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Old 10th March 2014, 05:51 PM   #1
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Default Bi-amp/pre-amp design help

Hi all,

I am designing a stereo amplifier with an additional channel for low frequencies (<200Hz). I have designed and am now testing the pre-amp/bi-amp which is shown as a schematic. The idea is to have the left and right input signal separated and filtered (>200Hz) so they can drive separate amplifiers, and then the average of the left and right is filtered separately to provide the signal for the bass amplifier. In the schematic the resistor pairs R4 and R5, R22 and R23, R35 and R36 form a 100k pot for each channel, used for volume control. The opamp used is the LM833M in each case.

After testing this design, I've found that not a lot happens. I get some noise at the output of the opamps that is of a couple of MHz which I think is because I haven't included any decoupling caps for the supply rails. Can anyone suggest a way to improve my design/see any major problems, or suggest a similar design to achieve the same thing?

Any help would be appreciated.

Barney
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Old 10th March 2014, 05:53 PM   #2
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Here's the schematic
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File Type: png Preamp.png (36.0 KB, 169 views)
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Old 10th March 2014, 06:16 PM   #3
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Decoupling at the opamps is a a first order of business thing.

The only real issue I see is the high crossover frequency. Unless your sub is dead center between the mains you are going to hear where it is. Can your L/R speakers handle down closer to 60 Hz?

Of course the primary online reference for active crossovers is Active Filters
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Old 10th March 2014, 06:36 PM   #4
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Yeah it completely slipped my mind to add the decouplers in, I will do that first thing tomorrow. The L/R speakers I was planning to use can go down to 200Hz, which is one of the reasons I chose this for the crossover frequency. Do you think this crossover frequency will be a significant problem then?

Barney
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Old 10th March 2014, 06:39 PM   #5
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Bo$* crosses that high. Lots of people have no problem with that. On the other hand, I find it fairly easy to localize my subs if they are rolled off >70Hz 4th order. It drives me nuts...
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Old 10th March 2014, 07:06 PM   #6
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Ah well I'll try at 200Hz and see how it sounds, I just want to get the circuit working first. Do you think the resistors in the 3rd order low-pass are too high?
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Old 10th March 2014, 08:38 PM   #7
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You can get lower R values by increasing the C value. I've used 100 nF for C to bring the resistor values a bit lower, or you could go on the order of 1 F.

Looking at it again, why R21 and R22? Why not one or the other? Similarly R34 and R24. What are you trying to do there? Drop the signal from the sum of the two channels? Why not use a pot there to allow you to precisely dial in the level needed? That's the perfect spot.

Then you put back the gain you just lost with U5. Just use it as a unity gain buffer unless your LF driver needs the extra gain.

Same thing with the multiple resistors between the high pass stages and their buffers. Resistors are available in just about any value you want, there's no need to parallel them to make a value.
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Old 10th March 2014, 08:54 PM   #8
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If you change the low pass to an MFB you can remove most of those extra opamps.
Is the bass speaker the least efficient and thus need the most signal?
Then remove the adjustment pot and use the MFB to feed direct to the Bass Power Amplifier.

Now just one opamp that does the summing and filtering and buffering. One third the cost and one third the added distortion.

What are the 15k doing just before the vol pot buffers?

Why is C17 & C16 so large. The Lows have already been removed.
Increase the 100k to 1M.
Add an RF filter at the input. Maybe 330pF to 1nF.
Add a 1M0 input grounding resistor before the DC blocking input capacitor.
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Last edited by AndrewT; 10th March 2014 at 08:59 PM.
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Old 11th March 2014, 08:47 AM   #9
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The 1K's after each filter are redundant and can be taken out, I'm not 100% sure why I put them there. I read somewhere that the 15K's in parallel with the pots should give a more linear volume control (I think), but this may be wrong.
I think I intended to make U2, U5 and U7 unity gain at first but there was reasoning for having some gain (I designed this a while ago).

AndrewT, I've not heard of an MFB before, can you please tell me what it stands for and I'll look into it. One op would be much better for me. The bass speaker is driven by an amp with more gain than the L/R channels so I intended for all outputs from the preamp to be the same. And the only pots I have are for the volume control and I would like to keep them.

Thanks for the replies
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Old 11th March 2014, 09:59 AM   #10
AndrewT is offline AndrewT  Scotland
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MFB is Multiple FeedBack.
It is a standard alternative to Sallen and Key which are non-inverting.
Ti's filter calculator and quite a few others have an MFB option.
D.Self and Others discuss them.
There are probably quite a few Threads that mention them here.

An audio vol pot is best if it has an audio taper.
Log law vol pots make an excellent alternative.
BUT,
these variable slope of resistance with rotation are very difficult to make such that the stereo outputs track each other accurately over a range of rotations.

To improve this stereo channel tracking, one can use a linear law vol pot and fake the log law by adding a resistor across the output of the vol pot.
eg.
a 100k lin pot with a 15k log faking resistor behaves a bit like a 15k log law vol pot over part of it's rotation (not over the full electrical rotation). This gives better channel to channel stereo matching.

The output adjustment attenuators of the active crossover, once set up for the crossover, amplifiers and speakers should never be changed. The log law does not apply here.

Use a volume control before the crossover to control the volume of ALL the output channels together.
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Last edited by AndrewT; 11th March 2014 at 10:01 AM.
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