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Old 9th January 2014, 11:47 AM   #1
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Default Designing a noob-preamp (single supply)

Hi everybody,

i want to design a sla battery driven active mono crossover.
i attached a circuit diagram below, i guess you can see from that what i'm going to do.
i want to use sallen-key topology with butterworth characteristic because of the obvious advantages.
single supply is kind of a problem especially with the low-pass.
maybe i should use mfb-design instead?

what i did:

input buffers
- does it work like that with single supply?
- from what i understand i create a virtual gnd in that stage. i needed that when i only did the low-pass. does it make sense like that?

adder
- i already used exactly that adder, but never with single supply

high-pass
- that high-pass usually works with single supply, but i have no idea if it makes sense together with the input buffer (vcc/2 and so on)

low-pass
- ?

other
- is an output-buffer necessary?

here is the diagram.
i tuned the r for 150hz crossover-frequency.
this my first ever attempt with this prog.
the ic are not named correctly and may have done a bunch of other mistake.
but i'm here to learn.
Click the image to open in full size.
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Old 10th January 2014, 06:20 PM   #2
Mooly is offline Mooly  United Kingdom
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You have a few errors.

1) The input buffers are OK.
2) IC1A has the opamp inputs reversed.
3) R11 and R13 need to go to the opamp + inputs.

Instead of a resistive divider for biasing, why not have one stable reference. It doesn't have to be exactly VCC/2 either. Take your supply voltage and divide it by 2, then pick a zener closest to that value. Use the zener plus a resistor and decoupling cap to create a clean and stable reference. Then use a single resistor to each opamp that needs biasing.
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Old 10th January 2014, 06:55 PM   #3
Mooly is offline Mooly  United Kingdom
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Like this. The zener and resistor depend on the total supply. And you need to AC couple the output from the final opamps.
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Old 10th January 2014, 08:08 PM   #4
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thank you very much.
i was a little hasty to upload this, so i even did the s-k-topology wrong.
and i also messed up the values of a few resistors.

your advice with the zener and not directly using vcc/2 is good news to me.
the perfect fit for the rest of the setup would be using three of the 6v sla batteries.
when the are fully charged, their are something around 6.8v and use them until they reach 5.8v or 5.7v volt.

the 100uF cap should be an elko?
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Old 11th January 2014, 01:37 AM   #5
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Yes electrolytics in the power supply are traditional. You can buy 10000 hour service life rated ones at farnell.com or mouser.com, farnell has the hours service life in the selector table. I hate changing e-caps over and over, I've done the ones in my ST70 tube amp 4 times.
I use two 1n5344 8.2 zeners and two resistors (typically 33 ohm) to make a split supply with about even characteristics. The signal ground is the middle of the two zeners. The plus and minus supplies of the op amps are the top and bottom of the two zeners, not the actual supply from the battery/transformer. You don't need three 6v batteries if you can get something like an 18 VDC wall transformer for a battery hedge clipper or race car. I paid $1 for mine at the charity resale shop. I'm using an 18v wall transformer with the two 8.2 v 1n5344's for my disco mixer. I use two capacitors, one across each zener diode. then if my op amps are faster than 5322 I use a ceramic 10 nf cap near each one. Lots of op amp kits like tubelab don't have holes for the electrolytic or ceramic caps.
Please Mooly, I've had a protection circuit up on PA amps thread for a couple of days, you mind taking a look? PV-1.3k vs PV2000? Sorry Buehgemeiste, this has nothing to do with your circuit.
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Last edited by indianajo; 11th January 2014 at 01:43 AM.
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Old 13th January 2014, 09:24 AM   #6
maouna is offline maouna  Greece
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i hope my question isnt a lot out of topic. in order to drive two different amplifiers with the same signal i designed the schematic below..should i add an input filter at the first opamp? and regarding the two buffers,will they be ok like this or it would be better to add some resistors and compensation caps even if they are stated to be unity gain stable?
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Old 13th January 2014, 09:35 AM   #7
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Basic error on the first opamp. No DC bias reference on the non inverting input needs and input resistor to ground the value of which will define Rin)

Input filters are generally regarded as a good thing although actual real world use has to be taken into account. In other words is it in a "noisy" environment and/or is it being fed from a source that could have noise/hash present.

Unity gain buffers are fine as they are... big question is whether or not they do "sound" better running away from unity gain. I don't as yet have a definitive answer to that but its worth a try.
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Old 13th January 2014, 09:54 AM   #8
maouna is offline maouna  Greece
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thank you.will correct the no dc bias reference on the first opamp and i will most probably include an input filter. the input source will be the line out of a laptop. And i will use ne5532
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Old 13th January 2014, 10:07 AM   #9
Mooly is offline Mooly  United Kingdom
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I was going to say definitely use a FET opamp because of its imunity to hf hash (laptops have plenty). And definitely use a filter.
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Old 13th January 2014, 10:16 AM   #10
maouna is offline maouna  Greece
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the problem is that the circuit is for a friend of mine and has a lot of unused ne5532 and we think of using 6 of them for a stereo version. i remember most of your advice you gave me in my other threads.
So... A better improve over the last one. is attached below.. what do you say?....other wise we can use a tl074 and a tl072 to make a stereo version...
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