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Old 18th February 2013, 07:41 PM   #3971
PauloPT is offline PauloPT  Portugal
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Quote:
I think you should try a buffer between your computer and cable.
I did try my preamplifier between the pc and the amplifier. Although the oscillations decreased and the sound got better it didn't make the problem go away completly. In fact when I raised the volume knob the oscillations also raise in intensity.
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Old 18th February 2013, 07:55 PM   #3972
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Quote:
Originally Posted by PauloPT View Post
I did try my preamplifier between the pc and the amplifier. Although the oscillations decreased and the sound got better it didn't make the problem go away completly. In fact when I raised the volume knob the oscillations also raise in intensity.
Was this before inserting the input network or after? A quick, short comparison with Symef amp would be appreciated. Thanks.
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Old 18th February 2013, 08:17 PM   #3973
PauloPT is offline PauloPT  Portugal
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Quote:
Was this before inserting the input network or after?
After inserting the snubber. Without it it's even worst. Maybe I should try a different RC constant.

Quote:
A quick, short comparison with Symef amp would be appreciated. Thanks.
Subjectively speaking, both the amplifiers are very, very good sounding. The Symef has a warmer sound - perhaps more "tubey" - sweet, with lots of "texture" and "body". It reminds me of the cosiness of a fire place in the winter. The SSA is cleaner, more forward sounding, more transparent, more "fierce" (strong deep bass) and with highs more extended than the Symef. The soundstage of the SSA is bigger than the Symef's. It's also less forgiving of bad recordings.
It's easy to fall in love with any of these amps but I believe they will appeal to different people.
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Old 19th February 2013, 03:08 AM   #3974
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Quote:
Originally Posted by PauloPT View Post
Subjectively speaking, both the amplifiers are very, very good sounding. The Symef has a warmer sound - perhaps more "tubey" - sweet, with lots of "texture" and "body". It reminds me of the cosiness of a fire place in the winter. The SSA is cleaner, more forward sounding, more transparent, more "fierce" (strong deep bass) and with highs more extended than the Symef. The soundstage of the SSA is bigger than the Symef's. It's also less forgiving of bad recordings.
It's easy to fall in love with any of these amps but I believe they will appeal to different people.
Thanks Paul. This is one of the best posts here; honest and giving credit where it is due. Keep up the good work. Now I've got work to do.
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Old 19th February 2013, 10:36 AM   #3975
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Lazy cat,

Very nice design, lots of great sonics to be found in Current feedback designs. I've been an adherent to the symmetrical CF circuits as of the last year plus, and dual feedback loops, one for each positive and negative half. Though after reading the nocompensationaspect, even though it may not be external compensation is always internal due to in any semiconductor cob or fet it's output capacitance which is the equivalent of a cap to ground. Once these cf circuits are nailed its some almost ethereal sounds!


Colin
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Old 19th February 2013, 04:41 PM   #3976
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Absolutely. Ethereal...

It's just hard to turn off an SSA once turned on.

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Old 19th February 2013, 05:52 PM   #3977
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The behavior of Shaan's schematic changes a lot depending on which BD139/140 is used. ST, Phillips, Fairchild, OnSemi are all different, some of them markedly inferior. The size of Cob on some of them makes the 5pF miller caps laugh. Putting these parts on a schematic without specifying the vendor is a bad idea.

My initial guess of 20pF/3.3k was pretty darn close. Depending on the VAS you use, the best value ranges from 2.2k-3.3k.

However, Shaan's original 1k/10k//22p input network works well enough that there should not be any potential for oscillation with an input cable, so I think the problem must be somewhere else. Counterfeit transistor? Perhaps Shaan used Fairchild or OnSemi VAS and you used Phillips VAS resulting in a faster, less stable amplifier?
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Old 19th February 2013, 07:48 PM   #3978
PauloPT is offline PauloPT  Portugal
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Originally Posted by keantoken View Post
The behavior of Shaan's schematic changes a lot depending on which BD139/140 is used. ST, Phillips, Fairchild, OnSemi are all different, some of them markedly inferior. The size of Cob on some of them makes the 5pF miller caps laugh. Putting these parts on a schematic without specifying the vendor is a bad idea.

My initial guess of 20pF/3.3k was pretty darn close. Depending on the VAS you use, the best value ranges from 2.2k-3.3k.

However, Shaan's original 1k/10k//22p input network works well enough that there should not be any potential for oscillation with an input cable, so I think the problem must be somewhere else. Counterfeit transistor? Perhaps Shaan used Fairchild or OnSemi VAS and you used Phillips VAS resulting in a faster, less stable amplifier?
I'm using CDIL BD's. Are they inferior? Could they cause oscillations?

I replaced the 20pF cap by a 680pF one to serve as input low pass filter (with the 1k resistor). Them I added in parallel with the 680pF cap a 3.3k resistor in series with a 20pF capacitor to serve as snubber. Am I doing this right? Should I keep the 680pF as input filter or only the snubber parts? And where should the snubber be placed - at the pcb or soldered at the input RCA's?

Thank you.
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Last edited by PauloPT; 19th February 2013 at 07:53 PM.
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Old 19th February 2013, 08:01 PM   #3979
shaan is offline shaan  India
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Hi Keantoken.

I use either CDIL or BEL parts for VAS. And for the input and cascode, Philips.

My protoboard and later the PCBs didn't cause any oscillation or any noise in my radio(yes I use this trick too), even without any Cdom at all, so the 5pF silver mica caps were just luxory, for me .

Initially I used a 22pF at input, but the ear-on-speaker test showed evidence of some CRT buzz. So decided to use a decent 2KV ceramic 100pF and all's well since then(with a well shielded cable which is more than 5m long, from PC to amp).

Also, I decided not to use the -16(high gain) suffix labeled parts for the BD pair, went the cheap way.

However, as I recently posted, the amp can rectify a lot of MW/SW signals when the input is disconnected from source and touched with finger or my twizzers.

Happy listening.
shaan
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Old 19th February 2013, 11:34 PM   #3980
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So, I would really like to build the very basic BJT version, based off of the schematic posted. Just wanted to ask a few questions first:
1. The three 1uF caps look like electrolytic's. Is that right?
2. The BC550/560's need to be strapped face to face?
3. The BD139/140's need to be on main heatsink?
Now for the big question. I already have a few different power transistors. They are:
1. Toshiba 2SC3281 / 2SA1302 (original)
2. Toshiba 2SB755 / 2SD845 (original)
3. NEC 2SD588 / 2SB618 (original)
I was hoping not to have to buy new power transistors. I'm also not to the point technically where I can tweak values in the schematic to be able to use these transistors, (still a newbie), but getting much better at following a schematic and building. I was hoping someone could point me in the right direction. I'm thinking the 3281 / 1302's would be the best bet. Advice?

Tom
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