Can I modify an opamp buffer to add 3dB of BSC? - diyAudio
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Old 19th August 2012, 07:47 PM   #1
goody75 is offline goody75  United States
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Default Can I modify an opamp buffer to add 3dB of BSC?

I am in the process of building one of the popular gainclone kits that will be paired with some diy speakers that need about 3dB of baffle step correction. Since their sensitivity is only 86-87dB, I would rather use active BSC than shaving off dB on the upper frequencies.

The gainclone has an opamp buffer after the volume control (preamp output). A pic of the relevant schematic section is below. It seems like I can modify the buffer to get my 3dB BSC correction (bass boost) if I cut the existing feedback trace and replace it with an appropriate resistor & capacitor.

Can anyone comment on whether this is a good/bad idea and what the potential downside is?

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Old 20th August 2012, 12:15 AM   #2
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The trouble with this is that active filter circuits need to be driven by low source impedance to work correctly, driving one from the output of the pot will not work correctly, this is one of the reasons the buffer opamp is used after the pot in the first place, so that the pre-amp has a low output impedance suitable for driving a cable + amp.

Another option would be to use a passive line level crossover for baffle step, these aren't active, but you cut the high frequencies down before the amplifier, rather then at speaker level, so you aren't losing any power, all you need to do is turn it up louder so to speak.

Your pre amp already has a passive line level xover of sorts on its input. The 22k resistor to ground sets the input impedance and the capacitor in parallel to it acts as a shunt to ground at high frequencies creating a low pass that filters out very high frequencies. The 2.2u cap in series with the the input does the opposite and forms a high pass with a very low corner frequency that blocks any DC from entering the input. Both of these filters need to be driven from a low source impedance (ie your CD player has a low output impedance) to work correctly, but their corner frequencies are going to be sub and ultra sonic so even if they are driven by a relatively high source impedance they are still going to have a minimal effect on any of the audible range.

The passive line level xover shown over at this website

Baffle Step Compensation

will work in conjunction with the current input filter, but this is only if the output of your source has a low output impedance. Now it is customary to use ~100 ohm resistors in series with the outputs of solid stage (opamp) based line drivers, in other words most modern stuff will have a low output impedance so on the whole you don't have to worry.

As is mentioned with the above link the circuit will work better if the circuit after it has an input impedance of around 100k. Your preamp has an input impedance of 22k, which will work, but not with complete accuracy. Increasing the 22k resistor to 100k will sort this out, but it will have the effect of adjusting the corner frequencies of both the ultra sonic and subsonic filters. The sub sonic filter you can ignore as it will only push the frequency from very low already to even lower, it is there to block DC and that it will still do. The ultra sonic filter though, you would be wise to lower the 100p cap down to 20p otherwise that will probably be audible.
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Old 20th August 2012, 12:34 AM   #3
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Quote:
Originally Posted by goody75 View Post
Can anyone comment on whether this is a good/bad idea and what the potential downside is?
Its a good one - you're not adding additional active stages. Just be careful where you take the signal ground connection from - you need a nice quiet 0V.
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Old 20th August 2012, 12:41 AM   #4
sreten is offline sreten  United Kingdom
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Hi,

Yes you can. Its a doddle. I can't see what the fuss is about.
There is no downside. Simplest is to modify the x2 stage
before the volume control for the bass boost.

rgds, sreten.
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Old 20th August 2012, 03:08 AM   #5
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Sreten I assume you mean putting something like the Rod Elliot circuit before the pot. This would work but we do not know the value of the pot. If it was a standard 10k, then you'd be forced into using resistors of ~1k in the compensation circuit which would then be = to that of the output impedance of the previous stage and surely you'd run into a problem. Maybe it would help to know what value the pot is first.
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Old 20th August 2012, 03:30 AM   #6
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Its not that complicated really. To take up sreten's suggestion - change the 20k feedback resistor on the first opamp stage to 30k, this gives the +3dB, then parallel the new 30k resistor with a series RC network where the resistor is 62k and the C is chosen to suit the BSC frequency desired.
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Old 21st August 2012, 12:00 AM   #7
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Oh by x2 stage sreten meant the stage with x2 gain, I thought he meant the second stage which comes after the pot, so that didn't make much sense ;/
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Old 21st August 2012, 01:25 AM   #8
sreten is offline sreten  United Kingdom
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Hi,

the only complication is choosing the right value of C, its not intuitive.

The impedance seen by C is not 62K, its 62K + 30K in parallel with 10K.

So its 69.5K. Not a huge difference I must admit, subtle in this case.

rgds, sreten.
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