better amp configuration

[AndrewT]

Hi,
bridging requires twice as many voltage amp stages.
Bridging also requires four times as much smoothing capacitance.
The heatsink and output devices are very similar, although the same number of different devices will be chosen to suit either topology.

[longthrow]

Originally posted by hienrich

bai, do you have ideas as to where to buy better extruded heatsinks.?

surplus centers here bay(5 jays;pier3 or side walks along colon area)
bay,for simplicity reasons i usually build single amps/ch..the one that i frequently use is bridgeable(i included a phase splitter),56V per rail..
this new project is for car audio,thats why i'll use smps...

Originally posted by AndrewT
Hi,
bridging requires twice as many voltage amp stages.
Bridging also requires four times as much smoothing capacitance.
The heatsink and output devices are very similar, although the same number of different devices will be chosen to suit either topology.

exactly,my option is either to use a higher transformer secondary voltage with less filter caps or a lower voltage secondary at higher current with more filter caps; which costs more...

[djk]

"bridging requires twice as many voltage amp stages."

Sort of, maybe.



(conceptual drawing)

The right-hand amplifier need only have a gain of -1, and it doesn't have to be high quality. IRF type FETs will work here too (otherwise you need a tripple EF for each device shown). Bias details omitted for clarity. See the Pass Zen v5 for ideas. The left-hand amplifier controls the sum, and thus the distortion of the whole amplifier (it must be high quality).

Forgot to label the inputs of A1, the main input is inverting, and the feedback comes to the non-inverting input.

[AndrewT]

Hi Djk,
there are some car audio chip amps that do something similar.
Maybe they could be designed to pass good quality sound, but my little experience with bridged car amps shows that the sound quality is dire.

[jcx]

Originally posted by djk
"bridging requires twice as many voltage amp stages."

Sort of, maybe...

...The right-hand amplifier need only have a gain of -1, and it doesn't have to be high quality. IRF type FETs will work here too (otherwise you need a tripple EF for each device shown). Bias details omitted for clarity. See the Pass Zen v5 for ideas. The left-hand amplifier controls the sum, and thus the distortion of the whole amplifier (it must be high quality).

Forgot to label the inputs of A1, the main input is inverting, and the feedback comes to the non-inverting input.

looks like you forgot a couple of resistors as well, as shown your circuit's A1 has no idea what the sum across the load is (relying on the inaccurate "it doesn't have to be high quality." inverting stage) so A1 doesn't correct for those inaccuracies

the whole fast accurate amp correcting a cheap inaccurate stage approach has fundmental problems - when the "cheap stage" is Class B you have high order crossover distortion that the fast accurate amp corrects by virtue of high loop gain error feedback
but the feedback is limited by the added delay of the cheap stage and the effectiveness of negative feedback at reducing high order distortion is limited - if poor bias thermal compensation in the Class B stage causes transient underbias of the cheap stage the resulting dead zone causes a huge spray of high order distortion products as the fast amp tries to push harder against the deadzone

and yes I am quite aware of "current dumping" and various "error correction" schemes - I beileve effort put into making low distortion output stages is much more quickly repaid than trying to fix up a higher distortion "cheap" output stage

[deleveld]

"Bridging also requires four times as much smoothing capacitance."

What for? A bridge is less sensitive to power supply variations because the output is equally affected on both sides thus creating PSRR. This is something a single ended cant do.

[djk]

Crown used to make a bridge model the put out 7KW into 0R5 that only had a single 10,000uF cap.

[djk]

People interested in bridge amps should read the two-part article in Wireless World circa 1981. It gives a very good overview and presents two practical designs.

What I was trying to show here (from memory) was the general concept, and that the design of the B amplifier need not be elaborate. While the Zen v5 would work, I probably wouldn’t choose it due to its low PSRR. Even so, the Zen v5 had only about 0.1% distortion mid-band with an AV=-10. With about 20dB of gain for feedback it might be OK (since this design only requires an AV=-1).

The original from the magazine had a single Vas transistor with a bias spreader and a bootstrap current source. It drove an EF output stage. Measurements showed that the A amplifier reduced the distortion in the sum output by an order of magnitude (20dB or better). It should be remembered that a pair of matched outputs will have less than 0.05% distortion (open loop), and don’t forget the feedback from the excess gain of the Vas stage. Current limiting (if desired) only need be done in the B half (unless you expect a short to ground).

[AndrewT]

Originally posted by AndrewT
bridging requires twice as many voltage amp stages.
Bridging also requires four times as much smoothing capacitance.
The heatsink and output devices are very similar, although the same number of different devices will be chosen to suit either topology.

Originally posted by deleveld
"Bridging also requires four times as much smoothing capacitance."

What for? A bridge is less sensitive to power supply variations because the output is equally affected on both sides thus creating PSRR. This is something a single ended cant do.

Hi,
a push-pull ClassAB amplifier without bridging requires the PSU to supply half waves of the output signal alternately from either polarity rail.
If you accept that the main high pass filter is located at the amplifier input and that to ensure LF stability, then the two filters associated with this, namely the NFB DC blocking filter and the PSU/Load impedance filter, should be scaled to be some multiple lower in frequency. I have adopted the ratios of square root (2) as suggested by others and it works for me.
I set the input filter to ~90mS and the NFB to ~130mS and the PSU/load to 160mS to 200mS. For a 8ohm load this equates to +-20mF to +-25mF on the supply rails.
If the load impedance is reduced to 4ohms then the smoothing capacitance should be +-40mF to +-50mF if the same ratios of time constants are to be maintained.

Now let's look at the bridged amplifier.
Each amplifier in the pair is loaded by an effective impedance of half the actual load.
The supply rails supply half wave versions of the output signal.
This time the halfwaves are continuous rather than from alternate supply rails. (it looks like a full wave rectified version of the output signal).
The supply rails still supply the current to meet the demand from the effective load on each amplifier. But this time there is no rest period between half waves for the smoothing to recover. The transformer and rectifier work hard to deliver sufficent current during the small recovery period between adjacent halfwaves rather than alternate halfwaves and to compound this both rails must be charged simultaneously.
So I believe the smoothing capacitance needs to be doubled to take account of each amplifier feeding a half impedance effective load and doubled again because there are two amplifiers taking power from the supply. The effective load seen by the supply is one quarter of the actual load, due to two parallel amplifiers working in anti-phase but with half impedance loads on each.

If you separate the PSUs for each amplifier and treat each one as a PSU + amplifier + half load impedance it becomes more clear that each amp needs double the smoothing and we now have double the number of PSUs requiring a doubling of smoothing.
That in my book is four times as much smoothing for a bridged amplifier compared to a single amplifier.
It might even be worse than that because I have not made any allowance for the very much shortened recharge period.

[djk]

That makes sense.

I always high-pass filter the inputs at a very high frequency, usually the Fc of the speaker system, and with a Q of at least 1. This does reduce the demands a bit for pole-staggering.

A couple of random notes:

The Crown 10K amp mentioned earlier has 3 phase AC, so it doesn't need much filtering.

The Crown 5K amp (vz5000) has a double integrator on the input and using feedback around the amp to act like an 18dB high pass filter at 40hz (it looks like a 6dB filter on the schematic until you consider the ground reference). They call this filter a 'Loudspeaker Off-set Integrator'.