marchel said:
The input impedance to the amplifier may be constant but it may also change with volume setting, this has to be checked. Not to mention that a first order filter does very little to protect small speakers from deep bass, second order with adjustable Q gives much more chances of success.
You should not fear op-amps, an op-amp doing an useful filtering/EQ job always sounds better than no op-amp (and thus no useful task done
)"The input impedance to the amplifier may be constant but it may also change with volume setting,"
True.
cowanrg ,
You should check the input of the power amp, to see if the impedance changes by turning the volume control when connected, If so , this setup will not work for you.
The only place you'll hear the inferiority of the SQ of opamps, Is in a expensive and revealing system in a properly acoustically treated room, Using critical ears, Other wise, The degradation intruduced by opamps are inaudible 99% of the time.
So if you think your system is not reavealing enough or not dear enough, Using commercially available xo should be just fine.
True.
cowanrg ,
You should check the input of the power amp, to see if the impedance changes by turning the volume control when connected, If so , this setup will not work for you.
The only place you'll hear the inferiority of the SQ of opamps, Is in a expensive and revealing system in a properly acoustically treated room, Using critical ears, Other wise, The degradation intruduced by opamps are inaudible 99% of the time.
So if you think your system is not reavealing enough or not dear enough, Using commercially available xo should be just fine.
OTOH, THe cutoff freq. of the passive hi-pass line level filter should not change, regardless of the volume control's position before it, cause it is connected in series with the amp's input after it.
only a lowpass passive line level filter will be affected by the position of the potentiometer connected before it, cause it is connected in parrallel, So it will take the combined impedance of both the amp's input and the volume control.
So, cowanrg, I think it will work for you.
only a lowpass passive line level filter will be affected by the position of the potentiometer connected before it, cause it is connected in parrallel, So it will take the combined impedance of both the amp's input and the volume control.
So, cowanrg, I think it will work for you.
actually, for what it's worth, this is how the welborne labs active crossover works. their low-pass is a traditional op-amp based circuit, but the high pass is simply a cap in line with the input.
http://www.welbornelabs.com/acm.htm
http://www.welbornelabs.com/acm.htm
Maybe this can help. I built an active x-over based on the OPA2134 OpAmps for a 2.1 system. Complete with power supply this cost me about 55$ Canadian and it is dead quiet with no hum at all. It uses LR4 slopes. As you can see in the pictures I built it on a Bread-board. The board was so inexpensive I decided to keep it on the board permanently and avoid making a PCB for it. This leaves me the option to change the x-over frequency in the future.
http://www.htguide.com/forum/showpost.php4?p=376458&postcount=27
http://www.htguide.com/forum/showpost.php4?p=376805&postcount=30
Complete thread: http://www.htguide.com/forum/showthread.php4?t=28300&highlight=active+2.1
Hope it helps,
Peter
http://www.htguide.com/forum/showpost.php4?p=376458&postcount=27
http://www.htguide.com/forum/showpost.php4?p=376805&postcount=30
Complete thread: http://www.htguide.com/forum/showthread.php4?t=28300&highlight=active+2.1
Hope it helps,
Peter
its not so much a hum or a hiss, but that low level noise that you would get if you turn up your amps to full blast and listen to the tweeters. however, when the gain of the marchand is at around half (~0db gain) i can hear it a few feet away. in my main system, i can hear it almost back at the listening position (8ft away).
there is no ground hum or anything. when i short out the inputs, it doesn't change. and when i disconnect the input, it stays the same as well. it's being generated by the unit, and is definitely NOT a humming.
phil marchand told me that it's most likely the opamps im using (LM4562's). he suggested the OPA2134's or whatever. im going to try those. who knows.
there is no ground hum or anything. when i short out the inputs, it doesn't change. and when i disconnect the input, it stays the same as well. it's being generated by the unit, and is definitely NOT a humming.
phil marchand told me that it's most likely the opamps im using (LM4562's). he suggested the OPA2134's or whatever. im going to try those. who knows.
ah ok. i have a few of them showing up this week, and i'll plop then in and see.
my system (without this crossover) is dead quiet. literally. i can turn the gain up to 0db, and put my ear up to a 5' tall ribbon and NOT hear anything unless its 3am and there is no ambient sounds. if the fridge is off, the heating/cooling isnt going, and there is no wind, i can barely hear a small hiss. but other than that, i can't hear anything. and with the crossover in there, i can easily hear something from 8ft back.
so, its WAAAAAY too noisy. hopefully these other opamps fix that.
my system (without this crossover) is dead quiet. literally. i can turn the gain up to 0db, and put my ear up to a 5' tall ribbon and NOT hear anything unless its 3am and there is no ambient sounds. if the fridge is off, the heating/cooling isnt going, and there is no wind, i can barely hear a small hiss. but other than that, i can't hear anything. and with the crossover in there, i can easily hear something from 8ft back.
so, its WAAAAAY too noisy. hopefully these other opamps fix that.
If you can manage it: bypass caps for all the V+ and V- inputs on the OPA2134. 10nF ceramics is what I used and kept the distance between chip and cap as short as possible. Some people have use 100nF with success. The spec sheet for the OPA2134 mentions a minimum of 10nF so you have a pretty wide selection. Another thing that comes to mind is the power supply feeding your OpAmps. Could it be the source? Here is what I used: http://sound.westhost.com/project05.htm
This guy also sells PCBs and parts for a newer version which is supposed to be even quieter: http://sound.westhost.com/project05b.htm
REF: http://dkc1.digikey.com/Redirect.aspx?url=http://focus.ti.com/lit/ds/symlink/opa134.pdf
Let us know,
Peter
This guy also sells PCBs and parts for a newer version which is supposed to be even quieter: http://sound.westhost.com/project05b.htm
REF: http://dkc1.digikey.com/Redirect.aspx?url=http://focus.ti.com/lit/ds/symlink/opa134.pdf
Let us know,
Peter
Let us know... in the mean time: Eh... D'oh!
I think I need to correct my self. My OPA2134 x-over is not as quiet as I originally stated. The high pass output has some hum to it but it does almost completely disappears when I connect something (any source) to the input of the x-over... It is still noticeable but only if I stand within 8 feet of my speakers, directly in front and with the house dead quiet... heating system off, PC fan off and no traffic on the street... Since the low pass doesn't exhibit the same hum, there is a chance it might not be the OPA2134 but rather the input cable or source.
Are we ready for balanced inputs????
I think I need to correct my self. My OPA2134 x-over is not as quiet as I originally stated. The high pass output has some hum to it but it does almost completely disappears when I connect something (any source) to the input of the x-over... It is still noticeable but only if I stand within 8 feet of my speakers, directly in front and with the house dead quiet... heating system off, PC fan off and no traffic on the street... Since the low pass doesn't exhibit the same hum, there is a chance it might not be the OPA2134 but rather the input cable or source.
Are we ready for balanced inputs????
hum, that doesn't make me too happy. that is a significant jump in noise for my system. i never turn anything off, and one of my speakers is right in a pathway. even when walking by it late at night with gain at -10dB (from watching a movie the night before), you can't hear anything.
so, i would hate to go backwards in terms of noise.
so, i would hate to go backwards in terms of noise.
Ok I've narrowed down the source of the noise...
If I connect my x-over's high-pass output to the amp... and disconnect everything from the OPA2134's input I get a big improvement. By everything I mean I disconnect the connection that feed the low-pass input, I also disconnect the shielded wire that brings in the input signal that feed the entire x-over... It is now almost dead quiet! What ever is left I think can be attributed to the PSU because the Low-pass can filter it out, so the OPA2134 shouldn't be the source.
As for solutions for me, I guess I should try with a better shielded input cable. The one I used was a crappy super thin shielded that probably has the wrong impedance/capacitance for it. Same would apply to the connection to feed the low pass as it was creating hum in the high-pass by picking up noise from somewhere.
I will report back when I get a chance to get a new cable.
Peter
EDIT: What I describe above reduces the hum to make it noticeable only at 5 feet, directly in front of the speaker with volume at MAX.
If I connect my x-over's high-pass output to the amp... and disconnect everything from the OPA2134's input I get a big improvement. By everything I mean I disconnect the connection that feed the low-pass input, I also disconnect the shielded wire that brings in the input signal that feed the entire x-over... It is now almost dead quiet! What ever is left I think can be attributed to the PSU because the Low-pass can filter it out, so the OPA2134 shouldn't be the source.
As for solutions for me, I guess I should try with a better shielded input cable. The one I used was a crappy super thin shielded that probably has the wrong impedance/capacitance for it. Same would apply to the connection to feed the low pass as it was creating hum in the high-pass by picking up noise from somewhere.
I will report back when I get a chance to get a new cable.
Peter
EDIT: What I describe above reduces the hum to make it noticeable only at 5 feet, directly in front of the speaker with volume at MAX.
Double D'oh,
never post while testing and playing around with a DiY circuit! Things change quickly and useless post can be generated.
All I had was a loose ground jumper. I think my jumper wires are just too thin and the bread-board connections are week. I can safely say that this thing is dead quiet!!! Even with my existing low quality RCA connectors, needless to say I am very happy. The OPA2134 will not be a problem. Let us know how yours turns out.
Regards,
Peter
never post while testing and playing around with a DiY circuit! Things change quickly and useless post can be generated.
All I had was a loose ground jumper. I think my jumper wires are just too thin and the bread-board connections are week. I can safely say that this thing is dead quiet!!! Even with my existing low quality RCA connectors, needless to say I am very happy. The OPA2134 will not be a problem. Let us know how yours turns out.
Regards,
Peter
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