Logitech X-230 Subwoofer volume knob - too much bass - add resistor to pot? - diyAudio
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Old 14th December 2012, 06:28 AM   #1
sam562 is offline sam562  United States
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Default Logitech 2.1 bass is too heavy - Will replacing the pot with a higher value fix it?

I submitted this post a few days ago but I guess I didn't provide enough information for anyone to respond. Also the pictures I provided had glue covering key components. -Sorry!


So let me try this again.

First of all, I'm not very experienced with electronics other than soldering obvious things like broken traces, loose resistors, capacitors etc... To give you an idea, I have to google Ohm's law everytime I need it.

Also, I know these are ridiculously cheap speakers with most-likely designed with a planned obsolescence, but I do love them

This time I made a detailed wiring diagram (click to enlarge):

Click the image to open in full size.


Despite keeping the subwoofer level volume all the way down, they've always tended to still be a little boomy in the bass, which I would fix by rolling off the lower frequencies in winamp's EQ.

I would really like to fix the bass problem at the source so that I don't have to compensate via software, especially for connecting them directly to a tuner/mp3 player.

The subwoofer level control is a 10 kOhm linear taper pot that has all three legs leading to somewhat complicated (to me) circuits:

Both Leg 1 and Leg 2 (wiper) of the pot connect to the upper two opamps of the TL074CN from opposing directions via an array of resistors, capacitors and a couple of diodes. It really does look like they are opposite sides of the same circuit. I'm guessing that this circuit is the preamp for the subwoofer alone since you can see where both the left and right channels lead into the lower half of the TL074CN for the satellite speakers.

Leg 3 leads to the one of the outputs of a JRC4560D opamp.
The negative input for that amplifier gets fed from both the left and right input channels. The positive input for that amplifier comes from the two upper (subwoofer) opamps of the TL074CN via a complicated route that is connected to the base of the lone 2N3904 transistor.

Is the JRC4560D some sort of active EQ circuit?
If so, is it already defeated when the knob is set at "0" (as it always is)?

As you turn the subwoofer level knob up, does the pot simply apply the output of JRC4560D to both of the other Legs (1 and 2) in order to boost the bass?

If this is the way it works then it seems like my original idea of simply increasing the resistance of the pot won't work because the pot is already all the way down at zero.

Is there another solution that would (slightly) decrease the bass? Perhaps there is a resistor on the board that I could replace?

Thanks in advance for any expertise you can offer.

Last edited by sam562; 19th December 2012 at 09:58 PM. Reason: Reformatted question and resubmitted with better description and picture
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Old 19th December 2012, 11:11 PM   #2
sam562 is offline sam562  United States
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Here are a few close-up pictures from different angles.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.


IMGUR ALBUM VIEW:
Logitech X-230 2.1 circuit board - Imgur

Last edited by sam562; 19th December 2012 at 11:27 PM. Reason: fixed image attachment mistake - added back of circuit board image
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Old 20th December 2012, 12:35 PM   #3
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Probably the most frequent complaint about that speaker system. It shows two things.
- You don't get perfect sound for that amount of money.
- You should also listen to a speakers before you buy them.

If you want to intervene into the electrical circuit, you have two options.
- Get a service manual from Logitech.
- Draw the schematic with all component values.

Other than that try to stuff the subwoofer's port with something, e.g. a piece of cloth. Experiment with its position in the room. Keep the sub as far as possible away from walls. Put it on a pedestal to remove it from the floor, if possible.
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Old 20th December 2012, 02:49 PM   #4
sam562 is offline sam562  United States
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Quote:
Originally Posted by pacificblue View Post
Probably the most frequent complaint about that

- Draw the schematic with all component values.
Thank you for your response and suggestions.

If I were to label all of the component values on the wiring diagram above would that suffice? - or do I need to make a proper schematic?

Also, do you have any recommendations for software or instructions to help a novice with creating a well-organized, concise schematics and wiring diagrams?
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Old 20th December 2012, 04:26 PM   #5
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Logitech will most probably not give you any service documents or other useful stuff.

I personally think it will be best to draw a schematic, it is easier for everybody who wants to understand the circuit and tries to help you.

Perhaps not a complete one, but start with the surrounding of the pot and the opamp it is connected to.
Maybe that will already suffice. If not, someone will request more...

For creating schematics and layouts, I use KiCad EDA Software Suite - Kicad EDA - KiCad EDA
It is open source, free, has no limitations in pins or pcb size, and works stable.

I have used other tools before (Eagle and "Target 3001!"), but the free versions did not convince me.
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Old 20th December 2012, 05:13 PM   #6
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Just decrease or short r48 resistor near JP5 gumper.
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Old 20th December 2012, 05:17 PM   #7
sreten is offline sreten  United Kingdom
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Hi,

I've found nearly all such systems sound much better with the sub
at about the same level as the satellites rather than on the floor.

Open the sub up and stick in a decent thick piece of acoustic foam,
rockwool or fibreglass behind the driver, keep the port opening clear.

(For mine the acoustic foam runs from front left to rear
right, jammed beside the left side of the port at the front).

If the bass is still boomy with the above changes detune the port.
Line the port with a 3mm to 5mm thick piece of open cell foam,
around 3 times port diameter x the port length, this will deepen
and tighten the bass and is far better than stuffing the port.

I'd be very surprised with all 3 of the above that the sub level
would still be too high, on my logitech system the level is very
near perfect at the 12 o'clock setting, as you would expect.

I have the X140 2.0 and the S220 2.1 systems.
IMO both rather good for the price.

Click the image to open in full size.

rgds, sreten.
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Last edited by sreten; 20th December 2012 at 05:46 PM.
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Old 20th December 2012, 05:17 PM   #8
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I find stuffing a rolled-up pair of socks into the port does the job nicely.

If not, looks like Moon Track has found an electrical means of solving the problem.
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Old 20th December 2012, 05:54 PM   #9
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by chris661 View Post
I find stuffing a rolled-up pair of socks into the port does the job nicely.
Hi,

For most speakers its not half as good as proper detuning,
either by foam lining or extending the port length. Only
for speakers that should never have been vented in
the first place will stuffing the port be the best.

rgds, sreten.

Vent tuning quadruples the effective driver size at vent tuning. Not
something you want to throw away by stuffing if you can help it.
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Last edited by sreten; 20th December 2012 at 05:57 PM.
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Old 21st December 2012, 12:20 AM   #10
sam562 is offline sam562  United States
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Quote:
Originally Posted by Moon Track View Post
Just decrease or short r48 resistor near JP5 gumper.
Thanks for this solution!

First I tried shorting the resistor with a piece of wire but it just made the subwoofer go completely silent. The satellites kept playing as normally though. I then tried to jump across the resistor with a 225 ohm resistor and it essentially did the same thing, although I think I could faintly hear the subwoofer.

I figured I could use a volume pot to dial in a resistance value by jumping across the resistor with some test clips. I ended up with a nice bass level at about .18 or 180 ohms on the volume pot.

I measured the resistance of R48 and it is actually 3.16 kOhms.

The "Resistors In Parallel Calculator" over at

PARALLEL RESISTOR CALCULATOR

took those two values and and says that I need a .17 or 171 ohm resistor.

I hope that is correct because the math looks terrifying and completely counter intuitive.


I'm glad I measured the R48 resistor because it looks to me like the resistor should be a 49.8 kOhm. I mean the colors are super difficult to see over the blue paint, but I see: Yellow / White / Grey /Maroon / Maroon:

Click the image to open in full size.

Luckily I've got this nice sized Mom & Pop store "Tanner Electronics" that has a whole wall of bins full of tiny components so I don't think I'll have a problem finding a resistor.

Thanks again! :-)
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