[CButterworth]

Hello,

I finally decided to install the Nirvana upgrade to my AKSA55.  I still have to tweak a few things (the speaker / earth offset).

However, I noticed a discrepancy in the instructions on the adjustments page:

OK, so the bias needs to be set to the usual setting of 55mV.  The probes are placed at TP1 and TP2 - the extreme ends of the large output resistors.  This is straightforward.

The testing then says to play music quietly for around 20 minutes, then recheck the bias, and set it back to 55mV using the wound pot.  The instructions say to use TP1 and TP2.  OK, no problem.

However, the testing then says to play loud music for 20 minutes, then let the amp rest for 10 minutes (switched on, but no music), then recheck the bias.  It says something like "now place the probes again on TP1 and TP2 (the right legs of the emmitters)..." to make sure that the quiescent bias does not fall below 45mV and adjust the wound pot to bring it up to that value.

This is what I don't understand - TP1 and TP2 are at the Left and Right extremes of the emmitters NOT at the Right legs of the emmitters.  This is confusing.

SO....do I use the same test points for all the bias measurements?  If, the answer is "yes", then are these test points the ones shown in the diagram in the instructions?

Confused,
Charlie

[AKSA]

Charlie,

The emitters of the output devices and TP1 and TP2 are one and the same thing;  check under the pcb and you will see all roads lead to Rome.......

When measuring from above, however, the component side, a probe on the emitter of an output device becomes a disaster waiting to happen, particularly if the probe slips.  Nonetheless, access to TP1 and TP2 once installed is not always easy, so I offer both options to check bias in the instructions.

Since the instructions are so comprehensive, and go on forever, I don't like to explain absolutely everything because it just makes it too long winded, actively putting people off.  One should be mindful of the kitbuilder's prime directive:

'When all else fails, read the instructions'.  

However, I should amend the instructions briefly to reflect this!

Cheers,

Hugh

[CButterworth]

Hugh,

Thank-you for the response.  After I asked the question, I thought about checking the circuit diagrams to see if they were the same.

Everything on the Nirvana seems to check-out OK.  Although I did not notice a drop to 45mV after the "loud" playing for 20 minutes followed by 10 minutes with no sound - the bias stayed at around 55mV.  Maybe I did not play the music loud enough to demand the amplifier.

Anyway, if this is not a problem, then I will not bother retrying the test.  Is it a problem?

It sounds very good, and I am pleased at having success in the upgrade.

Regards,
Charlie

[AKSA]

No problem, Charlie.

The business of dropping to 45mV after a heavy music session reflects the thermal stability of the amplifier.  With very careful dimensioning of the bias generator values I have gradually improved this performance, and while it is possible even with the better design of the last 18 months to get it to drop to 45mV, this requires poor convection cooling at the heatsink and a 4R load in a high ambient temperature around 45C.  This is a 'Death Valley' test, of the type used by US auto makers for their cooling systems, but I considered nothing less was required to make the design robust across all climates of the world.  Of course, the fact that even Melbourne hits 45C once or twice each summer helped......

Delighted you like the sound, stick around, hombre, it gets better!

Cheers,

Hugh

[CButterworth]

Hugh,

I may get to test extreme temps in my amp, as I will move to Phoenix in the summer     Well.....I hope not, we'll likely make good use of the AC.  I will however, get to test RF noise rejection in the amp, as I'll have a 1 acre yard in the countryside and be able to install a powerful HAM antenna system and make a return to my large Kenwood tube transmitter.  Having said that, a good antenna will likely mean far lower spurious RF emissions, so it may not be a fair test.

The next item on my agenda is to buy good speakers, preferably a kit.  This will be followed by either the DAKSA, or one of the AKSA preamps.

Anyway, I will listen to the Nirvana break-in and begin a testimonial of the AKSA, which I hope you'll be willing to put on your website.

Regards,
Charlie