Sunday, January 25, 2009


Something else - Logitech Z680 mod for lower Bass output

NOTE: If you execute the modification suggested below and you blow-up your set, you are on your own! I can give no warranties of any kind whatsoever...


Ever since I can remember I have a Logitech Z680 5.1 audio set which I use to watch movies and listen to my music. It was the top-of-the-line audioset for PC's from Logitech way back, there's a follow-up top-set now: The Z5500.

This (Z680) is an audioset with integrated Dolby digital and DTS decoder and it has a remote as well. You can input audio using 3.5Inch jacks, but also using coaxial or optical cable. You can choose from 3 input signals using the remote so I have the set connected to my stand-alone DVD player, but also to an old stand-alone audio set with CD player.

On the Z680 (one of) the downside(s) is that you cannot set the sub-woofers volume between 0 and 100% in small steps. Instead you can set 0%, and about 60-100%. On the Z5500 this limitation was removed BTW.

A lot of Z680 users including myself found that the 60% setting of the subwoofer is much too loud for most (living) rooms: Neighbours will complain about pictures falling of their walls, and the sound is not optimal for the user him (or her)self either.

So, I decided to take my set apart and modify it to be actually useable. I did two modifications: for one I added a RS232C interface in the hope I could (re)program the set some way. This RS232C interface was there on the board already, it just missed a few components placed though. Unfortunately the set's firmware does not have RS232C connectivity I found out.

The second modification, which is much more low-level, worked like a charm for me. I'll decribe that modification below.

Z680 Modification for a lower bass setting range.

The control POD of the Z680 has all digital inputs and the decoders. After all signal processing is done, the audio channels are converted to analog signals. These analog signals are then transferred to the bass speaker housing via the 15-pins thick black signal cable. In the bass speaker housing sits the analog amplifier that cranks up the signals so you can hear them from the speakers.

This means lowering the bass signal range can be simply done in the control pod at the exiting spot to the amplifier. I just located the signal wire for the sub-woofer speaker and took the wire loose. I then added a simple resistor network which divides the voltage to 1/3rd of the original level, and I attached the signal wire to the output of this divider. (The input of the divider is connected to where the wire was before.)

Since the voltage is now 1/3rd of the original, the current through the speaker is also at 1/3rd of the original which means output power is 1/3 * 1/3 = 1/9th of the original for a specific volume setting.

Now I can set the volume perfectly for my taste!

Photo's and a more detailed description.

Before you start make sure you have the following:
- a screwdriver
- a soldering iron
- soldering tin
- two resistors: one of 1k-Ohm and one of 2k-ohm or there about. 1/8th Watt versions will do nicely.

First, remove mains power and take the Z680 control POD loose from the set and any extra wires. Now remove the four black screws at the backside: make sure you removed them completely (take them out).
After you did this, you can gently pull of the front panel. Please note that a wire sits in between the front panel and the mainboard which will remain in the back part of the housing: so be gently indeed!

Picture 1:
The four black screws removed completely and the front cover removed gently.

You can see the dark-grey ribbon cable that connects the frontcover part with the POD's mainboard. Note that the light-grey ribbon cable will not be there in your case: this cable belongs to the RS232C modification I did.

This RS232C modification is not needed.

Once you have the situation as shown in picture 1 you can pull-off the dark-grey ribbon cable gently so the front-panel part nolonger bothers you while you do the modification. Please note the red wire in this cable as it denotes PIN1 of the connector (has a small triangle pointing at it on the mainboard if you look closely).

OK. Now you should gently pull the mainboard from the back part of the housing. Please note that at the backside of the mainboard there is a soldered connection to the thick black wire that normally connects to the Bass speaker's housing. Lift up the mainboard (including the other boards that sit on it) and gently turn it over.

Once you did that you'll be in the situation of Picture number 2 below.

Picture 2:
mainboard removed from back part of the housing. Again you can see the light-grey ribbon cable that belongs to my RS232C mod.

You'll not have that in your situation.

Below there's a closeup picture (3) of the situation in picture 2. Please note that the area of interest is where the black cable is soldered too. Please note the white characters printed on the board denoting (among others) the analog channels that are outputted on the respective wires:

+8 - +8V 'logic' power supply
G - 'logic' supply ground (return)
SB - ??
G - signal ground
RR - Rear Right speaker channel
RL - Read Left speaker channel
C - Center speaker channel
S - Subwoofer speaker channel - this signal we are going to modify
FR - Front Right speaker channel
FL - Front Left speaker channel
G - Signal ground
G - Signal ground
-18 - -18V analog power supply
G - analog power supply ground (return)
+18 - +18V analog power supply

Picture 3.
The area of interest, specificially the 'S' wire and the two G wires next to each other next to -18V.

Now flip the board so the other side faces up. Now Heat up your soldering iron and remove the brown wire that's soldered to the 'S' connection. Solder the 2K resistor (or 2 1k resistors in series as I did) to the S connection of the board leaving the other end free in the air.

Solder the 1k resistor with one side to the 'G' connection(s) next to the -18 signal as these G connections are closest by. Make ABSOLUTELY SURE you don't connect the -18 with the G next to it by accident! You'll probably blow-up the power supply in your set if you'd turn it on in such a case...

The other end of the 1K resistor needs to be soldered to the free end of the 2K resistor. Also connect the loose brown wire to this point. If you do it as neat as in picture 4 you can leave it as it is: it will fit nicely in the housing this way without making shortcuts.

Picture 4:
The board is flipped over showing the modification done to decrease the subwoofers power level.

The next picture shows the same modification from a different angle. Note BTW that I removed some of the white transparant 'glue' to be able to solder decently..

Picture 5:
Same spot viewed from a different angle.

OK, that's it!

Now put the control POD back together gently, keeping an eye on the modification and where you put the black cable. Re-attach the grey ribbon cable with the red wire at the same spot is was before. make sure all pins are indeed in the connector: no pins should be visible next to the connector's sides!

Gently put the top side on the backside: it should fit perfecly. If it does not, you probably made a mistake somewhere and you should just do it once more.
Put the four black screws in the backside and screw everything back together.

All set? Reconnect mains power and try your set. If it doesn't work remove mains power quickly and trace back all your steps to see if you made any mistakes, maybe it can be fixed.

If all is right it should work without a problem.


You could also use other resistor values for more or less attenuation to fit your needs. I suggest to keep the values in the range from 1k to 10k Ohm and not much higher since the impedances of the circuitry around it are 'low' as well. It's said in the electronics that you get the best power-transfer if the impedances match..

Good luck!


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