F5 meets Buzquito

[juma]

I tried using Hitachi/Renesas lateral MOSFETs in F5 but they lack transconductance and can not provide sufficient gain as drain followers without harming the sound.
Since they perform great as source followers it's obvious that another gain stage is needed between input JFETs and output laterals. The choice fell on 2sk2013/2sj313, well known and respectable pair (estman, thanks for providing j313s ).
Schematic resembles Buzquito but feedback network is F5-like. I think of running 50-100mA through k2013/j313 and 0.5 to 1A through each lateral.
Thermal stability shouldn't be an issue for output MOSFETs (negative tempco) but k2013/j313 should have adequate heatsinking too - although they don't drift too much - I measured about 17mA increase of Id for change of temp. of 30 degrees Cels. and that's not a problem, they are easy too cool since they dissipate only 1.5 to 2.5 Watts.
Since OLG is higher this way I can increase the amp's gain to 16 V/V - enough to avoid preamplifier. I built one channel point-to-point and it behaves and sounds impressive. Serious build will follow as soon as I get rid of previous two amps
Here are the schematic and PCB layout (both open for discussion ) :

[Zen Mod]

excellent , as always .

while you're fresh with it - will you try it in B class ?

I'm interested to toss somewhere those old T03 cans ........ maybe as PC amp for my precious

[juma]

Originally Posted by Zen Mod

...while you're fresh with it - will you try it in B class ?...

Of course I did - I played with a bias from 0.1 to 1A per MOSFET and although you cannot consider single channel testing a "critical listening" it's obivous that with small bias current the amp sounds a bit "thin". With 0.5A it's really good and marginally better with 1A per MOSFET.

BTW, PCB pic shows copper side, dimmensions are 160 x 26 mm and I made room for caps parallel to feedback resistors (150R 2W) in case of oscillations (p2p model didn't show any signs of instability but...)
Of course, old 2sk135/2sj50 and similar laterals can be used as output devices.

[Zen Mod]

yeah ....... that's A class damnation ;

entire world is satisfied with B class amps , but we always think in 500mA units



I remember Magnum amps ....... ditto from Hitachi app. note ( OK - little tweaked ) ,smallish heatsink ...... and Iq under 50 (fifty!!) mA


they were good for that time

[osscar]

I agree - this is neat. Do you have any measurements are taken with a scope? (square etc.)
It would be interesting to see.

Thank you.

[njepitt]

Hi juma,

I can't help myself - If I see a cool circuit like this I want to build it, even if I don't understand it competely... Of course, I don't know whether I'll end up doing this, but can I throw in a couple of beginner questions?

Originally Posted by juma

...Schematic resembles Buzquito but feedback network is F5-like.

Can you elaborate a little? I realise it's a different type of mosfet on output, but what is the principal advantage you see in this design?

Originally Posted by juma

...Since OLG is higher this way I can increase the amp's gain to 16 V/V - enough to avoid preamplifier. I built one channel point-to-point and it behaves and sounds impressive. Serious build will follow as soon as I get rid of previous two amps

"OLG" is open loop gain? (sorry if it's clear to everyone except me...)

What sort of size of amp is this? Output? Are F5 size heatsinks more-or-less adequate?

Cheers

Nigel

[juma]

Originally Posted by Zen Mod

yeah ....... that's A class damnation ;
....

Unfortunately, yes...
The only AB cl. amp I can listen to more than a couple of days is SuSy with opa1632 and inverted opa549 - somehow it manages to sound good for months

Originally Posted by osscar

I agree - this is neat. Do you have any measurements are taken with a scope? (square etc.)
It would be interesting to see.
Thank you.

I use scope just to check on oscillations and noise. I avoid to indulge in 100kHz square wave shapes - it tends to influence one's brain when it comes to amp's true evaluation i.e. how it sounds

[juma]

Originally Posted by njepitt

....
Can you elaborate a little? I realise it's a different type of mosfet on output, but what is the principal advantage you see in this design?

Output devices are spared from voltage gain - laterals act as source followers only. Voltage gain is left to k2013/j313 which are more adequate for this task and they don't have to provide much current - driving two pairs of laterals is an easy task.

Originally Posted by njepitt

...."OLG" is open loop gain?

What else?

Originally Posted by njepitt

....
1. What sort of size of amp is this?
2. Output?
3. Are F5 size heatsinks more-or-less adequate?

1. I'd say acceptable
2. I used a PSU from another amp (+/-25V) so there is a bit more swing possible than with F5 but roughly, that's it.
3. If biased to same current, yes
Using two pairs of output devices allows heavier biasing and driving lower Z loads deeper into class A but increased heat is the price to be paid for that.

[njepitt]

Thanks for the answers, juma.

3. If biased to same current, yes
Using two pairs of output devices allows heavier biasing and driving lower Z loads deeper into class A but increased heat is the price to be paid for that.

Can I rephrase to see if I understood this? (Sorry if it's obvious to everyone else...) F5 in standard form has 1.3A current through a single pair of output devices, so when you say "same current" this would mean about 0.65A through *each* output device, since they come in two pairs in your schematic. In other words, "same current" means same total current, and therefore about the same total amount of heat. It follows that each device is operating farther from its limiting values, so you could bias "more heavily" if you wanted to, but obviously this means it gets hotter...

So, did I get it?

Cheers

Nigel

[VladimirK]

Do you really need source resistors for output laterals ? With idle currents above 150mA the laterals balance themselves very good.