Muff's Modules & More

cleaninglady

I have a friend who needs the output of his Arp 4075 filter module boosted so i am doing this for him on perf board.

I'm building with R1 and R2 @ 10K for both resistors to give 2 x gain.

I'm also putting 10uF electros on the input and output as suggest by someone else. (not pictured...)

Have tested it on breadboard. 2V in gave me 4V, so it obviously works.

I'd just like anyone who is familiar with this to give it the once over and advise me if you think it needs changing for this particular application.

Does this seem ok ? Does anyone have anything else to add ?

cheers thumbs up

stk · Posted: Sat Jun 30, 2012 8:33 am Post subject:

I don't know a lot about these things, but isn't it better to use both chans of the tl72, one as an inverting amp, the other to revert the signal?

Please somebody learn me better if i'm wrong (but do tell me why)!

mOBiTh · Posted: Sat Jun 30, 2012 8:48 am Post subject:

I think you can get better stability using this latter method, especially at high frequencies?

I'm still learning too but I'd go for a standard buffer amp configuration with the gain set to two.

the simple virtual earth schem at the bottom of this page demonstated it:

http://www.all-electric.com/schematic/simp_mix.htm

not sure whether you'd need feedback caps in your situation?

we need an expert hihi

ringstone · Posted: Sat Jun 30, 2012 8:56 am Post subject:

BananaPlug · Posted: Sat Jun 30, 2012 10:17 am Post subject:

Stick a 1K resistor on the output so that if you have a patch cord flailing around and it hits some grounded metal you don't draw enough current from the op amp to kill it. 1K is plenty.

L-1 · Posted: Sat Jun 30, 2012 10:37 am Post subject:

Maybe easer to modify a bit the module than to add second board.

Dave Kendall · Posted: Sat Jun 30, 2012 10:39 am Post subject:

I'm *not* an expert, but some rules of thumb I've picked up that may be useful...

1/ A single non-inverting amp is very slightly lower noise than dual inverting. (One reason why pre-amps often have non-inverting amp stages in the input signal path)
2/ Non-inverting amps can't have a gain less than one.
3/ An inverting amp allows mixing inputs (summing) without the different inputs interacting and affecting each other.
4/ The maths for working out gain is more straightforward for inverting amps/mixers.
5/ TL07x usually don't need a capacitor in the feedback path to be stable (to prevent HF oscillation in certain situations), whereas older OPAs like 741s and LF353s and so on often do.
6/ If an OPA output is going to the outside world, it's worth adding a series resistor on the output, or in the feedback path between the OPamp's output and the output jack. 1K used to be the standard for this, but 330R seems to be pretty common now. IIRC there was an [SDIY] thread where Dr Sketch-and Etch calculated 220R as the optimum minimum value for the series resistor that would protect the OPA's output in the advent of an output-to-output short.

Any corrections/additions/clarification to the above from an OPA expert are welcome
Hope it helps a bit smile

cheers,
Dave

megaohm · Posted: Sat Jun 30, 2012 11:16 am Post subject:

daverj · Posted: Sat Jun 30, 2012 2:07 pm Post subject:

I believe the Arp 4075 already has a capacitor on it's output. So you just need a resistor to ground on your input (maybe 20K). No need for a capacitor on your output either since the output will be centered around ground. But a 330 ohm to 1K resistor in series with the output is not a bad idea.

Keep the resistors close to the chip and you shouldn't need a feedback cap.

Don't forget to add a pair of .1uf caps on the power supply pins of the chip (right up at the chip). Put a pair of 10uf to 22uf caps on the power rails nearby. They don't need to be as close to the chip as the .1's

If the second half of the op amp isn't used, on that half tie the output to the minus input and tie the plus input to ground.

mOBiTh · Posted: Sat Jun 30, 2012 2:45 pm Post subject:

ooh, lots of good tips in this thread smile

@cleaninglady: have you looked at the filter output on a scope with various level inputs to see what it actually kicks out? this might be interesting I think

cleaninglady · Posted: Sat Jun 30, 2012 6:16 pm Post subject:

Thanks for all the replies ! I'm really new to this stuff.
Even though i have build many modules and BoCGS panels , anything of this nature is new territory for me.

It's a lot of info to try and digest all at once but i'm trying... hmmm.....

I don't have a program for drawing circuit diagrams so i'm just taking examples from the net.

I'm attaching a project notes file that shows the schematic.

L-1 , if this could be done without using a daughterboard (by just changing some resistors or something) , i'd really like to know how.

mOBiTh , i don't have the module with me yet , so i hasn't been up on a scope.

daverj , would it be possible you could upload a drawing of what you mentioned ?

megaohm , in fig 2 of the link you sent me would i be using 10K for R1 if both my other resistors are 10K ?

Dave Kendall , would i just better avoiding the TL072 and going for a TL071 instead ? (With suggested circuits herein...)

stk , BananaPlug , ringstone many thanks for the advice.

daverj · Posted: Sat Jun 30, 2012 7:43 pm Post subject:

megaohm · Posted: Sat Jun 30, 2012 7:55 pm Post subject:

cleaninglady · Posted: Sat Jun 30, 2012 8:02 pm Post subject:

cleaninglady · Posted: Sat Jun 30, 2012 8:08 pm Post subject:

Thanks for that Dave ! I'll try to make it.

Must...make...brain...work. Dead Banana

cleaninglady · Posted: Sun Jul 01, 2012 1:46 am Post subject:

So...i have built up Dave's circuit above on Breadboard and when i feed it 2V from a source it gives me a bit over 4V on the output.

If i am trying to double the output volume of the Filter module is this what i'd be after ?

I assume so... hmmm.....

daverj · Posted: Sun Jul 01, 2012 3:44 am Post subject:

Feed it multiple voltages. If 2v in gives 4v out, and -2v in gives -4v out, then that's 2 times gain. With only one measurement you don't know if you are looking at gain or bias change.

cleaninglady · Posted: Sun Jul 01, 2012 3:51 am Post subject:

Cheers Dave

I'd like to try that but i taking voltages from a 9V battery and 100K pot setup. Maybe i could send it something from a module with a polarising output ?

L-1 · Posted: Sun Jul 01, 2012 6:06 am Post subject:

I would change 360K at input to something smaller and increase C2 in this case.

daverj · Posted: Sun Jul 01, 2012 3:18 pm Post subject:

Is the circuit only being powered by the 9v battery? Or do you have bipolar power supplies? If using a single 9v battery as the power, the circuit needs to be different.

cleaninglady · Posted: Sun Jul 01, 2012 5:33 pm Post subject:

On the breadboard , I'm using my 15V bipolar power supply (TL072 pins 4 and 8) and the 9V battery to simulate an output signal from the Arp 4075 filter (2V at TL072 pin 3). I'm doing it this way because i don't have to module i'm fixing yet and want to have it right before i start on it.

daverj · Posted: Sun Jul 01, 2012 9:15 pm Post subject:

Well, in that case, turn the battery around and you'll have a negative voltage. Then you can feed -2v in and see if you get -4v out to match the +2v in and +4v out.

Or hook the pot between +15 and -15 and then you can get any voltage between the two.

cleaninglady · Posted: Mon Jul 02, 2012 2:34 am Post subject:

that all checks out , pos and neg.
going to build it up on perf board now.

thanks all for the help. we're not worthy