What is an output stage?

The output stage is the part between the analog converter and the cinch sockets on the back. Here is the final processing of the now analog audio signal of the converter instead. Depending on the type of converter, there are different components that are necessary here. The Philips converters TDA1540 differ from the converters of modern design not without reason by their natural sound:

Modern analogue converters only occasionally require active decoupling since they already provide a ready-processed audio signal with embedded filters and amplification stages - this can no longer be influenced by changing the filters and operational amplifiers. Active decoupling takes place in the part of the output stage called the buffer. After using such a transducer to buffer a tube, it only happens for marketing reasons. Mostly this function is performed by a chip OP-amplifier. With the principle of a voltage follower of the buffer, there is no amplification and at best no change of the audio signal. The converter and buffer also produce DC voltage, which is finally filtered by a capacitor. The music signal is otherwise shifted from the zero axis, which results in a higher load on the following components - too high level to the point of damage to filterless speakers. This coupling capacitor allows only the part of the audio signal consisting of alternating current.


The output stage of a TDA1540 converter is more elaborate because no finished audio signal is delivered and this still has to be electrically converted. Due to this design, the analog converter also allows interference in the analog signal. Below are the possible modules in CD players, depending on the model, in order of the analogue converter.


Tweeter correction. The De-emphasis is an analog high-tone filter. This filter is activated by the digital decoder when it receives a signal which is in the subcode on the CD to be played - which is not usually the case. This primarily inactive circuit requires no special attention.

I/U Conversion

With the I / U conversion, the electrical conversion is completed, the audio signal is available thereafter. The I / U conversion runs in the first half of a dual OPV. Not all chip OPVs, commonly referred to as a good-sounding buffer, provide the optimal features. An optimal mix of passive and active I / U conversion, which has the best features due to its speed and high virtual mass, is adapted to the respective operational amplifier.

Analog filter

The signal is reduced to the audio band. The analog filters are placed between the standard double OPV. They separate the audio signal with an 18db slope from the inaudible frequency range.


The signal is stabilized. The decoupling by a buffer occurs in the second half of a double OPVs. The full range of a TDA1540 transducer without "swallowing", without analytical sharpness, with the largest power reserves and that without typical discoloration of a tube output may be expected in the metronom.

Coupling capacitor

DC filtering. A coupling capacitor behind the active components in the signal path is the simplest method to prevent the unwanted DC voltage, but always represents a compromise. Here, a capacitor forms increasing loss factor and microphony with increasing size. This important task is for a bipolar electrolytic capacitor with the utmost expediency - no foil capacitor.


For muting, the signal is left as standard to a high-quality reed relay. For base units in which this is to be done by a transistor, this area can be bridged.