Audio Question

BuffaloEngineer · Feb 23, 2009

I'm making an audio mixer - the output goes to the line in of another device. Do I need to match the voltage range on the output of the mixer to the input range of the device it's going to?

The mixer runs on 12VDC and goes to a DVD burner, which has an RCA jack for a line in. If I see an RCA jack, is there a standard voltage range for the input? There's a capacitor on the output of the mixer, so DC isn't going to be passed.

Thanks from Buffalo!

Nigel Goodwin · Feb 23, 2009

Yes, you need to feed it a suitable voltage range, normally it's about 700mV or so - that's why you have a volume control on the output of the mixer, so you can set it to the level you want.

98formulaLS1 · Feb 23, 2009

note, try and avoid using carbon resistors as attenuation with audio. Carbon resistors can generate noise.

TheNthDegree · Feb 24, 2009

Hi,

I'd give the "line level" page at Wikipedia a read:
Line level - Wikipedia, the free encyclopedia

Basically, RCA jacks are almost always used to carry a "line level" audio signal between audio devices. Unfortunately, there is no strict standard as to what "line level" is, and it tends to vary from device to device.

In the US, all "professional" grade audio equipment usually uses a +4 dBu signal level (1.228 Volts RMS) and "consumer" grade equipment usually uses a -10 dBV signal (0.316 Volts RMS). Portable devices, such as iPods, laptops etc, will vary greatly depending on device, but will usually be in this range.

Generally, as long as you keep the signal level below 2 Volts peak, you will not damage "line level" compatible equipment. And yes, coupling capacitors to remove any DC in the signal are highly recommended!

Hope that helps,
-Nick

BuffaloEngineer · Feb 27, 2009

Thanks guys - I've got a preliminary setup working on 5V on a breadboard. I hooked up the output to my computer to listen to it, and right now the quality is... decent, but it could be better.

Would using a solderless breadboard affect quality? Do I need to choose an op amp specifically for audio? Right now I'm using the MCP6042 because I had it lying around.

I haven't switched to metal resistors yet, I'll have to buy some.

Thanks very much!

TheNthDegree · Feb 27, 2009

BuffaloEngineer said:
Would using a solderless breadboard affect quality? Do I need to choose an op amp specifically for audio?

Hi, yes... both could be negatively affecting performance. Keep in mind that you ideally want to reproduce the spectrum from about 20Hz to 20kHz with minimal distortion. So if quality is of concern to you you need to make sure your op-amp(s) have a bandwidth at least this wide at the gain levels you need.

I had actually tried posting to this thread a few days ago, but it seems it got lost in moderation purgatory or something?

The basic idea I was trying to convey to you was that "line level" audio is a subjective term, and multiple "standards" exist to define it. For example, consumer-grade audio devices in the USA use a line-level of about 0.316 Vrms, and pro-grade devices use a line-level of about 1.228 Vrms. Portable devices like laptops, media players, etc can be anywhere in this range...

Basically, as long as you keep your output signal(s) under about 1.5 volts rms, you should be safe to connect to any "line-level" input (at least on US equipment!). Also, coupling capacitors to block any DC are indeed very very important to include on all outputs (even more so if you are feeding into a power amplifier).

The "line level" page on wikipedia has even more info...

Hope that helps,
-Nick

BuffaloEngineer · Feb 28, 2009

That does help - I was sending the input to my computer and it kept maxing out the computer's line in. I think I need to limit it to 0.316V for that application.

I was thinking of using 1 dual op amp for the inputs, then another op amp for the outputs, which are 1 headphone and 3 line in connections. Now I'm looking for op amps that can do both... maybe something from New Japan Radio - their parts seem to be in all the audio devices I've taken apart.

BuffaloEngineer · Mar 1, 2009

Actually, I have another question - I've got a sound on the output of the mixer that I didn't notice before. When I listen to the playback, it sounds like a constant low volume sine wave, but I don't see anything like that when I scope the output... Any ideas?

I attached the noise - an mp3 in a zip file (12 seconds).

audioguru · Mar 1, 2009

The tone you hear could be the opamp oscillating because the capacitance of the tracks on the breadboard to other tracks is too high and the oscillation frequency is beating with the sampling frequency of your sound card.

An opamp with a good bandwidth will oscillate if it directly drives the capacitance of a shielded cable. Adding a 100 ohm resistor from the output of the opamp (or from its output coupling capacitor if it has one) to the cable fixes it.

The MCP6042 opamp is the slowest I have ever seen. With a gain of only 10 it barely produces a frequency as high as only 1kHz. with a gain of 100 it barely makes it up to 100Hz.

A TL071 audio opamp produces a frequency up to 30kHz when its gain is 100 and goes higher when its gain is less.

BuffaloEngineer · Mar 8, 2009

Thanks for all the help so far - the mixer sounds great now that I have some audio amps and metal film resistors.

The last hurdle is the headphone amplifier, which I thought would be easy. I've been using the schematic attached from this website:
https://www.electro-tech-online.com/custompdfs/2009/03/cmoy-tangent-sch.pdf

That seems to be the most popular headphone amp design, so I followed it, but I'm not getting any output... Should I be using a different design?

Thanks!

98formulaLS1 · Mar 9, 2009

What OpAmp did you use? I dont think you really require a -9V for the ground. I think you should use 0V GND. I don't think you'll need the attenuation resistor on the output.

This should be super simple for you, its really just a basic non-inverting opamp setup.

audioguru · Mar 9, 2009

The opamp circuit with a gain of 11 uses a dual-polarity supply because its inputs are referenced to ground. The negative battery can be eleminated if the input is biased at half the supply voltage with two resistors in series from the supply. Then the negative feedback resistor to ground must be capacitor-coupled to ground.

Headphones are from 8 ohms to 600 ohms. Most headphones are 32 ohms.
Ordinary opamps can drive 2k ohms. A few opamps can drive 600 ohms.
Use a power amp IC to drive low impedance headphones. Most power amp ICs have inputs that work at 0V and have built-in negative feedback.

BuffaloEngineer · Mar 10, 2009

That's the problem - I wasn't paying attention to the output impedance - I just figured it would be enough for the headphones.

I happened to have a few TDA7052, but these don't seem to be working either - I got it to work with an 8ohm speaker, is it possible to get it to work with headphones? Should I hook the headphone ground to ground or Vcc/2?

audioguru · Mar 11, 2009

An opamp does not have enough output current to drive low impedance headphones.
The TDA7052 amplifier has too much power, too much gain and too much distortion.
What is the impedance of each ear of your headphones?
Do you want stereo or mono?

BuffaloEngineer · Mar 11, 2009

The headphones are 32ohm - this problem seems tricky - I have quite a few op amps, many can drive loads down to 100ohm - that's a graph I didn't think to check when I purchased them.

Is there a way to decrease the power of the TDA7052? I just need a mono output.

The last ace I have (before breaking out the BJTs) is the LM386:
https://www.electro-tech-online.com/custompdfs/2009/03/LM386.pdf
This is an audio power amplifier, which is also used for speakers - could I put a ~32ohm resistor at the output, or use the application circuit?

Nigel Goodwin · Mar 11, 2009

You don't want an opamp, you want a power amp - but you don't want a bridged amp either, that would be useless to you. It's not at all 'tricky', it's dead simple and trivial.

An LM386 is perfectly fine (I used LM380's back in the 70's for this).

You could simply use the LM386 circuit as above, and in to 32 ohms this will give you a quarter of the power than it would give to 8 ohms. But, to prevent it been loud enough to damage your ears, you should add a series resistor between the coupling capacitor and the headphones. The value depends how loud you want it to go, the larger the resistor, the quieter it will be - try 100 ohms, and adjust from there.

audioguru · Mar 11, 2009

The TDA7052 has a gain of 98 which is probably too high and has too much output power.
The LM386 has a gain of 20 which is good and has a reasonable amount of output power.

Adding a 220 ohm (or less for more power) resistor in series with the output of the amplifier to the headphones will reduce the output power for headphones.

BuffaloEngineer · Apr 7, 2009

This is the circuit I came up with for this project - I tested the headphone output with the NJM4558 amp, and it works even though I thought it wouldn't from the info in the datasheet.

Let me know if anything looks fishy - all comments and criticism welcome/needed. Thanks!

audioguru · Apr 7, 2009

The NJ4558 will not drive 8 ohm headphones well in this project because its max output current is too low and the 100 ohms output resistor will severely limit the output level.

The mic will not work without a preamp. The value of the 2.2k resistor that powers the mic is too low. Use 4.7k to 10k.

audioguru · Apr 7, 2009

The mic will not work without a preamp. The resistor that powers the mic must be 4.7k or 10k.

The circuit will not be very loud with 8 ohm to 32 ohm headphones because the opamp cannot supply enough current and the 100 ohm resistors limit the output level.

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