Bridge amplifier load

[hentai]

First of all i want to say hello to everyone.
Now to the matter at hand. I want to know if i put a 4 ohm speaker to a bridge output each amplifier will see a 2 ohm load? Is it some kind of virtual ground for the amps "in the midle of the speakers impedance" (sry for my choice of words.. not so good with english) ?
Thx in advance.

 

[Nigel Goodwin]

You are correct, each amp essentially sees half of the load - so with a 4 ohm load each amplifier has to be able to feed a 2 ohm load.

 

[hentai]

this explains all the smoke then...
Thank u very much for ur quick reply.

 

[audioguru]

Most bridged amp ICs for cars deliver about 14W to a 4 ohm speaker at clipping with a 14V supply. The TDA7240 for example.

 

[hentai]

i wanted to make a small amp for my pc... i have some tda2030 so i looked in their datasheet and i saw that they can only deliver around 10W in 4 ohms with 0.1% THD so i thought ill bridge them. At first when i put the speaker i didnt played it loud so there was no problem but when i turned on the volume... . then the thought occured to me that 4 ohm is too low for them.
i might consider the car ics...thx

 

[Nigel Goodwin]

Bridging is all about speaker impedance - it doesn't provide any more power, it just provides it to a different impedance. So your TDA2030 amp can provide 10W per channel into 4 ohms, that's a total of 20W into two speakers. If you bridge the two amps, and put the speakers in series, you get exactly the same 20W into the exact same speakers, but now mono instead of stereo.

So it's only useful if you want to provide more power into one speaker of a higher impedance - as Audioguru mentioned, it's main application is to get more power from a low voltage supply.

 

[hentai]

it's main application is to get more power from a low voltage supply

that was my goal because i only had a 30V power supply available

 

[Nigel Goodwin]

that was my goal because i only had a 30V power supply available

With a 4 ohm speaker, and a decent discrete design, you should be able to get 25W to a 4 ohm load with a 30V supply.

 

[hentai]

I chose the tda because i had them around and i still have 4 pairs undamaged but since i cant rely on them i might just as well go for discrete design. So while i search and simulate schematics whould u be kind enough to post a recommendation? Thx for ur help.

 

[hentai]

Discrete power amp design

well i found some 2n3055 from HTC i dont know since when do i have them but they look pretty good so i decided to take ur advice and go discrete. i found this schematic in a book rom 198x don't know or sure... i simulated using Spice and it outputed about 19W rms in a 4 ohm. the output voltage swing isnt as great as i expected only 11.2 V poz and 12.4 neg. what do u think about this circuit?.. any sugestions are welcomed.

 

[Nigel Goodwin]

It's old and crude!

It also makes no attempt to try and maximise the output swing - you really need to design it with the collectors of the output transistors together, rather than the emitters.

 

[hentai]

well when i saw the bootstrap i thought i'll see what it can do...

you really need to design it with the collectors of the output transistors together, rather than the emitters

u mean that Sziklai final stage? i wouldn't want to use complementary pairs because i only have these 2n3055.

 

[Nigel Goodwin]

well when i saw the bootstrap i thought i'll see what it can do...

u mean that Sziklai final stage? i wouldn't want to use complementary pairs because i only have these 2n3055.

That would be one option, but obviously not if you're only using NPN transistors.

 

[hentai]

yea i'll stick to quasi-complementary output
can u recommend something ?

 

[audioguru]

You have a 4 ohm speaker on an amp that is designed for an 8 ohm speaker.
they should make sim software that makes smoke when a circuit is overloaded.

 

[Nigel Goodwin]

Pretty much what you've already got, but adjust the potential divider feeding the base of the first transistor tp give symmetrical clipping at the output - that will give the highest possible output for that configuration.

 

[hentai]

i will thank u very much for ur assistance.
take care