Bridged and paralleled (not in the same design) question

[Adam M.]

2 More questions...

For a bridged design, the impedence the chip sees is equivalent to 1/2 the impedence of the speaker if I understand correctly. If a bridged amp is run into a 4 ohm nominal speaker with appropriate cooling, will it provide more power than a single chip?
Intuitively it seems to me that adequate cooling for this situation would need a fan and heatsink.

For a parallel design, the impedence the chips sees is equivalent to 2x the impedence of the speaker if I understand correctly. If a paralleled amp is run into an 8 ohm nominal speaker will it produce more power than a single chip? National's Application notes seem to suggest that a paralleled amp will not provide much power into an 8 ohm load.

The reason I ask is because I build a new set of speakers every few months, and often I will parallel the woofers giving a 4 ohm impedence. I'm not interested in active crossovers (yet, 1 step at a time) but would like to have an amplifier that could drive both, and I don't think I'm ready to jump directly to a bridge/parallel solution yet, I want to understand what I'm building before I build it. Thanks,
Adam

[GregGC]

If you have a load less than 4 ohms use paralelin and for loads more than 8 ohms use bridge config. For loads between 4 and 8 ohms just a normal single amp will do unless you need a lot more power. When paralelling you get twice the current capability and you need to use less PS voltage. That why it's suitable for loads lower than or 4 ohms when you need more power. The bridge config. is the oposite you apply twice as much voltage across the load with the same PS compare to a single amp. So if you need more power over a 16 ohm load you'd need more voltage across it and the bridge config will give you that, but parraleling will not provide the same benefit.

I hope I didn't confuse you.

/Greg

[leadbelly]

Originally posted by Adam M.
The reason I ask is because I build a new set of speakers every few months, and often I will parallel the woofers giving a 4 ohm impedence. I'm not interested in active crossovers (yet, 1 step at a time) but would like to have an amplifier that could drive both, and I don't think I'm ready to jump directly to a bridge/parallel solution yet, I want to understand what I'm building before I build it. Thanks,
Adam

You want a 4 ohm capable chip amp, no bridging or paralleling. An LM based GC is 4 ohm stable as long as you meet the specs for heat sinking and power supply voltage. I believe I have read posts reporting that these specs have been exceeded slightly with good results as well, but you should search for some posts on the subject. The attached graph is from the LM3875 datasheet:

[sreten]

the suitability of a chip for bridging or paralleling depends on
on its output capability into various load impedances /resistances.

Many car audio chips give maximum output into around 2R loads,
and a bridged pair with an 8R load will give substantially more
power than a single chip into 8R, also more but not as much with
4R loads. Bridged output is likely to be the same into 4R, whilst
single output into 4R is more.

As you say paralleled amplifiers are a very minor improvement
for 8 ohm loads, they are more suited to low impedance loads.

The LM1875 is rated at 20W/4R +/-25V supplies and 30W/8R +/-
30V supplies.

Simply put, this is load sensitive design, and nowhere near the
ideal hi-fi amplifier which doubles power into 4 ohms.

For simple 4/8 ohm capability use :

One with 25-0-25 V rails.

Two in parallel with 30-0-30V.

Four/Six with 25-0-25V rails, paralleled and bridged.

Eight/Ten with 30-0-30V rails, patrolled and bridged.

I have put greater emphasis on current capability
the higher the power.

The LM3875 looks very similar but with higher supply voltages.

What comes out of this is a single pair bridged is not an option,
so given your original question, a pair paralleled with high supply
voltage is your only extra power option.



sreten.

[Adam M.]

As you say paralleled amplifiers are a very minor improvement
for 8 ohm loads, they are more suited to low impedance loads.

Is there a way to calcualte this? I'd be interested to run the numbers and see what comes out. My current amp says its 75W which is plenty for normal listening, but some of my 8 ohm designs are kind of inefficient, and I'd like to have the option to play louder than necessary. the 30-0-30 paralleled sounds interesting.
THanks,
Adam