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Old 18th April 2013, 07:23 AM   #381
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Originally Posted by danielwritesbac View Post
On powerside, you quadrupled the load, but it isn't safe to quadruple the transformer because you only doubled the number of chips and therefore may only double the transformer current. SO, this bridge amp idea operates starved for current, wastes a lot, and you've then got a big audio compressor with poor bass.
When building a current limited chip bridge amp, you want to cut down on the voltage to bring the power output right in line with twice the output. In other words, choose a voltage for a 4 ohm single ended amplifier for a bridge driving 8 ohm loads. Then double the current = double the power. This raises efficiency and produces far less heat.

If you want to go hog wild, you could bridge-parallel and get 4 times the power. I always wanted to do this and I just might.

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In addition to those troubles, bridged adds an additional power amp to the chain, effectively doubling the amplifier distortion
You've got that right.

Has anyone ever built a bridge amp that uses a balanced differential driver and precision resistors? Would there be a disadvantage to this, other than complexity and the expense of 0.1% resistors?
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Old 18th April 2013, 07:33 AM   #382
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Daniel thank you so much. I was looking to your tda7293 parallel ebay circuit.Can I apply the schematics to build TDA7294 paralleled ?
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Old 18th April 2013, 05:11 PM   #383
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Originally Posted by Fast Eddie D View Post
When building a current limited chip bridge amp, you want to cut down on the voltage to bring the power output right in line with twice the output. In other words, choose a voltage for a 4 ohm single ended amplifier for a bridge driving 8 ohm loads. Then double the current = double the power. This raises efficiency and produces far less heat.
So, a 22+22 transformer instead of a 25+25 transformer?
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Originally Posted by Fast Eddie D View Post
Has anyone ever built a bridge amp that uses a balanced differential driver and precision resistors? Would there be a disadvantage to this, other than complexity and the expense of 0.1% resistors?
You could sharpen up the soldering iron to a really fine point and try the OP275 bridge adapter for hi-fi. It is possible to spread the pins a bit so the very tiny thing can be soldered. For DIY, the precision resistors don't cost more money, they just cost more time, with the ohmmeter.
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Originally Posted by Fast Eddie D View Post
If you want to go hog wild, you could bridge-parallel and get 4 times the power. I always wanted to do this and I just might.
Bridging chip amplifiers has the disadvantage of putting on-chip limiters series with more on-chip limiters.

Can block the limiter by paralleling more:
Parallel amp to 8 ohm speaker, or.
4 chips parallel to 4 ohm speaker.
Now the limiter doesn't work because threshold is higher than output.
Likewise, 5 chips parallel amplifiers can help make the bridged chip amp that doesn't set off a doubled limiter.

Question:
If we build an ordinary TDA7293 with bootstrap, mute and standby and after that, do the input section exactly like the Slave mode configuration (disable the onboard front end) so that we could drive it with any amplifier (and get regulated front end). . . I wonder if the limiter is still engaged?

Can block the limiter by buffering the output:
There's a wide variety of ways to put high end, yet inexpensive, output devices on TDA7294 and TDA7293. Then the chip amplifier doesn't see a high load and doesn't set off its limiter. Before considering that, maybe consider that the LME chip amplifiers are higher resolution.

Or, a more streamlined alternative:
However, with that much effort, you might as well build the Honey Badger discrete amp instead. Since the PCB boards are availabile in the diyaudio.com store, building the huge amp is almost as easy as soldering practice. And it has enough power to X-Max most speakers.
Quote:
Originally Posted by simply14prem View Post
Daniel thank you so much. I was looking to your tda7293 parallel ebay circuit.Can I apply the schematics to build TDA7294 paralleled ?
If you can find TDA7294S, with all 15 pins active, just like TDA7293.
The Master/Slave parallel method is easy, low cost and extremely low loss. It is for use with speakers that are very difficult to drive and also for when you need a sturdy amplifier with zero output impedance.

Otherwise, the normal TDA7294, can be be paralleled very similar to LM3886's PA100. This method of paralleling uses matched resistors (via ohmmeter) and sturdy resistors at the output--that is the hi-fi method because output device noise goes into ballast resistors instead of into speakers.
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Old 18th April 2013, 06:26 PM   #384
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Originally Posted by danielwritesbac View Post
So, a 22+22 transformer instead of a 25+25 transformer?
That sounds like an uninspired guess to me. I will show you a simple "cookbook" method of determining optimum supply voltages for your bridge amps.

For illustrative purposes let's say we want to build a bridged 3886 amplifier. A look at the datasheet reveals these specs that are relevant to our discussion.

68W Cont. Avg. Output Power into 4Ω at VCC = 28V
38W Cont. Avg. Output Power into 8Ω at VCC = 28V
50W Cont. Avg. Output Power into 8Ω at VCC = 35V


If we build a bridge amp with Vcc = 28 volts, our bridge amp will deliver 136 watts into an 8 ohm load with optimum efficiency and lowest heat generated. Raising Vcc will not raise the output of the amplifier, but it will significantly raise the operating temperature of the chips.

If we build a bridge amp with Vcc = 35 volts, we can deliver 100 watts into a 16 ohm load with optimum efficiency.

If we build a parallel-bridge amp (4 chips per channel) with Vcc = 28 volts, we can deliver 272 watts into a 4 ohm load with optimum efficiency.

All of the above configurations are subject to the usual caveats and represent absolute best case scenarios. Prudent design would dicatate that we set our goals a little below what these figures suggest. A safe "fudge factor" would be to reduce Vcc by 5-10% which would reduce generated heat but also reduce available power.

It is really up to the designer to juggle all the factors.

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Last edited by Fast Eddie D; 18th April 2013 at 06:32 PM.
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Old 18th April 2013, 06:52 PM   #385
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Quote:
Originally Posted by danielwritesbac View Post

You could sharpen up the soldering iron to a really fine point and try the OP275 bridge adapter for hi-fi. It is possible to spread the pins a bit so the very tiny thing can be soldered. For DIY, the precision resistors don't cost more money, they just cost more time, with the ohmmeter.
Now that's what I'm talking about.
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Old 18th April 2013, 07:24 PM   #386
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An 18+18 dual secondaries transformer /w dual schottky bridge rectifiers will probably make that 28+28vdc or very close to it. However, that's the unloaded rails. Yes, that will run very cool with Bridged TDA7294. I'm not sure about the 136 watts.

P.S.
There's actually some advantages to this. You get 6db more without having to increase the gain. And, subjectively speaking, I though the chip sounded a lot better at lower voltage.
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Old 18th April 2013, 08:13 PM   #387
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Originally Posted by danielwritesbac View Post
I'm not sure about the 136 watts.
Best case scenario with 3886. I don't know about TDAxxxx.

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P.S.
There's actually some advantages to this. You get 6db more without having to increase the gain. And, subjectively speaking, I though the chip sounded a lot better at lower voltage.
You're getting it.

A chip amp run on lower voltage will be subject to less instantaneous thermal modulation. This is not necessarily the only dynamic at work here.

Now try one of those bridge adaptors and see if it sounds better. It is a simple way around a short list of caveats.
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Old 19th April 2013, 01:16 AM   #388
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Quote:
Originally Posted by danielwritesbac View Post

<snipped>

For DIY, the precision resistors don't cost more money, they just cost more time, with the ohmmeter.

<snipped>
Well, almost. But if you buy some real 0.1% resistors, you usually also get a much-better temperature coefficient, so that they will stay within 0.1%, or stay better matched, even when their temperature changes.

Some standard 1% metal film resistors that I just looked up (e.g. Xicon) vary their values by up to 50 ppm (parts per million) PER DEGREE Celsius. Some others are listed as +/-100 ppm.

Some 0.1% 1/8th-Watt metal film resistors that I just looked up (e.g. the Vishay/Dale PTF series) vary their values by 5 ppm or 10 ppm (depends on which particular value) per degree C, i.e. 1/10th or 1/5th as much. Their cost in my 2006 Mouser.com paper catalog was $0.81 for qty 1 of the 10 ppm values and $1.30 for qty 1 of the 5 ppm values.

The 0.1% ones also tout their very low noise and very good high frequency characteristics, and they note that they are epoxy-sealed for superior moisture protection.
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Old 19th April 2013, 02:29 AM   #389
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Quote:
Originally Posted by danielwritesbac View Post
Basically, bridged makes a guitar amp, but parallel makes a hi-fi. Which one do you want?
You don't have to choose - why not both? At least that's my ideal solution.

You did omit some important advantages of bridged which are related to the power supply. Running bridged is akin to full-wave rectification in terms of the supply caps, it doubles the frequency of the supply ripple for a given output frequency. In my experience, power supply noise has most often been the limiting factor in chipamp SQ, not THD caused by heavier loading of the output stage .

@Fast - why do you want to go to 0.1% resistors? They're needed for gain setting when paralleling amps or they'll fight one another, but I can't see another application...
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Last edited by abraxalito; 19th April 2013 at 02:34 AM.
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Old 19th April 2013, 03:51 AM   #390
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Just because a resistor is precise at DC doesn't mean it is at AC. As I recall you can get precision resistor that distort with AC. I don't know where to find information on this though.
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