[Audio Empire]

On Tue, 13 Jul 2010 06:48:58 -0700, Arny Krueger wrote
(in article ):

"Audio Empire" wrote in message


On Mon, 12 Jul 2010 03:31:13 -0700, Trevor Wilson wrote
(in article ):

Audio Empire wrote:
In an earlier post, I said that my Martin-Logan
speakers fell below 2 Ohms impedance, but that this
should be inconsequential. I forgot to add that this
would prove inconsequential for most Solid-State amps
(which was what I was thinking about when I made the
comment).

**Not so. In several ways:

1) Few solid state amps can cope with sub-2 Ohm loads
without entering into current limiting.

The assertion is patently false given that there are so many amps that are
rated for 2 ohm operation, 20-20 KHz, that are readily available for
reasonable prices.


2) Most solid
state amps utilise an output inductor, which causes a
rising output impedance relative to frequency. This can
cause frequency response issues under certain
circumstances.

This is real. The usual design rule is to make this inductor as large as
posible consistent with minimal (usually 0.5 dB) loss at 20 KHz and minimum
rated resistive load. Since most speaker loads even planar and
electrostatic end up with rising impedance at the highest frequencies, they
generally perform better than this in the real world.

At 20 Khz and above? What recordings have enough 20 KHz
high frequency energy to trigger current limiting in any
amplifier?

An excellent point. Spectral analysis of most recordings proves this
assertion to be correct.

And, frankly, I haven't seen an SS amp with an output
inductor in series with the outputs for a long time.

The purpose of the inductor relates to stability and longetivity. As output
device technology and other forms of protection improve, there's less need
for it.

However, I see a lot new product with the inductor and also many without it.
A blanket statement one way or the other seems unwarranted.

I agree. None of the SS amps I have on-hand seem to have an inductor in
series with the output load, but I'm equally sure that many do.

Of
course, it would likely be very consequential for most
tube amps, as they, generally, do NOT like highly
reactive loads.

**On the contrary. Most valve amps are untroubled by
highly reactive loads. Current limiting is gentle and
unobtrusive. Frequency response abberrations, with
highly reactive loads, is, however, a real issue.

Most tubed amps will have greater frequency response aberrations due to
reactive loads simply because they present a much higher source impedance.
If you call that "untroubled" that would probably speak to some other
preferences.

Agreed, but due to these frequency response abberrations
(which is what I was referring to, rather than any nasty
stuff such as oscillation or current limiting), most
electrostatic speakers don't "like" tube amps (unless
they are direct-coupled with NO transformers, like the
old Beverage speakers from the 1980's where the plates of
the output tubes are connected directly to the stators on
the ESL element. Then, tubes and ESLs are a match made in
heaven [or some such place]). While my M-Ls sound fine
with my VTL 140 monoblocks, they sound much better with
my Krell SS amp.

This all makes sense, again due to the typically lower source impedance
provided by good, modern SS amps.

I once had a pair of Innersound ESLs here and they hated
my VTLs - sounded lousy with them. In fact, Innersound
recommended that one NOT try to use them with valve
(Tube) amps because the Innersound ES speakers were very
reactive. Innersound then sent me their "companion" SS
amp and it worked fine as did my two bridged
Rockford/Hafler "TransNova" SS amps.

FET-based amps were among the first SS amps to do well with tough loads.
However modern BJT output devices are more than up to any reasonble usage
sitaution.


That's more or less what I suspected.

I still suspect
that unless that reactance caused some kind of
non-linearity in the driving amp, that this would
likely affect top-end frequency response rather than
distortion.

**Of course. That said, sib- 2Ohm loads is likely to
trigger current limiters in many amps. Current limiting
can be viewed as a very serious form of non-linearity.

Again, there are a ton of modern SS amps that are rated for and work well
with 2 ohm loads.

....and even lower. Many SS amps use MOSFETS or, as you stated above, modern
bi-polar transistors such as ring-mesa types which tend to have astonishingly
low "on" impedances of less than an ohm. They have no problems with very low
speaker impedances.