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Constructive wanted criticism for op-amp circuit design.

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I’ve attached a PDF of a project that has been in the works off and on for the past few weeks.  As I am definitely playing outside my usual prevue, I would like constructive criticism from the fellow board members.  I took a dc class back in high school in 1998 and in the spring a more formal dc class at a local community college.  I wanted to take ac this semester toward my EET, but my work schedule wouldn’t play nice.  I understand the very basics of op-amps, the idea of feedback, and the concept behind working from a single pole power source.  I have the educational version of Multisim 11 and modeled the TL074/TL071 part of the design and it is stable and acts as I would like it.  I have built the LM384 side of the circuit on a breadboard and it seems to also work as I would like.   Where I am having a bit of difficulty is choosing cap values and type (ceramic, electo, film, polarized, non-polarized).  I know Multisim says it should work, but I have a feeling the values I chose are not optimal given I was just choosing a number at random.

- The basic concept of the circuit is to monitor four audio signals on a local small speaker.
- Power (+30VDC) comes paired with each audio signal from a shielded twisted pair cable.  Shield as ground, red wire is power, black wire is signal.
- The diodes are there to prevent power from one line backing up onto another.
- The audio signal is speech (only so say about 500Hz to 4kHz) and maxes out around 1Vpeak.
- From time to time a positive DC signal (+15VDC) will be present briefly (1-10 seconds) on the audio line that needs to be filtered out reasonably well from the op-amps and power amp.
- I must be careful how much load I place on the signal line as it is a bussed system that many many components share.  For that same reason, I need to be careful about adding anything (like DC for instance) back onto that bussed signal line.
- VR1 through VR4 act as volume controls for each individual channel. VR5 will be a trim pot to reduce the strength to of the signal going into the LM384 as it has no gain control.
- As power is from a big beefy central power supply, power consumption within reason isn’t a big deal and is very well filtered.

Let me know your thoughts, but please be nice…  Thanks!

- EM

C4-C7 set the low frequency rolloff.  They also will be responsible for blocking the DC pulses, and the bigger they are the bigger the thump when the DC switches on or off.  Out of curiosity, why is there a switching DC level on an audio signal line?  Anyway, the lower cutoff frequency is 1/(2*pi*R1*C4) or 16 Hz.  Since you are only handling voice and probably don't have much in the way of bass response in any case, you can bump that up.  Try a smaller capacitor.  330 nF will give you around 50 Hz high-pass.

Your virtual earth could be a bit better filtered.  I would make the C15 10 uF and probably make R14 and R15 10 kohm.  That will reduce the coupling of any supply ripple onto the audio.

U3 has the inverting and non-inverting terminals switched.

U4 shows the non-inverting input connected to GND, I think that should be V/2 unless the LM384 is internally capacitively coupled (I have never used it).

Your volume control is not really optimal.  It gives a gain range of only 2:1.  For audio work you usually want more like a 10:1 ratio or more using a pseudo-log pot so you have useful sensitivity across the whole adjustment range.

C8-C11 and C13 are unnecessary.  No reason not to use a DC coupled signal path here.  As for the other capacitors, C4-C7 can be any type.  Hi-fi people generally prefer film caps for the audio signal path, but it doesn't really matter for AC coupling capacitors (the AC voltage across them is very small), and you aren't doing hi-fi in any case.  If you use electrolytics, make sure to put them the right way around so that when the DC voltage is present they are correctly biased.  Most of the rest of your capacitors are power filter/bypass caps and should be electrolytic for large values and ceramic for small values.  C18 is fine as shown, and gives you a 50 Hz high pass on the output.

U3 needs both inputs connected. Put a small cap in parallel with R5-R8 and R13 to minimize hiss and increase stability.

The DC pulses will be a problem if the edges are sharp. If so, you'll have a full volume bang and no easy solution. Think it like a 15x full volume square wave (although only the first part of it). You do want a good (40-60dB) attenuation at that frequency. If the pulse raises and falls in 1sec, the project has a change. If the rise time is 1ms, you are in trouble.

There's something wrong with U3 - it has no negative feedback and a dangling input. Connect it like U2 (swap "+" and "-" inputs, ground the "+").

As mentioned above, C8-C11 and C13 aren't really necessary. You already have C4-C7.

I'm not sure LM384 doesn't need negative feedback, please check the datasheet.

You may improve performance by adding an active voltage follower at V/2 betveen the R14 R15 C15 joint and rest of the circuit. To do that, replace TL071 with TL072 and you'll get one more opamp. This is not very necessary but may reduce noise.

Another possible approach is to use 7812+7912 instead of 7824. So you may get rid of C18 and avoid click at power on. You also may use BTL output for the same purpose.

Replacing LM384 with TDA2030 or similar would simplify the output stage.

I was going to say "You've come to the wrong place for constructive criticism" ;D,but all the posters are on their best behaviour!



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