… susceptibility to RF…
the main virtue is a _true_ galvanic isolation between console and dozens of effects. Inserting the effect in one line can cause hum and crosstalk in completely independent line. I was confronted with such problems. The explanation is on Jensen site:
http://www.reed-electronics.com/ednmag/archives/1998/060498/12df_03.htm
http://www.reed-electronics.com/ednmag/archives/1998/060498/12df_03.pdf
the main virtue is a _true_ galvanic isolation between console and dozens of effects. Inserting the effect in one line can cause hum and crosstalk in completely independent line. I was confronted with such problems. The explanation is on Jensen site:
http://www.reed-electronics.com/ednmag/archives/1998/060498/12df_03.htm
http://www.reed-electronics.com/ednmag/archives/1998/060498/12df_03.pdf
Asbjorn said:
Okay.
I think this is a myth.
I did some comparing spice tests with differential inputs.
Lownoise JFETs vs. Lownoise BJT.
Note: My test used ONLY input pair, with collector resistors.
They were tuned to do exactly same AC current job.
for example: 5 uA current AC output across 1 kOhm resistors
or 5 /50 mV ac across same resistors.
This means
- open loop
- one transistor base/gate tied to GND
- the other fead with a few mVolt input
I did tests without Emitter/source resistors.
In this case I adjusted mV input, to do same job across collector load
( BJT had like 4-5 higher gain than JFET. uA AC current / mVolt input )
The THD harmonics ( 2nd + 3rd ) was at very similar level of magnitude.
At same job done.
This might come as surprise.
But actually this was so in my test.
I also tested with same input level: 1mV / 5 mV
In this case I adjusted with Emitter/Source resistors
to get equal job done.
Of course the bipolar in this case can afford more resistive degeneration
as they have like 4-5 higher AC current output per milliVolt AC in.
This gave like 12 dB lower level distortion than JFET pair. ( -149 dB vs. -137 dB )
At same level in, same collector job done.
=======================
Remember, not only low THD is a positive amplifier feature.
Also we have low bias current and DC Output offset to consider, many times.
Lineup
----------------------------
Attachment.
2SC2240, The Fourier result.
Attachments
You will find a thesis on exactly the same subject Audio Frequency Noise Characteristics of JFets AN74-4 by Siliconix. JFet design catalogue 1983! Also composite Op-Amp for high performance AN81-3.
Unfortunately both no longer in print.
I have no software means for copying/ sending both articles.
richj
Unfortunately both no longer in print.
I have no software means for copying/ sending both articles.
richj
It depends. Your question is too general. Either add specifics or better, upload a circuit.
I have a Cary Audio headphone amp, Nighthawk, which is now discontinued, bought it 7 years ago. It came with AD825 opamps socketed on browndog adapters. I was actually thinking along the lines of the discrete opamps from Burson audio or Sparkos labs. They claim you can just install, replace them with any audio opamp. But is this really true?
Hard to tell without knowing more. The Cary seems to be a discrete poweramp with an opamp at the input. Very likely the opamp is responsible for maintaining the dc offset at output and as such it's dc performance is critical. The 825 has 1mV of input offset voltage and practically nil input offset current. You should aim for a similar or better input offset voltage and the effect of possibly elevated input offset current will depend upon the resistor values.
Perhaps the easiest is to plug in an alternative opamp and measure the offset voltage at output.
Even without knowing the exact dc properties of the discrete opamps i am not optimistic. Very unlikely any discrete opamp will beat an AD825 at DC.
Perhaps the easiest is to plug in an alternative opamp and measure the offset voltage at output.
Even without knowing the exact dc properties of the discrete opamps i am not optimistic. Very unlikely any discrete opamp will beat an AD825 at DC.
Very much depends on the source impedance, and the requirements for bias current. If the source impedance is very high, JFET or CMOS opamps can provide that. If the bias current has to be very small, again its a job for a JFET or CMOS opamp.
One situation commonly seen is when bias current passes through a potentiometer wiper (for instance in volume tone control) - this will risk horrific crackling if you use a bipolar opamp. If the pot wiper is protected by a capacitor and the bias current has a separate DC path, either bipolar or FET will work.
For low source impedances where very low voltage noise is required bipolars tend to dominate (AD797 for instance). If current noise is an issue, FETs tend to win out (some bipolars have pretty low current noise though).