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Old 11th March 2012, 04:26 AM   #1
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Default editing old Kenwood amp-board

This is a 30 watt sub-woofer, plate amplifier from the early nineties, by Kenwood. I have put a new driver into the sub-woofer box and tuned the port and box volume, but it still sounds terrible. My worry is that there is an undesirable bass boost circuit built into the chip. I am wondering is if any one can take a look at it and tell if there is a bass boost circuit on the board, and also how I might bypass it by making some simple soldering edits.

The first image here is of the amp board.

Click the image to open in full size.

The second image is of the back of the amp-board.

Click the image to open in full size.


The third image is of the front of the switch-board, as I am calling it, that has the LED light, the power switch, and the RCA jacks on it.

Click the image to open in full size.

The last image is of the face plate itself, showing the volume pot, LED, and RCA jacks.

Click the image to open in full size.

Any Ideas?

Thanks for any advice or information.

Last edited by birdyfoot; 11th March 2012 at 06:49 PM.
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Old 11th March 2012, 06:47 PM   #2
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Can you tell me if this circuit is too simple to have a bass boost circuit be a part of it?

From what I can tell bass boost circuits can be quite complex.

For some reason this sub-woofer is just too loud in the 30-50 Hz range, and every sound even up 100 Hz seems to produce a rumbling note in this range.

Could it be a ill tempered bass boost circuit!?!

Maybe I should add a few more soda bottles full of water in the box to see if the volume is still off ;-) Also, I might invest in couple dozen tennis balls for this reason.

Thoughts, Please,
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Old 11th March 2012, 07:10 PM   #3
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hi , the only thing you can do is to follow the 'maze' and figure it out by yourself !
The 'amp' board contains also the PSU (without the transformer ) and the output protection . What you call 'the switch board' is indeed the suspect .
After the RCA plugs there are 4 ICs deputed to : mix the l and R channels ,make a 4th order lowpass , give a special targeted slope and equalization which basically may also cover the old driver's deficiencies .
So you may skip the 'special EQ' or 'ill tempered bass eQ' stage .
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Old 11th March 2012, 08:25 PM   #4
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picowallspeaker - Thank you.

This is very hopeful. I know that there is a low-pass filter set to about 100Hz. The slope on that seems pretty sharp... I would want to keep this. As far as mapping things out to discover a "special EQ", I am at a loss.

Based on this information, however, I have done the only thing that I really can do and taken better pictures of the "switch-board".

Here is a composite image of the front and back.

Click the image to open in full size.

Every little bit helps guy's, thanks.

In the mean time, I will do my best to sketch a circuit diagram...

Last edited by birdyfoot; 11th March 2012 at 08:29 PM.
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Old 11th March 2012, 08:53 PM   #5
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you're welcome . hhhmmm ( not very skilled ,sorry)
At the right of the RCA input plugs : mixing ,lowpass ,buffer ; the output from the buffer goes to amp input AND to all the circuitry located at the left of the RCA plugs , which rectifies the signal and send it to a comparator that changes the status from standby to on, indicated by the 2 colors LED .
I would firstly change the two capacitors at the input, to better non polarized types .
Then....ugh !!
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Old 12th March 2012, 11:49 AM   #6
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If you have a model number, we can hunt on the interweb for a schematic - much easier than drawing it from a picture of the circuit board.
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Old 13th March 2012, 09:36 PM   #7
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Well, here is my best effort to sketch the circuit diagram, so don't laugh. I tried to color code some things to make it easier to look at.

I believe that the blue, red, and orange lines are for power.

I think that the circuit which contains chip #3 and #4, as I have them indicated in the diagram, might be my special EQ. I am thinking that it might be possible to bypass it by shorting out the board almost all the way from the volume pot to the output #3 (pink). I will need plenty of advise how to do this, if that is correct.

Click the image to open in full size.

I was looking at the amp-board again, and I think that the big square thing is an inductor, and that this may comprise the low-pass filter all by itself. Is this reasonable?

Thanks,
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Old 14th March 2012, 06:34 PM   #8
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I went ahead and tried to bypass the circuit that contains chips three and four by shorting out the connections shown here on my previous diagram. It didn't work, but it's okay because I put everything back.

Click the image to open in full size.

Here is a picture of the wire that I used to short the board itself. I just used what I had...

Click the image to open in full size.

Now I am thinking about trying this again but in a different spot.

Click the image to open in full size.

Really I'm not even sure which way the signal is running through the pot!?!
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Old 14th March 2012, 06:54 PM   #9
GoranB is offline GoranB  Poland
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Birdyfoot, as Ingenieus suggested its better if you tell us the model of the amplifier, so we could look for a schematic on net.
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Old 14th March 2012, 07:07 PM   #10
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Quote:
Originally Posted by GoranB View Post
Birdyfoot, as Ingenieus suggested its better if you tell us the model of the amplifier, so we could look for a schematic on net.

I have looked without any luck, but here it is anyway.

Kenwood Sub-woofer model: SW-30

Part Number on the back of the Plate-Amp: WF-30K

Thanks,
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