Butterfield Audio: Budget Speakers

NOTICE: Introducing the B.A.B.B.S.!!!

The purpose of this page is to share the designs I have made for some inexpensive 2-way speakers. The first design I came up with was intended to act as my surround speakers. Around the same time I was working on the design, I found out about the DC Do-It-Yourself speaker competition of 2001. I designed the speakers to cost under the $150 limit, and entered them in the compeition. I didn't judge the budget category, but I am told the competition was pretty tough, with around a dozen pairs of speakers. In the end, this design was awarded first place, based on the scores of three judges. The judging was blind judging , and based only on the sound of the speakers. The speakers were measured afterward, but these measurements were not taken into account in the competition.

After the competition, I decided to replace the unshielded mid-woofer with its shielded counterpart, and to increase the size of the capacitor in the zobel network, since I was no longer restricted by a $150/pair limit of the competition. My friend uses this incarnation as the monitor speakers for his home studio, which he has built around his computer. I am told that he has friends interested in duplicating his monitors, and this page is intended to allow them and anyone else to do so. Here are the frequency response curves, and also the impedance curves. The minimum impedance is 5.37 ohms at about 6000 hz, but generally the impedance is a little over 6 ohms. This is typical of what is usually called an 8 ohm speaker.

The speakers on this page are fairly simple, and should make for a good first speaker project. Everything required to build my budget entry is pictured HERE. The tweeter has a 2nd order crossover with l-pad attenuation network. The woofer also uses a 2nd order crossover with zobel impedance compensation network. Both drivers are wired in phase. HERE is a simple schematic of how the crossover should be wired. In case the values are difficult to read, I have put them in the following table:

Tweeter High Pass Tweeter L-Pad
C=6.6uF L=0.35mH RS=2.7 ohms RP=6.8 ohms
Woofer Low Pass Woofer Zobel
L=0.67mH C=6.0uF C=20uF R=6.8 ohms

The speakers are built out of 3/4" MDF. The external dimensions have typically been 15.5" high by 9" wide by 10.75" deep. This gives an internal volume of 0.56 cubic feet, or 16 liters. The port is 6" long, with a 2" internal diameter. This is tuned a bit low. For a flatter response that roles off just a little earlier, use a port that is only 3.5" long with 2" internal diameter, or use a sealed box of 0.4 cubic feet.

I order most of my parts from Madisound Speaker Components. An order for all the parts necessary for a pair of these budget speakers would look something like this:

Quantity Item Description Unit Price Total Price Sale Price
2 Seas H690 Woofer $53.85 $107.70 $72.00
2 Audax TMO25F1 Tweeter $19.10 $38.20 $38.20
4 3.3uF Capacitors ** $1.05 $4.20 $4.20
2 6uF Capacitors $1.50 $3.00 $3.00
2 20uF Capacitors $5.00 $10.00 $10.00
2 0.35mH Inductors $2.00 $4.00 $4.00
2 0.67mH Inductors $2.50 $5.00 $5.00
4 6.8 Ohm Resistors $0.60 $2.40 $2.40
2 2.7 Ohm Resistors $0.60 $1.20 $1.20
2 3.5" by 2" Vents $2.00 $4.00 $4.00
2 GB Cups $2.75 $5.50 $5.50
Total Price: $185.20 $149.50

** Capacitors are wired in parallel to obtain a value of 6.6 uF.

The B.A.B.B.S

The Butterfield Audio Budget Bookshelf (Shielded)

I have re-designed my budget speakers to use the Seas H625 in place of the Audax TM025F1. The reason for doing this was to move the crossover point lower, without pushing the tweeter past it's linear excursion capabilities. The Seas H625 has five times the linear excursion of the Audax. The new crossover point is about 2000 hz. At this frequency, the Seas H690 midwoofer still exhibits wonderful off axis response. Another consideration was the small faceplate of these drivers. The wavelength of the crossover frequency is 6.75 inches, while my center to center spacing is only 5 inches. This close spacing should create a more even vertical off axis sound, with less severe lobing issues.

Here is a look at the on axis frequency response curves. This speaker measures very flat from 200 hz to 13,000 hz, and is within +/- 3 db from 200 hz to 20,000hz. With a/b testing against my original design, I hear more micro-details from this speaker than the original. Perhaps that is due to the even smoother and better balanced response, or perhaps the lower crossover point further limits any effect from the woofer's cone break up. Here is a look at the impedance response. Besides the extreme low end, which approaches the 6.1 ohm impedance of the H690, the minimum impedance is 6.7 ohms at 4,500 hz, and on average, the speaker is around 7.5 ohms. This should be very easy for any amp to drive.

The crossover is rather simple, utilizing only 8 pieces. Crossing the H625 very low, and close to it's 1800 hz resonance frequency didn't prove to be much of a problem, on account of the tweeter being mostly mechanically damped, and thus having a small, broad impedance peak. The B.A.B.B.S. parts list (pair):

Quantity Item Description Unit Price Total Price Sale Price
2 Seas H690 Woofer $53.85 $107.70 $72.00
2 Seas H625 Tweeter $22.95 $45.90 $45.90
2 3.9uF Capacitors $1.15 $2.30 $2.30
6 1.0uF Capacitors $0.60 $3.60 $3.60
2 12uF Capacitors $2.75 $5.50 $5.50
2 1.75mH Inductors $4.40 $8.80 $8.80
2 0.35mH Inductors $2.00 $4.00 $4.00
2 0.15mH Inductors $1.65 $3.30 $3.30
2 6 Ohm Resistors $0.40 $0.80 $0.80
2 2.7 Ohm Resistors $0.60 $1.20 $1.20
2 3.5" by 2" Vents $2.00 $4.00 $4.00
2 GB Cups $2.75 $5.50 $5.50
Total Price: $192.60 $156.90