Thanks Ouroboros.Ouroboros said:
The URL is:
http://www.national.com/an/AN/AN-346.pdf
(written when I was just four years old!)
Ok, looks like I've managed to secure a lift to Bardwell's 
I will buy:
- some TL071
- 'turned pin' type, dual op-amp socket
- 15-20K linear dual gang (so I can vary the x-over fr.)
- 9v+ regulator (seems about right for my rail voltage)
- 9v- regulator
- LEDs
- diodes
Then I can hopefully do a 'proper' job and finish this nicely.
I will buy:
- some TL071
- 'turned pin' type, dual op-amp socket
- 15-20K linear dual gang (so I can vary the x-over fr.)
- 9v+ regulator (seems about right for my rail voltage)
- 9v- regulator
- LEDs
- diodes
Then I can hopefully do a 'proper' job and finish this nicely.
IMO get a 10k linear dual pot as then with 10k fixed resistors you will get an octave of adjustment.
I very much doubt you will pick up any 9V regs. As your secondary is only 9V you will not have enough voltage to run common 12V regs, so you will have to make do with an unregulated supply. This is no bad thing as in this application there will be no noticeable difference.
I very much doubt you will pick up any 9V regs. As your secondary is only 9V you will not have enough voltage to run common 12V regs, so you will have to make do with an unregulated supply. This is no bad thing as in this application there will be no noticeable difference.
Hi, I'm back!
I got 7808 and 7908, so +/- 8v regs.
TL071 x4
socket x1 (no turned pin in stock, so I thought I'd just try one)
Some small bridges
50k lin pot (they didn't have anything smaller that was suitable, so there won't be much useful range on it)
+ other fun parts
Just playing with the socket in the car, I realise why you recommend the nicer type - trying to remove an op-amp once in I pierced a finger tip with the socket.
And then using a pcb screw-down connector I tore through some skin on another finger.
Why is electronics so dangerous???
I got 7808 and 7908, so +/- 8v regs.
TL071 x4
socket x1 (no turned pin in stock, so I thought I'd just try one)
Some small bridges
50k lin pot (they didn't have anything smaller that was suitable, so there won't be much useful range on it)
+ other fun parts
Just playing with the socket in the car, I realise why you recommend the nicer type - trying to remove an op-amp once in I pierced a finger tip with the socket.
And then using a pcb screw-down connector I tore through some skin on another finger.
Why is electronics so dangerous???
Cool, those regs sound ideal even though I've never heard of 78xx/79xx in those voltages.
Actually, with a 50k pot you will have a massive range if you stick with your 10k resistors for the filter, 6x frequency span in fact
You could use 47k resistors to regain a more usable 2x span, but you would need to reduce the caps proportionally.
Removing the chips from the sockets is no harder with wiper sockets, it's getting them in that's easier with turned pin. You will find it many times easier to remove the chip once you have soldered the socket in the board You are supposed to put the chip in after you have soldered the socket
Actually, with a 50k pot you will have a massive range if you stick with your 10k resistors for the filter, 6x frequency span in fact
You could use 47k resistors to regain a more usable 2x span, but you would need to reduce the caps proportionally.Removing the chips from the sockets is no harder with wiper sockets, it's getting them in that's easier with turned pin. You will find it many times easier to remove the chip once you have soldered the socket in the board
You are supposed to put the chip in after you have soldered the socket
Well, I asked for 9v, which they didn't have, but the man said he had 8v, so I'm happy. My unregulated DC is actually over 13v, so maybe 12v regs would work, but I'm sure it's very unadvisable to cut it so fine.
Re. the pot: I feel I am only likely to need the x-over to work at a range of 40-55hz. I can't see the optimum being either side of that. But for the sake of fun, and possible future changes, it makes sense to have it quite variable.
I cannot picture what to do if using resistors as well as the pot. My plan was to just solder in the pot, and adjust it in the range of 8-12k or there-abouts, wired as a variable resistor. I would tune it by ear, and mark on my enclosure where it should be positioned.
"You are supposed to put the chip in after you have soldered the socket"
Yes, yes, very good, but I'm a fiddler, I can't help but mess about with stuff!
So now I might have pin-damage-scarring to add to the soldering iron burns on my hands..
Re. the pot: I feel I am only likely to need the x-over to work at a range of 40-55hz. I can't see the optimum being either side of that. But for the sake of fun, and possible future changes, it makes sense to have it quite variable.
I cannot picture what to do if using resistors as well as the pot. My plan was to just solder in the pot, and adjust it in the range of 8-12k or there-abouts, wired as a variable resistor. I would tune it by ear, and mark on my enclosure where it should be positioned.
"You are supposed to put the chip in after you have soldered the socket
"Yes, yes, very good, but I'm a fiddler, I can't help but mess about with stuff!
So now I might have pin-damage-scarring to add to the soldering iron burns on my hands..
SimontY said:
40-55 is quite a narrow band and IMO you will struggle quite badly to achieve that adjustment with that pot alone and your existing caps as it will be too coarse.
All you do is put one 47k resistor in series with each pot track. This also avoids you ever going down to zero ohms and possibly damaging something. The downside is that you need smaller caps to achieve the same frequency, so back to Bardwells unless you have something suitable in stock
I realise just using the pot alone would be difficult, so I'll try to avoid that, and follow your advice.richie00boy said:40-55 is quite a narrow band and IMO you will struggle quite badly to achieve that adjustment with that pot alone and your existing caps as it will be too coarse.
All you do is put one 47k resistor in series with each pot track. This also avoids you ever going down to zero ohms and possibly damaging something. The downside is that you need smaller caps to achieve the same frequency, so back to Bardwells unless you have something suitable in stock
I love buying and salvaging capacitors, infact it's becoming a hobby in itself (sad or what?!), so I'm sure I've got something suitable
Just so happens I couldn't resist the £1 bag of mixed caps (mostly film types, some 'lytic, some ceramic too) when I was in Bardwell's...And I have ~200 82nF polyesters which might do the trick nicely (not all at once!). Also, they're much smaller than the huge 330nF ones I used, which did get in the way a bit.
I think you must have got something a little mixed up. 10k fixed and 50k pot will give you and adjustable range of 6x ([10 + 50]/10) the lowest frequency. You claim 10x.
In your calculator, decide what you want the lowest frequency to be, I'll offer 30Hz. In the resistor box enter 60k as this is what you will get with a 10k fixed + 50k pot. Now see what cap it tells you to use. Your max frequency will be 30 x 6 = 180Hz and at this point the pot will be set to zero ohms, leaving just 10k of resistance.
In your calculator, decide what you want the lowest frequency to be, I'll offer 30Hz. In the resistor box enter 60k as this is what you will get with a 10k fixed + 50k pot. Now see what cap it tells you to use. Your max frequency will be 30 x 6 = 180Hz and at this point the pot will be set to zero ohms, leaving just 10k of resistance.
Hmm, it's possible I made a mistake. I can only choose the cap value and fr. in WINISD, not the resistor value.
Looks like I cocked something up
Putting in 0.08uF as the cap value and 33hz as x-over fr. gives 60k resistors, and 195hz gives just over 10k. This seems to fit better with your theory.
Looks like I cocked something up
Putting in 0.08uF as the cap value and 33hz as x-over fr. gives 60k resistors, and 195hz gives just over 10k. This seems to fit better with your theory.
SimontY said:Hmm, it's possible I made a mistake. I can only choose the cap value and fr. in WINISD, not the resistor value.
Looks like I cocked something up
Putting in 0.08uF as the cap value and 33hz as x-over fr. gives 60k resistors, and 195hz gives just over 10k. This seems to fit better with your theory.
Have a look at the 2nd-order Low Pass Filter Designer on my utilities page, you may find it some use.
I built a new one using TL071 and IT WORKS!! 
Thanks for helping everyone
I will take a picture if anyone is curious to see what the little blighter looks like, but I'm sure you've all seen a crossover before
I still don't know why the other one didn't work, but I don't care anymore.
I just have this running directly off a cdp, then into the sub. The pot control works beautifully, and the difference between 12 and 24 db/oct is staggering playing the sub solo - gets rid of voices etc. leaking through - just pure (as pure as a Tempest can sound) low bass!!
Next step is to try it coming off my passive pre/pot that feeds my chip-amp....
Thanks for helping everyone
I will take a picture if anyone is curious to see what the little blighter looks like, but I'm sure you've all seen a crossover before
I still don't know why the other one didn't work, but I don't care anymore.
I just have this running directly off a cdp, then into the sub. The pot control works beautifully, and the difference between 12 and 24 db/oct is staggering playing the sub solo - gets rid of voices etc. leaking through - just pure (as pure as a Tempest can sound) low bass!!
Next step is to try it coming off my passive pre/pot that feeds my chip-amp....
OOPS! I smell danger... 
Running the circuit (as shown on the pic/schematic you posted earlier) directly from the pot will cause problems. In effect you will just create a series connection with your LP filter potentiometer and the volume pot, causing an unknown resistance.
You will not attenuate correrctly, but move the cross-cover frequency!
I would strongly suggest a buffer (x1 gain) between the volume pot and your filter circuit.
Jennice
Running the circuit (as shown on the pic/schematic you posted earlier) directly from the pot will cause problems. In effect you will just create a series connection with your LP filter potentiometer and the volume pot, causing an unknown resistance.
You will not attenuate correrctly, but move the cross-cover frequency!
I would strongly suggest a buffer (x1 gain) between the volume pot and your filter circuit.
Jennice
Richie,
As I understand, Simon used a potentiometer to adjust the cut-off frequency of his filter. If I recall vorrectly, the input stage of this filter is a RC filter itself (of which he adjusts the R).
As long as his CD player (source) has low output impedance, there's no problem, and the cut-off frequency is well defined.
Now comes the part that scares me. Simon wrote:
"Next step is to try it coming off my passive pre/pot that feeds my chip-amp....".
If I understand the situation right, he previously used his CD player firectly as a source. Now he wants to use his "passive pre/pot" in between. Being a passive preamp, basically a source selector and a potentiometer (If my guess is right), there will now be a variable output impedance of the "source" that drives the filter. Effectively, Simon just puts two potentiometers in series with his filter capacitor. This doesn't cause attenuation, but an undefined cut-off frequency for his filter. The only way this will work predictable is by inserting a buffer, effectively making it an active preamp with low output impedance
Someone please correct me if I got it all wrong
Jennice.
As I understand, Simon used a potentiometer to adjust the cut-off frequency of his filter. If I recall vorrectly, the input stage of this filter is a RC filter itself (of which he adjusts the R).
As long as his CD player (source) has low output impedance, there's no problem, and the cut-off frequency is well defined.
Now comes the part that scares me. Simon wrote:
"Next step is to try it coming off my passive pre/pot that feeds my chip-amp....".
If I understand the situation right, he previously used his CD player firectly as a source. Now he wants to use his "passive pre/pot" in between. Being a passive preamp, basically a source selector and a potentiometer (If my guess is right), there will now be a variable output impedance of the "source" that drives the filter. Effectively, Simon just puts two potentiometers in series with his filter capacitor.
This doesn't cause attenuation, but an undefined cut-off frequency for his filter. The only way this will work predictable is by inserting a buffer, effectively making it an active preamp with low output impedanceSomeone please correct me if I got it all wrong
Jennice.
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