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Old 21st June 2011, 07:17 PM   #1
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Default Help with DIY Sub Pre-Amp

I have a Klipsch Promedia 4.1 Sub that I want to use as a home theater sub. I have repaired the power supply and upgraded the wattage on the resistors to fix the flaws in the design. The sub runs in class D and doesnt sound to bad for an entry level setup.

The system origionally had an external pre-amp based on op-amps. I wanted to create my own pre-amp loosely based on the origional design with some improvements. My idea was to add controls for low and high pass filters to add some tuneability to the sub.

My initial idea uses a 10x gain with 2 saleen-key filters. This may not be the best approach so I am requesting input here. I dont need anything too complicated or fancy. This current schematic has the following specs:
High-Pass - 20-50Hz
Low-Pass - 120-200Hz

Please have a look and provide input.
Click the image to open in full size.
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Old 22nd June 2011, 09:12 PM   #2
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I'm not too good with actual audio theory, but perhaps you would take interest into something like this?:

Subwoofer (12HZ-48HZ)+(28HZ-112HZ) preamplifier board | eBay

Controls crossover, phase, volume and more.

Decent value considering the cost. Components are actually decent, and don't appear to be knock offs.
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Old 23rd June 2011, 09:47 AM   #3
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I like the simplicity of a completed board but theres 2 problems.
1 The gain is not specified
2 The low pass is a little low for the sub I am using

I could modify the components to my specs but if I am going to do that I might as well just make my own PCB. I am very good with Eagle schematics and PCB's.

I am more interested if my choice of filters is appropriate and the method that I am using to adjust them. Thank You for showing me that alternative.
Still looking for some input as far as my circuit choice.
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Old 23rd June 2011, 10:43 AM   #4
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
can you look at the gain and phase on the output of each of the active filters?
If you can, set the VRs to lowest value then to highest value and compare the two sets of curves.
You will find that using single VR in each filter that you are changing the frequency and the Q (roll-off shape) for both filters.
If you want to hold Q at near the same value throughout the range of adjustment you must use a ganged pot for the adjustment of both parts of the filter resistors.

Do you need to sum the bass from two channels (or more) to feed a single bass amp & speaker?
If so then use an MFB active filter.
It has gain and filter frequency and Q all set separately as well as the ability to add two or more signals together without interfering with the stereo separation of the remainder of the audio signal.
regards Andrew T.
Sent from my desktop computer using a keyboard

Last edited by AndrewT; 23rd June 2011 at 10:46 AM.
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Old 23rd June 2011, 05:02 PM   #5
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Oh right I forgot to tell you. That is a summer board. You input 2 channels, comes one 1 channel. =D

I think the other knob could actually be adjustable gain, but I don't know for sure.
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Old 23rd June 2011, 05:41 PM   #6
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The input is just going to be a simple LFE output from my Yamaha reciever. I am following what you are saying about the ganged pot. I will update the schematic so that the resistance of R9 and R10 are both summed with pots.

Is it recommended to add an adjustment for phase? I still have a free I/O from one of the op-amps.
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Old 23rd June 2011, 06:28 PM   #7
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ALSO...Should I use Linear or Log Tapered pots?

And maybe some links where I could buy them?

Last edited by crazifunguy; 23rd June 2011 at 06:51 PM.
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Old 23rd June 2011, 08:39 PM   #8
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For volume, Log.

I believe you'll want linear for things like phase, crossover...though I could be totally wrong on that.


Mouser and Digikey stock tons of pots, from cheap to really expensive. You can also get some more "exotic" pots from other websites. Depends on what you are after. Mouser has Alps and Bourns which are nice.
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Old 24th June 2011, 06:04 PM   #9
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I have revised the design. This is version 2 using double pots and keeping Q at a constant value of .5

Click the image to open in full size.

Gain - 10X
High Pass - 22.5Hz-67.7Hz
Low Pass - 88.4Hz-198.9Hz

Last edited by crazifunguy; 24th June 2011 at 06:07 PM.
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Old 6th July 2011, 12:52 AM   #10
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Stage 1
Click the image to open in full size.
Stage 2
Click the image to open in full size.

So I am working out the final details on the preamp that I am trying to create and I have run into a few questions.

The schematics above are the actual origional preamp. From what I gather this is what happens.

1 - Volume level is sent from the pot to the gain stage.
2 - Gain is variable adding another level control
3 - High pass filter

Cut-off frequency
fc = 41.5300501346[Hz]
Quality factor
Q = 0.699321882176

4 - Another gain/buffer
5 - Low pass filter

Cut-off frequency
fc = 59.6817235356[Hz]
Quality factor
Q = 0.741334463644

I find it odd that the frequency range is so narrow and that Q is not maintained. I also have no idea how to calculate the gain in the first stage. I have never seen a circuit like that before. Also why does it appear that the signal is amplified 2 times and inverted 2 times? Is it ok to have variable gain based on opamp feedback or prefered to have constant gain? Are the filter caps on the gain stages really necessary?

I have a feeling that the first gain stage is some sort of method to obtain high gains with low value components....maybe to reduce DC offset? I breadboarded a simple opamp gain stage and the sub didnt start to sing until about 50x gain

I am also considering adding a phase control 0* to 180*

Would my proposed circuit be an improvement?

Both filters are 2nd order 12dB/Oct
Q maintained at .5 which makes the filter 'sub-Bessel' (Values can be changed
if Butterworth (.707) is more prefered)

Sorry for all the questions. I just want to try and understand what I am doing vs just slapping something together. Plus I want this thing to sound good too.

I have been using the origional design as well as this as a guide. I don't need auto on/off and would prefer to not have boost or Q adjustments. Volume(and or Gain), Low pass and maybe phase are all I want. I am going to update the schematic and set the high pass at 40Hz 12dB/Oct
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