[ctviggen]

So, what do the pots (aka "level controls") actually do?  Are they adjusting slope of the frequency response of the crossovers?  Are they adjusting the crossover point(s)?  Some combination of these?

[Sedona Sky Sound]

Brian/John/Jim might know something that I do not, but I believe the pots are strictly attenuators. I have played with them more than I care to admit but have noticed no phase difference, slope change, etc. The only difference I can think of from a crossover perspective is that the overall frequency response curve will change slightly due to the difference in attenuation at the center of the frequency band covered by the pot (i.e. the frequencies between 500Hz and 5000Hz will always be flat no matter what you do to the mid pot) versus the crossover region overlap (will create a ramped frequency curve centered on the crossover point but the slope of the crossover is technically still the same). Hope that makes some sense.

Julian
www.sedonaskysound.com

[ctviggen]

Hi Julian,

That makes sense.  Perhaps when I get my RM40s, I'll trace the circuit and see how it works (assuming that I can remember anything from my electrical engineering classes, that is).

[John Casler]

I think Julian is "on the money".  It is a device to "boost" or "recess" the driver(s) it supplies.

It has no affect on the crossover and will quite simply adjust the "gain" of the frequency spectrum associated with the drivers it controls.

example: the HF pot will increase or decrease the gain to the tweeter.  The MR pot will increase or decrease the gain to the Neopanels.

Gross adjustments are in "clock form" as in 12:00 or 1:15
Micro-adjustments are in "ticks".  That is, when using a small screwdriver you can "feel" the wiper runing over the individual coils, and each coil is a "tick"

Imagine a "tightly coiled spring" where there is no space between the coils.  It is enclosed in a circular case with a shaft in the center that has a metal wiper, that runs tightly against that coil.

Turning the shaft moves the wiper along the coils like the hand of a clock.

As the wiper moves along the coils, it has a tendency to stop in between each one and those are the "ticks"

At first you may not be able to "feel" the ticks, but just move the shaft a very small amount (you must use a screwdriver and not your fingers or you won't feel them) and you will feel each one.

I might also suggest that every 3-6 months you twist the shaft clockwise and counter clockwise 10-15 times to keep the contacts clean.  Then reset to your preferences.

[DFaulds]

Any component added to a speaker has an affect, and an "active" component like an adjustable L-Pad will certainly affect performance more than a "passive" component like wire, or binding posts.

In crossovers without adjustable L-Pads, to pad down (lower the output) a driver, you would insert a resistor in series with the driver (tweeter, mid) to lower the sensitivity.  Since this resistor will raise the impedance of the tweeter as seen by the crossover (resistance of the tweeter + the resistance of the resistor in series), there is usually another resistor added in parallel to the driver to lower the overall impedance to a target point, usually around either 8 or 4 ohms.  The crossover is designed with this imepdance for the tweeter/mid circuit, and allows for a pretty good degree of accuracy in the crossover, especially when using close tolerance parts.

I think that with an adjustable L-Pad, the L-Pad attempts to duplicate the series and parallel resistance by allowing for a continously adjustable series and parallel resistance to simulate the fixed values usually found in a crossover.  Obviuosly, this works pretty well, but in general is much less accurate than using high quality, close tolerance resistors to accomplish the level matching of drivers.  I think this why there are few commercial speakers made with adjustable L-Pads.  While there are very high quality adj. L-Pads available (Fostex makes an extremely nice transformer based one for $170) that would have negligible affect on the crossover, I'm aware of no main stream US speaker manufacturers using them, since it's more cost effective to simply use high quality resistors.

None of this is to say that adjustable L-Pads are bad, but to think that they have no affect on the sound other thanto adjust the output level of a driver just doesn't make any sense.  Especially when speaker designers are so picky with the brand and tolerances of other components in the crossover/speaker interface.

[bkwiram]

I doubt that anyone would say the quality of the L-pad used has no effect on the sound.

It seems to me the more important question is, which is worse:
1) a nonadjustable speaker being used in a room it is not suited to, or
2) a speaker that can adjust to that room using an L-pad that is not as good as it might be?

B.C. seems to think that situation 1 is worse than situation 2, at least given the assumption that 90% of rooms are not good matches for any given nonadjustable speaker.

BK

[DFaulds]

BK

Good point.  However, response abberations caused by room interactions are usually way too complex to be accounted for with adjustable mid and tweeter level controls.  While they may provide some help, room reflections and nodes are usually quite narrow in frequency and require much finer adjustment to tame than is possible with adjustable mid and tweeter L-Pads.  Most likely the adjustable mass on PR provides the most benefit with combating these problems, which are usually quite noticeable in the lower frequencies.

BTW, I was only trying to point out that the adjustable L-Pad has to have more affect on the speakers sound than the simple adjustment of mid and tweeter output levels.  In fact, I rather like their inclusion in speakers, but I think the eventual use of them is most likely more to make fine adjustments so as to satisfy the owners preference in sound.  Not that this is a bad thing.  Actually, it makes sense.  If you spend good money on speakers they should good to you.

[azryan]

"-However, response abberations caused by room interactions are usually way too complex to be accounted for with adjustable mid and tweeter level controls.-"

Exactly.

Esp. in the neo panel section covering ~166Hz-7 or 10kHz. This is a HUGE range and adjusting the gain here can't possibly 'fix' any problem room peaks or dips.

Yet 'adjust to your own room' is often the reson people give for what the pots are there for and why they're such a great added 'feature'.

Being able to adjust the top octave or octave and a half on the planar or ribbon supertweeter is sort of like treble controls on a Rec./pre/pro or buying certain cables that e.q. the high (or low) end.

These pots aren't just there to allow added tweaking.

In the larger models where the neo panels act more like a line source, but the woofers and supertweeters still are a point source and depending on your distance from the speakers it will make the two types of outputs shift levels unlike a normal point source or true line source speakers.

You also need them 'cuz the drivers aren't level matched.

It's hard to believe vmps doesn't even put knobs on these pots.

The stems look the same as guitar volume/tone pots. I'd think you could use those same type of knobs?
Cheap and have a point to so you could mark where your settings are/were.

If they made a white dry erase-like ring sticker around the pots you could mark these spots easy too.

It's also tough (impossible for most owners it seems) to know where to set the pots to make the speakers themselves flat, so you can tweak it/unflatten it from there if you want.

I've been told that this doesn't matter because of the damage rooms always cause, but I feel that's a weak answer.

"-at least given the assumption that 90% of rooms are not good matches for any given nonadjustable speaker.-"

Treat room probs by treating a room, not by adding signal degrading parts to inherently very transparent drivers (using planar drivers is why they can get away w/ the loss from the pots IMO).

Note, you still need room treatments in room using vmps speakers as much as you would with other speakers (generally speaking).

Having pots doesn't at all negate a room's effects in any way.

[tkp]

To me, these pots are used to match the level of the line source (Neo pannels) and point source (tweeters and woofer) for a particular listening position because line source and point source attennuate at different rate from their origin.  I would love to get a very fine adjustment stepped attennuator instead of the L-pad because this will allow me to go back to optimal setting for certain listenning once I found them a high quality stepped attennuator is a lot less intrusive to the signal.  I understand that Brian is designing to a price target so a very fine stepped attennuator is out of the questions.  One thing I think Brian can do is to allow the post of the L-PAD to be accessible from the outside so some one with a multimeter can measure its resistance to get:

a)  The same L-PAD setting for both left and right speakers.
b)  Be able to go back to the certain pot setting if desired.

I do understand the danger of exposing the pins of the L-PAD to the outside could potentially damage amps/speakers if some one try to take measurement while the system is playing.  Maybe this was the reason why Brian decided not to provide this.

[Brian Cheney]

Between 5 o'clock and 12 o'clock on the Lpad (4 dB attentuation) there are over 100 individual steps available, the smallest of which don't register on a precision Ohmmeter and don't changing the frequency response measured by a mic, but which are clearly audible.  Each step is about 1/20 dB or less.

You can't get this level of precision with a stepped attenuator or a resistor network.  In fact even 1% tolerance Vishays (which we use elsewhere in the xover) do not permit this level of precision, which can be very important when you switch equipment or IC's/speaker wire.  

Our Lpads allow great flexibility and microadjustments that nothing else approaches.  Rather than a compromise, the Lpad gives the user essential freedom to adjust the system to taste.  Contact John Beavers over at www.hometheatertalk.com who has been shown what 1/20dB level adjustment on the mid and treble can mean to ultimate listening enjoyment.

If I could make a resistor array at the exact value needed it would sound better than the Lpad.  Unfortunately I can't, and no adjustments would be possible.

The reason other manufacturers don't provide Lpads or similar precision adjustments is, I would guess, because they don't trust the user to match the tonal balance the designer feels is right in his personal listening environment, which is where most commercial speakers are voiced.  Of course, if your room and equipment precisely match the designer's you will not need any adjustments, via Lpads or otherwise.

[bkwiram]

I wasn't trying to suggest that L-pad adjustments could make a speaker flat into any room, nor that they would somehow render room treatments, etc. irrelevant.

A 4db range of attenuation is pretty big, however (3db = double power), and adjustments within that range of tweeter vs mid vs woofer could no doubt be used to make a speaker in a given nonideal room sound better or worse.

Which brings me back to my point, which was: which is better? Adjustable speaker with L-pad of quality X, or nonadjustable speaker with resistors of quality X+Y? (where Y is assumed to be a positive quantity

That's what's called a design choice...

regards,

BK

[James Romeyn]

So, what do the pots (aka "level controls") actually do?  Are they adjusting slope of the frequency response of the crossovers?  Are they adjusting the crossover point(s)?  Some combination of these?

For anyone curious of the internal workings: the crossover network requires a constant impedance to avoid changes in the hinge point.  The control has two internal pots.  A parallel resistance is varied inversely with a series resistance, producing a constant impedance with variable output voltage.  The driver level changes while the circuit impedance & hinge point are constant.  

Years ago in a listening test, even with low to moderate power applied, I thought the 100W controls sounded better than the smaller 15W versions.  Slightly opening a cooling vent to spray Caig ProGold onto the contacts, & applying silicone sealant to the backs for mechanical damping, is, as Martha would say, "A good thing".

[ted_b]

For anyone curious of the internal workings: the crossover network requires a constant impedance to avoid changes in the hinge point.  The control has two internal pots.  A parallel resistance is varied inversely with a series resistance, producing a constant impedance with variable output voltage.  The driver level changes while the circuit impedance & hinge point are constant.  

Years ago in a listening test, even with low to moderate power applied, I thought the 100W controls sounded better than the s ...

Is what you said "good" or "bad"?  I didn't understand a word!   Sounded great, though....

[Q]

Dont forget one of the other huge advantages of being able to control the level of each driver section...you can use slightly different gain amplifiers in a bi-amp system, and adjust to achieve balanced system gain without the need for adding anything else to the signal chain.

[azryan]

"-To me, these pots are used to match the level of the line source (Neo pannels) and point source-"

Depends on model.

A single neo in the 626R or LRC isn't a 'line source' driver.

If the panels effi. matches the woofer (I dunno if it does, but I bet it's close and a woofer 'could' be made to match exactly if vmps's don't) then you could eliminate the neo pot all together and need no resitors either -'cept on the spiral or FST supertweeter section to pad it down as the whole speaker should act as a point source.

Gets tricker on the larger neo panel versions.

There's a formula for what's a line source or not, but I'm not sure about the transition between point source (-6db per double distance) to line source (-3db per double distance).

I'd think level matching  a line and point source so they match at about 11' would work well and there'd be little change in output for anyone sitting 9'-13' away which I think most people fit into. 8'-14' even wouldn't make much of a change.

If the speaker was designed to be it's flattest at 11' it could still be called 'flat' within that wide seating distance range while getting the benefit of eliminating veiling parts in the signal path.


"-The reason other manufacturers don't provide Lpads or similar precision adjustments is, I would guess, because they don't trust the user to match the tonal balance the designer feels is right in his personal listening environment,-"

That.... or they spent a lot of time to level match their chosen drivers and carefully measured the speaker's quasi-anechoic responce to get the speaker's range as flat as they can, and intended the x-over to be as pure a signal path as they could find.

But you're probably right. You must be the only speaker designer that 'trusts your customers'. And seeing as these adjustments are a breeze for all your customers, then I guess that helps your point too.

Oh wait... a lot of people have a very tough time tweaking these. Nevermind. None of these people know if their speakers themselves are as flat as then could be.
If they did, then they could look to their components and/or room if there are problems or slight tonal qualities they'd like to change.

And how is the gain adjustment of a 5 and a half octave bandwidth neo panel considered a 'tone control'?

I certainly wouldn't consider that a tone control at all.


"-Which brings me back to my point, which was: which is better? Adjustable speaker with L-pad of quality X, or nonadjustable speaker with resistors of quality X+Y? (where Y is assumed to be a positive quantity-"

If it's the same speaker in both cases.... it's the latter.

Your front end choices, speaker placement, and room treatments are more than enough variables to drastically alter the sound of a non-adjustable speaker to 'tweak to taste' IMO.


"-Dont forget one of the other huge advantages of being able to control the level of each driver section...you can use slightly different gain amplifiers in a bi-amp system, and adjust to achieve balanced system gain without the need for adding anything else to the signal chain.-"

Using diff. amps IMO isn't typically a good idea for reasons other than diff. gain.
There could be phase and tonal problems.

If anything... bi-amping vmps w/ diff. amps I'd rather have a pot on the bass woofer section and none on the neos and super tweeters, but the woofer sections are the lowest effi. section so you can't attenuate them to match the neos and FST.

This could be redesigned though so the woofer section is the higher effi. well you'd still probably be padding down that 96db FST, but I'd like to hear a vmps w/o pots who's drivers are level matched for about 11' distance and compare the transperency to a 'potted' version.

[James Romeyn]

Dont forget one of the other huge advantages of being able to control the level of each driver section...you can use slightly different gain amplifiers in a bi-amp system, and adjust to achieve balanced system gain without the need for adding anything else to the signal chain.

True.  But this method of gain matching converts wattage into useless heat in the spkr control.  To reduce gain at the input of the necessary amp will increase wattage = to the dB of attenuation, a better approach (each 3dB = a doubling of amp power).  Email me for notes on this.  I promise to update my website to have notes like this readily available.

[DFaulds]

azryan -- speaker manufacturers seldom have systemes where all the drivers have the same efficiency.  Usually you find that the tweeters have to be padded down to match the efficiency of the woofers/mids.  As I stated previously, this is done by inserting a fixed L-Pad which is a resistor in series with driver, and then usually also consists of another resistor paralelled with the driver so that impedance presented to the crossover has very little, if any, measureable change.  As Brian explained he has chosen to match the levels with variable L-Pads (pots is actually an incorrect term) instead of the fixed L-Pad that is more commonly found in quality loudspeakers.  I personally think there would be less undesireable affect on the systems performance with a fixed L-Pad of high quality resistors, but that's not how Brian has designed his systems, and they are generally getting great reviews.

While it would be an interesting excercise to compare a VMPS design with the variable L-Pads replaced with fixed L-Pads, it's unlikely to happen, and would alter the wishes of the designer.  Let's keep in mind that the variety of ideas presented by great designers like Dick Shahinian, Brian Cheney, Jim Thiel, etc., are often contradictory, but still they all seem to create great designs.  Go figure...

[maxwalrath]

I wonder if Brian would try making a fixed L-pad design?

[James Romeyn]

Never.  The controls have been integral to his designs since the beginning.  VMPS tuning features contrast with most competitors, whereby users tailor the rest of the system to complement the permanent voicing of the speakers.  VMPS owners may become familiar with the audible difference of turning a control only one detent either direction.

[John B]

I for one am glad I have these controls, as without them I could not have adjusted the 626Rs tonight to a level that satisfied my ears.  This is my first experience with ribbon speakers and the adjustible controls that come with them, but after Brian's demonstration of the inner workings of the pot controls I understand how they do what they do.  As I worked with them tonight for the first time, I got a feel for the "balance", I'd guess you call it, that can be dialed in.   I've got one speaker set at the 12 o'clock positions, and a little beyond that on the other, as I need to compensate for how one speaker is next to a corner wall, which is creating a different tonal signature from the other which is next to an open area that leads to a long hallway.  This was creating a diffenent tonal balance for each speaker.  I still haven't got it quite where I want it yet, and what with room treatments on the way, that will probably neccesitate some more adjustments.   I've had speakers before that started out like this, and I tried to live with them, but eventually got tired of the mismatch.  Only one speaker that I've owned has overcome my room limitations, and that I think was due to the unique layout of the drivers.   Now with the pot controls and some room treatments I think I'm on my way to dialing in the most balanced sound I can obtain in the room situation I'm in.