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Old 17th July 2002, 10:57 PM   #1
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Default Sound of bridged amplifier

Hello all,

Why is the sound of my amplifier so much better bridged than individual?


Gabe
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Old 17th July 2002, 11:08 PM   #2
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Potential advantages of bridged operation:

* Reduced audio frequency modulation of power supply
* Enhanced common-mode rejection of residual PS noise
* Cancellation of even order distortion products
* Reduced voltage induced distortion in gain stages
* Increased output voltage swing (may improve bass control)
* Increased overall power output (may improve dynamics)

Potential drawbacks of bridged operation:

* Increased current induced nonlinearity in gain stages
* Increased risk of overcurrent damage to output section

IMO, the benefits of bridged operation are most likely realized in a high bias (class A or high class AB) design. What amp are you using?
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Old 17th July 2002, 11:34 PM   #3
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Look at the thread on STK amplifiers and particularly rljones posts.

STK chips

No specific answers to you question, but thoughts similar to yours...
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Old 19th July 2002, 06:37 PM   #4
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Hi Joe and Pmkap,

Yes, I saw rljones' posts, but as you say he notices the same without giving much more of an explanation.

Here is my situation: The ones I have heard are both car amps. The one I have now is an Optimus (Radio Shack) four channel amp bridgable to two channel with a real (RMS) power output of about 250 watts per channel into 4 ohms (they claim 400). The other one I tried was a Jensen, also with about 200 watts bridgable into 4 ohms. They both also have a truly balanced inverter stage, like rljones has.

Both sonded very close, with the Jensen sounding a bit metallic on the high end. On the other hand, I did modify the Optimus by replacing the 4558 op-amps with TLO82 Bifet op amps, and using 18 gauge wiring from the power supply rails right to the output transistors, and 16 gauge wire from the power resistors of the output transistors to the binding posts for the speakers.

However, both also sounded very close to... and don't cringe folks... a tube amp!

So, I have difficulty with one of your thoughts, not because you are wrong, but because I don't understand:

"Potential advantages of bridged operation:

* Cancellation of even order distortion products"

How, since the amp is pretty much all odd order harmonics. Differential input and complementary symmetry output all but eliminates all even order harmonics? Bridging, if it eliminates the rest, should sound hard and sterile, not smooth, revealing and sweet like it does.

If that is the case, then a good tube amp is more linear than a good solid state amp... yet measured distortion figures say that tube amps produce even ordered distortion, even in push pull amplifiers, where a good SS amp can have virtually no distortion.

You Wrote:

Potential drawbacks of bridged operation:

* Increased risk of overcurrent damage to output section

The Optimus amplifier has current limiting at the output stages. Presumably, the design engineer took this into consideration when choosing output transistors. The ones in this amp are rated for 25 amps. The amp may put out 17 into 4 ohms bridged. If that is the current limiter doesn't kick in way before then.

I am going to do some bench tests in the near future to take a closer look into this. It is fascinating that bridging would make an OK sounding SS amp into a truly wonderful sounding one.

Thanks!
Gabe
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Old 19th July 2002, 07:36 PM   #5
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Quote:
Originally posted by Gabevee
Differential input and complementary symmetry output all but eliminates all even order harmonics? Bridging, if it eliminates the rest, should sound hard and sterile, not smooth, revealing and sweet like it does.
A typical complementary circuit has some residual even order distortion due to imperfect mathing of the P and N type devices. If you bridge two such circuits, you can get side-to-side symmetry which is even stronger than the P-to-N symmetry, resulting in further cancellation of residual even order products.

Quote:
If that is the case, then a good tube amp is more linear than a good solid state amp... yet measured distortion figures say that tube amps produce even ordered distortion, even in push pull amplifiers, where a good SS amp can have virtually no distortion.
Well, as they say, welcome to the next level. ;-)
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Old 19th July 2002, 08:10 PM   #6
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Interesting. Thanks for the input. I guess I need to stop thinking "ideal" and realize that there are always imprefections.


Next level. Hah! LOL!


Thanks,
Gabe
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Old 22nd July 2002, 01:23 AM   #7
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Balanced drive of speakers or bridged amps can sound better for many reasons. First of all you have symmetric drive of the speakers which is more natural. You also have more power for some extra head room.

Itīs not perfect though. If the amps are not totaly the same you may have more distortion. Also bridged amps see half the load.
If your speakers are 8 Ohms the amps see 4 ohms. This means more THD and more need of power that can cause clipping and so on.

Bridging should be done when the two amps are totaly identical, from a stereo amp for instance. Are capable of driving at least 2 Ohm loads with low THD and high power. The circuit that makes the balanced signal in the inputs should be of good quality with perfectly symmetrical output.
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Old 22nd July 2002, 02:08 AM   #8
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Default Optimus amps

Built by Pyramid Audio (Sound Around)
Low bias current - operates mainly in Class B
Underwhelming power supply: The 250WPC of which the outputs are capable is limited by the sorry power supply.
VI limiters kick in way before you get there anyway.

That said. I have a Pyramid amp that uses STK devices - sounds good, but again, limited by the power supply into any funny loads. Clips easily.
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Old 22nd July 2002, 02:10 AM   #9
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I donīt think the STK devices are so suitable for bridging. Maybe a few pieces are but only with a good PSU and only in 8 Ohm loads of course.
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Old 23rd July 2002, 03:46 PM   #10
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Veeeery interesting. Thanks.

BTW, if you noticed that I mentioned that I also have a Jensen that sounds this way.

The Optimus/Pyramid uses discrete components all the way through the power amp section. The phase splitter section is all OP-Amps.

Yes, my impression about the power supply was YUK also. However, I plan to replace the filter caps with bigger ones. Should help the occasional burst of power through the music. I also plan on removing the current limiter circuit. I had a receiver that had those in and when I removed them the sound got much better dynamically. The thermal limiter will remain, but I am going to put a fan in there.

Class B? Looking at the circuit did not give me that impression. Is that typical of car stereo amps, or just Pyramid? If so, I will throw in an extra diode in the bias of the output transistors. I think those hefty beasts can take it.

I will let you all know how it turns out... if I don't blow it out first!

Gabe
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