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| Just how bad is it? Audio mixer with headphone amp. |
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| paulca:
And... on the inverting amp. I put 47R resistors on all the outputs. So to aim for a gain of 1 of the inverting mix amp I put a 47R on the feedback. I gather from what you said this probably isn't going to work well. Do I need resistors between the amps at all? If I put the OUT of an opamp directly into the IN- of the inverting amp and hard link it to unity will it still function as a unity gain summer or will the virtual ground on the - input kill the upstream opamps output? |
| Audioguru:
If the output of the opamp must have a maximum swing of 7V peak then the output current in a 47 ohm feedback resistor needs 7V/47 ohms= 149mA peak! But most opamps can drive 2000 ohms minimum for a current of 3.5mA including the current in the load. Why the extremely low value of 47 ohms? Why not 47000 ohms instead? Please learn what an inverting opamp is. It has a series resistor to its - input and another resistor fronm its - input to its output. Its voltage gin is the ratio of the two resistors.If one opamp drives the - input of a second opamp that has a negative feedback resistor then the output of the first opamp is almost shorted and the second opamp has a voltage gain of a few hudred thousand so it will be clipping the signal like crazy. Each input needs its own series input resistor whether it comes from a signal source or from the output of another opamp. Yes, the - input of an inverting opamp is a virtual ground, the series input resistor is very important. |
| paulca:
I found myself a copy of Small Signal Audio Design. Interestingly, Chapter 1, page 6, Gain Structures, describes exactly what I am aiming to do. --- Quote ---1. Don’t amplify then attenuate. 2. Don’t attenuate then amplify. 3. The signal should be raised to the nominal internal level as soon as possible to minimize contamination with circuit noise. --- End quote --- It then goes on to suggest that 1 and 2 are compromises when you have an attenuating gain control, you have to do one or the other. 1 risks clipping, 2 risks signal to noise loses. As my input stages will not be getting anywhere close to the amps VPP peak of 20V (a gain of about 30-40) I have plenty of headroom to lift it considerably, gain of 10 or 20. The idea is to lift the level once (rule 3) and then only buffer it or attenuate it for output level. Not this attenuate, gain, attenuate, gain series stuff, that just adds noise. It also mentions that active gain controls should be used ideally, although I have not read the chapter on the pros and cons of their use/design yet. I'm sure there are dragons there. |
| paulca:
Well, had another session with this. Decided to try something different and implemented one of these: Source: Small Signal Audio Design The positives are that it's the most stable outputs I have seen on the scope. Plenty of gain, although when running my headphones the gain fell to 1, which has me a little perplexed. The downsides are it's an opamp killer on a breadboard. Two 5532s died. The first one was most likely because I forgot the GND reference on the second stage amp. That chip now does nothing but howl with a 13.5V DC offset. The second chip died when I had a lose connection on the breadboard. I was (probably foolishly) running the output into my headphones (34 Ohm IIRC), so a total output load of only 134 Ohms. I had fissling and was poking various connections to find it then WWOOOOOOOOOO! The opamp broke into full song howling. If I swap in another 5532 directly everything comes back to working, I conclude both chips are dead. They seem to work in isolation as voltage followers, but as soon as they go back into this circuit the each behave incorrectly. I'm not sure what it is about the circuit that makes it easy to kill opamps, maybe the low value resistors and 5532s are not exactly expensive, but I'd rather not have to order more. I was using my new PSU an XP Power +-15V 200mA. It seemed to be pulling around 40mA idle, 66mA running the circuit which was a single 5532 chip + an LED with a 2.2K limiter across the 30 volt rails. This does not sound like the efficiency in the datasheet of 84%. Anyway. I'm a bit concerned about proceeding to create two inputs and a summer from this if I have other issues. I'll retest the PSU in isolation to make sure it is functioning and I haven't wired something wrong. 40mA to power an LED which should be drawing around 12mA. The small signal audio design book claims the resistors are small value to keep down the noise, if I take them up a factor of 10 would that should make the circuit safer against breadboard glitches? |
| paulca:
Actually checking the datasheet for the XP PSU, it states 25mA idle current. I believe the 40mA I was measuring included the LED, so 25+12mA = 37mA and not far off the measured 40mA. That leaves the rest of the circuit pulling 26mA (20mA account from the PSU efficiency). The 5532 has a max input current of 10mA, but it's hard for me to work out how much is dropping on the various resistors and not going into the opamp. |
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