Help With Buffer Amplifier/Filter Type Circuit

Thread Starter

Frank Spigner

Joined Mar 17, 2018
1
I’m having trouble with a circuit I designed and I would be greatly appreciative if I can get some help here.

The circuit I designed is a buffer amplifier type design intended to take two channels of audio input from a passive monitor controller, sent to either the resistor circuit, or low pass filter (as seen in the schematic), and then to the second opamp (which has a trimpot to control DC offset for each channel), and then sent as an audio output at unity gain (or near unity gain). The circuit is powered with +/-15VDC, taking a 36 VDC power supply and sent through a virtual ground type divider and regulated with 7815 and 7915 voltage regulators (although I’ve tried disconnecting the power section and using a bench power supply setup to provide + and – voltages with the same problems).

I designed the circuit using the falstad applet and I’ll attach the text for the circuit so you can see for yourself how I imagined it would work, and I’m attaching images of my schematic, screenshots of my oscilloscope, and images of my soldered perfboard circuit.

When I try to input a 120Hz, +/- 2V sine wave with my function generator, all I get from the output is a mostly flat voltage. It wiggles and shows occasional noise and whatnot when I try tuning the trimpot, but what you see in my image is basically the gist of it. In addition, the opamps (OPA2604APs) get a little warm, which I don’t think is a huge problem, but the 7815 gets pretty hot.

https://imgur.com/a/Q97WvE3


The following are URLs for the relevant datasheets:

media.digikey.com/pdf/Data%20Sheets/Texas%20Instruments%20PDFs/OPA2604.pdf

hep.upenn.edu/SNO/daq/parts/lm7815.pdf

hep.upenn.edu/SNO/daq/parts/lm7915.pdf

vishay.com/docs/28193/mkt373.pdf

globalsources.com/gsol/I/Slide-switch/p/sm/1142899729.htm#1142899729




Falstad circuit simulation (copy the following and select "import from text"):



$ 1 0.000015625 4.252108200006279 68 5 50

a 448 16 592 16 8 12 -12 1000000 0.000026274481766352875 0 100000

a 1152 32 1296 32 8 12 -12 1000000 -0.000012183066700374307 0 100000

r 448 -48 592 -48 0 10000

w 336 0 448 0 0

w 448 -48 448 0 0

w 592 -48 592 16 0

w 592 16 640 16 0

S 640 16 720 16 0 0 false 0 2

r 720 80 784 80 0 5600

c 784 80 784 144 0 2.2e-7 4.849103899492971e-81

g 784 144 784 176 0

w 848 80 928 80 0

w 976 80 976 32 0

w 976 -64 976 0 0

r 1024 16 1152 16 0 4700

w 1296 32 1376 32 0

w 1152 48 1152 128 0

r 1152 128 1152 192 0 1000

g 1152 192 1152 208 0

r 1152 128 1088 128 0 4700

174 1056 96 1088 160 1 220 0.5 Resistance

R 1056 96 1056 64 0 0 40 15 0 0 0.5

R 1056 160 1056 192 0 0 40 -15 0 0 0.5

R 240 0 176 0 0 1 220 2 0 0 0.5

g 416 144 416 176 0

w 720 0 720 -64 0

w 720 32 720 80 0

w 912 -64 976 -64 0

r 448 32 448 144 0 10000

g 848 144 848 176 0

c 848 80 848 144 0 2.2e-7 4.618125257029856e-81

r 784 80 848 80 0 13700

S 1024 16 976 16 0 1 false 0 2

r 448 -80 592 -80 0 10000

w 448 -48 448 -80 0

w 592 -80 592 -48 0

r 384 32 384 144 0 10000

w 416 144 448 144 0

w 384 32 448 32 0

w 384 144 416 144 0

w 1296 -64 1296 -32 0

w 1152 -32 1152 -64 0

w 1296 -32 1296 32 0

w 1152 -32 1152 16 0

r 912 -64 912 0 0 5100

g 912 0 912 32 0

r 720 -64 816 -64 0 10000

r 976 80 976 144 0 100000

g 976 208 976 240 0

g 928 176 928 208 0

r 928 80 928 176 0 470000

w 928 80 976 80 0

r 1152 -64 1216 -64 0 10000

r 816 -64 912 -64 0 10000

r 1216 -64 1296 -64 0 10000

r 976 144 976 208 0 100000

r 1504 96 1504 176 0 20000

w 1376 -32 1376 32 0

w 1376 32 1376 96 0

w 1376 -32 1504 -32 0

w 336 16 336 32 0

w 336 48 336 32 0

w 240 48 240 32 0

w 240 16 240 32 0

r 336 16 240 16 0 10000

r 336 48 240 48 0 10000

r 336 -16 240 -16 0 10000

r 336 -48 240 -48 0 10000

w 240 -48 240 -32 0

w 240 -16 240 -32 0

w 336 -16 336 -32 0

w 336 -48 336 -32 0

w 240 -16 240 0 0

w 240 16 240 0 0

w 336 -16 336 0 0

w 336 16 336 0 0

g 1504 176 1504 208 0

g 1504 32 1504 64 0

r 1504 -32 1504 32 0 20000

w 1376 96 1504 96 0

w 1376 448 1504 448 0

r 1504 320 1504 384 0 20000

g 1504 384 1504 416 0

g 1504 528 1504 560 0

w 336 368 336 352 0

w 336 336 336 352 0

w 240 368 240 352 0

w 240 336 240 352 0

w 336 304 336 320 0

w 336 336 336 320 0

w 240 336 240 320 0

w 240 304 240 320 0

r 336 304 240 304 0 10000

r 336 336 240 336 0 10000

r 336 400 240 400 0 10000

r 336 368 240 368 0 10000

w 240 368 240 384 0

w 240 400 240 384 0

w 336 400 336 384 0

w 336 368 336 384 0

w 1376 320 1504 320 0

w 1376 384 1376 448 0

w 1376 320 1376 384 0

r 1504 448 1504 528 0 20000

r 976 496 976 560 0 100000

r 1216 288 1296 288 0 10000

r 816 288 912 288 0 10000

r 1152 288 1216 288 0 10000

w 928 432 976 432 0

r 928 432 928 528 0 470000

g 928 528 928 560 0

g 976 560 976 592 0

r 976 432 976 496 0 100000

r 720 288 816 288 0 10000

g 912 352 912 384 0

r 912 288 912 352 0 5100

w 1152 320 1152 368 0

w 1296 320 1296 384 0

w 1152 320 1152 288 0

w 1296 288 1296 320 0

w 384 496 416 496 0

w 384 384 448 384 0

w 416 496 448 496 0

r 384 384 384 496 0 10000

w 592 272 592 304 0

w 448 304 448 272 0

r 448 272 592 272 0 10000

S 1024 368 976 368 0 1 false 0 2

r 784 432 848 432 0 13700

c 848 432 848 496 0 2.2e-7 4.976304002642837e-81

g 848 496 848 528 0

r 448 384 448 496 0 10000

w 912 288 976 288 0

w 720 384 720 432 0

w 720 352 720 288 0

g 416 496 416 528 0

R 240 352 176 352 0 1 220 2 0 0 0.5

R 1056 512 1056 544 0 0 40 -15 0 0 0.5

R 1056 448 1056 416 0 0 40 15 0 0 0.5

174 1056 448 1088 512 1 220 0.5 Resistance

r 1152 480 1088 480 0 4700

g 1152 544 1152 560 0

r 1152 480 1152 544 0 1000

w 1152 400 1152 480 0

w 1296 384 1376 384 0

r 1024 368 1152 368 0 4700

w 976 288 976 352 0

w 976 432 976 384 0

w 848 432 928 432 0

g 784 496 784 528 0

c 784 432 784 496 0 2.2e-7 5.225197196101461e-81

r 720 432 784 432 0 5600

S 640 368 720 368 0 0 false 0 2

w 592 368 640 368 0

w 592 304 592 368 0

w 448 304 448 352 0

w 336 352 448 352 0

r 448 304 592 304 0 10000

a 1152 384 1296 384 8 12 -12 1000000 -0.000012183066700374307 0 100000

a 448 368 592 368 8 12 -12 1000000 0.000026274481766352875 0 100000

207 144 0 16 0 4 Left

207 144 352 16 352 4 Right

207 1520 -32 1600 -32 4 Left\s1

207 1520 96 1600 96 4 Left\s2

207 1520 448 1600 448 4 Right\s2

207 1520 320 1600 320 4 Right\s1

o 23 16 0 21771 5 0.0015625 0 2 23 3

o 59 16 0 21771 5 0.00078125 1 2 59 3

o 79 16 0 21771 5 0.00078125 2 2 79 3
 

AlbertHall

Joined Jun 4, 2014
8,381
Nothing in that circuit should get even vaguely warm so something is drawing way too much current.
The way the supply voltage is connected does not guarantee that the voltage will be evenly spread between the two regulators. Supply each regulator with its own supply voltage to avoid this problem. If you still have the heating problem then check all the connections and polarities very carefully.
 

DickCappels

Joined Aug 21, 2008
5,895
There are buffers in the schematic to which the link leads.

@Frank Spigner I see many rows of numbers (a few lines are copied below) that may be code for drawing something. Is this supposed to be a schematic? Whatever it is it does not make much sense to me.

207 144 0 16 0 4 Left

207 144 352 16 352 4 Right

207 1520 -32 1600 -32 4 Left\s1
 

shteii01

Joined Feb 19, 2010
4,667
There are buffers in the schematic to which the link leads.

@Frank Spigner I see many rows of numbers (a few lines are copied below) that may be code for drawing something. Is this supposed to be a schematic? Whatever it is it does not make much sense to me.

207 144 0 16 0 4 Left

207 144 352 16 352 4 Right

207 1520 -32 1600 -32 4 Left\s1
From first post: Falstad circuit simulation (copy the following and select "import from text")
 

Audioguru

Joined Dec 20, 2007
11,251
You have two photos of the oscilloscope. I assume that the first one is the output of the first opamp that is oscillating at a high frequency and the second photo is the DC output (but no 220Hz signal) of the second opamp?

The OPA2604 opamps are good up to 20MHz so the capacitance between your wires that are all over the place on your perf board is probably causing the high frequency oscillation.

Did you know that many good audio opamps (TL072 for example) oscillate at a high frequency when they have the capacitance of shielded audio cable or a 'scope probe connected directly at the output? Add a 100 ohm resistor in series with the opamp output and the cable or 'scope probe to see.
 
Top