Simplistic NJFET RIAA

[Salas]

What brand are your BF FETS, and what type DMM you use?

[RCruz]

The fets came from reichelt they have a Fbb42 printed... Fairchild ?... Measured 17mA idss....I am using a TENMA 72-7930

[RCruz]

Originally Posted by Rush

OK, received the caps, Mundorf 0.1 uF Silver and oil.
Took out the direct coupled JFET, added a 0.1 uF cap with 1 meg to ground on the RIAA board. Then ran to an out board DCB1 sub board and pulled power and ground from the Mezmerize B1 Buffer. I am not fretting about large caps anymore and no hum issues, sounds fantastic.
This is a must do, actually the Mezmerize B1 Buffer board could have double set of DCB1s before and after the volume as I have essentially done and it wouldn't hurt. Four JFETs are a lot cheaper than high quality, large value, coupling caps.

Thank you Salas,

Wonderful stuff!

Rush

Hi Rush

Can you post a schematic of what you have now.... very interested I am.

[Salas]

Originally Posted by RCruz

The fets came from reichelt they have a Fbb42 printed... Fairchild ?... Measured 17mA idss....I am using a TENMA 72-7930

Its 0.8% +1 count for DC mA which is decent. One 0.5% +10 count meter I got was still showing shy by 0.2mA to my best 0.15% meter for BL. So it must be IDSS and meters.
If you see a tiny f somewhere not in in part number line on your 245's face its FAIRCHILD.

[Rush]

Originally Posted by RCruz

Hi Rush

Can you post a schematic of what you have now.... very interested I am.

Yes, here is an earlier schematic that shows the output of the cap (0.1 uF) and 1 meg to ground (just the output is the part I used, not the RIAA from this early one) and the schematic I chopped up posted earlier. Also, the ground for the DCB1 is to the Mezmerize board.

Rush

[buzzforb]

I just happen to have that very thing. Any change or diminishing ofsound, or did you get to listen, pre DCB1?

[Rush]

Originally Posted by buzzforb

I just happen to have that very thing. Any change or diminishing ofsound, or did you get to listen, pre DCB1?

It is better. Better sound stage, smoother. Still trying to find time to listen more. Only have 2 hours of low level listening now.

Rush

[andyr]

Originally Posted by Rush

Yes, here is an earlier schematic that shows the output of the cap (0.1 uF) and 1 meg to ground ...

Rush

Hi Rush,

Can you tell me why you went for a 1Mohm res off the 2nd JFET Gate ... rather than, say, half that? IE. what advantage does 1Meg have here, compared to 500K?


Thanks,

[Rush]

Originally Posted by andyr

Hi Rush,

Can you tell me why you went for a 1Mohm res off the 2nd JFET Gate ... rather than, say, half that? IE. what advantage does 1Meg have here, compared to 500K?


Thanks,

159155
Divide your resistance then your uF into the above number and you have the 3 BD down roll off frequency. Multiply the 3 DB down frequency by 10 and that is how far the phase shift effects the frequency response.
So, 159155/1000000/.1 = 1.59 Hz x 10 = 15.9 Hz. All good, below 20 Hz
If, 159155/500000/.1 = 3.18 Hz x 10 = 31.8 Hz.

As the resistance goes down, the cap size goes up for the same roll off.

In the original circuit we had 2.2 uF and 1 meg, but I am using a DCB1 buffer with a 25k volume pot and a 220k input resistor. So an effective 22k resistance following the output cap.
The math, 159155/22000/2.2 = 3.29 Hz x 10 = 32.9 Hz phase shift
Really need to double the output cap, now that is getting pricey.

I see no advantage of using a lower resistance into a B1 buffer and I see great reasons to use the smallest cap that will do the job. The 0.1 uF coupling cap I used are $34 each, if I was looking a 4.7 uF of the same quality, would be $111 each and who knows if the bigger cap would sound as good as the smaller one, some folks would by pass the 4.7 uF with a smaller value and the cost keep going up.
I also don't have the physical space in my preamp for a larger cap anyway.

Rush

[andyr]

Originally Posted by Rush

159155
Divide your resistance then your uF into the above number and you have the 3 BD down roll off frequency. Multiply the 3 DB down frequency by 10 and that is how far the phase shift effects the frequency response.
So, 159155/1000000/.1 = 1.59 Hz x 10 = 15.9 Hz. All good, below 20 Hz
If, 159155/500000/.1 = 3.18 Hz x 10 = 31.8 Hz.

As the resistance goes down, the cap size goes up for the same roll off.

In the original circuit we had 2.2 uF and 1 meg, but I am using a DCB1 buffer with a 25k volume pot and a 220k input resistor. So an effective 22k resistance following the output cap.
The math, 159155/22000/2.2 = 3.29 Hz x 10 = 32.9 Hz phase shift
Really need to double the output cap, now that is getting pricey.

I see no advantage of using a lower resistance into a B1 buffer and I see great reasons to use the smallest cap that will do the job. The 0.1 uF coupling cap I used are $34 each, if I was looking a 4.7 uF of the same quality, would be $111 each and who knows if the bigger cap would sound as good as the smaller one, some folks would by pass the 4.7 uF with a smaller value and the cost keep going up.
I also don't have the physical space in my preamp for a larger cap anyway.

Rush

Terrific - thanks, Rush ... now I understand your choice.

What caps cost $34 for 0.1uF? I have used Relcap RTX at about $9 in this position ... I presume yours sound a lot better?


Regards,

Andy