Simplistic NJFET RIAA

[Salas]

What does your transformer ask for fuse on its label? Does it have a printed value there?

 

[merlin el mago]

120VA / 230VAC = 0.521A

 

[merlin el mago]

As dummy load for 45V and 200mA I need 220R, will be enough 10W or it's a bit just, calculations gives 9.2W?

 

[Salas]

What does R4 do (on the Tea-Bag FSP board)?

It needs adjustment depending on gain, I am building for MM 5mV input, the guide says 1.8K. I have 1.68K and 2.21K, or should I make a series pair to get the exact value?

R4 is the load resistor of the folded cascode. It directly influences both the gain and overload margin of the input stage. With 1.8K there you will get 470mV RMS output from a 5mV nominal cart at 1kHz 0dB (5cm/sec) groove modulation.
Changing to 1.68K expect about 440mV i.e. less than 1dB down. Avoid much sensitivity with higher values than 1.8K if the cart seems to challenge THD in transients and the tone starts to sound thicker in general.
This discussion assumes all other values and parameters are loyal to the construction manual for MM mode.

 

[Salas]

120VA / 230VAC = 0.521A

Use 1A fuse.

 

[Salas]

As dummy load for 45V and 200mA I need 220R, will be enough 10W or it's a bit just, calculations gives 9.2W?

It will go smoking a little soon. If a power resistor without sink is receiving more than 1/3 its nominal for long enough it really gets hot. Why 200mA? Using single common supply? Not recommended, hope it won't hum. If it is one Tx but two secondaries and two PSU, then OK.

 

[merlin el mago]

You said FSP consumes 200mA

I designed my PSU for 200mA, have I to change?

Yes is one tx with the two 21VAC in series, only one transformer for the two channels.

 

[Salas]

100mA+100mA=200 total for two channels.

 

[Salas]

Yes is one tx with the two 21VAC in series, only one transformer for the two channels.

Better two smaller ones with less VAC secondaries, but if you will manage safe Vin level to regs with CRC, no hum and good sound in the end, OK. In this case you test raw DC Vout for 200mA.

 

[merlin el mago]

So all is ok only I need a dummy load of 220R 20W or more, right?

 

[Salas]

Right.

 

[merlin el mago]

What value for the fuse just after last main filter cap in the psu?

 

[Salas]

I don't fuse there but to the primary so I haven't looked at surge current to both the regs. Start at 250mA slow blow?

 

[merlin el mago]

OK, I have to wait till tomorrow morning because I don't have any 220R of this huge power.

 

[merlin el mago]

I guess the fuse 0.25A in the attached pic is slow, right?

 

[Salas]

If it has T (time) symbol etched somewhere.

 

[Salas]

OK, I have to wait till tomorrow morning because I don't have any 220R of this huge power.

You need test with dummy so to know raw DC is no more than 52VDC with your CRC loss under 200mA load and at average day mains.

 

[merlin el mago]

If it has T (time) symbol etched somewhere.

No. Slow I only have 500mA or tomorrow I will buy your advice for 250mA?

 

[merlin el mago]

You need test with dummy so to know raw DC is no more than 52VDC with your CRC loss under 200mA load and at average day mains.

I designed for 45VDC with the CRC and the 200mA of load.

 

[Salas]

No. Slow I only have 500mA or tomorrow I will buy your advice for 250mA?

Buy for 250 and 350 bcs I haven't tested in that position you want to fuse as I wrote.