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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 2nd February 2018, 10:04 PM   #1
DonVK is offline DonVK  Canada
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Default Budget active 3 way crossover - buy, eval, mod

I've always seen these active filter / cross over boards for sale from the far east vendors. Most advertise they are Linkwitz-Riley and may even add a data point or two (ie. XO points @300Hz and @3Khz) and maybe the supply voltage (AC15). Everything else is omitted, but the price looks attractive ($40 CDN. Certainly better than me designing a PCB, ordering all the parts, and assembling it.

It's not trendy, using op amps instead of a mini-DSP or an RPI+DAC. However, I have a project in mind, and I wanted to play around with an active filter design. I don't care what the specific XO points are, as I'll probably change them anyways. I wanted a board full of potential (2nd order LPs and HPs, occasionally cascaded.) to start with. So here goes.
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Last edited by DonVK; 2nd February 2018 at 10:06 PM. Reason: price
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Old 2nd February 2018, 10:12 PM   #2
DonVK is offline DonVK  Canada
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Default power supply

I need a power supply. Normally these boards call for a transformer as they have a rectifier, filter, and regulator on board (probably). Unfortunately, transformers are heavy (shipping) and expensive so I'm going to use a small switch mode power supply. I can get a small SMPS for $10 that provides +/-15VDC @0.55A. Given the board has 9 op amps, and they use typically 10mA per op-amp, I should need 100mA approx. So plenty of headroom with this supply.
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Old 2nd February 2018, 10:27 PM   #3
DonVK is offline DonVK  Canada
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Default powering it up

A quick check on the polarity of the power connections, and the outputs (+/-15VDC) of the SMPS, then remove all the op-amps before connecting power to the board.

Hmm,.... no lights come on for the XO board. There is +13.6VDC and -13.2VDC from the boards regulator. It doesn't seem to be regulating very well. Current draw is 80mA. I could live with those voltages, but it should be better.

I trace/buzz out the circuit, and it appears to be a pass transistor with a shunt regulator (not my choice). The series pass transistor is working but the shunt regulator is set at too high a voltage for my +/-15VDC input. Not a problem we'll set it lower to +/-12VDC. I don't have a set of precision resistors to replace the resistors used to set the shunt voltage. So instead I placed a 22K resistor in parallel with R5 and another in parallel with R8 to lower their resistances. Both barnacles are placed on the backside. The shunt is there so I might as well use it.

I now have +/-12.1VDC board supply rails and the lights are on. Current draw is 110mA.
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File Type: jpg active 3way feb 02 20180001 reduced.jpg (207.5 KB, 376 views)
File Type: jpg active 3way feb 02 20180002 reduced.jpg (217.0 KB, 375 views)
File Type: jpg DSCN2374 reduced.jpg (535.0 KB, 371 views)

Last edited by DonVK; 2nd February 2018 at 10:30 PM. Reason: typo
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Old 2nd February 2018, 10:44 PM   #4
DonVK is offline DonVK  Canada
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Default input stage

A quick buzz/trace out of the input stage reveals its DC coupled, singled ended, with some gain, and a noise filter.

There are enough component connection points there to float the input connector and make it into a differential input.
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File Type: jpg active 3way feb 02 20180004 reduced.jpg (175.9 KB, 99 views)
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Old 2nd February 2018, 11:29 PM   #5
Nelson Pass is offline Nelson Pass  United States
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Budget active 3 way crossover - buy, eval, mod
Quote:
Originally Posted by DonVK View Post
I've always seen these active filter / cross over boards for sale from the far east vendors. Most advertise they are Linkwitz-Riley and may even add a data point or two (ie. XO points @300Hz and @3Khz) and maybe the supply voltage (AC15). Everything else is omitted, but the price looks attractive ($40 CDN. Certainly better than me designing a PCB, ordering all the parts, and assembling it.
Looks pretty nice from the photo, what appear to be polypropylenes and
Silmic electrolytics.
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Old 3rd February 2018, 02:50 AM   #6
DonVK is offline DonVK  Canada
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I'm happy with the assembly. Lots of plane fills and yes it uses polys.
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Old 3rd February 2018, 02:54 AM   #7
DonVK is offline DonVK  Canada
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Default Filter cross over points

After assuring myself it wouldn't blow up my sound card. I ran a quick test through REW to plot the FR. No surprise it crosses at 300Hz and 3Khz. It also appears (no surprise) to be an LR4, pass flat with -24db/octave attn.
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Last edited by DonVK; 3rd February 2018 at 02:56 AM. Reason: filter order
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Old 3rd February 2018, 09:36 AM   #8
AndrewT is offline AndrewT  Scotland
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I think you are showing that each filter is using a gain block.
If that is the case then you don't have a unity gain S&K.
Instead you have an equal component value S&K.
This gives a gain of ~+8dB when set up for LR4.
Fortunately ECV filters have separate control of frequency and Q (Damping)
Look up an ECV S&K design app (Ti have a few).
You will find that each (2pole) Butterworth must be set up with Q values that are specific to it's location in the string of filters.
It is worth checking that the PCB uses this correctly.
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Old 3rd February 2018, 09:41 AM   #9
AndrewT is offline AndrewT  Scotland
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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?
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Old 3rd February 2018, 10:53 AM   #10
mt490 is offline mt490
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I don't think all those caps are PP types, the light yellow ones are likely a rather common polyester capacitor.
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