Bottlehead Fix for Full Range Speakers

The Bottlehead Fix is a passive baffle step corrector and treble compensating device that sits between your source and your integrated amplifier, or between your pre-amp and amp. Baffle step correction (if the baffle is narrow) and treble compensation can benefit the sound of full range speakers in particular, so if you are into Fostex or MarkAudio drivers (or even Planet10 / Feastrex modified drivers), this is a great device to check out. Do note, in some cases, the compensation is already being handled by a network of capacitors, inductors and resistors within a full-range speaker.

There are four switches in total. As stated on the Bottlehead website: two of switches on the Fix controls the adjustable baffle step corrector that compensates for the bass roll-off in narrow baffled speakers. The first switch is used to set the bass turn-over frequency for the baffle width of the speaker you are using (selectable in inches). The second switch controls magnitude of the cutoff frequencies for passive bass compensation (which flattens a falling frequency response below baffle step frequency).

The third and forth switch compensate for treble roll-off with typical full range drivers. One switch selects turnover frequency, and the second switch chooses 0,2,or 4 dB of treble compensation above that frequency.

Curious if your speakers could benefit or not? The Fix is only $75 so it's not a big gamble, and given the popularity of full range speakers lately it wouldn't be a tough sell either. 

Normally, the Fix comes with a random colored piece of plexiglass (or similar material) and you either make the base yourself, or order one of Hammond or Bottlehead's for a few extra bucks. The build should be pretty quick as there aren't many parts, and would take the average person a couple of short nights to put together. 

Since I like everything on my rack to be "matchy matchy", I'll be using some alternative materials to put mine together, namely the typical copper leafed wood base and a copper plate. The copper plate I ordered was a remnant online, nice and heavy at 1/8" thick. I taped the plexi plate to the copper one and used it as a drilling template on the drill press. 

After drilling the holes, I sanded with 60, 180, 300 and 600 grade sandpaper, then spray painted it with a matte clearcoat from Krylon, otherwise it would be oxidized with hand-prints the very next day. 

Drilled copper plate

The base is made from 2 inch tall decorative molding from the local hardware store, cut with a miter saw, adhered with wood glue, and braced in the corners with square pieces of pine. It's then copper leafed using generic copper leaf and Mona Lisa Speedball base coat, adhesive size, sealer and antiquing glaze. These items can be found at your local Michaels craft store or online. 

Sitting in a copper leaf base

The first step in assembling the Fix is fastening on the switches and RCA female connectors. A socket wrench is especially handy in this step, however if using the plexi base, caution is advised in the manual as it can crack. In addition, the switches have plastic threading, so they could strip easily if too much pressure is used. 

Switches mounted

Once these items are secure, it's then time to mount the solder tabs. The manual shows corresponding letters and numbers for each solder point, so it's great to write these directly on the plate for ease of assembly. 

Labeling the solder points

I ran a solid piece of Cardas magnet wire for the ground plane and tied any solder tabs that needed to be grounded to it using bus wire. The polystyrene caps supplied are very nice quality. Polystyrene was commonly used for capacitors in the 50s up through the late 70s. You'll still find them today, many of them surplus from overseas. In vintage tube amps you may find them in glass-sealed tubes. This variety has the leads laid-in, and you need to be careful in higher values as they will create inductance based on the way they are built. Also polystyrene caps can melt, so don't put them in areas of high temperature! Those are the cons, but many feel that polystyrene caps are second only to Teflon in sound quality, so it may be worth it to experiment with them.

The 15,000pF caps in the Bottlehead kit are polypropylene, I opted to order some polystyrene to match the rest of the build. These came from Israel and are a little larger as they are 125V (no particular reason, just the smallest value I could find).

I opted to use some Cardas hookup wire and carbon film resistors for the build, just based on my own preferences. The ethernet wire that Bottlehead supplies is extremely high quality, so go with that unless you are a die-hard Cardas fan ;)

Bottlehead Fix interior

With the 15000pF caps installed

Bottlehead supplies a nice piece of black vinyl that has the functions printed on it, as well as an "I" and "O" for input and output. As a final touch, I added the iconic Bottlehead badge to the lower center.   

The completed Bottlehead Fix

I hope you enjoyed this post. Please visit Zynsonix if you need some sweet sounding interconnects for your Bottlehead gear, or any other custom audio cable. Until next time! 

The Fine Print:

The above steps detailing the build of a Bottlehead Fix are for entertainment purposes only, and not intended to be used as assembly instructions. Zynsonix Audio LLC is in no way affiliated with Bottlehead. The owner of this blog and all associated parties can not / will not be held responsible if you attempt the process posted and cause physical harm to yourself, your surroundings or your property.