Simplistic NJFET RIAA - Page 770

Milos P. · 27th May 2012, 08:59 PM

Hi
today I finished Salas-RIAA-V1102F0. It sounds very nice and detailed, thank you Salas, but at the moment I think I over done it with takman carbon resistors, it is a bit to mellow. I like it but I'll change 47k in the signal path to metal film (PRP). Also I started with 1uF(Ampohm paper in oil) at the output and that took away some very low end. Paralleling another 1uF brought the low end back. I'll avoid PIO as well, and go for the polypropylene at the output (100n is Mundorf Supreme).
The only thing that is a bit of a problem is that I'd like to have more gain. My R2 is 100ohm (Grado black 5mV). I'd like to get closer to the CD output level so I don't need to change the volume so drastically when switching from TT to CD.
Any ideas how to get more gain? Another gain stage? Or changing R2 to 39ohm?
Thank you for this great pre.
Cheers

Salas · 28th May 2012, 01:02 AM

Nice initial outcome and room for tuning to your liking then. Congrats. Try lower R2 as you mentioned, in steps though, down to the point you will feel the gain has raised significantly but before audible overload distortion is kicking in.

AndrewT · 28th May 2012, 10:06 AM

Salas,
can the supply voltage be raised a bit or quite a lot, to allow more headroom in a high output RIAA stage?

If the output voltage is raised to ~600mVac, then that requires a lowish distortion output of ~6Vac for only +20dB of overhead. That is much lower than the usual >30dB overhead allowed and expected in a RIAA stage.
600mVac and +30dB of overhead allowance would require a non clipped output of ~27Vpk.

That would be an almighty RIAA stage. No wonder we see valves/tubes used in some RIAA stages.

Salas · 28th May 2012, 10:37 AM

We do use rather high B+ choices in our 300mV RMS output builds here, which allows higher value drain resistors, good for easy swing and first stage SNR even. The main restriction of going higher and higher B+ & swing is the distortion rising in those simple JFET cells.
Maybe you remember this alternative MC&PRE combo I had shown once? Its a matter of Rf/Ri on the OPAMP the output gain and 2nd stage won't shake for THD.

Milos P. · 04:47 PM

I went for 28ohm R2. No distortion, sine wave looks good on o'scope (pre fed with 7mV sine 100Hz). Now I'm much closer to CD out (~1.6V).
I also paralleled 100n with 330n Russian pio, and changed the output cap to 4.7uF. I was short of very low sub frequencies

. No hum, buzz rumble etc.
When music is not playing nothing at all comes out of the speakers.
This pre sounds fantastic, thank you very much again Salas. Another improvement and not small at all IMHO, are 10uF PIOs bypassing 10000uF PSU caps, but I'm not sure I'll be able to fit them in the box.

Salas · 28th May 2012, 04:52 PM

Very good. Enjoy.

RCruz · 28th May 2012, 07:09 PM

Nice enlightening experiments Milos

Salas
How low can we go for R2 in a stock 48dB build (28V) ?

Salas · 28th May 2012, 07:22 PM

Depends on cartridge output and +gain needed VS THD. Should experiment in each system.
Maybe with a trimmer and then replace to a good quality resistor. If gain is OK, better not.

RCruz · 28th May 2012, 08:31 PM

So the lower Rs, departing from optimum value, the higher THD right ?

Salas · 28th May 2012, 09:10 PM

Yes for same mV input, not significant initially for small steps, but it really takes off when the second stage is starting to get pushed as a result.

28th May 2012, 07:09 PM	#7697
RCruz diyAudio Member Join Date: Sep 2006 Location: Saxon Blog Entries: 1	Nice enlightening experiments Milos Salas How low can we go for R2 in a stock 48dB build (28V) ? __________________ RC

28th May 2012, 08:31 PM	#7699
RCruz diyAudio Member Join Date: Sep 2006 Location: Saxon Blog Entries: 1	So the lower Rs, departing from optimum value, the higher THD right ? __________________ RC

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27th May 2012, 08:59 PM	#7691
Milos P. diyAudio Member Join Date: Mar 2008 Location: Belgrade	Hi today I finished Salas-RIAA-V1102F0. It sounds very nice and detailed, thank you Salas, but at the moment I think I over done it with takman carbon resistors, it is a bit to mellow. I like it but I'll change 47k in the signal path to metal film (PRP). Also I started with 1uF(Ampohm paper in oil) at the output and that took away some very low end. Paralleling another 1uF brought the low end back. I'll avoid PIO as well, and go for the polypropylene at the output (100n is Mundorf Supreme). The only thing that is a bit of a problem is that I'd like to have more gain. My R2 is 100ohm (Grado black 5mV). I'd like to get closer to the CD output level so I don't need to change the volume so drastically when switching from TT to CD. Any ideas how to get more gain? Another gain stage? Or changing R2 to 39ohm? Thank you for this great pre. Cheers

28th May 2012, 01:02 AM	#7692
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	Nice initial outcome and room for tuning to your liking then. Congrats. Try lower R2 as you mentioned, in steps though, down to the point you will feel the gain has raised significantly but before audible overload distortion is kicking in.

28th May 2012, 10:06 AM	#7693
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Salas, can the supply voltage be raised a bit or quite a lot, to allow more headroom in a high output RIAA stage? If the output voltage is raised to ~600mVac, then that requires a lowish distortion output of ~6Vac for only +20dB of overhead. That is much lower than the usual >30dB overhead allowed and expected in a RIAA stage. 600mVac and +30dB of overhead allowance would require a non clipped output of ~27Vpk. That would be an almighty RIAA stage. No wonder we see valves/tubes used in some RIAA stages.

28th May 2012, 04:52 PM	#7696
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	Very good. Enjoy.

28th May 2012, 07:22 PM	#7698
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	Depends on cartridge output and +gain needed VS THD. Should experiment in each system. Maybe with a trimmer and then replace to a good quality resistor. If gain is OK, better not.

28th May 2012, 09:10 PM	#7700
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	Yes for same mV input, not significant initially for small steps, but it really takes off when the second stage is starting to get pushed as a result.