Hi all,
I would like to build the phono preamp from the LT1115 datasheet, the schematic shown on the first page of the document. This will be used with a Dynavector 20X-L (.3 mv - moving coil cartridge).
I have a few questions about the circuit:
The 2N4304 FET used to bias the LT1115 output in class A is no longer available, which component could I use to replace it? what about replacing it with a current source diode like the 1N5305 or even using a LM334 adjustable current source.
If I change the FET how can I calculate the value for the resistor in series with it? I can always go by trial and error...
Which component would be the best option in order to keep the noise level as low as possible.
What is the purpose of the 3n9 capacitor connecting the output to the ground? What about the 25K resistor marked as RL?
There is no coupling capacitor in the output of this circuit, should I add one, or is this not necessary with this topology.
What is the purpose of the 2200uF and 4.7uF (film) caps in parallel that connect part of the feedback loop to the ground?
Can I power this with +12 / -12 instead of 18 volts? what would be the drawbacks? If 12 volts is possible I could try to power it with batteries....
Thank you in advance,
Luis
I would like to build the phono preamp from the LT1115 datasheet, the schematic shown on the first page of the document. This will be used with a Dynavector 20X-L (.3 mv - moving coil cartridge).
I have a few questions about the circuit:
The 2N4304 FET used to bias the LT1115 output in class A is no longer available, which component could I use to replace it? what about replacing it with a current source diode like the 1N5305 or even using a LM334 adjustable current source.
If I change the FET how can I calculate the value for the resistor in series with it? I can always go by trial and error...
Which component would be the best option in order to keep the noise level as low as possible.
What is the purpose of the 3n9 capacitor connecting the output to the ground? What about the 25K resistor marked as RL?
There is no coupling capacitor in the output of this circuit, should I add one, or is this not necessary with this topology.
What is the purpose of the 2200uF and 4.7uF (film) caps in parallel that connect part of the feedback loop to the ground?
Can I power this with +12 / -12 instead of 18 volts? what would be the drawbacks? If 12 volts is possible I could try to power it with batteries....
Thank you in advance,
Luis
http://www.analog.com/library/analogDialogue/archives/39-05/op_amp_applications_handbook.html
section 6 Signal Amplifiers - has chapter on RIAA preamps that should answer some questions
section 6 Signal Amplifiers - has chapter on RIAA preamps that should answer some questions
I have made a pcb for this design.
It's not very critical the JFET. The resistor should be 200-500 ohms I should think.
Low noise level is achieved with a low ohmish feedback network as it is now!
The 3n9 is a filter to remove a zero which has to do with that the amp is non-inverting. Beyond 2122 Hz the gain should fall 6 dB/oct but it will stop doing that around 50 kHz or so just because of the non-inverting nature. The filter will cancel this.
The 2200 uF cap will set the DC gain to 1 and also the offset, around 1 mV or less. My design uses a DC servo whch takes care of that.
+- 12 V is OK. The possible drawback is less output voltage and less headroom.
It's not very critical the JFET. The resistor should be 200-500 ohms I should think.
Low noise level is achieved with a low ohmish feedback network as it is now!
The 3n9 is a filter to remove a zero which has to do with that the amp is non-inverting. Beyond 2122 Hz the gain should fall 6 dB/oct but it will stop doing that around 50 kHz or so just because of the non-inverting nature. The filter will cancel this.
The 2200 uF cap will set the DC gain to 1 and also the offset, around 1 mV or less. My design uses a DC servo whch takes care of that.
+- 12 V is OK. The possible drawback is less output voltage and less headroom.
Hello everybody,
I have built such a phono stage using the LT1115 producer suggested circuit (LT1115 & LT1010).
It works just fine, BUT I feel the I/C's (all of them) are getting a little too hot.
Does anyone have experience on this matter? Do they usually work hot? I am not sure what temperature they have, but I can keep my finger on them 7-8 seconds.
Thank you for help,
Marius
I have built such a phono stage using the LT1115 producer suggested circuit (LT1115 & LT1010).
It works just fine, BUT I feel the I/C's (all of them) are getting a little too hot.
Does anyone have experience on this matter? Do they usually work hot? I am not sure what temperature they have, but I can keep my finger on them 7-8 seconds.
Thank you for help,
Marius
I went to a friend who is an engineer (I am just an amateur) and he tested the circuit to see if it oscillates or not. It does not oscillate so he said most likely the ICs work this way: warmer than other ICs.
Then I decreased the power supply voltage from 18 to 15, as you are feeding the circuit with 15 V and so says the datasheet, despite the generic circuit from the datasheet is powered with 18 V.
Now everything seems to be just fine.
Thank you!
Then I decreased the power supply voltage from 18 to 15, as you are feeding the circuit with 15 V and so says the datasheet, despite the generic circuit from the datasheet is powered with 18 V.
Now everything seems to be just fine.
Thank you!
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Hi everyone,
peranders , can you please tell me why you chose 15V for supplying your RIAA project? In the datasheet it is powered with 18V.
I am asking because I have a variable regulator and I can set one voltage or the other, or any voltage between.
Please let me know and thank you very much.
Marius
peranders , can you please tell me why you chose 15V for supplying your RIAA project? In the datasheet it is powered with 18V.
I am asking because I have a variable regulator and I can set one voltage or the other, or any voltage between.
Please let me know and thank you very much.
Marius
Hi Guys
You can build the same circuit using the LT1028, which has a heftier output stage.
I modified the 1028's phono preamp to be inverting for better high-frequency accuracy. I added a second LT1028 as a buffer at the input, driving the 10R input to the EQ stage. The DJ who wanted this circuit had heard about the LT1028 and needed two stereo preamps for his two tables. I told him about the inaccuracy of the treble response and how to fix it, which he approved, and he was very happy with the results.
Inverting EQ stages at least provide accurate EQ. Their phase inversion can be corrected with a second unity-gain inverter.
D. Self does not like the LT1028, and by association would not like the 1115, because of the input bias current cancellation system. This feeds near identical currents into both inputs and represents a correlated noise that can add to THD.
Have fun
You can build the same circuit using the LT1028, which has a heftier output stage.
I modified the 1028's phono preamp to be inverting for better high-frequency accuracy. I added a second LT1028 as a buffer at the input, driving the 10R input to the EQ stage. The DJ who wanted this circuit had heard about the LT1028 and needed two stereo preamps for his two tables. I told him about the inaccuracy of the treble response and how to fix it, which he approved, and he was very happy with the results.
Inverting EQ stages at least provide accurate EQ. Their phase inversion can be corrected with a second unity-gain inverter.
D. Self does not like the LT1028, and by association would not like the 1115, because of the input bias current cancellation system. This feeds near identical currents into both inputs and represents a correlated noise that can add to THD.
Have fun
Hi Guys
If you must use integrated regulators, you can reduce hum further by cascading two sets of regs. That is, if you want 15V out, use the 7815-7915 for the output regs, but feed them from some higher-voltage units, like 7824-7924. The hum rejection multiplies.
A very big negative factor in using IC voltage regulators is that they have a very poor frequency response and this is imparted to the circuit they support. Fortunately, the situation is easily fixed by using much larger decouplers than the app notes show - 1mF per rail minimum. The apps show 10uF which is what is needed for stability - horrific for audio.
Better than that is simple discrete regulators or hum filters.
Even though the LT1028-1115 have very high PSRR, the signals in a phono stage are tiny and very susceptible to contamination. Anything you can do to help it out is a good thing.
Have fun
If you must use integrated regulators, you can reduce hum further by cascading two sets of regs. That is, if you want 15V out, use the 7815-7915 for the output regs, but feed them from some higher-voltage units, like 7824-7924. The hum rejection multiplies.
A very big negative factor in using IC voltage regulators is that they have a very poor frequency response and this is imparted to the circuit they support. Fortunately, the situation is easily fixed by using much larger decouplers than the app notes show - 1mF per rail minimum. The apps show 10uF which is what is needed for stability - horrific for audio.
Better than that is simple discrete regulators or hum filters.
Even though the LT1028-1115 have very high PSRR, the signals in a phono stage are tiny and very susceptible to contamination. Anything you can do to help it out is a good thing.
Have fun
I am using this power supply S-Audio Systems - PSU for High end audio without electrolytic capacitors
It has no audible hum (with 4,700 uF plus 470 uF in front of it). The sound is different than using just a regulator.
I am comparing this phono stage withe a tubed one (E88CC + ECC808) with tube rectification and PP caps. I feel the solid state is marginally better in sound and better in hum terms.
It has no audible hum (with 4,700 uF plus 470 uF in front of it). The sound is different than using just a regulator.
I am comparing this phono stage withe a tubed one (E88CC + ECC808) with tube rectification and PP caps. I feel the solid state is marginally better in sound and better in hum terms.
One would use +/-15V if that is all that is available or they want to reduce device power dissipation. Having +/-18V allows for a larger dynamic range to prevent clipping for large input signals. I believe that the pops from a LP present a signal that could potentially clip a phono pre-amp the most. Some think even +/-18V is not enough and go for +/-24V or higher supplies in discrete designs. Some use a OPA604, 2604 as it is rated for +/-24V