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Budget active 3 way crossover - buy, eval, mod
Budget active 3 way crossover - buy, eval, mod
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Old 3rd February 2018, 12:55 PM   #11
DonVK is offline DonVK  Canada
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Quote:
Originally Posted by AndrewT View Post
I am surprised that the Low Pass roll-off is not showing an attenuation limit.
This occurs when the output tap is taken from the opamp output pin.
There is a solution: move the tapping point on the Low Pass filters from the output pin to the input pin.
It looks like they may have done that. Can you confirm?
I just listed the first input stage to make sure I could connect my sound card to it. I still have to map out the actual filter stages and configurations. I suspect the actual filter stages are unity gain.

I'm using a PC soundcard, and the levels were set midway as I just needed to check where the crossover points occurred. The FR graph only shows the top 80db of the curves so you don't see the actual floor. Once I get it setup properly I'll post more graphs related to performance.

Interesting idea about the op-amp tap points. I would have expected to always use the output pin. Thanks. I'll post the schematic(s) after I map it out.
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Old 3rd February 2018, 01:13 PM   #12
DonVK is offline DonVK  Canada
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Quote:
Originally Posted by mt490 View Post
I don't think all those caps are PP types, the light yellow ones are likely a rather common polyester capacitor.
Possibly. If there are, they will likely be replaced with PP at the values I need. I'll find out more detail when I map out the filter blocks and lookup the component type/values.
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Old 3rd February 2018, 02:16 PM   #13
bicefalo is offline bicefalo  Italy
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What about noise ?
Can You mesure it ?
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Old 3rd February 2018, 02:54 PM   #14
DonVK is offline DonVK  Canada
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I can do some limited noise measurements, with a sound card or an oscilloscope. When I get to performance testing, I'll post what I can measure.

I don't have access to a proper spectrum analyzer for noise measurements.
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Old 3rd February 2018, 09:08 PM   #15
DonVK is offline DonVK  Canada
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Default Floorplan and connector map

The silkscreen on the pcb has a number of different terminologies (ie. #1, right, treble, bass, low, etc.) and some are wrong. The midrange output labels are reversed.

So I mapped out the board. I use Channel#1 or Channel#2 for the stream, and [L,M,H) for the frequency range, and (LP, HP) for the filter type.
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File Type: jpg active 3way floorplan reduced.jpg (204.8 KB, 169 views)
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Old 4th February 2018, 07:03 AM   #16
Boden is offline Boden  Netherlands
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Really surprised to find DonVK here, after all his rather sophisticated omni experiments and optimizations.

May I kindly ask you what you have in mind with experimenting with a PCB layout, that clearly only provides for standard, non-optimized textbook transfer function filter sections?

Or is this a curiosity-only driven exercise?

Eelco
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Old 4th February 2018, 02:13 PM   #17
DonVK is offline DonVK  Canada
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Surprised? why? I'm an audio-omnivore . I enjoy listening to those Omnis nearly every day. I learned a lot from that exercise. It's a 2way passive LR4.

The current idea was related to active speakers and I needed a cross over. So I ordered a few items like this 3way for curiosity and evaluation. There's no documentation, so I posted it, in case others were interested. I suspected this 3way design was a "cut and paste" but you never know with this stuff. No promises made, so no promises broken So far I'm happy with it, its a vanilla filter design.

One use for it is performance testing against the passive XO in my Omni. The same amp, same speaker design, but compare XO before amp to XO after amp. The second use is a possible active n-way cylinder that needs multiple driver sizes to create very wide directivity and low diffraction. The speaker EQ and room EQ would be done in s/w in an RPi3B, but I still need multiple outputs for the amps.
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Old 4th February 2018, 04:06 PM   #18
DonVK is offline DonVK  Canada
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Default Crossover filter blocks

The filter blocks are standard LR4, the same form as the original SL ones at Active Filters The derivations and formulas are on that SL site as well. Its a great site.

I've provided the layout and schematic from trace/buzzing out the pcb. These are the filter blocks along the top row of the pcb. The lower row is exactly the same so I won't repeat it. Given the symmetric structures and repeated values it would be easy to adjust the frequencies. Given this board is a field of 2nd order LPs and HPs it would be easy to reconfigure and tap out what you need. All the intermediate filter stages are unity gain, the only gain change occurs at the initial input stage.
Attached Images
File Type: jpg active 3way filter blocks0001 reduced.jpg (223.8 KB, 143 views)
File Type: jpg active 3way filter blocks0002 reduced.jpg (209.6 KB, 141 views)
File Type: jpg active 3way filter blocks0003 reduced.jpg (290.4 KB, 142 views)
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Old 4th February 2018, 06:01 PM   #19
AndrewT is offline AndrewT  Scotland
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You use 33nF 20off, for two channels.
Measure them.
Select the lowest and the highest and pair them off to give ~66nF.
Select the next L + H and pair them. And again till you have 4 pairs, all near 66nF
The middle group of 12 off 33nF are hopefully close enough to being the same, you just insert them where singles are required.

Same for 3n3F

Once you have all those capacitors selected, you don't want to change them.

To change frequency you change the resistors. Since these are 1%, then selection is much less important.
But use DiL sockets on the PCB. Solder all the resistors to DiL sockets/headers. Then plug them in to set the frequency.
Make up a spare set of soldered socketed resistors to change frequency.
I see the Low Pass and the High Pass have the setting resistor arranged in pairs. That suits sockets.
The bandpass is a mix of singles and pairs. Might be better using SiL pins and sockets.
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Last edited by AndrewT; 4th February 2018 at 06:10 PM.
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Old 4th February 2018, 09:37 PM   #20
DonVK is offline DonVK  Canada
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All good suggestions AndrewT. Matching pairs using "select on test" certainly would work and I agree that reprogramming via resistors changes is the preferred route. Its just easier to get variety in resistor values. I'm just not at the point where I know exactly what I want it programmed to.

I plan on doing some performance measurements, now that I know what the circuitry is. In particular noise levels and harmonic distortion.
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