Testbench - Yamaha YTA15A Combo Amp
Test Report on: Yamaha YTA15A Combo Amp.
Date: April 1975, £97-20
The YTA15A is a small combination amplifier rated at 25 watts RMS. It is designed for a lower cost market than the amplifiers previously tested and should suit the needs of most small bands other than those playing 'heavy rock'. It is also a suitable size for house or studio use on electric piano or guitar.
Layout is conventional. A chassis control panel is mounted in the top of the case above a 12 inch speaker. Overall size is only 18½ inches wide, 17¼ inches high and 8 inches deep; which with a weight of under 25 lbs makes it very portable and convenient for small gigs.
The finish is 'spartan' but good. Grey leatherette makes a pleasant change from the normal black, and the matching protective mouldings on all corners and grey handle gives an overall attractive contrast to the black control panel and speaker doth.
The controls provided are: volume, bass, treble, distortion effect (fuzz), reverb level and an on/off switch. There are also two input sockets, one high sensitivity and one low, and a pilot light on the control panel. The back panel carries two fuses, a socket for an optional reverb foot switch, and, had we been provided with the correct model for this country, there would have been a voltage selector. The mains lead is of the retained type and has a moulded-on continental plug. A plastic strap is also thoughtfully provided for stowing the mains lead.
The quality of construction is good - an unusual feature on a low price amplifier. Access to the electronics is easy and all parts are well finished and fit together without difficulty.
The electronics are on a single phenol banded paper, printed circuit board. Components are all of good quality and correctly fitted but the 'domestic equipment' practice of mounting components on end has been used. This is naturally done to permit a smaller, therefore cheaper, board to be used but in this case there is plenty of space to lay the components flat on the board had they chosen, soldered joints are good and wiring tidy.
The circuit uses 5 transistors, one integrated circuit and a sealed module power stage.
Performance is good for this class of equipment as the following measurements show:
|Measured Performance||result||test conditions||remarks|
|Power Output to 8 L.S.||27.4 W RMS||@ 10% total harmonic distortion||Rated value 25 watts|
|20.16W RMS||@ 1 % total harmonic distortion||Mains volts set to 220V as on the label|
|Distortion @ 50% rated power||0.37%||1KHz sine wave 12.5W||Fair|
|Distortion @ 22W||1.7%||450Hz sine wave||Manufacturer's figure less than 3%|
|Sensitivity||Hi 49m V. RMS||Bass, Treb., Vol @ Mix|
|Lo 100m V. RMS||450Hz sine wave for 20W out|
|Bass Control||17.6dB swing||@ 70Hz||16 ±3 quoted by manufacturer|
|Treble Control||19.6dB swing||@ 7KHz||19 ±3 quoted by manufacturer|
|Noise||—59.2dB relative & 20 W||Vol, Bass, Treble @ max. Reverb & distortion @ min.||fair|
|Distortion effect control||20m V RMS input is required to add 20% to THD||Too insensitive for most guitars|
|Reverb||Subjective test only||Only a single spring gives bad echo from repeated reflections from the ends of the spring but otherwise the sound is dear and undistorted|
The design shows how a low price can be achieved by very careful consideration of the specification while maintaining good construction quality. This is far more satisfactory than offering the earth in the spec., but reducing quality, as often happens. The whole unit is very practical, workmanlike and will do most jobs where the status of huge pyramids of equipment is not required. 25 watts is only 6dB below 100 watts.
One point which I feel is very bad is that the wrong unit was supplied for use in this country. It has a two pin and earth continental mains plug moulded on the lead. This will inevitably be used in a shaver adapter resulting in an unearthed chassis. Also the unit provided was for 220 volt mains not 240 V. This can easily be connected by fitting a 3 pin mains plug and a minor change inside; then you have a safe unit and good value for money.
Oscillator; purpose built sine wave generator, total harmonic distortion measured at 0.005 %.
Total Harmonic Distortion Factor Meter, Sugden JE S1452.
Advance OS250 Oscilloscope with calibrator.
Resistive loads 2% tolerance, 4, 8 and 16 ohms.
Level TM3 milli-volt meter.
Power measurements are correct to ±10%
Sensitivity measurements are correct to ±0.5dB.
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