From: rsd@sei.cmu.edu (Richard S D'Ippolito)
Newsgroups: rec.audio.high-end
Subject: Re: Benefits of high-current design?
Date: Fri, 8 Jan 1993 13:44:38 EST
Organization: The Software Engineering Institute
NNTP-Posting-Host: 129.89.7.4
Originator: tjk@csd4.csd.uwm.edu



There are several misleading statements and outright inaccuracies in the
following article.  I will attempt to comment on them keeping my quotting to a
minimum.

Rich


In article <1ienrvINN5dq@uwm.edu>, lipp@mtu.edu (JOHN I LIPP) writes:

|> 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.

Here, bigger is not necessarily better.  Over the ranges of typical powers,
the design procedures are essentially the same.  In many respects, high
current transistors are worse in some parameters than lower current members of
the same family, but are easier to stabilize.

|> 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).  

This is misleading.  Current limiting can be activated when the amp is driving
a short, in which case the power output is very low (or nearly zero!).  It
depends on the load line of the output devices, always elliptical due to the
reactive loads.  There are cases where both overvoltage and overcurrent
protection can be activated while the amp is delivering no power at all (a
purely reactive load).


|> 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.

Inconsistencies about design effort aside, it is not trivial to add more
transistors.  Besides, for a few bucks you can get 250V 250W 25A power
transistors.  If you go brute force, the output circuit might be siimpler, but
having a separate output protection circuit can simplify the design of the
_audio_ portion of the output.

|> 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.

Sorry, these formulas apply only to purely resistive loads where the voltage
and current are in phase.  Otherwise (for non resistive loads, i.e.,
speakers), P=VIcos(theta).


|> 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.

The implicit assumption is that the speakers are the same model and have
identical impedance vs. frequency curves.


|>  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!

No, the most likely reason is that the power supply cannot deliver the
continuous current required while maintaining its output voltage - silicon
steel and copper are expensive.  Also, the internal impedance of the output
circuitry limits the output current.

Good protectection circuits are more than current limiters.  The best limit
the instantaneous current and voltage that the output transistors see to the
safe operating area to avoid second breakdown.  Thermal limiters are orders of
magnitude too slow to effect sound -- they just shut the thing down.

Finally, minimum distortion does not have to occur at the highest rated load
impedance. Quiescent biasing current has a role here.  Distortion increases
when the output devices approach current or voltage saturation and when the
limits of the power supply are reached.  Which occurs first is the choice of
the designer.

Rich