Tweaks of the high fidelity preamplifier Audioplan Homogen (old model)

We're talking about the preamplifier Audioplan Homogen which was in fact manufactured by B&K Electronics in the U.S.A. and which was sold by Audioplan in Germany in the mid eighties using the Audioplan brand name.
Please note that in the mid nineties Audioplan sold a new preamplifier with the same name, but that was a completely different model and (probably) manufactured somewhere in Southeast Europe (but that is just hearsay).

To all electrolytic capacitors within the signal path a smaller MKT foil capcitors will be connected in parallel to improve the high frequency behaviour. The operational amplifiers within the AF signal path will be replaced by better ones (even just this results in a noticable improvement). The power supply regulation within the external power supply will be removed and be replaced by a better regulation within the preamplifier housing.

All modifications are done on your own risk and lead to the cancellation of any warranty.

The modifications in detail:

Open the case:
1. remove the screws which hold the lid in place.
2. remove the lid.
3. remove all plugs from the pcb.
4. loose the screws on all knobs at the front side and remove them.
5. unscrew the nuts which hold the volume and balance poti and the power switch in place.
6. take the pcb out of the housing.
Carry out the modifications:
- Connect 100 nF MKT foil capacitors parallel to each of the 47 µF electrolytic capacitors in the phono and line stages.
- Replace all operational amplifiers by OPA2134 or OPA2604 (both made by Burr-Brown). This is easy as there are already sockets installed.
  In principle you can choose any dual operational amplifier. The one which is used for the first amplification stage in the phono stage should have very low noise. The DC precision does not matter as all stages are coupled with capacitors anyway.
- Remove the 7815/7915 voltage regulators in the external power supply and connect the outgoing cable to the (previous) input pins.
- Build a new regulation using LM317/LM337 with 100 Ohm and 1100 Ohm resistors (1% precision) onto a small laboratory pcb and fix it close to the amplifier pcb in the preamplifier housing.
- Connect the regulation board to the external power supply and to the preamplifier pcb.
remount the case:
1. put in the preamplifier pcb.
2. screw in the nuts which hold the volume and balance poti and the power switch in place.
3. put on all knobs at the front side and tighten their screws.
4. put on the lid and fix it with the remaining screws.

Have fun listening to your new preamplifier. The gain in sound quality is quite noticable and also depends on the type of the operational amplifiers.

Appendix

MKT foil capacitors are available e.g. from Wima.

URL: http://www.huennebeck-online.de/mod/Homogen-english.html
last change: $Date: 2003/05/01 21:49:33 $

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