Hi Sean,
Thanks for your post. Your problems are minor, nothing significant.
First question. Gain. More precisely the adjustment thereof. When you are gunning for your 55mV gain do you just take the initial gain at switch on (ie.cold) or do you let it idle for a bit? After all the surgery, the gain started at 43 and I bumped it up to 55. Checked T2 and all was sweet then went back to T1 and we were up over 60 and climbing.....eventually got it down to 55 after ten minutes or so but nearly wound P1 back to where I started (or so I thought). This happened to both boards. Feed it some tunes (Massive Attack's 'Mezzanine') and managed to get the heatsinks well over room temperature. After resting for ten minutes, checked the reading across T1........30mV punched it back up to 45mV as suggested. And that's about it. Have done no more adjustment. Should I? Do I need to? Basically I don't understand the gain adjustment process so I have no idea whether all is good/bad/indifferent. Can someone explain in laymans terms what the go is just to set my mind at rest?
I believe the word you want here is bias; gain is fixed by the feedback resistors and bias is the standing current which flows at idle in the output stage.
First, you must ensure that the thermal coupling between each and every output device and the heatsink ledge is good. You need a thin film of the paste on both sides of the mica washer, and the same torque on each device's 3 mm bolt which holds the transistor, ledge and pcb together. And the Vbe transistor, the BD139, must be on top of T7 and screwed down with the metal portion against the power transistor. (No paste required here.) So, check all are evenly tightened.
To set bias, switch on, let it idle for about an hour, THEN set the level. Ensure there are no strong draughts around the heatsinks; don't do it with the back door open and a gale roaring through the house!
Bias does change slightly depending on ambient temperature. Set it as near as possible at 25C (77F) as this is a good compromise.
Second question/observation. I'm currently using a Yamaha integrated amp as a pre-amp. The AKSA is now hooked up as the amp (previously I'd bi-amped using the Yammy for the high range). If the AKSA is on and I kick the Yammy over I get a very sharp crack through the speaker - not loud but not real soft either - definitely not pleasant. Same goes when I switch off. Turning the CD on produces the same soft whump it always has. Turning the AKSA on last stops the crack but turning it off last doesn't (that said though there's still power in the AKSA even after a couple of minutes). What the heck is going on? This crack is something I've not heard before despite using the Yammy for quite some time as a pre-amp. It appears to crack in time with the Yammy switch relay but why would this be a drama now and not before. And this feature did not present itself at anytime during testing. Any ideas?
The input cap on the AKSA is now larger. This lessens phase shift at very low frequencies, another way of saying you hear low frequencies better. Downside is that this makes the amp more susceptible to low frequency thumps at switch-on. Your Yamaha preamp has a voltage spike at its output at switch-on. That's all. It won't do any damage, and my advice would be to leave everything on all the time. It might cost you $5 a year in electricity, but the advantage is that your amp will always be hot to trot.
Now, be prepared for some strange sounds as the upgrade burns in. This takes about 120 hours and is normal. But the sounds the amp will make will be quite strange; muffled bass, strident top end, etc. It starts out fine, then goes bad. But at the finish it will be much better than now. And detail - you will be astonished.
The AKSA will have power for some time after switch off because of the considerable energy stored in the filter caps. This is normal. With the new power supply pcb you should have bleeder resistors (8K2) for each of the four rails. This will take about ten minutes to bleed off all voltage from these caps at switch off. Again, normal. The bleeders run warmish, of course.
Cheers,
Hugh
