New Bass Extender

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

Hi Kanwar,

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

Cheers,
Greg

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

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

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

Finally I've got around to measuring this design. First an FFT showing residual noise and harmonic artefacts.

This is done at the 0.5V soundcard max signal level and shows no robust harmonically related artefacts above the noise floor.

Here -

And here is the FFT of crosstalk in the other channel keeping in mind that the dual chip does two channels, uses 2BJT 80dB discrete regs for +/- supplies and the PCB is fully ground planed with a progressive layout.

Cheers,
Greg

And here's the crosstalk at the peak of human hearing acuity -which you'd need to discern that 1/50000 narrowing of the soundstage or change in timbre.

Cheers,
Greg

Ho Bob,

Thanks for the constructive comments. I'ver done the bass response earlier I was looking at the wideband performance as it has the full audio frequency range to not impose on.

Ah..You noticed the 870Hz +735dB spurious peak in the response. Guess you don't wantr one then?

Seriously it's a piece of cr...rubbish on the measuring system. Ask Mr Mueller. It ain't there when I change range.

Cheers,
Greg

Well Hi djk,

...."Be careful of the gain in the opamp. If it is too much higher it will oscillate"...

the dual op amp used is an 8MHz UGS (unity gain stable), 20V/us
FET cascode input type running at closed loop gain of 2.5.
Please enlighten me as to how it is on the verge of oscillation?

Bob, you must surely appreciate the value of a high pass filter to ensure cone control as does Kanwar (from another thread) making for better power handling and lower bass distortion.

Cheers,
Greg

Hi odysseybmx414,

The values are pretty straightforward. Let's see -

VR1,2 DG 50K
C1,2 100n
R3,4 27K define the range with the pots 27K->77K
R5 3K or 3K3 gain of filter and peaking
R6 2K
R1, R2 divider to restore unity gain or 8dB without. Add to >10K, for input loading.

Of course a considerable part of the design is the 80dB discrete regs and the progressive layout for reduced interactions through hole plated, ground planed and double side solder masked PCBs available through my website. A poor layout or common ground tracks could result in a reduced image, resolution.

Cheers,
greg