Sound of bridged amplifier

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?
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.


Joe Berry

2001-03-15 6:15 pm
Gabevee said:
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.

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. ;-)
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.

It´s 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.
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.
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! ;)

PWM supply...

Adding extra capacitance isn't going to help as much as you'd think. The PWM supply in that amp is a fixed oscillator - doesn't change pulse width as more current is drawn. Therefore, when the capacitors are changed out to larger ones and draw more current, the PS switching transistors will be opertaing more toward their limits of current instead of being compensated for by the oscillator. Of course, all this happens near full power on dynamic music. I believe the Optimus uses IRFZ44 fets for switching. Consider upping these to Z44V (50A) or even Z48s (60A) for reliability. They have a lot of smoke in them when they go. :p

The rectifier diodes should be fine, but monitor the primary windings after replacing the Fets - clamping might necessary. At the very least, add solder "strips" to the existing copper runs to increase current capacity.

Right. THIS amp and all the rest of the larger Optimus amps are discrete. They still operate in low bias Class AB - which is a lot closer to Class B, but certainly not purely so. I guess I misspoke above. ;)

Many low end car amplifiers are this way. They do it for longevity's sake here, too. Since their amps can't be pushed very hard, why start them off warm (or hot) to begin with? (rhetorically speaking). The manufacturers get by with this since most buyers of their equipment just want loud sounds in their car and don't know or care about x-over distortion.

I used to try conversions on Pyramid Super Blue (same as the Optimus) amps when I was about 14. I made one a high current amp (1 ohm stable) by removing a couple windings from the toroid and tweaking the oscillator and VI limiters a bit. It wasn't an increase of anything except low impedance stability. Like other high current amps, it delivered half the power into 4 ohm loads - but it was impressive at the time. :cool:
Bridge Is Good

I have said it before .....
Take a standard stereo amp, a Y-Lead and one channel signal source, and connect one speaker between the active outputs.
If you juggle the input level controls (or volume and balance controls) you can set both the amplifier operating level and loudspeaker output levels independantly.
If one channel is set at zero it acts as an active load.
This sweep of range operating levels is educational.

Regards, Eric.

Thanks for the tips... but...

"PWM is a fixed oscillator" :confused:

I thought the whole idea behind pulse width modulation was "current on demand"??? That the duty cycle of the pulse is greater for current draws?

I would think, as you probably surmised, that using larger reserve/filter caps would lessen the need for the pulse width to... widen.

Live and learn.

Otherwise I like the idea of upping the MOSFETs.

Any schematics on line for a PWM supply anywhere? I know about switching, and brute force (AK linear), but can only guess at PWM from my digital communications knowledge.

Exactly but one more point that is very important.
The frequency is not 60 Hz or 50 Hz like in normal transformers.
Usually high frequencies are used like 100KHZ or more because at high frequencies the request for current is low so the transformer can be very very small and the caps can be a lot smaller also. Usually small coils are also used to filter the ripple out. The overall size and weight is a lot smaller.
That´s why you can see for instance a 1000 Watt amp in a 1U box, that´s 3-4 cm height in rack systems.

Thanks for the tips... but... "PWM is a fixed oscillator" :confused:
Ah, I meant to say that as Pyramid/Optimus states their supplies as PWM regulated - it's not the case. They chose a 40-45% duty cycle at about 50kHz that is consistant, no matter what the current draw of the audio stages. So, in essence, it's just a fixed width, fixed frequency square wave generator. It works, but it's not very efficient.

Rod Elliot has one at his site: It's pretty basic and isn't a regulating design, either. It's very close to something you'd find in the Optimus without the soft start (SG3525) circuit. I believe the Optimus uses an SG3524 or similar.