SKA GB150D now public domain...

[bimo]

And with 500mA bias the midrange is to die for! Beautiful female voices! The thing is that the highs are less present and the sound is less "exciting". It's more a "fat" sound. Also the amp sounds less loud, although the volume is kept unchanged. Is all this the result of less distortion?

If you think the high is not good enough, I think it is because the slew rate is low. According to my simulation, the slew rate is 30V/uS.

For +-55VDC power supply, usually I made slew rate at least 55V/uS in simulation of my amplifier.

 

[PauloPT]

I didn't know the slew rate correlated with voltage. I'm using 50V rails. Maybe the input LP filter could be modified for better slew rate?

 

[weskoki]

If i remember correctly Greg recommended 120mA bias.

 

[bimo]

I didn't know the slew rate correlated with voltage. I'm using 50V rails. Maybe the input LP filter could be modified for better slew rate?

Distortion cause by limitation of slew rate is different than LP filter effect. LP filter is linier but slew rate distortion is not linier.

You can compare low slew rate of op-amp (like 4558) with relative high slew rate op-amp (like NE5532) with and without LP filter at the input.

 

[PauloPT]

If i remember correctly Greg recommended 120mA bias.

125mA

The midrange is better at 400-500mA but the highs are better at 125-150mA.

Should I go for a lower or higher bias?

 

[weskoki]

Best to consult Greg.
How big are your heatsinks?

 

[PauloPT]

Best to consult Greg.
How big are your heatsinks?

0.5C/W

 

[PauloPT]

Distortion cause by limitation of slew rate is different than LP filter effect. LP filter is linier but slew rate distortion is not linier.

You can compare low slew rate of op-amp (like 4558) with relative high slew rate op-amp (like NE5532) with and without LP filter at the input.

Reducing R24 and R26 would it do the trick?

 

[Bigun]

My version of this amplifier (TGM7), from what I remember, had nothing 'wrong' with the treble, just clean and undistorted. I need to listen to it again in case I'm missing something that you fellas find a concern.

 

[PauloPT]

My version of this amplifier (TGM7), from what I remember, had nothing 'wrong' with the treble, just clean and undistorted. I need to listen to it again in case I'm missing something that you fellas find a concern.

Have you tried running it at higher bias? Did you noticed some difference to the sound?

It's not something wrong with the teble, it's different bias values result in different sound, higher bias decreases the treble. Maybe this is because current at R24/26 decreases as bias is increased, resulting in less slew rate. Or maybe is my perception of less distortion. I don't know.

 

[SKAFAN]

Paulo,does increasing the bias from 120 to 150 makes any difference?

 

[PauloPT]

Paulo,does increasing the bias from 120 to 150 makes any difference?

Yes, it does. Greg recommends 125-150mA bias (1.25-1.5V across the 10R resistors at the fuse holders). I have mine at 250mA without noticing the treble affected, and the sound seems even better than at 150mA.

 

[PauloPT]

I would be very interested to know from those of you who build the GB150D, what's the bias you're running the amp. Maybe I should post a poll...

 

[Bigun]

Good question - I didn't try different bias levels. I would expect that higher bias has some benefits because these old-style vertical MOSFETs tend to get more linear - I'm not sure but I thought they have less of a distortion sweet-spot for Class AB than BJT outputs ?

 

[PauloPT]

Food for thought:

Remember Walt Jung article "Sources 101: Audio Current Regulator Tests for Hgh Performace" ?
Part 2 http://waltjung.org/PDFs/Sources_101_P2.pdf

Replace R24 or R26 by circuit 16A. Adjust current for 9 - 9.5mA.

 

[erikovsky]

Hi folks,

I made a GB150 board based on Zener mod and 2240/970BL, about 10*10cm.
I tried to use some better CCS for 9mA, but due to size limit, I gave up
So, LED and resistor are used as schematic do.

 

[PauloPT]

You could use a single LM317 (circuit 10A) with a 120R resistor and see if there is a change (improvement) to the sound.

 

[erikovsky]

You could use just a single LM317 with a 120R resistor and see if there is a change (improvement) to the sound.

Sure, let's see what it will be

regards,

E

 

[PauloPT]

Don't forget to post your conclusions

 

[AndrewT]

Hi folks,

I made a GB150 board based on Zener mod and 2240/970BL, about 10*10cm.
I tried to use some better CCS for 9mA, but due to size limit, I gave up
So, LED and resistor are used as schematic do.

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