Symasym - the next generation (supersym)

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ostripper

ostripper

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Joined 2008
I no longer use IE or firefox... What slow poorly designed browsers.

The google chrome is unreal.. 2 to 3 times faster with everything.
I do have the IE6 still installed to check pages for compatibility.

As far as the 22R lifted ground , without it you would just "float around" between + and - .7v , or the NFB dc cap would just modulate the amp's ground reference creating another FB loop ?

If one did have the "worst case" scenario (bad house wiring) , with
a full live potential , the 22r and the diodes would be fusable. Hey.. this might be a good app for mr. dunlops "picofuse". Inline with the signal ground right at the jack :cool: A one amp device would blow before the diodes.
OS
 
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pacificblue

Member
Joined 2008
AndrewT said:
I would think that the differential amplifier at the input does not need a connection to Power Ground.
The NFB ties the output to the input. The output is an amplified version of the difference at the input.
Do we really need the resistor?
Click to expand...
Using a 100 mV input signal, with the 27k/1k pair the output signal should be ~2,8 V. Without the diodes the signal at the inverting input would be 100 mV. Both inputs are at the same voltage and everything is fine.

With the diodes, but not the 22 Ohm resistor the input is referenced to any value between 0 and 600 mV. Worst case would be 600 mV, that means for 2,8 V output the voltage across the feedback resistor pair is only 2,2 V, because 600 mV drop across the diodes. That leaves ~79 mV across the 1 k resistor, when there should be 100 mV. The inverting section would correct for a 21 % signal difference that does not exist at the output. The same example for different output levels leads to different ratios, so there is no chance to compensate easily for that error.

With the 22 Ohm resistor the feedback pair becomes 27k/1022r. A 100 mV input signal leads to ~2,742 V at the output and to 100 mV at the inverting input. Both inputs see the same voltage and everything is fine again.

Yes, the resistor is necessary.
 
ostripper

ostripper

Member
Joined 2008
By alex - I have , read several times, my errors and I try to correct in my new PCB
Click to expand...

now you are down to 2 (errors) the 27K NFB "takeoff" should come
right off the output pad. move that star ground just 20mm to the right (center it).

This amp does not oscillate , so your zobel- off center should not be a factor. Your output coil has disappeared !!!

I like your green and red led PS indicators , I do the same on my external 100k uF 75-0-75v supply.
An externally hosted image should be here but it was not working when we last tested it.


It can alert you to deadly voltages.
OS
 
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tinitus

Ex-Moderator R.I.P.
Joined 2005
Re: PCB top side with silk screen...

alex mm said:
Hi OS ,

I have , read several times, my errors and I try to correct in my new
Thanks for comments , are welcome and I have learn a lot .

Next PCB in progres , in diferent size and shape .

Regards Alex mm.
Click to expand...


Looks good :)

But why those high volt 100V caps
With less voltage they could be higher capacity
Is lead pinout really that much
Ahh, ok I see now, picture is bigger than actual size

I would try and direct/control the supply lines more towards where you want it
Well, may be perfectly fine as is, but from another quick glance it appears than ground lines for power supply caps are way too small

There seem to be plenty room for making double "bridge", with each pair of supply caps :D
 
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Rafael L

Member
Joined 2008
AndrewT said:
I had forgotten that discussion.

I agree that the Signal Ground can benefit from some form of Audio isolation from the Power Ground.

If an unsafe voltage accidentally contacts the Signal Ground somewhere along it's route then the diodes conduct, with either polarity, to Safety Earth when the voltage >~600mV
Click to expand...


pacificblue said:
Without the resistor a potential difference between power and signal ground of up to ~600 mV can build up. That would offset the correction signal and affect the NFB function. The lower the signal voltage, the worse the effect.

E. g. with the feedback combination 27k/1k shown in the last schematic 60 V output signal should give 2,14 V correction signal, but can give anything between 2,14 and 2,74 (up to 28 % difference). 6 V output signal should give 0,214 V correction signal, but can give anything between 0,214 and 0,814 (up to 280 % difference). 0,6 V output signal should give 0,0214 V correction signal, but can give anything between 0,0214 and 0,6214 (up to 2800 % difference).
Click to expand...

Where are getting these values, which can measure in the simulation?

In my opinion, resitor creates a link to Ground

Fig:
 

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pacificblue

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Joined 2008
Rafael.luc said:
Where are getting these values, which can measure in the simulation?
Click to expand...
The resistor values were taken from the last posted schematic. R46 and R12.
The voltage values were simplified assumptions for output to ground and gate of J2 to ground. They will be different in a simulation and in reality. I did not want to calculate, which current actually flows and whether that makes the diodes conduct or not. It is also not accounted for, why and how big a difference between audio and power ground will develop, e. g. due to static charging. In real life there will probably be a point, where the diode starts to conduct and all output voltages above that point would be offset by the diode's forward voltage. Below that point any differences would probably be leveled out through parasitic resistance after a while.

The gist is that the feedback pair is first of all a voltage divider that provides a scale model of the output voltage for the inverting input of the differential amplifier. Without a resistor across the diodes that scale model is not linear.

Rafael.luc said:
In my opinion, resitor creates a link to Ground
Click to expand...
That is correct.
 
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pacificblue

Member
Joined 2008
AndrewT said:
I guess the only way left to convince me is to breadboard it using an opamp as the power section and see for myself why the output would float in that range +600mV to -600mV.
Click to expand...
It is quite probable that you won't see anything floating. Most of the time parasitic resistance will be sufficient to draw both grounds to the same potential. The only two situations, where this could become an issue are
- when the output voltage is high enough to produce sufficient feedback current to make the diodes conduct. Then the diode's forward voltage will be the offset. With an opamp the feedback pair needs a sufficiently low resistance to achieve that at the achievable output swing.
- when for some external reason there is a potential difference that is below the diode's forward voltage and there is no connection between those two grounds in the source to draw them back to the same level, e. g. with a battery-powered non-earthed source like an MP3 player. For a simulation of that specific situation there would need to be a voltage source between the grounds with <600 mV.
 
ostripper

ostripper

Member
Joined 2008
PCB revised again
Click to expand...

That is better , still .. won't NFB be under the heatsink (you could notch it out). Also , the 22k from the bases of the cascode (ksa1845's) could be a 6.2v zener , both ways will work nice.

2 little refinements (attached) and lacking a double sided board , one could just run a heavy gauge wire from output pad to Inductor.

very pretty board..
OS
 

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Rafael L

Member
Joined 2008
AndrewT said:
the 10r0 in the diagram should not be in the signal return line.

Is the ground AC link shown in the diagram real?

I have never seen this done.

The PE wire is usually in the three core mains cable/cord and connects to the mains socket outlet.
Click to expand...

In my figure, there are two rack (pre-amp and power amp), GND of the Power sumply of each equipment, is connected to 3 pin connector AC (ground).
Mains socket is the short circuit the two pins ground, placed two equipament the same electric potential, it is used in equipment such informatica, a potential difference can burn the entry of an equipment

This is called "virtual earth" if your home has no grounding


Fig:
 

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