Taken from 'Audio' magazine, November 1958

CONVERSION TO STEREO has become the principal problem to many audio fans - mainly because they are not yet completely familiar with what facilities they are likely to require. Either they have a complete monophonic system already and feel that from an economic standpoint they must use as much of it as possible, or else they plan to start from scratch, adding from time to time as they become more familiar with their needs.

The Harman-Kardon "Trio," Model A-224, serves - as its name indicates - in three roles. First, it is a complete stereo amplifier system with 12 watts of audio in each channel; second, it is a monophonic amplifier with 24 watts of audio (leaving the preamplifiers and tone-control stages unused); and third, it is a conversion amplifier which can serve as the input section of a stereophonic system and one of the two output amplifiers - one which provides 24 watts of power - with the other being one already in the user's monophonic system.

In its first role - that of a complete stereophonic amplifier - the Trio provides two equalized preamplifier stages, usable for either phono or tape head inputs, and also accommodates three high-level inputs, such as tuner, tape preamp output, or high-output phono pickup, such as a ceramic. In the latter inputs, the impedance offered to the cartridge is 2 Megohms, which is sufficient for good low-frequency response with a ceramic cartridge. The preamplifiers are 12AX7's, with feedback equalization, switchable on the rear panel of the amplifier to phono or tape positions. These are followed by one section of a 12AU7 in each channel to provide sufficient gain for the tone controls, and these are followed in turn by 12AX7's as gain and phase-splitter stages, which drive a pair of EL84s in each output channel.

Tone controls are ganged for the two channels, with bass and treble being separate as in all high-fidelity equipment. The volume-loudness control is made to serve in the chosen role by means of a slide switch, and another slide switch serves to control the rumble filter. The MODE switch provides for stereo, stereo reverse, and either right or left input channel to both outputs as a monophonic amplifier. The FUNCTION control selects the input source.

Two other slide switches provide an unusual form of control which would be ideal in many installations. One switch, with positions labelled ONE and ALL, controls whether one or two pairs of output lines are being fed, and the other controls which of the two output pairs is being fed when the first switch is set at ONE. Thus the user can feed one speaker system in his living room or another in his den, or he may feed both at the same time if he wishes. This provides a sort of flexibility in speaker control that is rarely found on an amplifier control panel. Similar control action works as well when the amplifier is used in the monophonic application.

For both monophonic and stereo-conversion applications, it is necessary to switch the operation from SEPARATE to PARALLEL. This is done by means of a slide switch located inside the amplifier on the apron behind the front panel. This switch connects both output amplifiers to the right input section, and feeds the left input section to an output jack located on the rear panel of the amplifier. When this is done, it is necessary to strap the speaker connections of both output sections together. Both amplifiers have output impedances of 8, 16, and 32 ohms, and the instructions indicate that when a 16-ohm speaker is used in the monophonic or stereo-conversion application, both 32-ohm terminals should be strapped together to feed a 16 ohm speaker and so on. In the usual manner, the second (left) channel is fed to the input of a second power amplifier and thence to its own speaker system. The speaker selector switches are inoperative in the stereo-conversion connection, but they still function in the monophonic use.

Sensitivity of the amplifier is such that a 3mV signal at the phono input or a 1mV signal at the tape input will give the rated 12 watt output, and a similar output is had from a 300mV signal at the AUX, TUNER, and high-level phono inputs. A tape output is provided ahead of both tone and volume controls, with a 1-volt signal for the indicated inputs. The output for the left-channel external amplifier is 0.5 volts. Tone control ranges are +12 db at 50 and 10,000 cps, with the rumble filter giving a rolloff of 12 db per octave below 50 cps.

The Trio may be used in its normal metal housing (at extra cost) or it may be mounted behind a conventional panel without the enclosure. In operation, it was found to be convenient, and to have adequate control flexibility for practically any installation. The only change we might suggest in the entire unit is that it might provide both tape and phono input jacks for the preamplifiers, and to have the FUNCTION control switch inputs from tape to phono (and change the equalization at the same time) as a front panel control, rather than making it necessary to change input plugs and throw the switch on the rear panel. This is an amplifier we could recommend heartily for any one going through the pangs of conversion from mono to stereo.