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Philips 22AH270 pre-amp
Simple build pre-amplifier made in Belgium in 1980.
All properties a simple high-end pre-amp should have are in this design.
However Philips made many design concessions to keep the selling price low, so there is some reason to improve it.
This pre-amp is a member of the Philips "Black Tulip" range audio gear.
Also this pre-amp is capable of driving Philips MFB speaker boxes.
Matching: tuner 22AH170, power amplifier 22AH370, MFB box 22AH586 and more.
There were 2 versions produced by Philips of this 270, the early 1 version and the more common later 2 version,
that seem to have some questionable improvements.
Output voltage: 1 Volt per channel
Frequency range: 15 - 25,000 Hz (+/- 1.5 dB)
Harmonic distortion: less than 0.1%
Intermodulation distortion: less than 0.1%
Signal-to-noise ratio: 70/80 dB
Crosstalk damping: 55 dB
Treble control: +/- 10dB at 10kHz
Bass control: +/- 10dB at 50 Hz
Loudness control: at 50 Hz + 10dB and at 10kHz + 3dB
Noise & rumble filter: -6dB / octave
Input sensitivity @ 1kHz and maximum output: Record player 2.3mV @ 47k Ohm. Tape A: 200mV @ 180k Ohm.
Tape B: 200mV @ 100k Ohm. Tuner 200mV @ 47k Ohm. Tape monitor 200mV @ 100k Ohm.
Output impedance: headphones 8 to 600 Ohm.
MFB: 2 x 1 Volt output @ 1k Ohm.
Power supply: 220/250 volts
Dimensions: 78 x 482 x 280 mm.
Click here to download the user manual PDF format in Dutch
click here to download the service manual of the Philips 270
Restore on the way inside, replacing the silly red/brown/black wire by shielded cable, to get the noise-floor down ^
Updating the tone amp section ^
To get a fine high quality pre-amp you have to do some upgrading: the power supply is noticeable simple.
The 33V & 26V power lines need to be stabilized to get the noise floor down to high-end levels.
I did single stabilization for both voltage lines, as on the schematic diagram, but if you do both lines separately:
each an own stabilizer, thats even better.
The used electronic parts are of good quality, but aging is an issue so some parts should be replaced.
Due to the very simple power supply some resistors are burnt and need to be exchanged.
The pcb design is rather poor, so extra measures need to be taken: some kind of separating by shielding the pre-amp section
from the final stage is needed to avoid noise, feedback & oscilation.
Since this is not a DC-coupled electronic design, the coupling caps between the stages should be of much better quality.
Heatsinking the final transistor-set is needed as temperatures of this BC548/558 output-set is near 65° C.
This final stage gets into saturation when driven full power, causing distortion, output transistors capable
of handling 1 Watt can be the better option, eliminating distortion at high levels.
Hitachi 2SD667 & 2SB647 may be a good replacement.
Set the transformer from 220V~ to 230 or 240 in Europe, due to the detrimental voltage rise from 220V to 240V~.
Add an extra capacitor on the 26V power line, it helps making the noisy start of the amp softer, when switched on.
If you design the stabilized power supply addition well, the amp starts very softly, quite different from the original design.
If you use shielded cable intern to the output sockets, hum & noise will be much less and meet "High-end" level.
Omit the 2x 10µF caps in the output line, I do not know why they are in the circuit, they are not in the 1st original design:
I think it makes no sense to put a 220µF el-cap in series with a 10µF el-cap in the ouput line.
Take a look at the photo: "Upgrade ready" and you see how your 270 should look after the upgrade.
Schematic diagram and photo below of the circuit to add to the power supply to get a stable and quiet power supply ^
Updating the power supply: 1st. testing stabilization of the 33V rail, 1 capacitor should have a slightly higer value ^
Updating the final stage ^
Restore & upgrade ready, this is how the amp looks inside after the upgrade ^
old caps replaced and burnt resistors exchanged. ^
Matching tuner 22AH170 ^
Philips MFB box 22RH544 is a possible companion of the 22AH270 pre-amp.
The sound of the restored Philips 270 is very good.
© PE1MMK Hans Hilberink, last update: 22-09-2019.