0
\$\begingroup\$

enter image description here

In the above schematic, the volume even though at maximum was very low, so I took R32 and R34 out, shorted the path and then was much better but still not so loud (subjective ok). The LM4880 is unity gain stable so I chose a gain of 1.

Being not happy I have decided to implement an active volume control instead of the voltage follower and of course I have taken RV pot out.

schematic

simulate this circuit – Schematic created using CircuitLab

The volume was now going too high, creating distortion. The gain was \$-R_4/R_3 = 7\$, so I changed those resistors to 2.2k and 1k, having now a gain of 2.2. Still experiencing distortion at some point, I then added the output resistors R32 and R34 again being happy with 27 Ohm instead of 47 Ohm.

How do I amplify properly? How do I calculate how much gain can the LM4880 can handle? How high can I go?

ADD ON: 1kHz Sine wave

Input Signal INPUT SIGNAL

Output signal NO LOAD OUTPUT SIGNAL NO LOAD

Output signal 17Ohms in hear headphones OUTPUT SIGNAL 17 OHM IN HEAR HEADPHONES

\$\endgroup\$
7
  • \$\begingroup\$ Is the active volume control circuit above implemented with MCP6L2T OpAmps or LM4880? Also distortion is relative - All amplifiers generate some level of distortion so quantifying what level of performance you would like to achieve would be helpful \$\endgroup\$
    – Graham
    Commented Oct 5, 2023 at 20:34
  • \$\begingroup\$ I have also just noticed that you are running the LM4480 from +/-2.85V = 5.7V which is outside the LM4480 specification \$\endgroup\$
    – Graham
    Commented Oct 5, 2023 at 20:36
  • \$\begingroup\$ The active volume control uses MCP6L2T (second schematic). LM4880 absolute maximum rating 6V \$\endgroup\$
    – Status Re
    Commented Oct 6, 2023 at 4:39
  • \$\begingroup\$ Whilst electronic parts can survive at their absolute maximum rating, the manufacturer does not guarantee any performance at that level. As such the part might behave in unexpected ways, which is why operating at abs max ratings is avoided in most cases \$\endgroup\$
    – Graham
    Commented Oct 6, 2023 at 7:44
  • 1
    \$\begingroup\$ The LM4880 is normally rated to +5.5V, as described in the electrical characteristics section. That means that there is a grey area between 5.5V and 6V abs max where the chip may or may not work in the way that you expect. +/-2.85V rails falls within this region. \$\endgroup\$
    – Graham
    Commented Oct 6, 2023 at 18:55

2 Answers 2

2
\$\begingroup\$

Starting with the LM4880 circuit.

How much gain is possible with the LM4880 while avoiding distortion?

Even the best op-amps have some level of distortion, even at unity gain, so a truely distortionless amplifier is so far elusive. The LM4880 THD+N specs (used to quantify distortion) are not great by modern standards, and they are certainly at a level that you would be able to hear it. Without a THD measurement or an oscilloscope capture of the output signal, its very hard to know whether the distortion you are hearing is fundamental to this part (and therefore unavoidable; better performance = choose another part) or whether there is an issue with the circuit or its construction. The LM4880 THD+N (like most amplifiers) is also dependent on the load impedance, with more demanding loads (i.e. requiring more current) producing worse distortion.

I would be interested in the load (headphones?) connected to the amplifier output. The fact that shorting R32 and R34 noticeably increases the volume tells me the load impedance of the headphones is in a similar ballpark. Remember that the lower the resistance of the headphones, the more distortion the amplifier will produce as it needs to work harder (greater output current) to deliver the same power.

I would also check your voltage rails with an oscilloscope. Depending on how the rails are produced, their output impedance and the amount of current they can deliver, its possible that they are sagging during output peaks, which could cause distortion.

Finally, I mentioned in the comments above the issue with operating outside the electrical specifications, even if you are within the absolute maximum ratings. TI makes no promises about how the device will operate with a total voltage rail greater than 5.5V; one of these effects could be that it produces additional distortion.

The Active Volume Control

There are a few potential pitfalls with the way this particular circuit has been implemented (rather than the overall topology itself).

Firstly, MCP6L2T is not an audio op-amp and there are zero specifications relating to THD+N in the datasheet. Therefore, its distortion characteristics are unlikely to be great, and certainly not guaranteed.

Secondly, depending on the output load, the MCP6L2T may be struggling to deliver the necessary current to keep a clean output.

Thirdly, the MCP6L2T is not a rail to rail input opamp which might cause issues with the unity gain buffer. The key specification is the common mode input range. As this is a unity gain buffer, the common mode input range is the same as the signal at the non inverting terminal. The common mode input range is -0.3V to 3.7V for a VDD=5V,VSS=0V supply. This is probably referenced to VSS, so when VSS=-2.85V, the maximum positive input signal to this opamp is VSS+3.7 = 0.85V. As you can see, that is quite low, and you may be exceeding this, which could cause distortion.

Finally, the previous comments about power rails are worth checking again, assuming they are the same for both the LM4880 and active volume circuit.

\$\endgroup\$
2
\$\begingroup\$

How much gain you can use in the active volume control depends on the level of the input signal and the allowable input level of the LM4880. It also depends on the load attached to the LM4880.

The simplest thing to do would be to refer to the AUDIO POWER AMPLIFIER DESIGN section on page 12 of the LM4880 datasheet. There's a detailed design procedure there that tells you what information you need and how to do the calculations.

You'll need to know the input signal level and the desired output power along with the impedance of the speaker. The example is for an 8 ohm speaker, a \$1 V_{RMS}\$ input signal and 200mW of output power per speaker. The design example actually ends up using negative gain (attenuation) to deal with the input signal level.

You need to determine the input signal level, then design for 250mW of output power. You'll probably also want to switch to 8 ohm speakers to get maximum power with minimum distortion out of the LM4880.

You probably don't need your extra volume amplifier. You should be able to design the needed gain into the LM4880 circuit - but you must determine the input signal level first.

Keep in mind that the LM4880 is a low power amplifier - it will never be truly loud. It is rated for 250mW into an 8 ohm speaker. Using higher impedance speakers will lower the achievable power output. 32 ohm speakers will only get 85mW of power output.


High impedance speakers are actually your enemy when it comes to higher power and lower distortion. The following chart shows that the distortion gets crazy high for relatively low power output for high impedance (32 ohm) speakers. The distortion stays much lower to a higher power output for 8 ohm speakers.

enter image description here

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.