Electronics: Cassette Decks, Tuners, Amps, Repairs, Failures.: September 2020

Headphone Amplifier Design & Experiment.

So far my circuit -

Headphone Amplifier V0.1

LM 741 OP Amp at Buffer Stage

The slew rate of the LM741 is limiting the amplifier to about 40Khz sinewave, at about 2 volts (peak) into a 33 ohm load.

This finding ties in nicely with the theory which states - the maximum frequency of a sine wave (before Slew Induced Distortion) delivered at the output is limited by the OP Amp's ability to dump charge Q into a load.

The 741's quoted slew rate is 0.5v/us, or 500v/sec.

Applying the accepted guide of ... SR = Vp·2𝝅·f

and rearranging for frequency f gives ... f = SR/[2𝝅·Vp],

that is ...

f = 500,000/[2*3.142*2] = 39,788Hz.

DC Voltage Offset

The unwanted (although non-audio affecting) DC offset at the output of the bi-polar transistor designed LM741 was reduced by applying two same-valued resistors both at the output, and at the V+ input terminal. This DC offset is caused by internal bipolar transistor bias currents - the effects of which are to produce undesirable DC input to the OP Amp. The OP Amp amplifier section also 'sees' this DC offset at the input and amplifies it. To overcome this problem, creating an equal quiescent DC bias voltage (thanks to these resistors) at both V- and V+ then cancels out the undesired effect, or at least minimises it.

The LM741 DC output offset was reduced to less than 2mV, typically around 1mV or less.

And the output across the 33Ω 'power' load, we have a DC offset of much less than 1mV .... excellent, typically 100uV or less.

LF411CN or TL071 at Buffer Stage

Replacing the LM741 with a JFET LF411CN or TL071 resulted in instability - overshoot on the application of a square wave. and sometimes uncontrollable amplifier oscillatons.

Strong overshoot of a 4Khz Square Wave

Somewhere within the circuit we have a situation where negative feedback from the effective loop gain '𝜷·A' had a near 180 degree phase reversal at approximately unity gain - which is undesirable.

Without the 1.5nF loading capacitor, the gain with feedback takes the form:

Aᵥ = A/(1+𝜷·A)

where A = Open Loop Gain of the OP Amp,

and 𝜷 = the feedback ratio from an impedance network, but 𝜷 =1 (not proven here) in this voltage follower configuration.

Instability in the form of heavy overshoot and/or oscillations are likely if a near 180° phase shift in the loop gain term 𝜷·A occurs, ie as 𝜷·A → -1.

With the 1.5nF capacitor present, the expression in the denominator changes (this will be studied in another article), and I suspect enough additional phase shift occured which induced instabilty, hence overshoot to square waves and eventual oscillations. Not so with the LM741 OP Amp, but I did note the LM741 still exhibited small, but significant overshoot (fast dying oscillations) on the application of a square wave or pulse.

Removal of the High Pass Filtering 1.5nF Capacitor

Now, removing the 1.5nF capacitor (part of the high pass filtering at midstage) eliminated all instabilities, and now we have a stable headphone amplifier which can deliver (if needed) 500mW into either 33Ω or 8Ω.