New Bass Extender

[amplifierguru]

Here's my new little (3" x 2") Bass Extender. It's a tunable Q=2 high pass filter that runs from a Plug-pak (Wall-wart) or can be wired into a PS feed of +/-18 to 25V.

It gives a 6dB boost at a tunable frequency from 20Hz to 50Hz to restore the 6dB down bass end of a speaker to 0dB and then rolling off at 15dB /octave to be 40dB down at 0.1fo. The response is wideband from cutoff to MHz and gain is unity or up to 8dB. It's stereo with a fully ground planed board and discrete 2BJT low noise 80dB regulators supplying the dual FET cascode op-amp chip.

LF extension can be almost an octave of cleaner bass with out of band LF modulation reduced dramatically resulting in better power handling and lower distortion. It takes care of unloading.

Cheers,
Greg

[Workhorse]

Greg,

Where's the DAMN schematic........


K a n w a r

[amplifierguru]

Hi Kanwar,

Actually I had it on a scrap of paper somewhere.. guess I'll sim it and save screen. Maybe tomorrow.

Cheers,
Greg

[clem_o]

Looks great! Reminds me of the KEF Kube series of LF equalizers...

Now if only someone can figure out the Kube 200 circuit - that's really something I'd like to see!

Cheers!

[amplifierguru]

Hi Kanwar,

Here it is. VR1,2 are a dual ganged pot to tune from 20Hz to 50Hz for the peak to restore the -6 dB response of a speaker back to flat. Since the -6dB standard speaker is well capable of output but less capable at lower frequencies, there is definite and considerable audible benefit in boosting this narrow band while filtering out the subsonics that are well outside the speaker design range and where the often unloaded cone movement causes distortion due to infrasonic modulation causing extreme movement from the linear range of the motor magnetic field.

Cheers,
Greg

[BobEllis]

I built myself something similar. My subwoofer is a 6th order system. Of course my perfboard project isn't nearly as pretty as Guru's.

Since I knew the woofer's response, I was able to fix the frequency and make the Q variable over a range of .5 to around 3. Anechoically my woofers need a Q of 2 at 20 Hz, but in room it is somewhat less. With a twist of a knob I can go from smooth response for music to kaboom for HT. Too bad pots to vary the Q wouldn't fit guru's form factor.

[Workhorse]

Hi GreG,

That's very nice approach in a very nice application......

Congratulations for that...

cheers,
K a n w a r

[amplifierguru]

Thanks Clem O, Bob, and Kanwar.

Bob, if it's used with a sealed box (-12dB/octave) it's 4th order. With a bass reflex (-24dB/octave) it's 6th order. Sure, a parametric would be cool but complex and I wanted to keep it simple - with only one op amp in the signal path. minimalist.

I produced one of these 12 years ago as an addend to some double bass reflex systems taking them from 40Hz down to 28Hz really restoring the SLAM of the live performance. The other really beneficial application was for compact and bookshelf systems at the upper limit - just amazing how much bigger the bass was, without needing to resort to a sub.

My wife wants the first one. She loves em. Restores the grunt she says! Such a way with words.

Cheers,
Greg

[BobEllis]

My subs are ported, therefore 6th order with the 2nd order eq.

no need for any more opamps - just replace your R5 with a pot, move the feedback connection to the wiper and voila, adjustable Q. Not quite a full parametric EQ, but it gets the job done with a single opamp.

[amplifierguru]

Hi Bob,

Yes I see that's possible - so I guess R5,R6 could be hardwired as a front panel pot. Gain, of course would be all over the place though as these two resistors set the op-amp gain at 2.5 and the input divider cuts the signal back accordingly for zero overall gain in the pass band. If it's in a tape loop for instance, it can be switched in and out without level shift - only the bass extension.

Of course you could make the input divider a volume pot and add your suggestion for Q varying - and there'd be no shortage of controls to play with!!

Cheers,
greg