604-8G cab alignment musings (FB)

[GM]

Steve Hayden

Picked up a pair of 604-8g’s today with the crossovers. Now I need some cabinets.

Lot's of choices depending on the sort of performance desired, room acoustics and where the speakers, listening position (Lp) are and especially how much series resistance (Rs) is added or 0.5 ohms for a typical Altec XO to find Qts', though of course ideally need to use the driver's (measured) specs once broken in or accept the ones posted here: http://alteclansingunofficial.nlenet.net/Thiele-Small.html

(Qts'): (Qts) + any added series resistance (Rs): http://www.mh-audio.nl/Calculators/newqts.html

(Rs) = 0.5 ohm minimum for wiring, so may be higher if a super small gauge is used as a series resistor and/or there's other series resistance.

22.05 ft^3 * ~28.32 = ~624.46 L

Assuming the published specs, a T/S max flat alignment:

Vb = 20*624.46*0.27^3.3 = ~166 L/5.86 ft^3

Fb = 0.42*25.8*0.27^-0.96 = ~38 Hz

At the other extreme if coupled to a matching impedance amp:

Rs = 8 ohms, Qts' = 0.522

Vb = ~1462 L/52 ft^3

Fb = ~20 Hz

FWIW, the pioneers concluded that the optimal Vas/Vb ratio for a vented alignment is Vas/1.44 (~434 L/15 ft^3) tuned to Fs. Driven with a typical low output impedance amp this creates a 'shelved' LF response that typically blends well in a typical HIFI/HT app due to room/local boundary gain.

[RonSSS]

Interesting. I know as I’ve followed you over the years you have mentioned Rs. I don’t recall ever seeing this in box modeling software. Maybe they build it in?

Very curious how this relates to tube amps with no negative feedback?

[GM]

None I'm aware of.

In Hornresp it's the 'Rg' field and it's been so long since I've used many of the other (decent) programs, don't recall, but pretty sure they do or (like to think) it would of 'stuck' in my memory, but if they don't, then substitute Qts' for Qts per the above post.

Elaborating on (Rs) = 0.5 ohm minimum for wiring or just to make the cab a little larger, so maybe higher if a super small gauge is used as a series resistor (popular among the 'FR' driver aficionados to ~mimic a high output impedance amp) and/or there's other series resistance such as some XO designs and especially high output impedance S.E., matching impedance tube amps and why coupled to a typical speaker designed for SS amps tends to sound 'bloated' in the (mid) bass due to being too small, tuned too high.

Anyway, this subject has caused more misunderstanding WRT to driver spec choices than any other IME in that in comparing what the pioneers used Vs what's been made over the decades is due to the ever reduction of amp/system output impedance (ever increasing damping (DF)).

IOW driver Qt needed to be increased with decreasing system output impedance to compensate/perform in existing cabs or use ever increasing power robbing EQ.

Here's an extreme example that many 'traditional' DIYers today are 'politely' ignoring.

Specs for a 20-80 Hz tapped horn subwoofer : T/S theory peters out at the driver's effective upper [Fhm], lower [Flc] mass corners, so any bandwidth response plot beyond these points are strictly due to [inputted] inductance [mH] and/or driver mass roll off, with on-line freeware generally just flatlining it if inductance isn't inputted:

Fhm = 2*Fs/Qts'

Flc = Fs*Qts'/2 [normally not used]

Qts' = 2*Fs/Fhm = 2*20/80 = 0.5 Qts', 20 Hz Fs

Now how many would choose a 'weak' sealed cab woofer for such a horn design? But if they went back and reverse engineered the pioneer's low QT driver + amp output horn systems as I did they often used a 0.7 - 1.2 Qt system! to keep it as short as practical, witness the throat size of Altec's horns, they used the appropriate driver specs, adjustable amps to fit the drivers, desired BW.

Those folks that so called had no clue before T/S left no engineering detail 'unturned' IME, they just seemed to be due to designing around the electronics of the times.

Anyway, if your amp doesn't list its output impedance (not tap rating unless specifically stated as such), then either calculate or measure it.

[RonSSS]

Thanks GM.

My amps are homemade push pull with direct heated triodes. No global feedback.
4 are PP 6B4G and two are 300B.

I can do that measurement.

Thanks,

Ron

[RonSSS]

I measured an amp. I get 6 ohms at the 8 ohm tap.

Ron

[GM]

OK, so in theory the cab would ideally need to be huge, tuned < Fs, but due to the amp's design it will make the most (mid) bass if tuned exactly to Fs per the pioneer's equal amplitude impedance peaks, so typically start with ~128"^2 area and block off as required.

Box size boils down to how big one can tolerate with (measured) Vas/1.44 the pioneer's optimal reflex tuned to Fs (based on < ~0.4 Qts') being the normal minimum.

All that's left is box dims, construction, baffle layout, so any dims restrictions, desired floor/ear height, etc.?