op amp Humming and Transmission issues

Description: When the connection to the PC speaker is plugged in alongside the telephone connection (located at the bottom-left), a humming sound is heard in the headphones. However, when only one of the connections is used, the sound is clear without distortion. Sound files demonstrate the humming issue. It is recommended to provide constructive criticism regarding the circuitry, suggesting replacements rather than simply stating that it will not work due to specific issues. The current circuit is being updated to avoid scrolling through previous replies for the latest status. The circuitry for the telephone connection is derived from a recording interface in the Tekniikan Maailma magazine, chosen to enable parallel use with the existing phone. Concerning the power supply, a dual supply has been created using two batteries, which have internal resistance and should be bypassed by capacitors. Any resistance in the return circuit can lead to ground loops. A dual battery supply may become unbalanced if one battery discharges faster than the other. The circuit contains several questionable component values. For instance, the virtual ground mixer at the bottom has an input impedance of only 100 ohms, which is low for the preceding op-amp to drive and loads down the 10K potentiometer. When the potentiometer is fully turned up, the impedance is effectively just 150 ohms. The impedance to ground of the potentiometer is irrelevant due to its larger size. When turned halfway, the impedance increases because a 5K branch of the potentiometer is in series with the input impedance. For a linear potentiometer, the impedance should be at least ten times lower than the input facing the wiper. If both the potentiometer and the input resistance are approximately equal (e.g., a 10K linear pot and 10K resistance), it becomes nonlinear in a useful manner, approximating an audio taper. The gain on the upper op-amp from the PC speaker is less than unity, as indicated by a 1.5K feedback resistor and a 2.2K input resistor. This low gain, combined with the loading presented by the next stage (especially when the volume potentiometer is fully turned up), likely contributes to the insufficient signal across the transformer. The 450uF coupler between the transformer and the op-amp circuit is excessively large. A smaller capacitor can be used if an adequate input impedance is achieved in the mixer. Given the dual supply, a coupling capacitor may not be necessary at all, as it primarily protects the transformer winding from DC flow if the two batteries become unbalanced, potentially causing a significant DC offset in the output of the upper op-amp at rest. Similarly, the coupler to the headphones may also be unnecessary except to mitigate the risk of unbalanced batteries. For headphones rated at 600 ohms, a suitable coupling capacitor value to achieve flat response down to 20 Hz would be approximately 130 uF. The specified capacitor is designed for a frequency response down to 5 Hz. Bridging op-amps can create an amplifier that does not require DC coupling to a speaker, and components capable of driving 600-ohm loads effectively, such as the NE5532 or LM4562, should be utilized. The NJM4556 is noteworthy for its ability to drive loads as low as 150 ohms. Regarding the observed hum, it is evident from the sound files that the hum arises when the phone is connected, regardless of whether the PC speaker is also connected, manifesting as a 50 Hz hum.

To address the humming issue, it is essential to examine the grounding scheme of the circuit. Ground loops can be mitigated by ensuring that all grounds converge at a single point, commonly referred to as a star ground configuration. This minimizes potential differences that can introduce hum. Additionally, the use of twisted pair wiring for the telephone connection can help reduce electromagnetic interference, which may contribute to the hum.

The choice of op-amps is critical; using devices with low noise characteristics and sufficient output drive capability is recommended. The NE5532 or LM4562 op-amps are excellent options, as they provide low distortion and high output current, suitable for driving loads like headphones. The design of the feedback network should be revisited to ensure that the gain is appropriate for the application, aiming for a gain greater than unity to avoid signal loss.

In terms of the capacitors used in the circuit, it is advisable to select capacitors with low equivalent series resistance (ESR) to maintain signal integrity, especially in audio applications. Additionally, bypass capacitors should be placed close to the power supply pins of the op-amps to filter out high-frequency noise effectively.

Lastly, ensure that the power supply design considers the potential for battery imbalance. Implementing a battery management system that monitors battery voltage levels can prevent one battery from discharging faster than the other, thus maintaining a balanced dual supply. This will help avoid issues such as DC offsets that could lead to further distortion in the audio signal.When connection to PC speaker is plugged in and Telephone connection(bottom-left) is plugged in, I hear humming in the headphones. When only one is plugged in I hear it without distortion. Sound files showing humming. If you criticise my circuitry, add something to act as a replacement, too. Just saying that it won`t work because of this and that is not as helpful as saying: "It won`t work because of X, which is why I recommend doing Y". I try to update this question with the current circuit so you don`t have to scroll through all replies to get the current status. PS: Circuitry for telephone connection taken from a recording interface from Tekniikan Maailma magazine.

I chose it to be able to use the circuit in parallel with my current phone. Regarding your power supply. You have created a dual supply with two batteries. Batteries have internal resistance and so should be bypassed by capacitors. Any resistance in your return circuit can create ground loops. When a dual supply is created using batteries, there is a risk that it will go lopsided if one battery loses voltage faster than the other one. You have a number of sloppy choices of part values in your circuit. For example at the bottom, you have a virtual ground mixer whose input impedance is is only 100 ohms.

First of all, that is a low input impedance to ask the other op-amp to drive. Secondly, it loads down the 10K pot that you have in front of it. Weird things will happen. When the pot is turned all the way up, the impedance is basically just the 150 ohms. The pot`s impedance to ground doesn`t matter because it is much larger. When you turn it halfway, the impedance goes up because now you have a 5K branch of the pot in series with the input impedance. If you want a potentiometer to be linear, it should have an impedance which is at least 10X lower than the input facing the wiper.

If you have approximately equal input (say 10K linear pot, 10K resistance) then it`s nonlinear in a useful way because it approximates audio taper. The gain on your upper op-amp, from the PC speaker, is less than unity, because you have a 1. 5K feedback resistor and a 2. 2K input resistor. That low gain, combined with the terrible loading presented by the next stage (at least when the volume pot is turned all the way up) is probably why you have inadequate signal going back across the transformer.

The 450uF coupler between the transformer and your op-amp-ology is quite overblown. If you achieve a decent input impedance in the mixer, you can use a much smaller cap. Since you have a dual supply, you should not need it at all. The only thing it does is protect the transformer winding from DC flowing across it, if your twp batteries become unbalanced (so that the output of the upper op-amp develops a serious DC offset at quiescence). Similarly, the coupler to the headphones should also not be needed except for the threat of unbalanced batteries.

If the headphones are 600 ohms, what would be a good coupler value, if we want low end down to 20 Hz (And do we ) For flat down to 20Hz into 600 ohms, the cap would have to be 130 uF. Yours is spec`d for elephant ears: flat down to 5 Hz. You can bridge op-amps to create an amplifier that doesn`t require DC coupling to a speaker. Be sure to use parts that can drive 600 ohm loads well, like NE5532 or LM4562. The noteworthy NJM4556 can drive 150 ohm loads. Regarding the hum; it is obvious from the sound files that the hum occurs when the phone is plugged in, whether or not the PC speaker is plugged in.

It is 50 Hz hum. (I didn`t notice you`re European until examining this file! When I play it side by side with a 50 Hz tone generated in Audacity, the beat frequency bet