Hi
I've got some questions about this amp:
What is R1 used for?
I made some simulations with spice, and with all default values, the output voltage is linear from 40 to 10kHz, from 10kHz to 30kHz, it drops from 1.8V to 1.6V
Bypassing it, or reducing R1's value from 4.7k to 470 totally cancels this.
Are there any risks to change it to a 470Ohm one? (except the risk of having a better sound
)
Another question: why do we need to use another output cap value when using the ccs modif?
I've got some questions about this amp:
What is R1 used for?
I made some simulations with spice, and with all default values, the output voltage is linear from 40 to 10kHz, from 10kHz to 30kHz, it drops from 1.8V to 1.6V
Bypassing it, or reducing R1's value from 4.7k to 470 totally cancels this.
Are there any risks to change it to a 470Ohm one? (except the risk of having a better sound
)Another question: why do we need to use another output cap value when using the ccs modif?
Bricolo said:Hi
I've got some questions about this amp:
What is R1 used for?
I made some simulations with spice, and with all default values, the output voltage is linear from 40 to 10kHz, from 10kHz to 30kHz, it drops from 1.8V to 1.6V
Bypassing it, or reducing R1's value from 4.7k to 470 totally cancels this.
Are there any risks to change it to a 470Ohm one? (except the risk of having a better sound
I can think of two reasons:
1 current limitation in input overvoltage conditions
2 Create some roll off at high frequenies.
If you have control over your signals I think you can omit this resistor without any trouble.
Attachments
we don't want roll off from 10kHz to 30kHz, do we?
Normally not in a buffert amp.
But say you want a more "round, soft" sound
and not too "crispy, overbright".
Then you can use a low pass filter.
Lowpass filter you usually do with one resistor connected to one capacitor. The resistor is like Rg in series with signal
and the capacitor is connected to ground (0v).
With 1 R and 1 C, you get a first order filter. It rolls off with -6dB/octave. Octave is to double the frequency.
If MOSFETs was without input capacitance we shouldn't get a filter
with Rg+ Gate of MOS. But the gate is like a capacitor, it is charged with current and disscharged. Current is stored and dis-stored.
We can say that the Gate sucks!
a small current, at AC-signals.
And spits it back out.
This is one reson to not have too high resistors around
the input of a bigger mosfet.
higher resistors values creates higher voltage drop
at higher freq.
If I remeber correctly, Nelson, who is well studied in this subject,
recommended 15 kOhm and 22 kOhm, to set the working point for the MOSFET.
This can also be expressed as:
Having a lower input impedance to the circuit.
Impedance is something like "resistance at AC-signals"
That current starts to create a voltage over Rg,
and that voltage drop make the out signal lower.
The higher the freq the higher the drop.
Any R+C filter have a rolloff point depending on resistor and capacitor value.
Lower resistor, higher f (freq)
Lower capacitor, higher f (freq)
There is a formula!
I do not know what US-patent number that formula has????
So with same resistor and another MOSFET you probably get another roll off freq.
Also if there is other resistors before in series with the signal, this is added for calculating the roll off.
That can be output resistance of the CD-player
Normally not in a buffert amp.
But say you want a more "round, soft" sound
and not too "crispy, overbright".
Then you can use a low pass filter.
Lowpass filter you usually do with one resistor connected to one capacitor. The resistor is like Rg in series with signal
and the capacitor is connected to ground (0v).
With 1 R and 1 C, you get a first order filter. It rolls off with -6dB/octave. Octave is to double the frequency.
If MOSFETs was without input capacitance we shouldn't get a filter
with Rg+ Gate of MOS. But the gate is like a capacitor, it is charged with current and disscharged. Current is stored and dis-stored.
We can say that the Gate sucks!
a small current, at AC-signals.
And spits it back out.
This is one reson to not have too high resistors around
the input of a bigger mosfet.
higher resistors values creates higher voltage drop
at higher freq.
If I remeber correctly, Nelson, who is well studied in this subject,
recommended 15 kOhm and 22 kOhm, to set the working point for the MOSFET.
This can also be expressed as:
Having a lower input impedance to the circuit.
Impedance is something like "resistance at AC-signals"
That current starts to create a voltage over Rg,
and that voltage drop make the out signal lower.
The higher the freq the higher the drop.
Any R+C filter have a rolloff point depending on resistor and capacitor value.
Lower resistor, higher f (freq)
Lower capacitor, higher f (freq)
There is a formula!
I do not know what US-patent number that formula has????
So with same resistor and another MOSFET you probably get another roll off freq.
Also if there is other resistors before in series with the signal, this is added for calculating the roll off.
That can be output resistance of the CD-player
thanks, I've understood the concept
I thought that it was a RC filter, now you confirm it.
But you talk about Rg, Rg is a low value, to prevent some oscillations
I talked abour R1, that is 4.7K
I must check my electronic lessons, but a low pass RC filter is, as you said, a resistor in serie with the signal, and a cap to the ground
here, we have both R1 and C1 in serie
Another thing I'll have to check: the hitachi MOSFET has a lower capacitance that the IRF510, I'll maybe heve to recalculate R1 according to this
But I'll need some help for this, I don' know the formula
I thought that it was a RC filter, now you confirm it.
But you talk about Rg, Rg is a low value, to prevent some oscillations
I talked abour R1, that is 4.7K
I must check my electronic lessons, but a low pass RC filter is, as you said, a resistor in serie with the signal, and a cap to the ground
here, we have both R1 and C1 in serie
Another thing I'll have to check: the hitachi MOSFET has a lower capacitance that the IRF510, I'll maybe heve to recalculate R1 according to this
But I'll need some help for this, I don' know the formula
Also if there is other resistors before in series with the signal, this is added for calculating the roll off.
So in this case R= resistens of potentiometer + R1 + Rg and so.
That should mean that the RxC is dependent of the setting of the potentiometer.
Instead of doing the math,
you can feed the circuit with different freq signals
at different potentiometer settings.
And then take actual messurments.
This is a much more accurate (easier) way
then the math!!
With math we can try to estimate the behavior
of a circuit, but due to the imperfectness of components, layout
especially when handling AC-signals,
messurments gives us the real figures.
So in this case R= resistens of potentiometer + R1 + Rg and so.
That should mean that the RxC is dependent of the setting of the potentiometer.
Instead of doing the math,
you can feed the circuit with different freq signals
at different potentiometer settings.
And then take actual messurments.
This is a much more accurate (easier) way
then the math!!
With math we can try to estimate the behavior
of a circuit, but due to the imperfectness of components, layout
especially when handling AC-signals,
messurments gives us the real figures.
This is the results I've got
Original diagram, with R1=470R
Original diagram, with the 100k pot to 100%
Original diagram, with the 100k pot to 50%
Original diagram, with the 100k pot to 90%
C1 before the pot, with the 100k pot to 100%
C1 before the pot, with the 100k pot to 90%
C1 before the pot, with the 100k pot to 50%
Original diagram, with R1=470R
An externally hosted image should be here but it was not working when we last tested it.
Original diagram, with the 100k pot to 100%
An externally hosted image should be here but it was not working when we last tested it.
Original diagram, with the 100k pot to 50%
An externally hosted image should be here but it was not working when we last tested it.
Original diagram, with the 100k pot to 90%
An externally hosted image should be here but it was not working when we last tested it.
C1 before the pot, with the 100k pot to 100%
An externally hosted image should be here but it was not working when we last tested it.
C1 before the pot, with the 100k pot to 90%
An externally hosted image should be here but it was not working when we last tested it.
C1 before the pot, with the 100k pot to 50%
An externally hosted image should be here but it was not working when we last tested it.
I would put C1 after the pot and R1.
If you have single supply, using 0 volt as GND,
C1 before pot and connected to GND, would
change the current in MOSFET, when you turn pot.
The potentiometer would be, like R3, connected to ground.
So the working point is changed.
That is the purpose of C1, to block DC-current.
So that only R2 and R3 set the working point,
to about half voltage at Source pin of MOS.
Also if you put C1 before the pot, the highpass roll off point
is affected by the pot setting.
The RxC set by C1+pot, is something like 10 Hz.
When you have it at full volume R= pot//(R1+(R2//R3))
at Zero volume R= pot
So the RxC constant is changed by volume setting
If you put C1 after pot,
in RxC1
R= R1+(R2//R3) all the time.
If you have single supply, using 0 volt as GND,
C1 before pot and connected to GND, would
change the current in MOSFET, when you turn pot.
The potentiometer would be, like R3, connected to ground.
So the working point is changed.
That is the purpose of C1, to block DC-current.
So that only R2 and R3 set the working point,
to about half voltage at Source pin of MOS.
Also if you put C1 before the pot, the highpass roll off point
is affected by the pot setting.
The RxC set by C1+pot, is something like 10 Hz.
When you have it at full volume R= pot//(R1+(R2//R3))
at Zero volume R= pot
So the RxC constant is changed by volume setting
If you put C1 after pot,
in RxC1
R= R1+(R2//R3) all the time.
C2 doesn't need to be changed.
The value of C2 and the impedance of Headphone, 30 Ohm
is the RxC constant of the high pass filter,
that sets how low freq that can pass to the headphone.
If C2 is too little, say 10uF,
the lower bass tones can not pass.
RxC is then: 10uFx30ohm= 300uFohm
if you use headphone with 600ohm
it will be 10uFx600ohm= 6000uFohm
and lower bass tones can pass.
The higher the RC (RxC), the lower freq you pass out; can hear.
So you can say a headphone with 20 times higher impedance,
need only 20 times smaller Capacitor,
to get same roll off point.
It is good if the roll off point is 20 Hz or lower
for headphones.
So in this case both C1 and C2 can effect the low roll off point.
One at the input, and the other at the output.
Roll off point, also called f: Where curve(volt) is at 70% of the max
f=1/(2 x 3.14 x RC)
RC=1/(2 x 3.14 x f)
The value of C2 and the impedance of Headphone, 30 Ohm
is the RxC constant of the high pass filter,
that sets how low freq that can pass to the headphone.
If C2 is too little, say 10uF,
the lower bass tones can not pass.
RxC is then: 10uFx30ohm= 300uFohm
if you use headphone with 600ohm
it will be 10uFx600ohm= 6000uFohm
and lower bass tones can pass.
The higher the RC (RxC), the lower freq you pass out; can hear.
So you can say a headphone with 20 times higher impedance,
need only 20 times smaller Capacitor,
to get same roll off point.
It is good if the roll off point is 20 Hz or lower
for headphones.
So in this case both C1 and C2 can effect the low roll off point.
One at the input, and the other at the output.
Roll off point, also called f: Where curve(volt) is at 70% of the max
f=1/(2 x 3.14 x RC)
RC=1/(2 x 3.14 x f)
some pspice simulations later... (very great tool!)
C1 would be optimal with 4.7µF, if you want better <10Hz response (I'm not sure it's usefull...)
This R1 is still annoying me, I find 4.7K is too much, with 1K my 10KHz->30kHz is pratically flat
with 4.7K, output voltage decreases from 10kHz to 30kHz
AN IMPORTANT POINT
I set R2 and R3 to 15k and 22k, as Mr Pass told me once
when I redraw my shematic in pspice, with default values (150k and 220k), output voltage went from 1.25V to 1.8V!!
Nice gain
Don't forget that R2 and R3 are a voltage divider for the gate, but R3 and R1 are a voltage divider for the input voltage going in the gate
the greater R3 will be, the lower the voltage drop will be (setting R2 and R3 to 1.5M and 2.2M gives an 1.9V voltage) Not bad for 2V input
(very great tool!)C1 would be optimal with 4.7µF, if you want better <10Hz response (I'm not sure it's usefull...)
This R1 is still annoying me, I find 4.7K is too much, with 1K my 10KHz->30kHz is pratically flat
with 4.7K, output voltage decreases from 10kHz to 30kHz
AN IMPORTANT POINT
I set R2 and R3 to 15k and 22k, as Mr Pass told me once
when I redraw my shematic in pspice, with default values (150k and 220k), output voltage went from 1.25V to 1.8V!!
Nice gain
Don't forget that R2 and R3 are a voltage divider for the gate, but R3 and R1 are a voltage divider for the input voltage going in the gate
the greater R3 will be, the lower the voltage drop will be (setting R2 and R3 to 1.5M and 2.2M gives an 1.9V voltage) Not bad for 2V input
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