From: lipp@mtu.edu (JOHN I LIPP)
Newsgroups: rec.audio.high-end
Subject: Re: Benefits of high-current design?
Date: Tue, 5 Jan 1993 19:22:13 GMT
Organization: Michigan Technological University
NNTP-Posting-Host: 129.89.7.4
Originator: tjk@csd4.csd.uwm.edu

>	While shopping at my friendly hi-fi dealer last weekend, the
>owner made a comment about amplifiers that can deliver a great deal
>of current. One of the benefits that he claimed for a high-current
>design was better sound, specifically in the midrange.

The benefits of a high current design are usually related to quality.
A higher current amplifier has to be better designed to acheive this
goal.  That means better transistors, more attention to good design
practices, and likely more listening of the final product.  The most
important quality of a high current amplifier is usually the
elimination of current limiting circuitry.  This circuitry is always
present in lower current amplifiers (and almost all IC circuits such as
op-amps and one-chip amplifiers) to protect the output stage of the
amplifier.  Without, an accidental short circuit of the output or other
abuse of the owner leads to Poof!  The end of the output transistors.
The downside to current limiting circuitry is that it will always
interfer with amplifier operation near full power (that is how it
limits the current).  The most likely signal to trigger the circuit is
in the bass and will produce distortion products of the bass signal.
Consequently, these distortions will be in the midrange region and mask
other midrange signals that are part of the music.

Designing a high current amplifier is not more difficult.  Generally,
all that needs to be done is (1) adding more transistors in the output
stage and (2) adding a more powerful transformer and filter
capacitors.  These things all cost money but generally do not change
the design of an amplifier even slightly.  In fact, it usually
simplifies the design because output stage protection circuits are
eliminated.

>	Does anyone care to comment on this statement? I don't know
>enough about the basics of electrical engineering to effectively
>critique it. (BTW-- if anyone could recommend a good book on the
>basics of electrical engineering as it relates to hi-fi component
>design, I would be avidly interested.)

>	The amplifier that I eventually purchased was a Parasound
>HCA-500. The manufacturer claims that it can deliver 50wpc into 8
>ohms and 75wpc into 4 ohm. (RMS output.) They also claim 10 amps
>average current and 20 amps peak. Does this strike anyone as being
>a particularly good current rating? If so, why?

To analyze current ratings, you need the following power/current/voltage
relations that are 100% true:

  Power = Voltage * Current = Current * Current * Resistance
                            = Voltage * Voltage / Resistance

where Resistance is the impedance that the power rating applies to.
All of the values above are RMS values.

You can reverse the second equation and find that the current for 50
watts into an 8 ohm impedance is 2.5 amps RMS.  For 75 watts into 4
ohms the current is 4.33 amps RMS.  However, a speaker rated at 8 ohms
is likely to dip to as low as 5 ohms and one rated at 4 ohms may dip to
as low as 2.5 ohms at certain frequencies.  Using these real life
impedances you see that for 50 watts into 5 ohms you need 3.16 amps RMS
and for 75 watts at 2.5 ohms you need 5.5 amps.  Hopefully the need for
at least modest current is becoming clear: in order for the amplifier
to drive small impedances, you need higher current capabilities.  And
the lower the impedance that you want to drive, the more current you
need.  This is the primary motivation of manufacturer's to increase
current capability.

Keep in mind that if you hook two pairs of speakers to an amplifier,
the effective impedance seen by the amplifier is smaller.  If Z1 and Z2
are the impedances of the two speakers, then Z the effective impedance
is

       Z1 * Z2
  Z = ---------
       Z1 + Z2

If Z1 and Z2 are equal, Z is half that of a single speaker.  Hooking
two pairs of 4 ohm speakers to an amplifer is thus like hooking one 2
ohm speaker to it.  Ideally, an amplifier is a voltage amplifier and
using the third relationship above it follows that the output power of
an amplifier should be inversely proportional to the resistance.  As an
example, consider your amplifer.  It is rated at 50 watts RMS into 8
ohms.  Into 4 ohms it should then be able to put out twice as much
power (since the resistance is half as much) -- 100 watts.  But the
manufacturer has rated it at only 75.  But why?  At 100 watts into 4
ohms the current needed is 5 amps RMS, well within the average current
capability.  The reason is either (1) the 75 watt rating is also given
with a certain distortion level (this is required by law); the
amplifier can go up to 100 watts, but it starts to distort quite a bit
at this level or (2) a thermal circuit will kick in if operated at this
level due to overheating of the amplifier.  The first reason is the
most likely.  Keep in mind that the 10 amp average rating specified
does not have a distortion rating given with it!

I would conclude that the current rating of your amplifier is
satisfactory, if not good.  Their are many amplifiers with higher
current ratings, but what is the point if you aren't going to drive
very low (less than 2 ohm) loads?  As long as the current capability is
great enough to allow the elimination of current limiting circuits, I
believe the primary goal has been acheived: better sound quality.

>					-Cal

Overly lenghty, but I hope this helps.

/***************************************************************************\
*  John Lipp   *  Loudspeaker design pseudo-expert and software developer   *
* lipp@mtu.edu * (Working on amplifier and surround-sound psuedo-expertise) *
\***************************************************************************/