D1080 Mk II 08 actives - initial listening and some refinements

Having disassembled the amp board from its backplate once and got heatsink compound in various undesirable places, I was hoping that the first round of mods would also be the last. No such luck

However, first listening was extremely promising, so I was definitely up for a second round of extreme messiness to gild the lily. I did notice that my grounding changes had introduced a slight hum (50Hz fundamental only it seems) which was only audible when no music was playing and was independent of volume setting. The main gripe I had was that the stereo image was shifted over to one side - this I decided was because the volume pot was being used towards the lower end of its range where the matching is poorest. The chip amps have rather high gain (32dB in bridged) and this can't be reduced. So the solution had to be modding the resistive dividers between the XO and the amp chips - in effect reducing the power amp sensitivity. I went for about a 9dB reduction, determined by the resistor values I had to hand.

In getting down and dirty with these changes I also re-checked the datasheet recommendations for input coupling caps to the TDAs and found these had been skimped on (only 220nF in balanced mode). On increasing these to 2.2uF polyesters the turn on clunk became a pesky annoyance. To fix this I tweaked the time constants of the AC supply detector (half-wave rectifier from the low current supply). Alone these changes didn't do the trick, but together with beefing up the chipamp half-supply decouplers (additional 100uF in parallel with the existing ones), the turn-on became a quiet 'tick' once again. Bliss

Some more extended listening revealed 'ticks' which were noticeable in relatively quiet musical passages. I wondered if these were indicative of parasitic oscillations in the amp chips (no local decouplers being present, just the main res caps). After fitting 100nF 1206 ceramics right across the supply pins (two pairs per chip), I no longer noticed these so I figure that was the problem.

Finally, for no other reason than I had one to hand and wanted to see if it would make any difference, I installed a common-mode choke prior to the transformer (2 * 10mH). The rationale behind this is to discourage HF currents from passing from the mains through the grounds of the audio input cables.

Next posting - listening test...