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Increasing the bias current on my NAKSA80s & Sorayas ... seems to be a good thing!!

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Regarding your hot amp. A class AB amp rarely gets hot under normal listening. However, if there is even a few mA higher bias or the power supply is a few volt higher on one amp it can run several degrees hotter than the other.

Bias power dissipation is constant and most likely to set the heat sink temperature of the amp. Say 60 mA and 60 Volt, you have 3.6 watt and using four output trannies is 14.4 watt. This is will generate the same temperature than your soldering iron.

Unlike dynamic dissipation when you are playing music you need a relatively high average dissipation for the amp to become noticeable higher temperature.

My guess is the amps are not set up the same. I would recommend opening both amps and set their bias and off-set together under the same conditions. Don't do one today and the other tomorrow, do them together and in the same ambient.
 
Hi Saf,
The quote is from your Post #44 in the Maya thread, and clearly describes your setup and situation.
Have you given thought to possibly rearranging things a bit and giving the HOT amp in the middle of the rack a bit more space for ventilation?
No matter what, more ventilation never hurts.


Hi Steve,

You are correct on assuming that ventilation can be an issue.
But, you missed my last implementation of the Maya case, where i have perforated top plate :

34721257222_0a8aa9f497_b.jpg


This is more than enough to ventilate them nicely.
I do not think that this is a "ventilation" and postition of the amp thing.
In fact, i made some scientific measurements this week, like i promised, and found that i was clearly "wrong" in some assumptions i made, regarding temperature...or part of it.

I assumed the normal Maya was raising 10 degrees Celsius above ambient : i was right .
I assumed the hot Maya was raising up to 30 degrees Celsius above ambient : i was wrong.

But let me show you.

This is a test made with the following scnearios :

0min : Turn on both amps with NO SPEAKERS connected
60min : Connected speakers but NO SOUND INPUT
120min : Started playing some music in very low SPL (late night , wife sleeping)

37097499174_84f561dee9_z.jpg


First, we can clearly see that before music started, both amps behaved the same, despite different locations. This was during 120min . :p

Second, after music started, the normal Maya seems to stabilize at 9 degrees above ambient....just like i suspected. :)

Third, the HOT Maya did not stabilize even after 260min. I went to bed and turned it off after this. I am going to test them again, with HIHGER SPL next time and without "delay" time. :eek:

Take in mind that temperatures were measured OUTSIDE of the case.
I did not measure the temperature directly on the heatsinks. My case is all metal so i get good heat dissipation, but i believe the inside temperature on the heatsinks was a good 5 or more degrees above this ! :boggled:

I own a couple of very precise thermometers and i trust them down to 0.1ºC . I am a beer homebrewer so i use them to keep my mash with very tight tolerances :)


Hugh,

That is a very very good looking daughter you have there!
Congratulations and my sincere and most hearted wishes to a happy new family ! If only we all could live in the same continent....
 
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.

I have found sometimes if the DC offset isn't close to zero that the amp puts out a little DC and this makes one transistor warmer than the other.
 
Hi Steve,

You are correct on assuming that ventilation can be an issue.
But, you missed my last implementation of the Maya case, where i have perforated top plate :

34721257222_0a8aa9f497_b.jpg


This is more than enough to ventilate them nicely.
I do not think that this is a "ventilation" and postition of the amp thing.
In fact, i made some scientific measurements this week, like i promised, and found that i was clearly "wrong" in some assumptions i made, regarding temperature...or part of it.

I assumed the normal Maya was raising 10 degrees Celsius above ambient : i was right .
I assumed the hot Maya was raising up to 30 degrees Celsius above ambient : i was wrong.

But let me show you.

This is a test made with the following scnearios :

0min : Turn on both amps with NO SPEAKERS connected
60min : Connected speakers but NO SOUND INPUT
120min : Started playing some music in very low SPL (late night , wife sleeping)

37097499174_84f561dee9_z.jpg


First, we can clearly see that before music started, both amps behaved the same, despite different locations. This was during 120min . :p

Second, after music started, the normal Maya seems to stabilize at 9 degrees above ambient....just like i suspected. :)

Third, the HOT Maya did not stabilize even after 260min. I went to bed and turned it off after this. I am going to test them again, with HIHGER SPL next time and without "delay" time. :eek:

Take in mind that temperatures were measured OUTSIDE of the case.
I did not measure the temperature directly on the heatsinks. My case is all metal so i get good heat dissipation, but i believe the inside temperature on the heatsinks was a good 5 or more degrees above this ! :boggled:

I own a couple of very precise thermometers and i trust them down to 0.1ºC . I am a beer homebrewer so i use them to keep my mash with very tight tolerances :)


Hugh,

That is a very very good looking daughter you have there!
Congratulations and my sincere and most hearted wishes to a happy new family ! If only we all could live in the same continent....

Hi Saf,
Yes I didn't see a photo with the ventilated top cover.
Good that you took the time to take the temperatures and all.
I don't really see anything in your temperatures that is abnormal. The SF mini monitors are a little harder load than your other SF speakers.

I know how Hugh sets up his Maya modules and he is very thorough in his procedures and results.
All seems well.
 
Thanks Filipe,

Your piccies and temperature analysis is magnificent!
Clearly the additional heat comes around only when the music goes through the Mini speaker.

This indicates to me:
1. The smaller speaker is not expectedly less efficient, so it needs more juice to play along size the larger speaker.
2. There may be insufficient flueing from the bottom to the top of the heatsinks.

You have plenty of top ventilation - more than enough - but perhaps a bit more needed the underside of the fins on the heatsinks. Air circulation must come from below; in your case there maybe be small ventilation UNDER the heatsink, though there is ventilation from the rear and the sides.

I was amazed at the beauty of my younger daughter. Actually, the elder, Soraya, is stunning too, though somewhat Amazonian, with a love for scuba diving, sky diving and marathoning and a tongue so sharp she can easily remove a good man's head bloodlessly from 200m....... attached are Mother, Alessandra and Soraya in a small shot at the reception.

But Alessandra is extremely personable, everyone loves her, and even for a crusty old Dad she is a huge pleasure to be with in any situation. She is fascinating; very feminine, and very sharp to any social situation, perhaps like your beautiful wife, huh? Alessandra charms and teases her parents with affection; but Soraya is so sensitive she loves to open a fight. Drives the boys away except the 45 year old womanisers!! Sri copes with Soraya well, but I have to walk away; I have not the energy for fighting and she is so articulate she wins every argument. She had a far better education their either Sri or me; I suppose all this tells us the adage that the best we can do is live long enough to have our own genes thrown in our faces....... you are a Father, you will know very soon!

Thank you for your fantastic response to Simon's email - a huge lift for both of us. You did a beautiful Maya, he is an experienced carpenter, and I believe he will do a beautiful Maya too! I find people who work with timber have an eye for form and proportion........

Thanks Steve, we are honing in to this thermal issue. I have found that your quiescent settings work beautifully once they are set up after the amp reachs operating point and always drops slightly with increasing heavy music.

Cheers,

Hugh
 

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Nah, Sam, much better than I am NOT in the picture!! I only add one things to my children; their height. My wife is a very gracious, charming lady. Actually, the temperament of Alessandra is from her Mom's Mom, and Soraya follows mine (when I was young and aggressive!!).

Here's an October 2017 photo with my customer Frank Hinton at his Melbourne home with my bike in the foreground. I designed a 170W proaudio amp for Frank a few years ago, and one of them is used in the ABC Southbank Studios for our Classical FM Station (National Broadcaster). Frank and I are both 66. I am too heavy and need to lose at least 25Kg. Him too! Difficult......

We should talk about the bias values, shouldn't we? I like at least 125mA.......!

Hugh
 

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Ha ha....Hugh that is a nice picture !
If i didn't knew better, i would say it is a perfect portrait of an Italian family Godfather and his loyal Hitman !

Loosing weight at 66....now that is a challenge. With the world filled with goodies and with so much to worry about besides the scale....well.....i think i feel ya.

I took another shot at temperature measurements. Let me know what you think at this.
For me, i can live with it. But if its shortening the lifespan of my amp...i will have to sort it out.

37165717163_5c38e3786e_o.png


Like i suspected, inside temp (measured in the heatsink fins) is about 7 degrees above outside temp (measure on the side chassis).

I played the system for 6h . Did not crank the volume up, just normal listening.
I belive that in peak summer (5 degrees above current temp which is 20C, inside my house) and cranking the system up....i would raise these values up to 10C.

So that leaves us with inside heatsinks temps of 45C in current wheater....and 55C in summer and cranking the volume.

Is this too much ? What would be acceptable ?
Cheers to all
Filipe
 
Hi Felipe,
Well, it is what it is.
Be sure to keep wires well away from the hot heatsinks, just to be on the cautious side.
And you might want to heed Hugh's advice about cutting a hole underneath the heatsinks. However that means taking the amp all apart to do so, but it won't make anything worse.
Or, for this one particular application with the mini monitors, perhaps you could rearrange the heatsinks inside the amp and move them out to the edges of the amp with the heatsinks facing outward.
 
Filipe,

I think 55C is too much, yes, and to guard against that, put a cutout under the fins on both heatsinks, ensuring the feet keep the base at least 30mm above the ledge.

Yes, losing weight, not easy...... but I am grateful to be alive! I will pass your comment to Frank, my hitman will find that hilarious!

Hugh
 
Hi Hugh

Cogratulation!!!You have a beautiful family.How times changes....many years ago you sent me a pcture of the family and yourself ....The children are all grown up and have all grown up so beautiful.In the last picture i dont know who is the hit man but you both are looking good!!!!.)
 
Peavey tend to use low bias too.

And Nelson tends to use high bias. :D

I rarely bias my amp designs with more than 10mA.

I assume the optimum bias probably depends on the circuit configuration but, with Hugh's N80, I found the increased bias (over how Hugh sets it normally) delivered a deeper soundstage.

So a worthwhile thing. :)

Andy


Andy
 
I have found sometimes if the DC offset isn't close to zero that the amp puts out a little DC and this makes one transistor warmer than the other.

Sorry for the late reaction. Does this mean I should provide a little DC load to the amp like a 47 ohm resistor or something like that? It's the tweeter amp that's getting warmer. My tweeters are connected via a DC protection cap so the amp sees only a AC load.
 
I find that mosfets need more quiescent to give a reasonable linearity at very low outputs. If you use a 480W mosfet, a minimum is 125mA, and on the 1040W device, I like to set them at 210mA.

Bipolar is different. An AKSA needs only 58mA for outstanding clarity, but I have not tried the very modern devices, like the TO264 MJL4281AG and MJL4302AG (both 230W, 350V, 15A, 35MHz) which purport to have beta minimum of 80 from 100mA to 7A, just the same as the C5200/A1943 I used in the AKSAs. That is impressive, actually.

Cheers,

HD
 
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...

Two questions:

* With standard bias: 80 watts at 8 Ohms. THD 0.1% or 1% ?

* With 60mV at R22, how much power at 8 Ohms and 4 Ohms, THD 0.1% ?

You know, my tweaked and modded KEF Q100 5.25" coaxial speakers have true sensibility of 85 dB/W/m and 3.9 Ohms about 230 Hz.

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And how much dB at 1 watt, 10 watts? H2, H3, H4 and H5.
 
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