Osprey-12 with Electronic Bass EQ (pt 1)

Osprey-12 with Electronic Bass EQ

In my previous post, I used a bandpass subwoofer for bass reinforcement. I have no issues with that. Performance is superb. However, not everyone has space for two rather large bandpass subs and the money for an electronic crossover and an additional power amplifier. 

This led me to find a simpler solution to get more bass out of 2-way designs. I’ve been toying with the idea of using a bass boost circuit to get more bass out of the Osprey-12. The problem I have with tone controls is whenever I boost the bass, the vocals loses clarity. Some are so bad the midrange and the boosted bass end up in one mushy mess. What I want is a bass boost circuit that doesn’t affect the vocals and at the same time, increases the loudness in the bass.

Fig 1 – Osprey-12 Frequency Response

Fig 1 is the Osprey-12 frequency response with the speaker in 4pi placement. In this measurement, the Osprey-12 is about 5ft from the back and side walls. This arrangement results in the clearest sound. The bass however, is about 5dB less than the midrange. A response such as this can be quite tiring to listen to when some recordings end up with too much midrange and treble. The circuit I’m working on will raise the bass by about 6dB, resulting in a flat response extending down to 50Hz.


Fig 2 – Bass Boost EQ

Fig 2 are spice simulations of the circuit. The plot on the left is the Bode plot. It shows how the bass increases with respect to frequency. To prevent the bass from contaminating the midrange, I designed it such that the bass boost starts below 100Hz. Maximum bass boost is about 9dB at 50Hz.

Another feature that I incorporated in the design is for the bass to roll-off after 50Hz. This is very important otherwise the frequencies below 50Hz will continue to be boosted, ending up with more than 18dB at 20Hz. That can damage the woofer due to over-excursion.

The plot on the right shows the amplitude. The input is at 1Vpk (Red). The output from my Bass EQ circuit is 3Vpk (Orange). Now that I’m satisfied with the simulations, I went ahead with prototyping the circuit. If it works as intended, I will proceed with designing a PCB and having them manufactured..

Test Board

Fig 3 – Bass EQ Test Board

Before I start drawing a pc board, I always construct a prototype for testing. Fig 3 is the test board with the Bass EQ circuit wired up. This is a crucial step in the development.  With this board, I can check whether the Bass Boost circuit have any adverse effects on the sound of the Osprey-12. Spice simulation cannot tell me that. 

Auditioning the Bass EQ Test Board

For this audition, I rewired my system back to one power amplifier driving one speaker. The electronic crossover and the bandpass sub are now out of the system.

To ensure that the music stays pure, the test board is connected directly to my CD player. No other electronics like a preamp, is in the signal path. The output of the test board is then connected to the power amplifier.

It is with a mixture of excitement and trepidation when I started the audition.

First off is If You Leave Me Now (Peter Cetera). Bass is definitely louder with the bass boost circuit. What I like about it is the new bass  doesn’t interfere with the vocals. So my bass boost corner frequency is spot on. There’s no bloom in the bass and the attack is actually slightly better.

Next is I Feel For You (Chaka Khan-Live). This track contains some serious bass. Fast and complex. Bass cut right through the mix.

Still with the disco scene, it’s Car Wash by Rose Royce. This recording has a very distinctive bass that runs throughout the song. It is loud enough to be clearly heard. Doesn’t try to grab center stage by over-powering the vocals.

And one for country music. It Takes A Little Rain (To Make Love Grow) by Oak Ridge Boys, plays perfectly. The bass is strong yet vocals came out clear.

Having listened to various genres of music with this bass boost prototype, I’m happy to say the Osprey-12 sounds complete now. It is an elegant solution for bass-shy 2-ways, in particular bookshelf speakers.  Looks like I’ll be developing a pre-amp with the bass boost in the near future.