following up on biamping, is this a valid concern?

I've been looking into + and -'s of passive biamping (using speakers internal crossover as filters, and placing two .01 caps in series pre tube amp to cut out the lowest frequencies ~117Hz) and found this comment on a forum...

'According to a builder, seller, designer & engineer of premium tube circuits, transformers & amplifiers: tubes require a load at all source frequencies. In other words, if a tube amp is fed a full-range signal (including bass frequencies), it must be loaded properly at its output. This means you may not feed bass signals to the amp & drive only the passive high-pass XOs & mids/tweeters. In this case, the amp has no woofer load to reproduce the bass frequencies at its input. The tube internal impedance will rise to dangerous levels, eventually arcing or worse. This explains the problems experienced by myself & others using their tube amps as described. This is consistent with the old warning that tube amps must always be properly loaded (only played with a load at the outputs). We just did not know the load must match all source signals.'

comments- thanks

tajanes wrote:
I've been looking into + and -'s of passive biamping (using speakers internal crossover as filters, and placing two .01 caps in series pre tube amp to cut out the lowest frequencies ~117Hz) and found this comment on a forum...

'According to a builder, seller, designer & engineer of premium tube circuits, transformers & amplifiers:  tubes require a load at all source frequencies.  In other words, if a tube amp is fed a full-range signal (including bass frequencies), it must be loaded properly at its output.  This means you may not feed bass signals to the amp & drive only the passive high-pass XOs & mids/tweeters. In this case, the amp has no woofer load to reproduce the bass frequencies at its input.  The tube internal impedance will rise to dangerous levels, eventually arcing or worse.  This explains the problems experienced by myself & others using their tube amps as described.  This is consistent with the old warning that tube amps must always be properly loaded (only played with a load at the outputs).  We just did not know the load must match all source signals.'

comments- thanks

correct & well formulated.

Seems like several ‘high-end’ speaker manufactures are supporting / promoting this practice by offering bi-wiring/ -amping binding posts on their products… Curious what, if any, comments are provided in their customer documents relative to this issue?

And the big prompt for bi-amping is the ability to use tubes for the mid/highs… so are the tube amp companies complicit, or where are there comments on this subject?

If anyone finds tube and/or speaker companies' links to this please post- thanks

Some logical fallacies seem to be occurring here:

a) A given speaker system may or may not have a crossover. Whether it has multiple drivers or not.
b) A given speaker may not be capable of a full-frequency response - any given Bose 901-series for instance.
c) Amplifiers are catholic in their output - they emit power relative to the impedance (active resistance) they see irrespective of the type of load they see. Dummy loads being one example, or many Magnepan speakers for another.
d) Using sub/sat systems as an example, some simply attach a sub-woofer to two otherwise modest "full range" speakers that could stand alone. Advent, AR and Revox did this commonly. Yes, there was a crossover in the subs, but the point is made.

So, where am I going off the rails?

There is the Vandersteen approach, which I find salutary.
Whether using a stand-alone sub-woof like the 2W series, or an integrated subwoof/driver stack like the 5As, the proper set-up is with an amplifier input impedance-dependent high-bypass filter which basically slices the low frequencies off the feed to the power amp, and feeds the entire signal to the subwoofer section whose x-over "resconstructs" the low frequencies denied the power amps on a curve that mirrors the filter's roll-off, then sends the remnants to the mid- and high-frequency drivers.
This has the effect of taking the low-frequency "heavy lifting" off your output tubes and sends that task low-bass task to an s/s amp inside the sub-woofer(s).
Sounds complicated, but it works. I guess it's bi-amping of a sort. Vandies are not efficient, and this is how Richard addresses that issue.

yes, some speakers are w/o x-overs/filters and meant to be actively bi-amped- not the concern at issue here

don’t believe the issue raised by the person making the statement I quoted is how well a driver can produce low freq output, but whether there is a driver in the system to accept the low frequency vs. a ‘low pass to nowhere’

it’s more about what, if anything, happens to a tube amp that amplifies a full range signal and yet hooked up to a limited range system - in this case via a high pass filter to a high range driver (or mid & tw combo) and absent a low range driver / resistive load?

does the rising impedance via the high pass filter to these low frequencies compensate / eliminate need for a low range driver?

tajanes wrote:

does the rising impedance via the high pass filter to these low frequencies compensate / eliminate need for a low range driver?

I am asking, directly, is the nature of impedance different based on the frequency range of the drivers involved? This is an electrical question. I know that my AR3a impedance range is from under 2 ohms to over 30 ohms depending on the frequency. Whereas the Maggies are at 4 ohms at any mix of frequencies.

Generally, I understand that lower frequencies are at a lower impedance. So, do tube amps suffer into high-impedance loads? That, I can understand. But, if there are no low frequencies to amplify - how does a long piccolo passage affect tube amps, for instance?

... But, if there are no low frequencies to amplify - how does a long piccolo passage affect tube amps, for instance?...


The issue raised / presented by the section I originally quoted is not where a tube amp has no low freq to amplify, but occurs where a tube amp does in a passage amplify low frequency content and is only presented a system post a high pass filter… (with no low driver connection)

I had maggies in a set-up where I feed full range from my amp, and supplemented with velodyne sub (with its internal low-pass), but this issue would be more in line with running a tube amp for the tw section of the maggies and a separate amp for the main woofer panel…

tajanes wrote:

... But, if there are no low frequencies to amplify - how does a long piccolo passage affect tube amps, for instance?...


The issue raised / presented by the section I originally quoted is not where a tube amp has no low freq to amplify, but occurs where a tube amp does in a passage amplify low frequency content and is only presented a system post a high pass filter… (with no low driver connection)

I understand that - but, electrically, is LF impedance different than HF impedance? How does an amplifier differentiate between a high-pass filter and a resistor electrically? Or a speaker of limited range? After all, there are speakers out there with 2" drivers pretending to be "Full Range". I am not trying to be snarky - I am trying to understand. The glimmerings I have on the subject are the very real differences between inductive (motor) loads (a speaker is a motor) and purely resistive loads - which, effectively is impedance. But within the universe of inductive loads and the feedback to the source (amp) generated thereby, is the nature of this feedback different based on its frequency? That could make sense if clarified just a bit.

By the way - and on the *POP*. *THUMP* subject, switching AC motors on and off generates surges back to the source. Given that a speaker is motor - how the sources (our amps) handle that surge could be significant. It really does help to read up on AC motors and how they behave - there is a lot of commonality to speakers.

bingo (well put), and that may be the underlying question... (raised by the posted original quote)
(btw, I'm not presenting a position, I'm looking for the answer, hopefully in the end supporting passive bi-amping with a tube amp)

'electrically, is LF impedance different than HF impedance? How does an amplifier differentiate between a high-pass filter and a resistor electrically?'

...or potentially put another way, is there an issue (directed at a tube amp in a passive bi-amp set-up) with taking out the low frequency post amplification and not presenting ultimately to a driver?