SKA GB150D now public domain...

[erikovsky]

I find yellow on white very difficult to see.
Is that my problem?

How about this, sir?

Forget K170, it's there for fun only

 

[AndrewT]

Why are you using very low gain Grade a input stage transistors? Even the b grade are low.
Amp Guru (if I have the correct name) arranged a group buy, on this Forum, of bc546c & bc556c specially for this amplifier.

Is the input current temperature compensated to the output stage?
That was a fairly important parameter of the original amplifier.

 

[erikovsky]

Why are you using very low gain Grade a input stage transistors? Even the b grade are low.
Amp Guru (if I have the correct name) arranged a group buy, on this Forum, of bc546c & bc556c specially for this amplifier.

Is the input current temperature compensated to the output stage?
That was a fairly important parameter of the original amplifier.

Sir, as mentioned, I use 2240/970BL because I have some stock, convenient to me to use them.

 

[AndrewT]

Why post the wrong sch and then ask question about another sch?

 

[erikovsky]

Why post the wrong sch and then ask question about another sch?

My apologies. Sorry if that bothers you a lot.

 

[Project16]

I understand AndrewT!
You post a diagram impossible to see, then you put another with further references, and you say it's for fun.
It's a big mess, how to find?

 

[erikovsky]

I understand AndrewT!
You post a diagram impossible to see, then you put another with further references, and you say it's for fun.
It's a big mess, how to find?

My apologies again.

 

[PauloPT]

Any thoughts about running this amp class A vs class AB ? Better sound or not?

 

[Bigun]

And how does the sound compare with the KSA50 if you can ?

 

[PauloPT]

Is it worth building a class A SKA? Greg says little, if any, sound improvement is to be expected from class A. I've been experimenting with bias higher than 150mA and I'm not sure. Should I go and buy class A suitable transformers or I'll be wasting money? Is there a sweet spot for bias? Mosfets like to run hot, the higher bias the better, right? What do you say?

 

[Bigun]

I don't expect there will be any difference.

 

[AndrewT]

............. Is there a sweet spot for bias?

Borbely made it very simple: bias the output stage so that Total Output Bias current exceeds 500mA (for mosFETs).

Mosfets like to run hot, the higher bias the better, right? What do you say?

mosFETs just like all other semiconductors do not like to run hot.
Parameters are measured at 25°C.
Most parameters are compromised when operating at higher than that.
Most semiconductors have a limiting operating temperature imposed by the manufacturer. This upper temperature limit is usually 150°C or 175°C or 200°C.
Reliability is substantially improved by using less than half that range, i.e. staying below 100°C
The closer to 25°C you can keep the amplifier operating temperatures, the less the performance will drift/wander/degrade.

 

[PauloPT]

mosFETs just like all other semiconductors do not like to run hot.

By 'hot' I didn't mean hot as in temperature, I meant with higher bias current (higher than bjts). I'm sorry for the confusion.

Borbely made it very simple: bias the output stage so that Total Output Bias current exceeds 500mA (for mosFETs).

So, higher than Greg's 125-150mA. 500mA per module gives 4W class A at 8 ohms. Are you running your SKA at this high bias?

 

[Bigun]

Andrew knew what you meant, he's just being pedantic. Naughty boy

I don't expect you're going to hear any real benefit from higher bias - this amp has miniscule distortion due to high feedback and high gain-bandwidth.

 

[AndrewT]

By 'hot' I didn't mean hot as in temperature, I meant with higher bias current (higher than bjts).

I mean the same temperature is a measure of hot and cold.

I'm sorry for the confusion.

no confusion, there is no benefit in running a stage too hot (temperature). There are benefits to using adequate heatsinking to keep temperatures closer to 25°C

So, higher than Greg's 125-150mA. 500mA per module gives 4W class A at 8 ohms. Are you running your SKA at this high bias?

Yes, read Borbely. He quickly moved over to all FET amplifiers. He has many articles on the web. Unfortunately some are difficult to find since he retired.
And no, despite much help from amp Guru my SKA blew up. The To92s started to frizzle and caught fire.

 

[fab]

Is it worth building a class A SKA? Greg says little, if any, sound improvement is to be expected from class A. I've been experimenting with bias higher than 150mA and I'm not sure. Should I go and buy class A suitable transformers or I'll be wasting money? Is there a sweet spot for bias? Mosfets like to run hot, the higher bias the better, right? What do you say?

For this amp I use about 450ma for the mosfet pair ( based on my heatsink size) and it sounds nice. Sound is not too analytic or exaggerated in the highs. However, I use different mosfet which are designed for audio instead of the IRF... I think I have already mentioned it in this thread earlier.

Good luck
Fab

 

[PauloPT]

no confusion, there is no benefit in running a stage too hot (temperature). There are benefits to using adequate heatsinking to keep temperatures closer to 25°C

Not what I asked. I asked about bias not temperature. What you say I already know and is quite obvious we should run electronics at a low temperature if possible.

 

[Mike Gergen]

I haven't found there to be enough of a difference to warrant the higher bias.

 

[AndrewT]

I asked about bias

Then ask your question about bias.

 

[PauloPT]

I'm rebuilding my SKA. The modushop HiFi2000 400mm dissipante chassis have two 200mm heatsinks per side. Not one big 400mm but two 200mm. Should I attach the mosfets at the center of one heatsink or should I attach one pair of mosfets to one heatsink and the other pair to the other? There is always a gap in between the two heatsinks so each pair would be at the edge of an heatsink. Thanks.