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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Budget active 3 way crossover - buy, eval, mod
Budget active 3 way crossover - buy, eval, mod
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Old 29th June 2018, 01:12 PM   #31
DonVK is offline DonVK  Canada
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That's correct, this XO is fixed gain, but there are a few other places you could adjust the channel gains. It depends on your system and what's more convenient for you. My master volume control is in the DAC, and I'm using Sure class D amps with "fixed" (preset) gain that is set via dip switch (MF, HF) or a pot (LF).

1) select the amp fixed gain (some Sure class D amps have [25.6, 31.6, 35.1, 37.6 dB] settings
2) use amp variable gain (some Sure amps have a volume control/attenuate board) adjust for the diffs and leave it
3) use amp gain offsets, often in HT amps if you are repurposing one, it allows a delta adjustment per channel
4) put an L-pad on the tweeter
5) EQ the channel gain differences at the signal source (using EQ-APO, or FIR filters), you may end up needing this due to unruly drivers, user FR curve preferences, enclosure or room behaviours.

Sure Electronics AA-AB33184 4x100W TDA7498 Class-D Amplifier Board
AA-AA11117 Digital Volume Control Kit
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Old 29th June 2018, 02:34 PM   #32
keilau is offline keilau  United States
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I have 3 old fashion linear power amplifiers (6 channels). They have no input volume control. The 2-way board I purchased does have LP, HP and master volume control. I will wait a few more days to see if the seller will provide more information.

Another option is this fully assembled crossover box. The seller promises customized crossover frequency.
2 Ways 24dB/Oct Linkwitz-Riley Active Crossover Electrical Frequency Divider | eBay

Last edited by keilau; 29th June 2018 at 02:38 PM.
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Old 29th June 2018, 02:55 PM   #33
DonVK is offline DonVK  Canada
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Usually, boards like the one you ordered have master volume and tone controls. There is not enough info to tell what the extra controls are actually doing.

It's very easy to add a simple 2 resistor signal attenuator (fixed gain) to the HF output of your XO before the amps. Or you could add a pot between the XO and amp input if you wanted it variable attenuation.
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Old 29th June 2018, 03:34 PM   #34
keilau is offline keilau  United States
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The 3 dual ganged potentiometers are marked bass, treble and master on the circuit board. It is possible that they are volume and tone control. No way to tell until the seller send a schematic. The 2 sided board is hard to trace without removing the components.
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Old 18th August 2018, 12:24 PM   #35
DonVK is offline DonVK  Canada
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Default Reprogrammed XO points

I finally got a chance to reprogram this to want I need. Only the resistor values were changed, .... lots of resistors. Luckily you can buy small 1% kits with a limited [1K-47K] value ranges to suite this.

List is per channel, so double this:
[R30,R34,R50,R51,R52,R53]=4K7
[R31,R32]=9K1
[R33,R35,R36,R37,R40,R42]=7K5
[R41,R43]=15K

New XO points [725Hz, 4550Hz] will allow domes midranges and small tweeters.
Attached Images
File Type: jpg Reprogram 3way XO [725Hz, 4550Hz].jpg (109.5 KB, 97 views)
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Old 8th September 2018, 03:33 AM   #36
keilau is offline keilau  United States
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Default Linkwitz-Riley 4th order 2-way

Quote:
Originally Posted by DonVK View Post
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.
I just downloaded the REW. Can you point me to a section/page of the REW user's manual on how to create the line level FR plot you show.

I just finish building a 2-way crossover using the PCB from a seller in China. It sounds fine first connected. It is time for some measurement.
divider-crossover-linkwitz-riley-filter-2200hz-24dboct-class-a.jpg IMG_4753.jpg IMG_4754.jpg
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Old 8th September 2018, 04:10 AM   #37
DonVK is offline DonVK  Canada
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Nice. What is the XO point set to?

First a wording of warning. The XO has gain of 4x (12db) if it's the same base design as mine (looks like it) and its output can go to 7V rms so you need to make very sure the signal level going into the XO is set to a low level. There is gain control on the soundcard output (via PC) as well as in REW (signal level). Start with both on low (-20db) setting and gradually increase to avoid clipping. REW allows you to check input levels. Assuming you are connecting your soundcard's Line Out or headphone (green) to the XO inputs, and the soundcard's Line In (blue) to the XO outputs.

The curves I show are actually 3 measurements. I sweep 20Hz-20Khz connecting the soundcard Line In Left to the XO output I want to test. REW requires both the XO's output (connected to line in left) and its own test signal Line out right (connected to line in right) to make the measurement. Set REW to output the test signal on both Line Out L&R, reference Line In right, measure Line In left.

First configure your settings via "preferences" section and then make the measurements via "making measurement" section of the help file (manual).
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Old 8th September 2018, 04:22 AM   #38
keilau is offline keilau  United States
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The filter is text book Linkwitz-Riley 4th order set at 100 Hz for subwoofer. The seller did the crossover frequency mod for me and used the correct R and C values that Linkwitz's calculation show. For my board, I have R=11.3K and C=100nF, with corresponding 2R and 2C. Component quality and construction look pretty good for me.

I drive the LR4 with a Harmon Kardon AP2500 pre-out.
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