Simple Symetrical Amplifier

[Nico Ras]

Hi John,

I think Shaan's amplifier has gone through much more refinement than mine did a year ago and was essentially used to evaluate quickly what the Cat dragged in.. There are a few problems I have noted which is can be summarised as follows:

The driver transistors should best be BD139/140, the small signal device2SA970/2SC2240 just smokes at higher voltages but performs fine at 24VDC.

I have also noted problems with the 5K pot forgetting that this amp is symmetrical and the adjustment of off-set is very small. in fact only tens of ohms off the 1K mark, thus starting at a pot setting of 0 Ohms has catastrophic results. and possibly pop every device on the PCB

The pot should be set to 1 K and small adjustments made left and right for those off-set critical maniacs. In fact I don't know it offset adjustment is really that necessary and I used a fixed 1 K resistor which worked fine for me.

I have changed the 330 Ohm resistance between MOSFET gates with a 1N 4007 (any rectifier diode will do) and it tends to track any ambient changes well keeping offset more stable than with the resistor.

The bias current with the diode is around 180 - 210 mA and optimum for these output devices, but one must keep in mind that this amplifier completes a full circle so to speak, adjusting anything has an equal and opposite effect elsewhere. For instance changing the gain setting shunt resistors impacts on the output bias current. Everything is intewoven and you must keep your mind clear of distractions

My amp is still the crows nest it was made a year ago and I believe that much refinements has taken place since the very early stages, thus I would recommend Shaan's refinements if he is comfortable with my vote of confidence. Mine needs some experimenting from the builders point of view and is not absolutely optimized but will gladly assist anyone who wants to tackle it from his own perspective or how he wants to tailor it.SSA is in effect a very simple symmetrical design but the emphasis is on symetrical not on the fact that it looks all balanced as drawn, The whole amplifier has to be in equalibrium or nothing, there is no half good.

To summarize and I hope Andrej does not object - SSA is both simplex as well as complex, it is for the DIY that wants to experiment and get things the way he would like to have them.

My SSA as driven nothing other than headphones and on occasion BBC LS3/5a monitors very successfully, but it was tweaked for my purpose and being supplied by 24VDC rails.

[JOHN BALI]

Originally Posted by Nico Ras

Hi John,

The driver transistors should best be BD139/140, the small signal device2SA970/2SC2240 just smokes at higher voltages but performs fine at 24VDC.

I have also noted problems with the 5K pot forgetting that this amp is symmetrical and the adjustment of off-set is very small. in fact only tens of ohms off the 1K mark, thus starting at a pot setting of 0 Ohms has catastrophic results. and possibly pop every device on the PCB

The pot should be set to 1 K and small adjustments made left and right for those off-set critical maniacs. In fact I don't know it offset adjustment is really that necessary and I used a fixed 1 K resistor which worked fine for me.

I have changed the 330 Ohm resistance between MOSFET gates with a 1N 4007 (any rectifier diode will do) and it tends to track any ambient changes well keeping offset more stable than with the resistor.

The bias current with the diode is around 180 - 210 mA and optimum for these output devices, but one must keep in mind that this amplifier completes a full circle so to speak, adjusting anything has an equal and opposite effect elsewhere. For instance changing the gain setting shunt resistors impacts on the output bias current. Everything is intewoven and you must keep your mind clear of distractions

Thanks Nico, great point there...
But it is quite mysterious why 2SA970/2SC2240 fail at higher voltage, maybe too high.

I will at least supply +/- 35VDC max to +/-50 if possible??? Need advice here...
Sometimes ago I have modified BJT amps that run +/-35VDC to +/-45VDC but why the lower voltage give better sound, I hope to this time I build SSA give better sound with a little higher voltage

So for some adjustment it just like we adjust 2 amps at same time, Ok I will very careful

Originally Posted by Samuel Jayaraj

The Laterals shown in the picture is a bit suspect. They could be fake or at best Chinese; the latter cannot handle the same current or dissipation as the Hitachi originals.

Hi Samuel, I'm a little suspect too...
but I believe they are original one, I trust the seller but don't know where it is came from...
btw they look just like ordinary transistor not special looking
I more like TO-264 & the big MT-200
just let it sound decide


Regards

[Esperado]

Originally Posted by JOHN BALI

Hi Samuel, I'm a little suspect too...
but I believe they are original one, I trust the seller but don't know where it is came from...
btw they look just like ordinary transistor not special looking

I believe this remark about your parts provenance was done because the white balance of your photo make them looking blue...

[danzrose]

Originally Posted by JOHN BALI

Hi Samuel, I'm a little suspect too...
but I believe they are original one, I trust the seller but don't know where it is came from...
btw they look just like ordinary transistor not special looking
I more like TO-264 & the big MT-200
just let it sound decide


Regards

hi john
i use this mosfet for sigma PA by dr. bora & minicrescendo PA, n everything is ok...
mr. naf also use this mosfet for his SSA project ....
this mosfet not from hitachi but from renesas
cmiiw

[Nico Ras]

Hi John, it is a question of power dissipation on the driver transistors, unless you are prepared to make other modifications such as adding emitter resistors of about 150 ohms and a few other minor adjustments they can be made to work in the application.

Only thing is you must not be a novice else you will run into problems. I used 2SCX2240/2SA970 rather than the medium power devices purely because I could increase slew rate and decrease simulated THD - no other reason.

[JOHN BALI]

Originally Posted by danzrose

hi john
i use this mosfet for sigma PA by dr. bora & minicrescendo PA, n everything is ok...
mr. naf also use this mosfet for his SSA project ....
this mosfet not from hitachi but from renesas
cmiiw

Hi Danzrose, glad to see you here...
No I'm not doubt about it genuine
& they all arrived save, only one leg bent
they just a little bit small that all

Do you want to build SSA too, right?
Which one do you wanna go?

btw Thank you Danzrose

Right now I'm reading whole thread
looking something I must know
that all almost 3500 posts

[naf]

hi mr. danz, bli John nice to meet you here.
yes i use the same Fet as mr.danz and you and it's fine.

[naf]

Originally Posted by Salas

My friend's point to point one is going well he tells me. After he bypassed the 2X1N4148 analogous IR LED with Black Gate capacitors he had kept from older days, the sound opened up. He also had to drop the current in the BD transistors because they opened at about 25mA getting hot. He put 270 Ohm instead of 220 Ohm from base to rail and the spare 220 Ohm he put in series with the 1K2 current feed resistors. Now he got ~10mA on the drivers and they cooled down enough. He never gets more than 30mV offset after it had played for long or stayed on overnight. I hope I will visit him in next week to listen and to show you photos even. Have happy holiday!

thanks for the info sir, i'll try it too

[Salas]

To try it too on my experimental board is a nice little phrase that translates to "much rework".

[quadtech]

Originally Posted by masood

dear shan,
I have etched this pee cee pee .but it is hard to me make holes .I tried to make holes but it is not finished .thanks for giving pee cee bee image(COPPER TRACK) thanking you yours ever masood

You seem to have a mirror image of the copper side.