Simple Active Low Pass Filter/Buffer

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Ray P
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#1 Simple Active Low Pass Filter/Buffer

Post by Ray P »

I'm looking for some advice/steer on an active Low-pass filter for my Open Baffle project. Crossover of bass to full range unit will be circa 150-180Hz.

I've covered some of this in the SLOB thread but, just as a reminder, I'm hoping to run the full range unit essentially without a separate crossover using its natural rolloff and driving it with a bandwidth limited amplifier (a MoFo) by adjusting the input cap and inductor load, I've built up a single MoFo module, so configured, but found my source compenents have insufficient output to drive it well enough to be able to take FR measurements of the drive units mounted on a make-do baffle and driven by the MoFo - fortunately I always expected to build a suitable preamp just not quite as soon.

Anyway, I'm now working on a 2P29L preamp with about 19dB of gain. After the volume control I split the signal, one connection goes to the 2P29L preamp stage and the other goes to a unity gain buffer using a BUF-03 device (we're talking about one channel here) so essentially this gives me a passive buffered volume control that will feed the bass amplifier (the bass amp has slightly more gain than the 2P29L/MoFo combination).

I need a LP filter in front of the bass amplifer (a chip amp) and this is where I'm hoping I can some help. I have some spare BUF-03 devices and I'm thinking of using the same buffer arrangement as I described above but preceded by a line level low-pass filter. Does that sound viable or am I missing something fundamental? Here's the BUF-03 buffer schematic:

https://www.stereophile.com/content/aun ... schematics

An alternative solution to the same general approach would be to use the JLSounds JG Buffer module with a suitably configured LP filter in front.

http://jlsounds.com/buffer.html

I hope to be able to explore the crossover specifics with MiniDSP before building the actual end LP filters.

Where is the gotcha?

If this approach looks viable I can sketch a schematic for further connebts.

Thanks chaps.
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rowuk
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#2 Re: Simple Active Low Pass Filter/Buffer

Post by rowuk »

The "gotcha" is building a perfectly neutral sounding multi stage buffer. That can be many things depending on what comes before and afterwards. If it needs to drive long cables to the power amp, you will need low impedance. If the power amplifiers are close to the preamp/crossover, it is easier.
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Ray P
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#3 Re: Simple Low Pass Filter/Buffer

Post by Ray P »

rowuk wrote: Thu Mar 04, 2021 11:09 pm The "gotcha" is building a perfectly neutral sounding multi stage buffer. That can be many things depending on what comes before and afterwards. If it needs to drive long cables to the power amp, you will need low impedance. If the power amplifiers are close to the preamp/crossover, it is easier.
I just realised I had brain fade when I started this thread, the title should include the word 'Active'.

Anyway, my starting point is that nothing is perfect though BUF-03s are pretty damn good in my experience. We are talking about audio frequencies only up to around 150-180Hz. Cable runs will be around a metre or so.

The preamp I'm building will have a buffered unity gain output using BUF-03s that I know from another project will perform very well so I'm looking at using a simple passive line level LP filter after it and then another similar buffer to drive the bass amplifier. Something like this followed by a buffer:
Capture.JPG
Capture.JPG (51.25 KiB) Viewed 1680 times
I'm conscious I'll also need some means of adjusting the volume level of the bass amplifier to match the gain of the rest of the speaker arrangement.
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Nick
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#4 Re: Simple Active Low Pass Filter/Buffer

Post by Nick »

Just be aware that even if the following stage has a 100k input impedance, that filter will have a 6dB insertion loss.
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Ray P
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#5 Re: Simple Active Low Pass Filter/Buffer

Post by Ray P »

Thanks Nick, I think I have that covered, the full-range preamp/MoFo will have about 20dB of gain and the Chip amp for the bass has 29dB.
Wolfgang
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#6 Re: Simple Active Low Pass Filter/Buffer

Post by Wolfgang »

With miniDSP and measuring equipment the process would be ideally:
1. OB speakers in the final(expected) room position connected to the amps that will be finally used
2. miniDSP connected to the DAC (ideally miniSHARC before PCM DAC with I2s connection, or external unit), first selection of filters for first test run (like 6dB Butterworth)
3. measuring the room response in sweet spot and nearfield response (also later left-right sweet spot difference for final clarity)
4. switching between filters to find the best looking/sounding steepness and type for the situation (for example: Bessel 12dB sounds best in my system but L-R 48dB also sounded very good). The measured response curve shouldn't show any kind of dip at the x-over frequency .
5. fine tuning the miniDSP with EQ settings to get the best sounding (even if not the flattest) response with real music after the decision has been made which filter sounds best
6. rebuilding the EQ settings in REW for the convolution filter in HQ player; can be fine tuned later in REW (.wav export)
7. building the final analog filter
8. measuring the final results with the analog filter (comparing it to the previously measured responses), readjusting the EQ settings if necessary .

On a personal note: The REW is very good but for testing the CARMA (free) software is much easier to use and leads to very good results. The only drawback is that the convolution filter has to be built later by manually setting the parameters in REW which is very easy.
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Ray P
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#7 Re: Simple Active Low Pass Filter/Buffer

Post by Ray P »

That's more or less what I have in mind Wolfgang.
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