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op-amp current source

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sb****33

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Joined 2009
#1
Hi

I'm still working on building this preamp.
I just can't figure out why TL071's inverting input is protected by D29 & D31
while MC34001's inputs is by D4,D5 and Z20.
Are they equivalent tricks or the does the reason is the op-amps different characteristics ?
 

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#7

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#14
LinuksGuru said:
For example, this one, from AudioXpress.
Click to expand...

I asked for the complete AudioXpress article because, on FIGURE 1, I saw "(see text)" near Rset resistor.
I thought I could find there some formula to calculate the resistor's value.
I found some examples with different current values.
I need some help here.
I my project (ARC SP10), what current should I use in B+1 regulator (2 x 11.3mA) and B+2 (2 X .4mA) and with what Rset resistor ?
And why do we have to adjust CCS ? I thought that current is set by the circuit impedance.
 
#15
Plenty of manufacturer's still make depletion mode MOSFETs

Example:
Depletion Mode Products - Microchip Technology Inc

DN2470 would seem to be a good choice (700V rating). There are a lot of designs on this and other tube amp forums which specify the DN2450 (500V rating)

The ARC SP10 is using the current source as a load (for very high AC impedance but low DC drop across it) not as a source of current as such.

Cheers,
Ian
 
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#16
sb****33 said:
I asked for the complete AudioXpress article because, on FIGURE 1, I saw "(see text)" near Rset resistor.
I thought I could find there some formula to calculate the resistor's value.
I found some examples with different current values.
I need some help here.
I my project (ARC SP10), what current should I use in B+1 regulator (2 x 11.3mA) and B+2 (2 X .4mA) and with what Rset resistor ?
And why do we have to adjust CCS ? I thought that current is set by the circuit impedance.
Click to expand...

Use LTSpice simulation to adjust resistor values, its best tool in this case.
 
#17
LinuksGuru said:
Directly from IXYS technical support in Europe.
Thy are sold because some stock still available.

I wish they reconsider this decision.
Click to expand...

It is just strange that neither Mouser or Digi-Key shows them as obsolete or even NRND if this is true.

Also, I see the following at the IXYS website:
 

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#18
gingertube said:
Plenty of manufacturer's still make depletion mode MOSFETs

Example:
Depletion Mode Products - Microchip Technology Inc

DN2470 would seem to be a good choice (700V rating). There are a lot of designs on this and other tube amp forums which specify the DN2450 (500V rating)

The ARC SP10 is using the current source as a load (for very high AC impedance but low DC drop across it) not as a source of current as such.

Cheers,
Ian
Click to expand...

OK, "as a load", if you say so.
Now, I'm a little bit confused.
I started my post asking about op-amp inputs protection.
No one answered the question.
OK.
Jackinnj & LinuksGuru suggested a better ccs than ARC's one.
OK, I followed that path.
Now it's seems that you think that mosfet's css is the wrong way for B+ regulator on this preamp.
Am I right ? Did I miss something ? Should I replace op-amp current source by a mosfet one like AudioXpress' one or whatever ?
And if yes, how ?
 
#19
sb****33 said:
I started my post asking about op-amp inputs protection.
No one answered the question.
Click to expand...
The inputs must not go outside the supply voltages of the op-amp.In one case an input is protected with two diodes at the supply and the other input with another two diodes connected to the first input.The second case has each input protected separately with respect to the supply with diodes.Not much difference.
Should I replace op-amp current source by a mosfet one like AudioXpress' one or whatever ?
Click to expand...
No, it is a current source but not a constant one 😕
It is a stabilised voltage supply with max.current limit.It seems that the R? you mension determins the max.current.But the schematic on entry 1 doesn't show that R.
Mona
 
#20
Ketje said:
The inputs must not go outside the supply voltages of the op-amp.In one case an input is protected with two diodes at the supply and the other input with another two diodes connected to the first input.The second case has each input protected separately with respect to the supply with diodes.Not much difference.No, it is a current source but not a constant one 😕
It is a stabilised voltage supply with max.current limit.It seems that the R? you mension determins the max.current.But the schematic on entry 1 doesn't show that R.
Mona
Click to expand...

Thank's Mona. This time your answers suit me fine.
The resistor I mentioned (Rset) yesterday 03:07 PM is part of DN2540 ccs schematic given by linuksguru in 15th January 07:56 PM post.
Effectively it sets the current value up.

So I'll keep building this preamp without modification.
 
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