can I use either one of the channels for input?

 

[NTK]

I think this is correct. May not match exactly your use case.

 

[Trdat]

I think this is correct. May not match exactly your use case.

View attachment 117254

This make sense, I can just use a female to dual male XLR and invert one channel internally.

 

[NTK]

This make sense, I can just use a female to dual male XLR and invert one channel internally.

This cable, when made per the schematic, already inverts one of the channels (ch 2). No modification to the amplifier is necessary. Just connect the speaker +ve speaker terminal to channel 1 +ve out, and -ve speaker terminal to channel 2 +ve out.

 

[Trdat]

I think this is correct. May not match exactly your use case.

View attachment 117254

So technically the whole BTL thing is simpler than what it seems at first look.

I use this female to dual male XLR which inverst one channel then use the two plus outputs.

The manual also says that the "Please note that the negative (GND) output terminals are tied together. On the NC-MP modules, this is already implemented in the design and therefor these pins can be left unconnected."

So nothing else needs to be done if I have understood correctly?

Lastly do they sell Female to dual male XLR' that are already inverted? Just to save me the hassle of making one.

 

[NTK]

Lastly do they sell Female to dual male XLR' that are already inverted? Just to save me the hassle of making one.

The only one I know of is a ripoff from Simaudio.
https://simaudio.com/en/product/xlr-bridge-bridging-adaptor-cable/

The easiest I think is to get a female to dual male y-splitter cable and re-solder one of the male connectors.

 

[sergeauckland]

So technically the whole BTL thing is simpler than what it seems at first look.

I use this female to dual male XLR which inverst one channel then use the two plus outputs.

The manual also says that the "Please note that the negative (GND) output terminals are tied together. On the NC-MP modules, this is already implemented in the design and therefor these pins can be left unconnected."

So nothing else needs to be done if I have understood correctly?

Lastly do they sell Female to dual male XLR' that are already inverted? Just to save me the hassle of making one.

I very much hope NOBODY does that. ANY cable that has inversion in the wiring will one day get used in place of a normal cable, with disastrous effects on stereo and mono compatibility. That's why any inverted cable should be made up in the privacy of one's own home, and be confined to one's family and intimate friends. It should be clearly labelled as such, and ANY release into the wild will end in tears.

S.

 

[Trdat]

The only one I know of is a ripoff from Simaudio.
https://simaudio.com/en/product/xlr-bridge-bridging-adaptor-cable/

The easiest I think is to get a female to dual male y-splitter cable and re-solder one of the male connectors.

So just inverting the one side is enough? I might add an extension to the Y cable that shouldn't be a problem right...?

 

[Trdat]

I very much hope NOBODY does that. ANY cable that has inversion in the wiring will one day get used in place of a normal cable, with disastrous effects on stereo and mono compatibility. That's why any inverted cable should be made up in the privacy of one's own home, and be confined to one's family and intimate friends. It should be clearly labelled as such, and ANY release into the wild will end in tears.

S.

Of course point taken.

And it will be labelled, I'll be honest I didn't think of that. So great advice. Thanks.

 

[solderdude]

I was thinking about buying two NAD THX 216s (https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Nad-216-THX-Brochure.pdf) amps and run them in bridge mode to power Revel F208s which seem to be pretty hard to drive as they dip down to 3.4 ohms multiple times.

View attachment 68797

Would the NAD THX 216s in bridge mode be able to drive the F208s?

The amp is specified for 4 Ohm balanced usage and can provide 800W this way.
In 8ohm this is 560W.
Note that the revel is recommended with amps between 50 and 350W so use at your own risk when throwing a party..
Strange part is that the F208 is specified as 8 Ohm while the measurements show about 6ohm average definitely not 8ohm.

 

[PiobGuy]

Resurrecting this thread for a quick measurement confirmation; on the BTL output would I expect the left and right channel to read the "proper" polarity on a volt meter? That is red+ black-. Reading positive voltage? Sorry if this seems a silly question.