Bass boost - dumb question candidate

[Dave Jones]

As a neophyte to speaker design and a newbie on this group it appears that I may have yet to ask my obligatory stupid question. (I recognize that appearances can be deceiving. The jury's still out on one question.) The stupid question is a rite of passage. Maybe this one is totally daft.

What's to keep me from boosting the sub bass in my office system with a passive filter between the amp and the crossovers? Muscle the notes out, as it were. The room is only about 14x14x10 feet. The amp is a solid 40 watts per channel and the speakers (Lynn Olson Ariels) are pretty efficient. Playing music CD's I can't ever turn the volume knob more than about 1/7 of its rotation.

When I use a test CD, 60Hz is loud, 40Hz is pretty good, 30Hz is weak, and 20Hz is virtually inaudible. But if I crank up the amp at 30 and 40Hz, the speakers don't appear to have excessive excursion, and they sound good. Say I put a low pass filter in parallel with a resistor or pot between the amp and the crossovers -- what then? Let's say the filter had a cutoff around 30Hz, phase correctness, and a sharp rolloff. Wouldn't that not only boost the weak frequencies, but also allow more fine control with the volume knob on the amp?

[SY]

There are no dumb questions, just dumb people asking questions. And since you're in the South Bay and you're still employed, how dumb could you be?

Anyway, you can eq at the cost of gain, distortion, and max SPL. But you'll probably want to do an active eq, so gain won't be the issue. And active eq lets you actually compensate for the rolloff with a mirror-image function, which you can't do with a passive circuit. If it were my system, I'd bung together an opamp-based Linkwitz transform circuit, plug it in, and see if the cone excursion looks to be too much. An afternoon's work and maybe $20 in parts is pretty low risk.

[7V]

I ask some pretty dumb questions myself from time to time.

If you're looking for another opinion, I agree with Sy's comments. Go active.

[Dave Jones]

Originally posted by SY
There are no dumb questions, just dumb people asking questions. And since you're in the South Bay and you're still employed, how dumb could you be?

Anyway, you can eq at the cost of gain, distortion, and max SPL. But you'll probably want to do an active eq, so gain won't be the issue. And active eq lets you actually compensate for the rolloff with a mirror-image function, which you can't do with a passive circuit. If it were my system, I'd bung together an opamp-based Linkwitz transform circuit, plug it in, and see if the cone excursion looks to be too much. An afternoon's work and maybe $20 in parts is pretty low risk.

Let's table the dumbness question. You may want to reconsider.

My amp is integrated. There's a pre-out, but no main-in. The eq will have to go between the amp and the speakers. No problem?

Assuming I'm still okay, where do I begin my quest for knowledge?

[swirv]

Does it have a tape loop. ie. a play and a rec set of rca's on the back? you could put the acitve filter there.

[7V]

Originally posted by Dave Jones
Let's table the dumbness question. You may want to reconsider.

My amp is integrated. There's a pre-out, but no main-in. The eq will have to go between the amp and the speakers. No problem?

Ah, now that IS dumb.

Seriously, I think that you might be looking at buying a power amplifier and adding the active eq between the integrated (used as a pre) and the power amps.

[Dave Jones]

Originally posted by 7V

Ah, now that IS dumb.

Yesssssss! I knew I could do it!

Seriously, I think that you might be looking at buying a power amplifier and adding the active eq between the integrated (used as a pre) and the power amps.

But, but, but... I can't bear the thought of "wasting" the power amp part. I only got it a couple of weeks ago. The return policy is 7 days. Besides, I really like it. It's a Rotel RA-02 with keen-o blue LED's and a matching CD player.

[Dave Jones]

Originally posted by swirv
Does it have a tape loop. ie. a play and a rec set of rca's on the back? you could put the acitve filter there.

Yes. I thought of that, but I didn't want to go way over my dumb question quota in one thread.

Okay, I tried it. My "eq" is a cheap, thin Radio Shack interconnect cable. I can't hear a bit of difference between "source" and "tape".

[SY]

Oh, yeah, we have a very strict dumb question quota. Watch it, Mac!

Yeah, you definitely don't want to do this at speaker level. The tape loop is the way to go, or if you're one of those people like me who says, "@!#$ the warranty!" you can add a set of power amp in jacks with a couple minutes of drilling and wiring. Or if you're ambitious, you could incorporate the eq board right inside the Rotel, snaking your power from it.

[Dave Jones]

Originally posted by SY
Oh, yeah, we have a very strict dumb question quota. Watch it, Mac!

Yeah, you definitely don't want to do this at speaker level. The tape loop is the way to go, or if you're one of those people like me who says, "@!#$ the warranty!" ...

Right arm, brother! I always lose the receipts anyway.

... you can add a set of power amp in jacks with a couple minutes of drilling and wiring. Or if you're ambitious, you could incorporate the eq board right inside the Rotel, snaking your power from it.

I like it! I was thinking maybe batteries, but digging into a brand new piece of equipment gets my heart racing. I had my factory fresh 2002 Z28 convertible two days before I was under the hood with a Drimmel, opening up the air intake box. That bad boy now has the torque converter it was born to have and double diamond cross braces on the subframes.

Woo. I spaced. Now I'm cruising the net looking for design info on active EQ's and op amps.