Frugel-Horn Mk3

Here is a simulation of the effect of the choke/low pass filter (with no damping). It extends the bass response (negligibly) and smooths ripple out (prime purpose).

dave
 

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Just to expand on this a little, as you can see from the 1/2 space plots Dave posted above, in fact the choke / axial baffle in the horn expansion has very little effect on the bottom end. What it's there for as Dave notes is to attenuate unwanted shorter wavelengths / higher frequencies from passing through to the terminus. The effect on the horn's upper corner-frequency is actually rather more apparant when the cabinet is corner-loaded or used near a rear boundary as intended, and the respective termini outputs are compared when so positioned.
 
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I was curious as to whether the 'choke' in the hypex horn could be raised to the point that it would be removed. I understand the choke acts essentially as a low pass filter but would increasing the line length (thus the height of the cabinet) do the same thing?

If I'm understanding your question correctly, not really, no. All other things being equal, by lengthening the horn, you'd simply by tuning it to a lower frequency.

No response to my querie regarding something similar but for the BOFU's?

Sorry, I'm up to my eyeballs with writing lectures for the courses I teach & attempting to write my PhD thesis, plus trying to sort Woden out & helping get the FH site revised in light of Fostex's complete range revamp. So I'm struggling to answer everything at present. Short answer though is yes, no reason why not in principle, but it wouldn't be a small box.
 
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to elaborate on Scott's above post, and comments I made earlier, the degree of boundary reinforcement may certainly need to be experimented with, depending on driver and amplifier type/mode.

When operating in triode, I found the combination of FE126E and Gregg's Dynamutt EL34 PP amp much more musical and "natural" than any of the 3 Mark Audios.

but pull the cabinets further from the wall and operate in pentode mode and the Alpairs / CHR & EL70s come into their own.
 
Addendum: no damping adjustments were made between the 4 drivers tried.

That they all worked within reason means that FH3 should be suitable for a wide range of drivers. The various elements of the cabinet all work to make the cabinet dominate response. Damping & relative placement wrt to wall/corner allow for fine tuning.

dave
 
It seems then that the design is quite tolerant and ideal for DIY !

I'm curious as to what elements of the design are sensitive/picky ? For example, you mentioned that you didn't like the first 2 builds / alpha ? - what did this teach us ?

Actually, only 1 previous alpha build (at least that I can remember) - with the same quick mount panels to allow testing different drivers.

The earlier design had a vertical panel with rather shallow clearance to the back of the driver, resulting in some of the same hollow / cupped sounding resonance artifacts most notable on female vocals that we experienced several years ago with the first go-round of Ron Clarke's A126, as well as the PAWO.

What this taught us? perhaps that even good modeling can only provide a starting point, and that not every "at bat" yields a home run.

OTOH, the Maeshowe is definitely not a swing and miss