Elektor Preamp 2012 (Douglas Self) Mini, low budget

[borderman]

My Icepower asx125-2 needed a pre-amplifier. I tried the Velleman K1803 but it produced too much noise, even with a low noise opamp.
I decided to give the elektor preamp of Douglas Self a chance. But to fit in the same cabinet (21x17x4 cm.) as the asx125-2 I had to undress it, so I removed:
1 Moving-Coil / Moving-Magnet PCB: I don’t have antique audio equipment anymore.
2 Level indicator PCB: Too much space and money to blink a LED
3 Source selector PCB: Source can be selected direct by the rotary switch
The Power supply is necessary but in a more compact form: 2 switching power supplies (NFM-05-24), the amplifier only consumes 0,14 mA.
The recifiers on the Power supply board are replaced by a currentlimiter to avoid activation of the overload protection on the smps due to the high inrush current .
Also a background illuminion around the knobs were added

The result was a compact low noise amplifier that perfectly fits to the asx125-2.
The crystal clear sound of the combination was much better than the asx-2 stand alone.
T(his is my first thread, don't know if pictures are uploaded.)

[Zero D]

Hi, i love the way you've managed to illuminate the controls, very symetrically under the knobs

I wouldn't have chosen a SMPS though

[borderman]

Originally Posted by Zero D

Hi, i love the way you've managed to illuminate the controls, very symetrically under the knobs

I wouldn't have chosen a SMPS though

What's wrong with a SMPS ? At least low volume and high efficiency.

[Ken Newton]

Originally Posted by borderman

What's wrong with a SMPS ? At least low volume and high efficiency.

The main concern is the large amount of high frequency noise they typically generate. Since this generated noise usually occupies an ultrasonic to RF spectrum it isn't, itself, audible, but it could drive the powered analog circuits in to non-linearity or provoke audible intermodulation distortion. Filtering this noise is a standard SMPS design requirement. For high performance audio, however, extra pains must be taken to implement a low pass filter which will suppress this noise by perhaps as much as 120dB, relative to the audio signal.

[Bonsai]

Dougs preamp uses the Baxandall volume control, so there are a few summimg junctions as well, and they can be quite susceptible to pickup if you dont take care.

One option you can explore is to mount the PSU in a separate box and then run it to the pre via a 3 way cable (often this approach is used in the high end). Try to separate the pre and the PSU by 10 feet or so.

However, you pre-amp looks great! well done!

[NicMac]

Bang & Olufsen

[atupi]

Very nice work. That front panel is amazing, can you describe how is built ?

[borderman]

Originally Posted by Ken Newton

The main concern is the large amount of high frequency noise they typically generate. Since this generated noise usually occupies an ultrasonic to RF spectrum it isn't, itself, audible, but it could drive the powered analog circuits in to non-linearity or provoke audible intermodulation distortion. Filtering this noise is a standard SMPS design requirement. For high performance audio, however, extra pains must be taken to implement a low pass filter which will suppress this noise by perhaps as much as 120dB, relative to the audio signal.

The smps complies to:
EMC emission: EN55011(CISPR11), EN55022(CISPR22) Class B, EN61000-3-2,-3
So some action action to suppress the noise has been taken. I will have a look with the oscilloscope what spikes and noise remains and what extra precautions are necessary. Thanks, also to Bonsai

Originally Posted by NicMac

Bang & Olufsen

BANG, you are right.

Originally Posted by atupi

Very nice work. That front panel is amazing, can you describe how is built ?

The construction is easy.
There are 2 holes 3mm. drilled to fit 2 flattop Leds. The nut for the potmeter is replaced by a tranparant plastic lightguide. This lightguide receives the light from the bottom and the sides. It emits the light to the knob and is reflected to the frontpanel. Painting the bottom of the knob white helps the reflection.
See the pictures.

[Bonsai]

So you replace the nut that holds the potentiometer or switch with a transparent nut? Where did you get the transparent nut from?

[borderman]

Originally Posted by Bonsai

So you replace the nut that holds the potentiometer or switch with a transparent nut? Where did you get the transparent nut from?

I turned it on a lathe. The material is probably PMMA I had some pieces bar material for a long time, don't know the source.