Bias adjustment Toshiba 1943/5200

[Max Martin]

In schematic for my Nad C272 it says adjust bias to 6-7.5mv.

Done,
but cooling fins basically never get any hot. It sounds good but after about 2h playing music it starts to sound much better i think.
My old 216, 214, 218 in the past and other amps i've owned before getting quite hot right away..

Only thing that gets really hot are the 68v regulator and all parts in pre and gainstage (suppose to run i class A so..) but the 4 pairs of Toshiba 1943/ 5200 doesn't really. after many hours of playing music putting the hand om the lid it barely is fingerwarm.

IS it any real danger of adjust higher like say 10 or even 15mv on bias? (my old 216 was biased at 18 i think but other trannies from Sanken)

Anyone experinced from this amp or another with this output trannies? Is the regulator or other parts take a beating if i adjust the bias upwards. Think really there is nothing wrong with the amp. Recapped and done. It was the same before this too.

 

[Rallyfinnen]

I can only say that in general I have not seen any negative effect on distortion when increasing current. In theory there is usually a optimal Iq, but from what I have seen in practice, distortion usually decreases with Iq higher than the theoretical set point.
Try it, but watch the temperature so it does not run too hot. With good airflow to the amp, you might be able to increase it quite a lot.

EDIT: you should be aware that the higher currents and temperatures will age the components faster.

 

[JonSnell Electronic]

6mV across the 0R05 resistor equates to 120mA
15mV will become 300mA, that would try and make the class B design run in class A.
Not good!


The 216 has 15mV across 0R44 (Tp301/303) 0R44 = 15mV at 0R44 is approx 28mA.

 

[Max Martin]

What would your suggestion be?

 

[Max Martin]

Do you mean that those 5w resistors would go too hot or what parts would suffer? They are stonecold btw.

On each channel there is (R444, R434, R344,334) 2.2k 3w replaced with 5w is running very very hot now. I’m not sure how they would react to adjusting bias higher.

 

[Max Martin]

I’m not a pro in this. Trying to learn and all help would be great. I’m not sure either how you calculate that i get 120mv across the 0.05R.

Bias now is currently at 8.00mv and big cooling fins are cold even with loud.

 

[Lojzek]

I’m not sure either how you calculate that i get 120mv across the 0.05R.

It is a simple equation, I=U/R but of course that does not mean everyone has looked into SM and checked what actually reads there.

 

[Max Martin]

It is a simple equation, I=U/R but of course that does not mean everyone has looked into SM and checked what actually reads there.

Ok thanks.

No idea what SM is though .

 

[Indiglo]

SM = service manual

From the manual measure the voltage drop across both 0.05R resistors (TP 1/3 and TP4/6), so 6mV across two 0.05R resistors in series = 0.1R, therefore current = 0.006/0.1 = 60mA

 

[Max Martin]

SM = service manual

From the manual measure the voltage drop across both 0.05R resistors (TP 1/3 and TP4/6), so 6mV across two 0.05R resistors in series = 0.1R, therefore current = 0.006/0.1 = 60mA

Sm = yes ofcourse haha!

So i should have about 80mA now then (0.008/0.1)

 

[Max Martin]

Am i right this parts will suffer from higher current draw? These i need to check carefully? These all are mounted on a pretty heft heatsink (and they get hot, 50-60 celcius degress atleast. See pic with red X on my amp

 

[Max Martin]

Better pic how each channel looks like

 

[Indiglo]

That heatsink may become hot, I can see at least 5 devices mounted to it. Try setting the bias to 60mA and see if the heatsink is any cooler.

 

[Max Martin]

Yes, five of them.

I will check temp and see what happens here when i slghty turn upp bias.