[afblaster]

I use PMC IB1 speakers (horizontally) bi-amped by two 4B-SST amplifiers, with the amplifiers' left channels connected to the bass and right channels connected to mid/top of each respective speaker.

Today as an experiment I reconnected the amps in bridged mode, and used a single cable from each amp to connect to the bass terminals on each speaker, which were connected to the mid and top with good quality wire jumpers.

Playing familiar music tracks and listening at my usual volume levels, I was rather surprised to find that the sound was much more detailed and tight, with much greater slam on drums and percussion, and with the performers set in a much improved soundstage.  I now hear tiny "sounds-off" which I had not heard previously.

I had expected to hear little or no differences due to the method of connection, since the maximum power available is much the same at 450W  per channel @ 4 ohms in normal mode, and 900W in bridged mode. I guess the voltage swing available in bridged mode is twice that of the the individual channels, but since I am listening at the same subjective volume levels would this account for the improvement, or am I deluding myself?

[James Tanner]

Using the 4 channels of the 4B amplifers through the passive crossover is not the same power wise as using it in the 2 channel bridge mode.

The bridge mode gives you much more voltage swing and 4 times the power available in the stereo mode. Also the slew rate doubles and it operates in a fully balanced mode much more like the 7B SST mono amps.

james

[afblaster]

Thanks James - I guess my ears didn't deceive me.
Drum kits now sound frighteningly realistic!

[Viajero5000]

Using the 4 channels of the 4B amplifers through the passive crossover is not the same power wise as using it in the 2 channel bridge mode.

The bridge mode gives you much more voltage swing and 4 times the power available in the stereo mode. Also the slew rate doubles and the it operates in a fully balanced mode much more like the 7B SST mono amps.

james

i was under the impression that such bridging increases distortion, but have seen a couple of people stating that performance is better by 'mono-blocking' 4Bs this way as opposed to 'normal' bi-amping. Am i missing something?

[James Tanner]

Hi,

I would use the 'Quotes" above but when you type it jumps all over the place --talk about jitter!

With our designs there is no increase in distortion when you bridge in fact things are very well behaved assuming certain parameters.

The only issue is that when you bridge a stereo amp the amp "see's"  1/2 the impedance it would see in stereo.  So the amplifier in bridged mode has to deliver much more current than in stereo mode. So caution needs to be taken if your speaker has a lower impedance curve. That's why at say 4 ohms a pair of 7B's mono amps would be a better choice than a pair of bridged 4's. The 7B's would 'see' the 4 ohms the 4B bridged would see 2 ohms!

Also the 'reactivity' of the load enters into this decision.  A low impedance load that has a fairly benign phase angle and reasonably flat impedance curve will not present as difficult a load on the bridged amp as a highly reactive load with weird phase angles.

james

[b5pt9]

Are IB1's relatively easy to drive?  I think the nominal impedance is 6 ohms but I'd like to see the impedance curve from 20Hz to 20kHz. 

[afblaster]

I'm aware that the 4Bs see half the load impedance when bridged.  PMC state that the impedance of the IB1 is nominally 4 ohm, but a recent review of the IB2i in HIFICRITIC (UK publication) states that while the impedance is generally 6 ohms, it is reactive (52 degrees) at 5 ohms at 70Hz, and is 3.6 ohms between 80 - 120Hz, and falls to 5 ohms in the upper treble.

Bearing this in mind I was initially very cautious and monitored the temperature rise of the amps while playing music with plenty of low bass, but in fact they did not seem to get as hot as when I was using them as 4 separate channels.  Because of the sound improvement I will continue using them in bridged mode and continue to monitor temperatures.  Maybe it's time to think about getting a pair of 7Bs (and IB2is)....

[James Tanner]

I'm aware that the 4Bs see half the load impedance when bridged.  PMC state that the impedance of the IB1 is nominally 4 ohm, but a recent review of the IB2i in HIFICRITIC (UK publication) states that while the impedance is generally 6 ohms, it is reactive (52 degrees) at 5 ohms at 70Hz, and is 3.6 ohms between 80 - 120Hz, and falls to 5 ohms in the upper treble.

Bearing this in mind I was initially very cautious and monitored the temperature rise of the amps while playing music with plenty of low bass, but in fact they did not seem to get as hot as when I was using them as 4 separate channels.  Because of the sound improvement I will continue using them in bridged mode and continue to monitor temperatures.  Maybe it's time to think about getting a pair of 7Bs (and IB2is)....

I read that same review on the IB2i's - no wonder you need a Bryston amp!

james

[vegasdave]

So, is bridged better than bi-amped? Is that in fact what's being said here?

[afblaster]

Well - I won't be going back to bi-amping - in my opinion bridged mode is significantly better.
Why not try it yourself and give us your opinion?

[vegasdave]

Ok, but I need another 4BSST! I don't have the money right now!