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Old 9th December 2012, 01:32 AM   #3681
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Quote:
Originally Posted by Jay View Post
How much of us need the matching properties of PNP and NPN? How much is our purchase in a year?
I was thinking from an industrial point of view: So many of the amps on the market use near the same input pair, most on both differentials. it is A LOT of devices.
In my research and development office, we would had compared the price to pair 2 transistors VS the price of a ready to use matched pair with better temp balance.
Manufacturers could take a part of the difference, increasing their benefits.
Well, it was the good old time.
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Last edited by Esperado; 9th December 2012 at 01:36 AM.
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Old 9th December 2012, 06:40 AM   #3682
Magna is offline Magna  India
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Hi LC,

Thank you so much for your comments. I'll do all those changes that you've mentioned.

The CCS you've designed is a bit spohisticated, probably that will require an "add on" board like the one you've designed. I've checked from the local market for components and those matched transistors (SSM 22xx) parts are unbotanium here. Will a matched BC's or similar transistors work with this scheme...? or else can you suggeest alternative parts.
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Old 9th December 2012, 07:17 AM   #3683
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Quote:
Originally Posted by Magna View Post
The CCS you've designed is a bit spohisticated, probably that will require an "add on" board like the one you've designed. I've checked from the local market for components and those matched transistors (SSM 22xx) parts are unbotanium here. Will a matched BC's or similar transistors work with this scheme...? or else can you suggeest alternative parts.
Magma, i believe you can set those CSS on board as well, i believe Lazy CAT did a separate board for them to can reuse them on future projects and developments.
Why don't you try Mouser of Farnel, if you cannot get the parts locally ?
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Old 9th December 2012, 10:36 AM   #3684
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Quote:
Originally Posted by shaan View Post
And here it is. Running smoothly.

Offset setting was pretty easy; within +-5mV.

Bias perfect with 3mA, 14mA and 160mA in input, VAS and FET respectively. Without any manual setting. The input BJTs are BC550C and BC559C.

Sounds just like an SSA.

Hmm... peeceebee works for me too...

Hi Shaan

Finally SSA BASIC PCB for you, congratulations. Looks you've designed layout well since there's no sound difference compared to test boards. Only thing that bothers me is a question why you never connect output transistors pins directly to PCB?

Regarding CCS compared to current injection resistor to input stage, I can assure you that using fet cascoded CCS is more correct and musical than resistor. Because we are dealing here with input stage where uA-s have big signature we have to use best CCS, simple two BJT CCS is simply not enough accurate for the job. Modulation suppression vs. freq bandwidth, noise, thermal stability all are very important for SSA input stage CCS, that's why I choose the one proposed in post #3034.

Hopefully you'll test one like in the future and share opinion. Best regards L.C.
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Old 9th December 2012, 12:48 PM   #3685
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Hello, I am following with great interest SSA . I have provisionally
SSA established an Amp. Unfortunately, there is a problem.
The amp only worked up to a supply voltage of + - 35 volts properly. I increased the gate resistors to 220 ohms, and the capacitances between the base and emitter of the T8 and T11 increased to 47PF. The 3db frequency is reduced to 500 Khz. The amplifier is now working to + - 45 volts properly. From about + - 50 volts, the amplifier oscillates. But I have a transformer with 600 watts + and - 55 volts, which I would like to use. The amp is loaded with 8 ohms. The oscilloscope is set with 5Volt/cm. The frequency of the test signal is 10kHz. What else can I do? Are there any oscillations? or is anything limited.
Thanks Gerd
Attached Images
File Type: jpg SSA Aufbau.jpg (71.4 KB, 340 views)
File Type: jpg U = + - 45Volt.jpg (154.0 KB, 335 views)
File Type: jpg U = + - 50Volt.jpg (161.1 KB, 315 views)
File Type: jpg Sinus U = + - 50Volt.jpg (151.2 KB, 291 views)
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Old 9th December 2012, 01:03 PM   #3686
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Hi Gerd

Please post exact sch of your SSA variant. It looks to me the amp is not properly compensated or you probably forgot to install some caps on certain locations. SSA should work normally at +/- 60V, or even more.
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Old 9th December 2012, 01:28 PM   #3687
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Hello Lazy Cat,
I had forgotten the wiring diagram. I use a DC Servo. The current through T7 and T11 is 14 mA. The current through the mosfets 100mA
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Old 9th December 2012, 01:29 PM   #3688
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The current through T7 and T11 is 14 mA. The current through the mosfets 100mA
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Old 9th December 2012, 01:37 PM   #3689
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The circuit diagram with all capacities
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Old 9th December 2012, 01:55 PM   #3690
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Gerd, VAS directly loaded with 3 pairs of mosfets simply doesn't have enough current capability to drive mosfet's input capacitance plus load at full scale 10kHz squares. Intermediate driver (mosfet or BJT) would certainly help.

First you can also:
- try to observe 10 kHz squares with/without load connected
- try to use one pair of output mosfets to see if the situation is still the same
- compensate each output mosfet with serial zobel between gate and drain

If any upper measure would not help, than you'll need to install the driver stage between VAS and the output mosfets.
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