Adding a little EQ to a chipamp

[tresch]

Hey fellas. Sorry if this is a redundant question but as usual it seems it's one of those that's kinda hard to search for. I'm not looking for a huge answer just a nudge in the right direction so I can make a more directed search on my own. Anyway, here we go:

For my next loudspeaker project I want to build a set of powered studio monitors for desktop use. I want these to be fairly small and have flat response AS LOW AS POSSIBLE. For the woofers I'm using some Adire Audio Extremis 6.8" midwoofers. I want to put these in smallish sealed enclosures and EQ them to get them flat as low as possible. The woofers have a fairly long linear throw (12mm) and should be able to do some decent sub-bass in nearfield. They don't have to play super loud, as they're going to be desktop speakers, just want them to be decent for reference.

I want to play around with building some chipamps, and have ordered the 3886 kit from brianGT (and have a few other PCBs and chips laying around).

The question is: what's the simplest way to add a little one-band sub-bass EQ to an already simple gainclone? Does this require a pre-amp? Can one simply build a small passive circuit that merely alters the gain on the amp relative to frequency?

[sangram]

A parametric EQ like this?

http://damien.douxchamps.net/elec/param_eq/

Or, if you already know the center frequency and want something simpler, a Baxandall will work fine if adapted for your use. That would be this...

...http://sound.westhost.com/dwopa2.htm

without the treble control components (three resistors + 1 cap). The center frequency is a product of the 47nF and 22K resistors, I suspect those values will change significantly for your application.

[bosse]

how to modify feedback loop to make some bass boost?
and its possible with chipamp?
hom to calculate?

sorry if i disturb the thread but i am very interested on that subject.

thank

[AndrewT]

Linkwitz transform.

[KRONOH]

On the subject of boosting bass using an op amp its is possible to do so. The booster is a modified Baxandall tone control with no bass cut and no treble control. Hey, it's a bass booster. It boosts frequencies from about 30Hz to 160Hz. It can boost by 14dB, so be careful. For each channel you would use an 8 dip op amp, the op is not critical, a a4558 would be just fine.

[Igla]

Hi tresch !

I was experimenting with passive tone controle a while ago on my gainclone (a resistor with a cap at the input to ground) - the results were OK but I didn't like the attenuation of the input signal. So I did an active bass boost as in picture. I have the same sort of bass boost in my Cmoy OPA132 headphone amp and I must say that I am totaly satisfied with the result.
I'm at work now so I did a quick sketch of the bass boost on my gainclone.
Instead or 8k resistor you could use 25k pot to adjust the boost (I doubt you will need more). Change the cap (0.15u) to adjust the the frequency of the cut. I suppose you have different values of feedback resistors in you gainclone and I think this will somehow change the values of the bass boost resistor and the cap.
And a little warning - the bass boost circuit raise the DC offset at the output so you would like to measure it before connecting the loudspeakers. If DC is too high (>100mV in my book) I would use Ci cap at the feedback.

Regards

[gfiandy]

Hi,

Are you planning to do a ported speaker or a sealed box one.

If ported then a 2nd order High pass peaking filer with 3 to 6dB peak at the port frequency provides about the optimum extension and then reduces the gain in the region where the diver and port go out of phase. (hence there is not much sound output just alot of cone flapping) For a smallish speaker this would typically be arround 60 - 80Hz.

If the speaker is sealed box you could still use a peaking filter but it doesn't match nearly as well. For this type of system you would be better off using a linkwitz transform. Which will alow you to use active extension to creat any alignment you want within the excursion and power limitations of the bass driver.

Both of these filters can be implmented with a single opamp - the linkwitz transform requires alot of support passive components the 2nd order peaking filter is somewhat simpler.

To experiment with these I can strongly recomend downloading the free speaker development program WINISD

It takes a while to understand how to enter a drive unit - read the help carefully. But once you have you can experiment with all kind of box sizes and filters and it will tell you the excusion as well as the SPL and many other factors for the input power you use. It will generate "recomended alignments for you" however these are rarelly the best option and don't include active equalisation. So experiment.

Regards,
Andrew

[danielwritesbac]

On a very much related question is. . . how to fix an undercompensated op amp? You know, the LM3886TF that's on the chipamp.com circuit board? Instead of a bass boost, howabout a de-screech?