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Old 11th July 2009, 06:19 PM   #1
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Question Question about different gnd's on a specific amp

Hi everyone, which kind of gnd do the (red) circled points need to be? Are those chosen correctly?
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Old 11th July 2009, 07:55 PM   #2
fjr is offline fjr
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These joints should connect to the power ground, as drawn on the schematic.
The other ground (R4-C7/D1) is the signal ground, that should separated.
Use a 10Ohm resistor as "separator" between power GND and signal GND. In paralel with this resistor you could use a pair of diodes connected anti-paralel (1N5404, 1N4004 are fine here) as protection, and a 100nF ceramic disk capacitor as filter.
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Old 11th July 2009, 09:20 PM   #3
jaycee is offline jaycee  United Kingdom
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You would take all of those circled points to a point on your board where you would connect "Power ground" - a connection back to your main star ground point near to the power supply capacitors.

You would take all of the flat bar grounds to a seperate point on the board, the signal ground point, and then connect this to your power ground point via a 10 ohm resistor as described by fjr.

The point of that is to prevent high current pulses getting into the signal ground and causing oscillation or noise.
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Old 11th July 2009, 09:30 PM   #4
jaycee is offline jaycee  United Kingdom
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That schematic seems familar but cant place it. Looks like an implementation of Self's Blameless with 2-pole compensation.

I'd change the input part a little. Put a 1K resistor between the + end of C2 and where R4 connects. Put a 330p in parallel with R4.
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Old 12th July 2009, 03:27 AM   #5
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Thank you for the kindly replies!

Can I use TIP35/36(C) instead of 2SC3281 & 2SA1302?
(The PS rails are +-49V)
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Old 13th July 2009, 05:48 PM   #6
fjr is offline fjr
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Quote:
Originally posted by Ermac
Thank you for the kindly replies!

Can I use TIP35/36(C) instead of 2SC3281 & 2SA1302?
(The PS rails are +-49V)

Big NO!
Safe operation area of TIP35C/36C is low for the application. Use a pair of 2SC5200/2SA1943. 2, or even 3 pairs would be better, to be on the safe side when using 4Ohm speakers...
That pair is a substitue to 2SA1302/2SC3281.
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Old 13th July 2009, 05:59 PM   #7
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Quote:
Originally posted by fjr



Big NO!
Safe operation area of TIP35C/36C is low for the application. Use a pair of 2SC5200/2SA1943. 2, or even 3 pairs would be better, to be on the safe side when using 4Ohm speakers...
That pair is a substitue to 2SA1302/2SC3281.

Well, Thanks for the reply.

I couldn't find 2SC5200/2SA1943 at local store. There are only 2SC5242 & 2SA1986 (which are not complementary ). Can they be used together?
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Old 13th July 2009, 06:44 PM   #8
jaycee is offline jaycee  United Kingdom
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You may also use:
MJ15003/4
MJ15024/5
MJ21193/4 (all in TO-3 cans)
MJL3281/MJL1302 (TO-264 flat pack)

but they MUST be real devices. If you are from a country where fake transistors are a problem, you will have a hard time finding real versions of all of these (expensive) power transistors.

I would not recomend using mismatched pairs - especially not in this circuit which has a CFP output stage... in fact you may get into trouble substituting the above devices (except the MJL pair) into this circuit because of it.

This circuit does not look suitable for the voltages you want to use, anyway.
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Old 13th July 2009, 07:23 PM   #9
taj is offline taj
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I recognize it too. It's from one of Randy Slone's books, not sure which.

..Todd
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Old 13th July 2009, 07:59 PM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
R20 passes ~6mA to the VAS
R22 will start to turn on the protection transistor ~8.5mA.

That means the VAS current can vary by +-2.5mA just as the protection transistor starts to turns on.

R22 should be split into two equal halves, maybe 24r each.
The protection transistor base should be connected to the junction of the split emitter resistor.
Now the VAS can pass 16mA just as the protection transistor starts to turn on.
This gives a VAS range of +10mA & -6mA around the 6mA Iq.
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