Phono preamp from the LT1115 datasheet - diyAudio
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Old 25th June 2007, 02:33 PM   #1
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Default Phono preamp from the LT1115 datasheet

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,
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Old 25th June 2007, 04:31 PM   #2
jcx is offline jcx  United States
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section 6 Signal Amplifiers - has chapter on RIAA preamps that should answer some questions
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Old 25th June 2007, 06:27 PM   #3
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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.
/Per-Anders (my first name) or P-A as my friends call me
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