# Is a parallel amp the same as a balanced amp?

Discussion in 'Electronic Basics' started by Nick, May 20, 2008.

1. ### NickGuest

Hello all,

The differences between a parallel amplifier and a balanced amplifier
is rather confusing. Are they the same, or are they different? Both
these circuits, and their advantages, appear to be very close to each
other, if not the same...

Just curious,

-Nick

2. ### EeyoreGuest

Is this what you mean ?
http://en.wikipedia.org/wiki/Bridged_and_paralleled_amplifiers

Graham

3. ### BobGGuest

'Balanced input' means it can take a differential signal on a 'cannon
mic connector'. 'Parallel' is just a stereo-mono switch that lets one
channel drive both amp channels in phase and in parallel. 'Bridge'
sticks an inverter in front of one channel so they are out of phase...
one red wire is going positive and the other one is going negative....
hook the speakers up to the two red terminals for more ooomph. I've
never heard the term 'balanced amp' but it sort of sounds like the
bridged definition I just tried to explain. Hope it helped.

4. ### Phil AllisonGuest

"BobG"

** Watch out !!!!

Having a "mono' switch does NOT = parallel operation of the power amps !!!

Only a very few amps ( notably some Crown models ) have the ability to
parallel the speaker outputs with safety.

The others will BLOW UP if you dare try it.

** Mostly, yes.

But it only takes one resistor, connected from ch A's output to ch B's
inverting input, ( virtual earth point ) to get ch A to drive ch B with
equal level & reverse polarity.

** The OP is a fuckwit - omitting all context from his asinine Q.

...... Phil

5. ### CampKohlerGuest

Parallel refers to the configuration of an amplifier's output circuit.
And now for something completely different.

Balanced refers to the nature of the either input or output signals.
Balanced means that the two lines of the output are referenced to each
other and not to ground; the input's or load's voltage is measured
between two lines and voltages present on either line (as measured to
ground) are typically unintentional, undesired and are to be rejected
as extraneous. This is good for long runs, e.g. between a mic and an
amp input, because noise is mostly referenced to ground and is so
theoretically rejected. Whatever noise hits one line also hits the
other and so is ideally cancelled.

Unbalanced signals are measured to ground, so are limited to short
runs where they won't be susceptible to noise pickup, e.g. between
adjacent audio components. (The run to a speaker is typically long,
but can get away with an unbalanced line by virture of a speaker's low
impedance, making it much, much less susceptible to noise. If someone
went crazy and designed a high-impedance speaker circuit, it would
benefit from a balanced output, too. Nobody's been that crazy yet.)

As you might guess, a mic input, say, that is both balanced AND low
impedance, is the double-whammy for quietness. Professional mic inputs
are that way. Cheapo mic inputs are high-impedance and unbalanced, the
worst case, so they rely simply on short runs to avoid noise. If you
have to carry signals between two buildings that use separate ground
systems that may have a small difference between them (or a large one
if lightning or other surges strike!), then a balanced system would be
preferable so that the signals ignore the ground as much as possible.
Whole books are written on signals, grounding and noise reduction
tecniques. Many books. They come with magic wands.

6. ### NickGuest

Thanks guys for the great input. The "parallel amplifiers" that I
am speaking of are for HF PAs and LNAs, and are in parallel with each
other and have Wilkinson dividers at their input and output. But the
"balanced amplifiers" I have seen are also in parallel, but have a 90º
hybrid power divider at their input and a 90º hybrid power combiner at
their output. They look so darn close to each other, both circuit-
wise and performance-wise, that it starts to get a bit confusing!

Best,

-Nick

7. ### BobGGuest

Who would have thought that 'amplifier' without 'rf' in front of it
meant audio? Only about 99 out of 100 I bet. Fooled me!

8. ### leggGuest

A parallel voltage amplifier can take the form of a 'compound'
amplifier, where one provides the power and another provides the
signal (cancelling the PA noise), whether the resulting circuit is
balanced or not.

An RF PA with a low frequency modulator is another form of parallel
amplification, whether the resulting output is balanced or not.

In fact you can't really apply voltages in parallel; they have to be
isolated by identifiable impedances. Operating them in series is
tricky in concept, but fairly common in practice; directly at low
frequencies and magnetically coupled at higher frequencies.

RL

9. ### EeyoreGuest

Well, it didn't fool me which is why I asked "is this what you mean ?".

Nevertheless, a classic example of why it's best to provide more rather
than less info when asking such a question.

Graham

10. ### Frank RaffaeliGuest

Sounds like a continuation from the other thread ...
An RF balanced amplifier usually refers to a 180-degree phase split,
not 90. The main reason to use a balanced amp is to suppress the 2nd
(and other even order) harmonics.

The main reason to use a 90-degree split is to match the amplifier
over a broader bandwidth, and to realize a good match at the optimum
noise figure (Zopt).

So ... you still didn't answer the question: what linearity do you
need?

Frank

11. ### TheMGuest

Its the same guy looking to build a small pirate TV transmitter, I bet.

Mark

12. ### NickGuest

Thanks for all the clarifying answers guys. Now it's finally clear!
(Frank, linearity is not an issue for me, since I was just trying to
fill in one of the many gaps in my electronics knowledge, and not
actually trying to build either a parallel or balanced amplifier).

Best regards,

-Nick