The Fix
The circuit’s designer, Paul Joppa, explains the rationale behind the kit:
“The impetus for getting involved several years ago was the observation that many home-brew single driver systems needed some baffle step correction, which was (and still is) usually done at speaker level with a choke/resistor parallel combination. This combination reduced the effective midband efficiency to less than half of what it had been, meaning you needed more than twice as much amplifier power. This was a big problem when used with flea-power SET amps, and caused a lot of 45-powered speakers to suddenly need 300Bs.”
By doing this bass and/or treble compensation ahead of the amplifier instead of inside the speaker one can use the volume control to adjust for the relative drop in the midband output level, thus retaining effective midband efficiency of your speaker.
How it works
One pair of switches controls an adjustable baffle step corrector that compensates for the bass rolloff inherent in narrow baffle speakers. One of these switches is used to set the appropriate bass turnover frequency for the baffle width of the speaker you are using – under 6 inches (15cm), 6-12 inches (15cm-30cm) or over 12inches (over 30cm). The second switch controls the magnitude of cut of frequencies above that, to effectively create a passive bass compensation which flattens out what is normally a falling frequency response below the baffle step frequency. Magnitude settings are 0, +2 and +4 dB per octave of bass compensation. Note that the baffle step loss in anechoic conditions is theoretically 6dB, however it is commonly accepted that in typical listening rooms the loss is smaller, hence the +2dB and +4dB settings.
The second pair of switches similarly allows the ability to compensate for the treble rolloff often experienced with full range drivers. One switch selects a turnover frequency of 5kHz, 10kHz or 20kHz, and the second switch chooses 0,+2,or +4 dB of treble compensation above that frequency (again, since this is a passive device what is actually happening is that the spectrum below the turnover frequency is being cut).
Since this is a passive device, i.e., there is no amplification the device has an insertion loss (how much the volume level drops when you install it in your system) of -4dB. So you will need to turn up your level control 4dB to reach the same listening level as you had before installing the Fix. The source impedance should no greater than than 4K (the output impedance of Eros, Reduction, and Quickie with PJCCS) and the load should be no less than 100K (the input impedance of most of our amps). Cables on the output are preferably 0.5 meter (1 meter if low capacitance).
This kit resembles our Quickie preamp kit in that it is assembled on an acrylic panel and the wood base shown in the photo of the prototype is optional. You can use just the bass compensation for a narrow baffled woofer, or just the treble compensation with a tweeter that may need just a bit of help at the highest end of its range. The Fix could be a really versatile tool for ekeing the last bit of goodness out of your favorite speakers.