If it's the 12AX7/6DJ8 circuit you are looking for there's a schematic on post 10 of this thread.
Hi,
I'm a DIY-er living in the Netherlands and I would like to build a RIAA phono amplifier.
So I have been looking around for schematics of RIAA phono amps and found one in this old thread on post #10
Here is presented a two stage phono amp with ECC83 and ECC88.
Somewhere in this thread is written that the fist stage with 1/2 ECC83 is using about 1mA of current and the output stage with 1/2 ECC88 is using about 14mA of current.
But when I look at the first stage, then there is a drop-resistor of 180k between the B+ (=250VDC) and the Anode-resistor of 100k.
The voltage drop over the 180k with 1mA current would be 180V. So the voltage before the Anode-resistor of 100k will be 250V - 180V = only 70V. But then there can not flow 1mA through the Anode -resistor of 100k, because the voltage drop over 100k with 1mA would be 100V. And there is only 70V above Ra.
So there can't flow a current of 1mA in the first stage with these values of resistors.
My question is: is there just made a mistake with the value of the drop-resistor of 180k and should it be much lower, or is there flowing much less than 1mA current through the 1/2 ECC83 of the first stage?
Kind regards,
Kees.
I'm a DIY-er living in the Netherlands and I would like to build a RIAA phono amplifier.
So I have been looking around for schematics of RIAA phono amps and found one in this old thread on post #10
Here is presented a two stage phono amp with ECC83 and ECC88.
Somewhere in this thread is written that the fist stage with 1/2 ECC83 is using about 1mA of current and the output stage with 1/2 ECC88 is using about 14mA of current.
But when I look at the first stage, then there is a drop-resistor of 180k between the B+ (=250VDC) and the Anode-resistor of 100k.
The voltage drop over the 180k with 1mA current would be 180V. So the voltage before the Anode-resistor of 100k will be 250V - 180V = only 70V. But then there can not flow 1mA through the Anode -resistor of 100k, because the voltage drop over 100k with 1mA would be 100V. And there is only 70V above Ra.
So there can't flow a current of 1mA in the first stage with these values of resistors.
My question is: is there just made a mistake with the value of the drop-resistor of 180k and should it be much lower, or is there flowing much less than 1mA current through the 1/2 ECC83 of the first stage?
Kind regards,
Kees.
"About xxx mA" is the key phrase.
Not "exactly xxx mA".
I rounded to the nearest mA. This figure was mainly meant to allow sizing resistors power ratings.
Thor
I would like to build this phono pre and have a question. Does the B+ after the 180k dropping resistor/10uf cap feed both channels? At one point Thorsten says you can split the 180k to 2 -91K/10uf feeding each channel separately. But in the simulation above, as well as calculations elsewhere it is saying approximately .6ma through the 180k drops the voltage about 108v-124v and dissipating .065 watts across the 180K suggesting that there are separate180k dropping resistors per channel???
John
John
