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Old 27th May 2012, 08:59 PM   #7691
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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.
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Old 28th May 2012, 01:02 AM   #7692
Salas is offline Salas  Greece
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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.
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Old 28th May 2012, 10:06 AM   #7693
AndrewT is offline AndrewT  Scotland
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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.
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Old 28th May 2012, 10:37 AM   #7694
Salas is offline Salas  Greece
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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.
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Old 28th May 2012, 04:29 PM   #7695
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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.
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Last edited by Milos P.; 28th May 2012 at 04:47 PM. Reason: pix
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Old 28th May 2012, 04:52 PM   #7696
Salas is offline Salas  Greece
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Very good. Enjoy.
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Old 28th May 2012, 07:09 PM   #7697
RCruz is offline RCruz  Switzerland
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Nice enlightening experiments Milos

Salas
How low can we go for R2 in a stock 48dB build (28V) ?
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Old 28th May 2012, 07:22 PM   #7698
Salas is offline Salas  Greece
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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.
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Old 28th May 2012, 08:31 PM   #7699
RCruz is offline RCruz  Switzerland
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So the lower Rs, departing from optimum value, the higher THD right ?
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Old 28th May 2012, 09:10 PM   #7700
Salas is offline Salas  Greece
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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.
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