Elektor Preamp 2012 (Douglas Self) Mini, low budget

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
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.)
 

Attachments

  • 20130325_144001.jpg
    20130325_144001.jpg
    222.5 KB · Views: 2,410
  • 20130325_144051.jpg
    20130325_144051.jpg
    233.6 KB · Views: 2,377
  • IMG_0687.jpg
    IMG_0687.jpg
    435.9 KB · Views: 2,308
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.
 
Member
Joined 2003
Paid Member
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!
 
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

Bang & Olufsen;)
BANG, you are right.

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.
 

Attachments

  • backside.jpg
    backside.jpg
    61.3 KB · Views: 1,874
  • frontside.jpg
    frontside.jpg
    64.8 KB · Views: 1,665
  • lightguide.jpg
    lightguide.jpg
    146 KB · Views: 559
  • sketch.jpg
    sketch.jpg
    152.3 KB · Views: 626
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.

I understand now what you mean. The oscilloscope showed a lot of high frequency at the input, as well the output of the power PCB.
It was even visible at the audio output. Although I can't hear it, I don't want this in the preamp.
Will look now for a suitable trafo which fits in the space.

Thanks for your remark
 
Hi,

The NFM supplies may be used very well in ths application. I had used them as supplies for even a low output MC pickup phono pre and got no issues.
You have to take care of the HF noise. But this is quite easy. A small LC filter (series inductance or ferrite bead followed by a shunt capacitor) may already be sufficient. If you add a simple gyrator You're on the safe side.
Apart from possible hum problems due to loops (which is a inherent problem to all supplies, not just SMPS) the big advantages of SMPS are that they are noise free below their (ultrasonic) clock rate and that the measurements against switching artefacts and noise require just small, compact and cheap parts.
The classic transformer supply on the other hand has hum and noise right within the audio band. This requires more effort and larger and more expensive parts.
The only drawback I experienced with the NFMs is that some specimen's transformer may radiate a high pitched sound. The PM05-xx are encapsulated brethren with the same PCB mountable pinout. Those are mechanically quiet, look much better and are safer too.
You might also look for XP-Power's 5-30W dual supply, chassis mount ECL-Series.

jauu
Calvin
 
Hi,

The NFM supplies may be used very well in ths application. I had used them as supplies for even a low output MC pickup phono pre and got no issues.
You have to take care of the HF noise. But this is quite easy. A small LC filter (series inductance or ferrite bead followed by a shunt capacitor) may already be sufficient. If you add a simple gyrator You're on the safe side.
Apart from possible hum problems due to loops (which is a inherent problem to all supplies, not just SMPS) the big advantages of SMPS are that they are noise free below their (ultrasonic) clock rate and that the measurements against switching artefacts and noise require just small, compact and cheap parts.
The classic transformer supply on the other hand has hum and noise right within the audio band. This requires more effort and larger and more expensive parts.
The only drawback I experienced with the NFMs is that some specimen's transformer may radiate a high pitched sound. The PM05-xx are encapsulated brethren with the same PCB mountable pinout. Those are mechanically quiet, look much better and are safer too.
You might also look for XP-Power's 5-30W dual supply, chassis mount ECL-Series.

jauu
Calvin

Hi Calvin, thanks for your proposal but feel myself now a bit between 2 worlds. I ordered 2 trafo’s MCFM70/18 at Farnell.
It has a diameter of 51 mm. which can be build in the current case. 194mA should be good enough to drive the amlifier (140 mA)
Fortunately this was my second build preamp. The first one is driving, 10 hours a day, 2 Icepower ASP1000 in the living room.
I will do 3 things:

1) For one preamp
Go back to the original design and build in the transformer.

2) For the other one
Build in a LC filter or a simple gyrator.
Calvin can you help a bit here ? What values do you propose for L and C? A gyristor is probably not an option. It requires more components and as I am not able to produce a PCB have to build the components “in the air”.
An alternative (low radiance) SMPS is difficult due to space limitations. I looked to many types.
The internal heigth of the case is 40 mm.
The XP Power ECL serie for dual output is too large. I have to take single output and stack 2 pcs.
The nearest XP power ECL would be the ECL05US24E dimension: LxWxD=52.4x27.2x23 and cannot be stacked within 40mm.
An alternative is the ECE05US24 25.4x25.4x15.4 can be stacked but will probably not much better for radiation than the current meanwell NFM05 SMPS.
The compliance to EMC emission for the NFM05 and PM05 are the same.

3) Have a look how clean the internal 18 v powersource of the 125ASX-2 is. This could be an alternative.

After that I will compare the results and choose for the best solution.
 
Very nice mechanics. I am sure that it sounds just fine.
To bad Elektor could not figure out enough to put everything onto one pcb and make life so much easier in the wiring dept, including connectors.
Also to incorporating the tone amp, as was done in the '96 pre-amp design would have made this design stellar, well a little bit better, sorry i like that tone amp design, that is if you need them in the first place. Doug is said to be working on a low noise version of this tone control ckt.I am thinking of trying to make it remote controlled = any ideas anyone?
Honestly, can you live in this day and age and not need remote control? and this is coming from a guy that does not own a cellphone. Every recording that I own seems to have a different level, even switching radio stations has a diff level:)
None the less good DIY stuff.
Rick
 
Last edited:
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.