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In Reply to: RE: phono stage RIAA question. posted by lovetube on February 25, 2011 at 20:24:13
It has calculations needed. I know the dude took info I provided for free, and then writes on his page:
"©2003, 2004 Jeremy Epstein. No reproduction, retransmission or web distibution without prior written permission."
Thanks, Jeremy, for your lack of citations and your illegal claim to having no one do "web distibution [sic]". It's already published to the web! Jeez.
-Kurt
Follow Ups:
Dear me.I'm just noticing this post, 6 years later.
I'm not sure who Kurt S. is or where he published his RIAA phono calculations, and since it's been 14 years since I wrote that article, I can't conclusively recall every source I used in gathering the necessary calculations, SEVEN YEARS before Kurt S. complained about my plagiarism. But it should be pretty clear that my main source was Morgan Jones, "Valve Amplifiers" which is cited at the bottom of the article.
I had no idea that a guy named Kurt S. had invented mathematics! Sorry dude, I'll be sure to cite you next time I add, subtract, divide or multiply anything.
I know full well I am standing on the shoulders of giants. I guess I just don't know all of their names.
2017 update: don't build the power supply exactly as it was shown in the "Rev 4" schematic: it will give you a low frequency "motorboat" oscillation. Every time. I have removed that part of the schematic. The lower image (main power supply drawing) is still Rev 4, that doesn't need changes.
I updated the power supply filter part of the main schematic up to Rev 5 to reflect this change, it's caused people pain every now and then for the past 14 years.
But they always like the way it sounds once they get it sorted out!
Edits: 02/18/17 02/18/17
not sure how it will sound yet.
working on RIAA is funs but finding the correct components is a pain in the butt...
Cheers
LT
The KAB calculator will get you close but not the right answer.
It assumes the source impedance is very very low and in most valve circuits it isn't. It also assumes the load which is the grid bias resistor of the 2nd stage is 50 times that of the source resistor. If these two conditions aren't met, the calculator will be off. Depends on how accurate you want to get this thing. Say your series R is 330k, then 50 x 330k = 16.5M for the 2nd stage grid. As Kurt days, grid leak bias can be handy and get you close but not all valves like it and that's not your circuit at present.
IME, the best way is to sim it. Don't bother messing around with calculations unless you know all the variables and how they interact. The EQ loads the first stage but not in a linear fashion as it's impedance changes with frequency. With high Z EQ networks, the Miller capacitance of the 2nd stage will affect the values.
You could measure it but you will go nuts trying to dial in an all-in-one EQ this way. A sim can get you there in very little time. I use them all the time to determine EQ values. Before sims were easy, I had a mammoth spreadsheet that would do it but even that was hard work and error prone. A few sums won't get you there very easily.
cheers,
Stephen
Hi Stephen.
Thanks . yes I see what you means . i did the KAB calculation and run it on the pre but when I scope the frequency response I now see the bass boost way down to under 10hz but the top end is off . it wouldn't pass 5khz without dropping 5~7db .
so your post definitely explain it quite well .
I have download the sim program from linear.com which you send me the link of , but haven't extract it yet as it seem to requires a different program than what i have to open it .
for now can you help me to sim the circuit on your program ? see how this circuit come up .
the circuit running one section of the 12AT7 first stage and 6CG7 in Parallel on second stage. the series R was changed to 220k and all the caps is as my other post to Kurt.
Thanks
LT
See links for sim of something like your RIAA. It's not totally accurate as I don't know the value of the cathode bypass caps or the cathode resistor for the 6cg7 so I used the sim to work out a value.
It sims 20 - 20k within a 0.08dB window. Use 1% parts and it will be close to +/- 0.1dB or so. You'd need very expensive test equipment and a meticulous setup to verify any of this to that accuracy IMHO.
If you want to take it further, download the .asc file and play around with it in LTSPICE. There's a lot you can learn by doing this that will be invaluable for the future. You'll need the drop down valve models from the Intact Audio site, Sim section.
cheers,
Stephen
Edits: 03/03/11
Sincerely,\
this gives a lot to play with in one package...
sincerely,
-3db
is the C2 is 40n right ? I can't see it clear enough .and R11 is 1000K not 100k right ?
the cathode on the 12AT7 is 1K at prerent .but the cathode on the 6CG7 is 2.7k .
I thinking of paralell the 12AT7 as well . what do you think ? it should have lower the impredance of the stage right ?
Cheers
LT
C2 is 10n. R11 is 1000k or 1M.
2.7k seems a bit high according to the sim for a paralleled 6CG7. What anode voltage does it run at? I had to guess an HT as well and used 300V. With 2.7k, I'd expect the anode to be about 260V on a 300V HT. Still sims OK. Maybe it sounds better that way? It's not a valve I have any experience with.
Paralleling the 12AT7 would lower the drive Z. It would change the series resistor to the EQ as well. I'm not a fan of paralleling things up. My preference is to chose the right thing in the first place.
cheers,
Stephen
Here's some tips: Start trying it with R1 = 220,000 ohms. Then many typical values will be close for you.
Correction to my other post: 0.01 uF / 2.916 = 0.0034 uF
= close to 0.33 uF.
Lipschitz' formula here expects a zero output impedance from the op amp. 220,000 ohms is the total source impedance including Rp and Rl in parallel at first tube's plate. (Rp || Rl) + R1 = 220K.
If you direct couple this, expect a weaker bass, as V2 grid current will affect the small signal's DC effect toward it.
If you choose to AC couple this, the bass becomes more accurate and can even make it sound better. The grid current then goes out of play. And the coupling cap quality becomes very critical.
If you choose to AC couple and need a grid grounding resistor at V2, then make it as high as possible because this calculator expects the load there to be infinite. This is where very high grid resistor values and grid leak biasing can get real close to that on V2.
You will never achieve a perfect RIAA. :-)
-Kurt
Hi kurt.
I did that already last night with the R1 of 220k and come up with r2= 31.989K and the C2 =0.0034uf and c1 = 0.009940uf.
so i'm not sure why your value is different ?
I'm AC coupled the first to the second stage /. however as to my drawing I think is might be best to parallel the 12AT7 for lower impedance .
what you rekon ?
Cheers
LT
Your results are correct from the calculator.
Where I went wrong was thinking C2/C1 = 2.916, and still missed; But in reality, C1/C2 = 2.916,
or C2 = C1 / 2.916.
C1 = 0.01 uF; therefore C2 = 0.01 uF / 2.916 = 0.0034 uF. That's the correct answer. Thanks for catching that.
-Kurt
Cheers
LT
"working on RIAA is funs but finding the correct components is a pain in the butt..."
Well, in a phono RIAA, the series resistors and coupling caps have greatest impact on the sound. You may want to use 0.01 uF and 0.027uF + 0.003 uF for the 2.916 exact ratio of capacitors ( <3% error at factor of 3.0), as they are available in high quality caps. 0.003 uF, or 3000 pF might be best to go to mica. For resistors, use same kinds but allow yourself some slack or trim with paralleling or series-connecting resistors.
Good luck.
-Kurt
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