newbie FR questions


2009-01-11 4:31 pm

Although I call myself a newbie in the title I have a lot of experience with Audio equipment from the amplification, signal processing etc. side of things. I also have a lot of previous experience with Speakers but from the car audio perspective which tends to not lend itself well to true "audiophile" standards.

So basically I understand a bit about Thiele-Small parameters, "Ported" and Sealed" box types, box construction etc., but very little on the merits of each type of enclosure, crossover frequencies to choose, driver selection, etc.

So here is my set of "newbie" questions.

Given the hypothetical "system" I would like to construct for my "man cave" listening room (approx. 10' x 12' x 7'6" in a basement/cellar) I am looking for suggestions on "budget" drivers/enclosures/architecture for the speakers.

I plan this to be a 3 way - Tri-Amped system. The subwoofer portion will be a "hybrid" tube/chip amp of my own design. Basically the "BPA300" using a vacuum tube as a phase splitter to drive a pair of PA150 boards in Push Pull.

The rest of the speaker setup is where I am lacking in some decision making.
The midbass/midrange amp will be all tube moderate power (approx. 20Watts RMS) and the tweater amp about the same.

I am looking in the direction of a pair of FR drivers w/tweeters integrated into the cabinet, assume I can put about $250 into the 4 drivers and have decent woodworking skills what is a suggestion for driver/cabinet to handle from about 60-65Hz on up? (high pass is 24db/octave L/R alignment not constructed as of yet so frequency and be done to suit) Remember also that I need about 90+dB efficiency to work with the tube amps.

Most of the "interesting" FR projects I have seen are Back loaded horns or Pipes most of which I gather are tuned to play down to about Fs of the driver. What happens when the signal is crossed over above that say at 65hz with a cabinet at say 40hz? Is there a negative effect?

All suggestions are welcome.
Hi coldcathode,

I think your plans sound great, more adventurous than my man cave (er, HT set up). I've never tackled a multi-way speaker project, I suspect there's a lot of dependency of the results on how you go about implementing the cross-overs and choice of drivers. Will this be active line-level cross over ?

Since you are focussing on the mid-range driver, being a full range with as much coverage as possible with sub-woofer support below and tweeter to assist on the top I'd be thinking about a mid range in the 6" size range as the optimal for achieving good freq. response and 90dB sensitivity. Your budget might set the choices here.
Also doubling the drivers would increase overall sensibility ,and you can use 4 or 5" ones ,resulting in having each lower mass so being quicker (good trebles
reproduction needs little Sd and low cone mass ) .That load may be difficult ,as the 4 Ohm taps of a secondary OPT has less windings so less 'tracking' :confused: correct me if I'm wrong..


2009-01-11 4:31 pm
A 4 ohm load is not really an issue for tube amps and OPT's, I build my own amps so as far as the impedance is concerned it's a non issue. BTW, you could also series the woofers and run a 16 ohm load and a cheap transformer could have LESS windings on a 32Ohm Tap than a quality transformer has on it's 4 ohm tap!

The primary impedance of a tube OPT is the turns ratio squared times the secondary load. I tend to use NOS or good "pulls" for OPT's and "mix and match" the impedances to the desired design. There is no real "magic" as far as number of turns of wire. A properly sized transformer, working with the proper taps to match the impedance and the desired load to the tubes is fairly simple. A multi tap OPT should sound and perform pretty much the same no matter the load as long as the proper tap ie 8ohm to 8ohm speaker is followed.

As far as the line level topology is concerned :

Source (typically) will be a Behringer FCA202 Sound card.
That will feed a Tube Preamp (simple ground gathode into cathode follower 6SN7) on that same chassis will be a unity gain 24dB/Octave Linkwitz-Riley alignment 2 way crossover (OpAmps) for the sub
From there the high pass will be split again in an ALL TUBE 24db/Octave crossover.
I would like the drivers to all be directly coupled to the amplifiers with no filters.

My musical tastes are all over the board so there is no particular "sound" or "feel" I am looking for. I think sensitivity is the most important aspect since the power is limited with the tube amps.
Ok ,so tell me why you focused on having mixed ICs+chipamps+tubes+longstrokesubs+FR+helper tweeters in one project.
I guess the love for tubes and FR was born by listening to an exponential profile big old paper cone driven SE .Yet ,being a social human and having made contacts with others experiencing different systems or listening to live music , you have realized that sub 100Hz music could be well handled by SS ,having much force as lowish damping factor...>>very low output resistance.
At the stage of having connected amplifiers with speakers ,and speakers in a box (or without ,in dipole ),things change . When saying of having mirrored LP and HP,you didn't take in account the natural pass-band behavior of a speaker.
A little (FR) speaker would naturally cut the lows mechanically ,so the HP filter would only help in limiting excursion which would bring the driver to die .
A simple single capacitor of the right size at amp's input would do the job.
Different approach for a woofer ,whose sensibility being defined by Sd and force factor BX*l ,and by the design the of voice coil and magnet system.
Of course you don't want the speech coming out by both sats and sub(s),so a high steep cut is recommended.Also ,but this applies to higher frequencies that the ones handled by a subwoofer , a good response is determined by the proportion of driver's area and maximum frequency allowed to be reproduced ,because you would incur in what is called 'cone break-up' ,because the membrane is no rigid at all ....:vampire2:
When saying of having mirrored LP and HP,you didn't take in account the natural pass-band behavior of a speaker.

By 4th order XO you probably are talking LR4.

A convienient way to achieve that in the high pass is to load the FRs sealed with Q=0.707 (butterworth). Add an active 2nd order Butterworth at the same frequency, and you have an acoustic LR.

Not many FRs are really suited to sealed... 2 that are in the sim below. To take max advantage of the FR a higher XO than 65Hz is likely better.

CHR70.2 has no need of a tweeter IMHO, Bob sells FR125SR with an ERT26 tweeter as a kit.



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2009-01-11 4:31 pm
Lots of smaller, more suitable cabinets for your app.


That's EXACTLY the info I was looking for, I tend to look at projects as a "Mount Everest" thing, the more complex and near impossible it looks the better. I often need someone to set me straight. I am guessing that FR would be better suited as just that. Full Range. I have always wanted to build a super high end SET amp and I guess once the funds are available an FR horn project with a low power SET amp would be in order.


The overall approach that I was going for came about as a natural progression from my early days in Car Audio. In true high end car audio (not the booming bass boxes we hear on the street) there are many obstacles about the car interior to over come. Small Volume, HIGH ambient noise level. Very restricted driver locations etc. To produce a nice sounding system in a vehicle requires that the drivers be fed a high power clean signal that only has the frequencies desirable for it. Because the mid bass drivers are typically in a door and not even close to "on axis" to the listening position as well as very different path lenghts from Left to Right you need to have a tweeter crossed over pretty low and as near to the midbass as possible.

Steep LR (4thorder) line level crossovers fit the bill nicely.

The chip amp/tube hybrid for subs is just plain old economics. A moderately priced set of satellites can approach an sensitivity of 93-93 [email protected]

Subs that play low enough to be worth while and be affordable will be more like 86db. Hence I need a LOT of power to keep up with the rest of the system. Do you have any idea what a TUBE amp would cost me to get into the 100W/Channel or more arena?

The Tube in the hybrid sub amp is there just because I can put it there, it is more for aesthetics than anything else and a conversation piece.

Placing caps on the inputs of tube amps can be a tricky situation depending on the topology and tube type. There are stray and inherent capacitances in tubes that can work with you or against you to this end.

I welcome all comments and suggestions but the overall, triamped tube/SS hybrid nature of the source/Xovers/Amps is not up for discussion it is in essence (my everest).

Perhaps, the FR forum might not have been the best place to ask these question now that I realize that FR might not be the drivers to go with, but some suggestions as to possible sealed/bass relfex two way projects would be appreciated.
And what happens if you reduce the capacitor after the first stage (if SE ) ?

If properly sized it becomes a 1st order hi-pass corner. Dedicating an amp and appropriatly resizing components to achieve as much of your XO as possible in the amp is the most transparent way to do it.

For instance, take one of the sealed boxes above, with a cap coupled 2-stage SE amp, reduce the cap such that it and the grid link form a HP at the same frequency the box rolls off. Then add a cap in front of the front tube to achieve the same thing, you have a 4th order HP with only 1 additional cap in circuit