Bridging Mono Blocks

[jwatts]

Quick Question. I want to bridge two mono blocks. Can I simply Invert the signal to one of them and tie the two negative leads together from the output and use the positive output lead from the non inverted block as the positive and use the positive from the inverted as the negative.

[Stocker]

leave the negative leads alone


if they are internally bridged already all you will get is some heat if you are lucky, explosions if not.

[AndrewT]

Hi Jason,
You are talking tubes with transformer output?
Remember bridging will give you twice the voltage into the load which means four times the power into the same load. This implies that each amp will deliver twice its rated power and IMO for valves this does not apply. I think you will have to adjust your load to ensure that each amp is delivering rated power into your new load value.
You can arrive at this by thinking bridging gives twice the power into twice the load.
If you had been on the solid state forum your idea may have had more relevance.
Can anyone confirm?
regards Andrew T.

[DigitalJunkie]

I was always under the impression that with tubes amps(using OPT's) you parallel the outputs,and inputs,and connect the speaker to the next higher impedance tap.(ie,for an 8 ohm speaker,you would parallel the 16ohm taps.)

Atleast that's what it says in my Dynaco ST70 manual.

[jacco vermeulen]

Like with solid state output devices tubes are biased.
The output transformer lowers the output voltage of the tube to a level that is acceptable for the loudspeaker, and raises output current to match.
If you bridge tube amplifiers the amplifiers will see an impedance that is virtually half of the real value.
Output voltage can be twice as high but you will need enough current.
The output transformer will need to deliver the higher current, and at the other end the tube outputs will have to deliver higher currents.

Which they cant.
Tubes can deliver only that for which they were designed for, same goes for the output transformer.
Resulting in heavy clipping once nominal output current is exceeded.

In parallel the amplifiers can deliver twice the output current.
Meaning that if a 8 Ohm load is virtually divided in two parallel 16 Ohm loads one amplifier delivers its output current in a 16 Ohm impedance.
The output of the amplifier can be set to 16 Ohm because that is what it sees.
At the 16 Ohm setting output voltage is twice as high.
This again leads to an output that is 4 times the value of one amplifier.
If you go beyond the output of twice the nominal output of a single amplifier expect clipping, but the tube way.

For bridging a tube amplifier a 4,6,8,12,16 Ohm output would be nice.
At a 4 Ohm load the amplifiers can be paralleled on the 6 Ohm output, at 8 Ohm on the 12 Ohm.

[planet10]

IIRC (not done it myself -- just seen it) on the Decware Zen the inputs are paralleled, and the outputs are run in series....

The scheme mentioned above for the ST 70 is more the norm thou... this is how Norman Thagard described monoing his Citations in his recent aXp article.

dave

[Frank Berry]

"I was always under the impression that with tubes amps (using OPT's) you parallel the outputs,and inputs,and connect the speaker to the next higher impedance tap.(ie,for an 8 ohm speaker,you would parallel the 16 ohm taps.)"

That may work but it won't work very well.

As Jacco Vermeulen stated, there will be a mismatch. It's worth a try, however.

Dave suggested paralleling the inputs and series-connecting the secondaries of the output transformers. That will only work if the secondaries are not grounded. In most tube amplifiers, the Common or 0 connection is grounded.

You might also try feeding the inputs out of phase, connecting the output Common or 0 terminals together and then powering your 8 ohm loudspeakers from the two 4 ohm taps.

I'd like to know the results of your tests. Please post them.

[planet10]

Originally posted by Frank Berry
Dave suggested paralleling the inputs and series-connecting the secondaries of the output transformers. That will only work if the secondaries are not grounded. In most tube amplifiers, the Common or 0 connection is grounded.

As is the case in the ZEN.

Thanx for adding that little idbit to my knowledge store :^)

dave

[Eli Duttman]

Jason,

If, as previously indicated, the 2 monoblocks are "normal", as opposed to already bridged, your scheme of tieing the speaker grounds of the amps together and driving the speakers from the "hot" terminals is OK. However, since the O/P voltage swings of the 2 monoblocks are summed in the speaker, each monoblock "sees" only 1/2 of the load. You connect 16 Ohm speakers to the 8 Ohm taps and 8 Ohm speakers to the 4 Ohm taps. You can't use 4 Ohm speakers with the bridged setup.

I'd use a trafo, perhaps the Sowter (www.sowter.co.uk) Model 8920 as the phase splitter. A trafo will allow you to break up ground loops. Ground the shields of your interconnect cables to the chassis and keep them out of the signal path.