Go Back   Home > Forums > > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

AKSA Builders of market proven kits and modules at sound quality to rival the best in high end

Reply
 
Thread Tools Search this Thread
Old 24th July 2017, 06:49 AM   #1
andyr is offline andyr  Australia
diyAudio Member
 
Join Date: Nov 2003
Location: Melbourne (Oz, not Florida!)
Default Increasing the bias current on my NAKSA80s & Sorayas ... seems to be a good thing!!

In my system (actively tri-amped Maggies driven by 2x NAKSA 80s (for mids & ribbons) and a Soraya on the bass panels), I suffer from a lack of soundstage depth. Unfortunately, this is due to the (lack of) distance between the Maggies and the front wall - which is caused by the room dimensions. I may be able to improve soundstage depth by treating the front wall with diffusion products but, in the meantime, with Hugh's advice, I decided to increase the biasing of the amps.

Hugh told me that increasing bias delivers more soundstage depth and suggested I could double the bias currents without causing any problem for the output devices - this means:
* 70mv across the 0.33 ohm res (R22) on the NAKSA 80s (instead of 35mV), and
* 120mV across the emitters of the outermost output transistors on the Soraya modules (instead of 60mV).

As increased bias means increased heat into the heatsinks, I decided to go slightly conservative - so a bit less than twice the stock bias:
* 60mv across the 0.33 ohm res on the NAKSA 80s, and
* 110mV for the Soraya modules.

I'm hoping my heatsinks will be able to cope with this in summer - as the result is delightful! Yes, a bit more depth ... but also more 'body' to the sound!

So this is something that AKSA owners might like to try out.

Andy
  Reply With Quote
Old 24th July 2017, 09:01 AM   #2
saf is offline saf  Portugal
diyAudio Member
 
Join Date: Sep 2009
Location: Porto
Wow...this is a very easy trick ! Hugh, does this apply to the Mayas too ?
  Reply With Quote
Old 24th July 2017, 09:38 AM   #3
AKSA is offline AKSA  Australia
diyAudio Member
 
Join Date: Sep 2001
Location: Melbourne, Australia
Thanks Andy!

Felipe, I have taken the Maya to 150mA (54mV across the two 0.18R source resistors, TP1 to TP2) and it does grow a little scale. But I'm reluctant to take it more because with 65V rails, the heat dissipation is starting to move up, particularly in a hot climate.

You could take it up to 200mA for a brief period if you wish, that is, 72mV. But it's dissipating 26W at each of the two outputs into a 0.27C/watt heatsink, and this would mean 15C above ambient at idle, and considerably more heat if it's working hard.

You Maya is going well, Felipe? Are you as happy as you were a month back? I do hope so...... today I visited a Melbourne customer, Russell, who has a 125W Maya on recent Harbeth large, stand speakers - beautiful speakers, absolutely wonderful. We compared the Maya with a DIY 2A3 SET amp with 3.5W per channel, and we come to the same conclusion that the Maya was almost as musical but a little more clear than the SET, with a similar sound stage, very good depth. I was pretty happy about that.....

Cheers,

Hugh
__________________
Aspen Amplifiers P/L (Australia)
www.aksaonline.com
  Reply With Quote
Old 9th October 2017, 07:37 PM   #4
Sjef is offline Sjef  Netherlands
diyAudio Member
 
Join Date: Jan 2003
Location: Amsterdam
Would this also work for the older aksa70 as I have two in my bedroom system that lack a little live and sparkle in general.
  Reply With Quote
Old 9th October 2017, 09:54 PM   #5
AKSA is offline AKSA  Australia
diyAudio Member
 
Join Date: Sep 2001
Location: Melbourne, Australia
No, do not do this with the NAKSA 70. Bias control is not to precise as the 80 and the Maya, and taking it out to high quiescent is dangerous. Best left around 70mA, no more.

Hugh
__________________
Aspen Amplifiers P/L (Australia)
www.aksaonline.com
  Reply With Quote
Old 10th October 2017, 06:59 AM   #6
Sjef is offline Sjef  Netherlands
diyAudio Member
 
Join Date: Jan 2003
Location: Amsterdam
Ah, great info, thanks. I'll leave it as it is.
  Reply With Quote
Old 10th October 2017, 08:31 AM   #7
AKSA is offline AKSA  Australia
diyAudio Member
 
Join Date: Sep 2001
Location: Melbourne, Australia
Sjef,

I sold the NAKSA 70 only about a year, and had a couple of failures as a result of enthusiastic fiddling with the bias. I thought about it over quite a bit of time, did some tests, and found that over about 110mA I could lose temperature control of the output stage in some situations, particularly hot weather and strong bass. It prompted me to release a major redesign, the NAKSA 80, and for nearly three years have had no failures of that model. I am extremely cautious about temperature compensation on my amps now, and take pains to ensure that increasing temperatures in arduous conditions the quiescent REDUCES, to prevent thermal runaway. I have honed this approach over a couple of years and my new amps have sophisticated temperature control, using direct sensors on the output device, to measure heat very quickly and react immediately. I have found this issue critical in powerful amps using very large mosfets, like my Maya, which uses very large 480W devices featuring huge transconductance, that is, small increases in gate voltage result in huge increases in drain source. Thermal management in large amplifiers is similar to controlling a nuclear reactor, get it wrong, and there is a lot of smoke and fire!

If you are worried about the top end of the amp, which is actually very good on the N70, examine the other areas of the system, particularly the tweeters on your speaker, or perhaps the source.

Cheers,

Hugh
__________________
Aspen Amplifiers P/L (Australia)
www.aksaonline.com
  Reply With Quote
Old 10th October 2017, 05:43 PM   #8
Sjef is offline Sjef  Netherlands
diyAudio Member
 
Join Date: Jan 2003
Location: Amsterdam
That might explain why one of the amps runs warmer then the other. I'm using them in an active threeway system for mids and highs. In fact I like their treble the best as it keeps good control over a pair of old ess sir motion transformers with can get a little harsh on lesser amps. I was more talking about midrange, while pretty good just not as lush as a real classe A. Nevertheless highly enjoyable.

What is your view for the long term regarding temperature management. I tend to keep my equipment for periods of more then 10 sometimes even 25 years. Should I be worried about it?
  Reply With Quote
Old 12th October 2017, 07:36 AM   #9
AKSA is offline AKSA  Australia
diyAudio Member
 
Join Date: Sep 2001
Location: Melbourne, Australia
Hi Sjef,

Long term should be fine if you ensure all quiescents are set at 70mA.
If one is hot and the other cool, they are unequal, and should be corrected.

OTOH, I have later, better models now; NAKSA 80 has better top end, and SAKSA 80 has better midrange, but of course more designs cost you (and me!) more money.......

Despite my constant attempts, I cannot apologise for my latest amps sounding better than the older models! It's just life...... like buying a new car, and peeling an onion.

Ciao,

Hugh
__________________
Aspen Amplifiers P/L (Australia)
www.aksaonline.com
  Reply With Quote
Old 12th October 2017, 09:15 AM   #10
Sjef is offline Sjef  Netherlands
diyAudio Member
 
Join Date: Jan 2003
Location: Amsterdam
Hi Hugh,

It's not that one amp run hot. In fact it only runs warm, around 30 degrees or fo while the other one remains at almost room temperature. Both amps are virtually doing nothing. They drive a 93db / 16 ohm midrange driver from 150hz and a 95db / 8 ohm tweeter from 3500hz on up.

No need to apologize for anything, offcoarse there is always something better. I don't have any problems with that.
  Reply With Quote

Reply


Increasing the bias current on my NAKSA80s & Sorayas ... seems to be a good thing!!Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Using LED bias for 4.5 mA cathode current, is it a good idea? rtsang Tubes / Valves 11 29th January 2009 03:53 PM
Increasing bias in amps. beppe61 Solid State 124 17th February 2007 02:26 PM
increasing the bias will cause the amp to..... frank2395 Tubes / Valves 2 1st June 2005 12:12 AM


New To Site? Need Help?

All times are GMT. The time now is 05:46 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki