Active vrs passive

tinitus · 2012-10-20 10:49 am

demeterart said:
agreed, provided there is no need to match source/load as in tubes that use transformers and rely on impedance reflection to transfer max. power, although the significance would not be tat important given the outrageous speaker behaviour - impedanc wise.

with tube amps, the problem is more the sudden impedance 'bumps/peaks' caused by the crossover itself
some are worse than others

pinkmouse · 2012-10-20 10:50 am

Simple, don't use valve amps.

demeterart · 2012-10-20 10:51 am

yes, some crossover types do behave like drunkards ;-)

tinitus · 2012-10-20 10:52 am

could well be, but they look so good

pinkmouse said:
Simple, don't use valve amps.

demeterart said:
yes, some crossover types do behave like drunkards ;-)

I wonder why noone places the active xo right at input of power amps
you shouldn't need to ad more than the passive components, caps and resistors

(and no Dave, Im not talking about PLLXO

)

planet10 · 2012-10-20 11:02 am

tinitus said:
I wonder why noone places the active xo right at input of power amps
you shouldn't need to ad more than the passive components, caps and resistors

Lars,

If it just uses passives it is a PLLXO. I use them quite often. If you are dedicting the amps to the speakers you can often just change existing values (ie for a HP, shrink the size of a coupling cap.

dave

demeterart · 2012-10-20 11:02 am

i guess you are referring to integrated units with probably hybrid circuitry?
i suppose it presents no feat other than carefully designed supplies and real estate
provisions/layout.

tinitus · 2012-10-20 11:06 am

planet10 said:
Lars,

If it just uses passives it is a PLLXO.

nope, it uses the input of the power amp as the active part, just like in active

I guess it only needs adding the feedback loop to work like any ordinary active xo
well at least 12db should be possible and simple

I have been shown it works with a chip amp, so why not other amps ?

Charles Darwin · 2012-10-20 11:39 am

Wavebourn said:
Do you reproduce DC? I don't. I assume SY does not. So what is that DC resistance about?

Isn't an inductors impedance made up of the DC resistance plus the reactance?

Boscoe · 2012-10-20 11:43 am

tinitus said:
could well be, but they look so good

I wonder why noone places the active xo right at input of power amps
you shouldn't need to ad more than the passive components, caps and resistors
(and no Dave, Im not talking about PLLXO )

I've never seen a good looking valve amp.

SY · 2012-10-20 11:43 am

You need to get out more.

demeterart · 2012-10-20 12:03 pm

valve amps look good 'cause ... they are naked

chris661 · 2012-10-20 1:08 pm

Boscoe said:
I've never seen a good looking valve amp.

Didn't look pretty, but sounded rather good.

833A SET.

1460v @160mA. At that point, the PSU caps wouldn't take any more volts. 74 watts/ch output, they reckon.

Chris

gk7 · 2012-10-20 2:00 pm

Wow, now these look impressive (and scary @1460V).

SY · 2012-10-20 2:25 pm

But let's not get distracted from the topic- the point is that an amp with a high source impedance (whether tube or transistor) WILL change the frequency response of either a passive or active system, for better or for worse. It's a factor that needs to be accounted for in design.

Charles Darwin · 2012-10-20 2:43 pm

I posted these in the solid state forum but it might be of interest here too:

CARVERaudio.com CarverFest 08 pt.1 - YouTube

CARVERaudio.com CarverFest 08 pt.2 - YouTube

CopperTop · 2012-10-20 3:31 pm

SY said:
But let's not get distracted from the topic- the point is that an amp with a high source impedance (whether tube or transistor) WILL change the frequency response of either a passive or active system, for better or for worse. It's a factor that needs to be accounted for in design.

I take the point that a speaker can be designed for a particular amplifier impedance; a speaker can be under-damped or over-damped with a particular amplifier, and its resonance can somehow be 'corrected' with a particular output impedance. Physically, how is this done in terms of the construction of the driver? Is there no limit to how how high an impedance it can be designed for? Intuitively (and therefore probably wrongly) an amplifier with low impedance driving a diaphragm with an electromagnet against a restoring force seems quite a simple idea quite likely to work. But to connect the amp through an effective resistance or impedance, the system begins to look a bit 'spongy'. How does the design of the driver change in order to cancel out the effect of the series impedance? Does it do strange things with phase changes/group delay etc. compared to the standard speaker designed for solid state?

SY · 2012-10-20 3:58 pm

CT, I'd strongly suggest you read the papers by Mills and Hawksford on current drive speakers. It's the opposite approach to what has become standard, but has its own advantages/disadvantages (I know, engineering is all about trade-offs, how wacky!). They should be available on-line. Nelson Pass has written some more simple articles about this, but the Mills/Hawksford papers are a bit more comprehensive and discuss both driver and amplifier design.

James Lehman · 2012-10-20 8:15 pm

Wavebourn said:
Do you reproduce DC? I don't. I assume SY does not. So what is that DC resistance about?

DC resistance in series with the woofer will destroy the near zero output impedance of some amps, leaving you with no damping factor. You end up with a flabby sounding low end because it rings like a gong!

Charles Darwin said:
Isn't an inductors impedance made up of the DC resistance plus the reactance?

In the real world it is because the wire it is made of naturally has DC resistance. But the mathematically perfect model of an inductor is not suppose to have any DC resistance.

James.

planet10 · 2012-10-20 10:24 pm

James Lehman said:
DC resistance in series with the woofer will destroy the near zero output impedance of some amps, leaving you with no damping factor. You end up with a flabby sounding low end because it rings like a gong!

You are making an incorrect conclusion. What you get is very dependent on the mechanical Q of the speaker system. It is entirely possible to build a loudspeaker that only has properly damped bass with an amplifier with damping factor less than 1. For every loudspeaker there is an optimum output impedance from the amplifier. That can vary from very high to very low.

I don't know how many times -- and in how many ways -- that has to be said before heads are pulled out of the sand.

dave

Pano · 2012-10-20 10:26 pm

Many more times, Dave. Or so it seems. You can lead a horse to water....

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