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Designing An Opamp Headphone Amplifier

Logatithmic Amplifier

Opamp Classes And Major Characteristics

Opamp AC Specifications

Opamp DC Specification

The µA748 OPAMP

The LM101A OPAMP

The LM101 OPAMP

The µA709 Opamp

OPAMP History

Phase Shift Oscillator

The Ideal Opamp

Opamp Output Impedance

Opamp Input Impedance

Opamp Voltage And Current Offsets

Opamp Negative Feedback

Opamp Active Filters

Integration Circuit Amplifier

OPAMP - Differential Amplifier

Opamp Current Summing Amplifier

OPAMP Inverting Amplifiers

Non-inverting Amplifiers

Powering Up The Op-Amp

741 Op-amp Pin Functions

741 Op-amp Pin Diagram

741 Opamp Internal Block Diagram

Opamp Absolute Max. Parameters2

Open-Loop Gain & Frequency

Absolute Max. Parameters

What Is An Op-Amp

30 topics total

Logatithmic Amplifier

Opamp Classes And Major Characteristics

Opamp AC Specifications

Opamp DC Specification

The µA748 OPAMP

The LM101A OPAMP

The LM101 OPAMP

The µA709 Opamp

OPAMP History

Phase Shift Oscillator

The Ideal Opamp

Opamp Output Impedance

Opamp Input Impedance

Opamp Voltage And Current Offsets

Opamp Negative Feedback

Opamp Active Filters

Integration Circuit Amplifier

OPAMP - Differential Amplifier

Opamp Current Summing Amplifier

OPAMP Inverting Amplifiers

Non-inverting Amplifiers

Powering Up The Op-Amp

741 Op-amp Pin Functions

741 Op-amp Pin Diagram

741 Opamp Internal Block Diagram

Opamp Absolute Max. Parameters2

Open-Loop Gain & Frequency

Absolute Max. Parameters

What Is An Op-Amp

30 topics total

Not widely available back in the heyday of glass audio, tube opamps are very hard to find today. The following circuits develop the tube amp-block concept with increasing complexity. They all have limited current output and may need an output buffer stage to comfortably drive headphones (see the section on output stages below). To adjust the closed-loop gain of any of these amp-blocks, simply add a feeback resistor from the output to the inverting input and an input resistor just as with solid state opamps. Some of these amp-blocks may prefer higher magnitudes of resistance than are typical of solid state opamps, so sample gain resistances are included. It´s a good idea to construct several of these at a time to have a handy supply for experimentation.

**Figure 2**

Eric Barbour´s "1+1 Cascade" amp-block is an AC feedback amplifier (figure 2), consisting of a common cathode gain stage and a cathode follower output stage. This amp-block has a single inverting input and limited open loop gain, but is entirely suitable as a front-end of a headphone amplifier. The open loop figures are less than spectacular: G ~ -50, Fh ~ 30 kHz, THD > 2%, Rout ~ 2K ohms. When configured for a closed loop gain of -10 (Rf = 100K ohms, Rin = 10K ohms), the situation changes dramatically: Fh > 100 kHz, Rout ~ 500 ohms and THD drops below 0.4%. Since the 12AX7 is a dual triode, this design uses only 1 tube per channel. 12AU7s can also be subsituted, but the open loop gain will be lower. Purists may want to add an inverting output stage for correct output phase.

**Figure 3**

Keywords : Opamp, Operational Amplifier, Headphone, P-amp, Configuring, Opamps, For, Voltage, Gain

27 Feb 2011 Mon

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