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Old 26th September 2011, 01:52 PM   #921
shaan is offline shaan  India
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Basic test result: Iq is very stable. No overheating anywhere. VAS bjts are equally warm.

Offset is NOT. Dancing like balle dancers... seems to have a mind of its own... sometimes +10mV, the next moment -80mV, then -2mV! I can increase the offset from 5mV to say 150mV just by breathing over the input BJTs. I guess the mains AC variations is the main culprit here, as I had the hFE of each input BJT matched before construction. Also I curse the single-turn pot; life is hell with it sitting there. I don't want to use a regulated supply with SSA, so, maybe I'll use my trustful servo with it. We'll see later.

Listening test: I quit LTP. Also, there is more to come(and go) so stay tuned.
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Old 26th September 2011, 02:03 PM   #922
shaan is offline shaan  India
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Update: Offset variation is a little better with Iq of 300mA; at the time of last post it was about 170mA. But now I have a little humming in the speaker. Ground loop may be, I'll check.
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Old 26th September 2011, 02:18 PM   #923
gannaji is offline gannaji  India
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Originally Posted by gannaji
Dear shaan,
Those Heatsinks look nice! Are they available anywhere in India? Can you give me the source in India for the Semiconductors also?
--gannaji

I found them in Kolkata chandni market; the old/used equipment sellers lane, for one-fifth the retail price! These are AMD cpu heatsinks and they can dissipate 100watts with medium air flow. I plan to use them in SSA with a thermo-fan control for the fans.

All the semiconductors used in SSA are available at Nahar Electronics in Chandni market Kolkata. They are very mature suppliers and everydbody knows their shop location, ask anyone in chandni and you'll get there.

Any address or Contact Number of Nahar Electronics? I am operating from Hyderabad. Thank you!

--gannaji
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Old 26th September 2011, 02:25 PM   #924
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If this is your sch than:
- R1 should be 10 kohm
- R2, R3 should be 100 ohm
- R9 should be multiturn trimer (one fixed value resistor + multiturn trimmer)
- R10, R11 zener supply resistors must have current set to 15-20mA so the zener potential is fixed
- R14 should be coupled with 100 uF
- C1, C2 should be 1000 uF
- D1, D2 also coupled with 0,1 uF
- Q1, Q4 thermally coupled

And your offset will be normal.

So music is already playing hehe
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Last edited by Lazy Cat; 26th September 2011 at 02:33 PM.
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Old 26th September 2011, 04:45 PM   #925
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Originally Posted by shaan View Post
People, I'm about to test it right now. Wish me good.

LC, do we need thermal joint between the cascode transistors?
Nice job Shaan - Good Luck
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Old 26th September 2011, 04:52 PM   #926
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Quote:
Originally Posted by shaan View Post
Update: Offset variation is a little better with Iq of 300mA; at the time of last post it was about 170mA. But now I have a little humming in the speaker. Ground loop may be, I'll check.
Shaan, the little humming on the speakers are probably due to the power supply now rippling a little. I had that problem but decoupled the front end with diodes in the power supply, this removed the hum significantly it is barely audible on headphones. My bias is pretty stable after about 10 minutes, but I run the amp in "near class A" at just under an amp.

Nico
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Old 26th September 2011, 05:00 PM   #927
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Sorry I meant to add, re-set bias and off-set when the temperature stabilized. It takes a while to get it right but once set it varies only about 5 mV either way. This is not serious at all, it is going to make no difference to music.
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Old 26th September 2011, 05:31 PM   #928
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Originally Posted by shaan View Post
Offset is NOT. Dancing like balle dancers... seems to have a mind of its own... sometimes +10mV, the next moment -80mV, then -2mVI don't want to use a regulated supply with SSA, so, maybe I'll use my trustful servo with it.
It is absolutely normal with an amp flat from DC with no servo. And will have no consequences, as Nico says, as long it is in the 150mv scale, OMHO.
I highly recommend you use a dynamically filtered power supply (Simple emitter follower, like the one i had suggested in this thread), It improve the sound on transients and noise of all amps as much than the amp itself. Cost a power trans or two and a zener by power line. + additional electrolytic capacitors where huge is never enough ;-)
http://www.diyaudio.com/forums/attac...ifier-alim.pdf
Replace zener with Tl 431 or so...

Quote:
Originally Posted by shaan View Post
But now I have a little humming in the speaker.
Are-you sure there is no very HF oscillation too, hum can be often the sign of it ?
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Last edited by Esperado; 26th September 2011 at 05:39 PM.
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Old 26th September 2011, 05:34 PM   #929
Salas is offline Salas  Greece
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Shaan, that verobuild is pure art, underside especially.

Nico, what is the Iq in the output lateral Mosfets, and is it stable enough, if I will follow your schema with 2SC2240/2SA970 compound driver and 330R R13? Also when you say decoupled the front end with diodes, just those, or an extra cap to ground on the node also? I was thinking of simple capacitance multipliers there even, since the cascode is certainly exposed to ripple.
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Old 26th September 2011, 08:43 PM   #930
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Hi Salas, a Capacitive multiplier would do the job even better than my diodes but my PCBs was made a few weeks ago and I opted for the minimum modifications I can get away with. My Iq is about 800mA, yes I run them hot but I find hot is good for two reasons 1) temperature is more stable and 2)harmonic distribution is a nice roll off starting with H2.

Now the amplifier that I compared it with is a single input unbalance current feedback topology, but the balanced version just brought out a few nuances that I did not know existed. Both amps has extended bandwidth, but I think the symetrical complimentary just has that edge better performance in being more transparent. In this topology the artefacts are almost always odd order, but as soon as you over bias you tend to bring out the even order artifacts so that H2 > H3 > H4 > H5.... Although THDis relatively high at 0,2% this does not interfere with a completely transparent performance.

Years ago I used an almost identical topology, it was called Le Gecko, but I over complicated it with current sources and mirrors to the extent that it sounded cynical and not transparent and the reason that I wanted to stick with SSA for its simplistic approach - complicating it diverges from it intrinsic transparent character.

Kindest regards

Nico
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