How do I increase gain in sub crossover?

[paulspencer]

I have this kit, and it is completed, working and has been in my system for a few years. The problem is it is lacking gain, especially if I want to cross lower.

Here is the kit schematic:
http://www.geocities.com/f4ier/xover.htm

Click on white schematic to see large version.

Can anyone tell me how I can increase the gain?

Since I'm always turning it up to full, I'd like more gain, perhaps 6 - 10db more.

[SpittinLLama]

The text below the circuit tells you what each stage does. You can increase the output level in the first stage but not the last. The last stage is just an inverter to get the other output and should be left at unity gain. If you want to increase gain then change R6 in the first stage by increasing the value. Looks like the gain of the first stage is adjustable using the 10K pot (RV1). Don't have enough time to go through and figure a value for you but you can double it (R6) to 100K and see if that does it for you.

-SL

[paulspencer]

From the kit instructions:

The gain is +/-12 db, varied by RV1, which is placed across the opamps inputs in a slightly unusual configuration. Here, the pot varies both the input attenuation at R4 and the opamps negative feedback at R6 in an inverse relationship.

Is it necessary to vary R4 as well?

What could happen if I change values to something that is not suitable? Just wondering if there is anything I should be wary of ....

[SpittinLLama]

I am not totally sure on what might happen but I think just changing the value of R6 will do what you need it too. I think the worst that could happen is the signal gets clipped through one of the stages and so the signal coming out of the crossover is clipped. I don't see how much more could go wrong. 12dB is a gain of 4. Sorry, not much help.

-SL

[rabbitz]

You do it by decreasing the input attenuation. Omit R3 and change R4 to a wire link... gives heaps more gain.

That info came from Leo at Silicon Chip in Oz.

Changing R5 to 7K5 will give you ±18dB adustment.

If you still need more, add another op amp with gain at the output.

[paulspencer]

Thanks Rabbitz, so how much is heaps? More than 18 dB?

When you say "Changing R5 to 7K5 will give you ±18dB adustment. "

Is that one resistor change giving me the 6db extra gain?

Why is it specified as plus or minus gain? To me 0 - 18 db makes sense, but - 18db?!

[rabbitz]

It would not be an 18dB gain but allowed me to use a P101 as a sub amp which only had a gain of 27dB and when combined with the original circuit, was not enough.

The ±18dB is on the volume gain control so you get an extra +6dB in control up as well as -6dB control down. That control is only a fine adjustment to balance the sub with the rest of the gear and doesn't turn the sound completely off like a normal pot.

It's beneficial to read the article from EA September 1999 to get the full workings of this circuit. Silicon Chip do have that info available or it's supplied with the Jaycar KA1814 kit.

[paulspencer]

I got the article with the Jaycar kit.

Now I'm confused. Which gives more gain? Changing R5 or removing R3&4?

[rabbitz]

Removing R3 and adding a link for R4 gives the highest gain.

Altering R5 gives a higher range of gain adjustment only and a tad more gain.

[paulspencer]

It didn't work!

I removed R3 and inserted a wire link to replace R4. I then tested it. To be safe I used headphones to test first with lowpass at its highest setting (200 Hz). It was not very loud, and sounded terrible as if there was some very heavy distortion added.

I decided that was a strange way to test, but at least it is working ...

Well, I inserted it back in my system as usual and ... nothing! I checked and double checked wiring. I will have to check the PCB again but am out of time now.

Very disapointing. I find this frustrating when something doesn't work! Argh!

Is there something wrong with this mod, or is there a mistake in my implementation?