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Old 5th June 2012, 05:10 PM   #11
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Noted with thanks.

Kevin, Can you please me more about miller capacitance and what exactly I should add to the schematic to get it right?

Thanks and regards
Andersen
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Old 5th June 2012, 05:48 PM   #12
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6DJ8 has a mu of 33, with a 12K plate load and assuming >100K in the next stage you might achieve a gain of perhaps 26 (28.3dB) miller capacitance is about 1.2pF, so 26 x 1.2 =31.2pF, plus about 10pF for inter-electrode capacitance between grid, cathode, shield, and cathode to plate.. So figure roughly 42pF from this source, round off to 50pF for convenience and because there are likely external strays as well. Not a big contributor overall.
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Old 5th June 2012, 08:00 PM   #13
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Getting the right values is almost impossible for a number of reasons when using any easy calculator. I've tried virtually all methods up to very complicated spreadsheets to calculate RIAA EQ values.

There is only one way, IMHO, to calculate it accurately and that is a SPICE sim. LTSPICE is free. Check out Intact Audio forum to get models and info on how to use.

In the end, it will save you so much time and effort that you will wonder how you got on without it. All the other guess work and assumptions will become just that.

cheers,

Stephen
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Old 6th June 2012, 12:08 AM   #14
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I'm a big fan of LTSpice and have been using it for about 8 years now, but I have also had no problem in the past calculating the correct values using first principles and Stanley Lipshitz's useful equations which are probably the basis of most of those online calculators.

Also spice is only as accurate as the tube models used to build the simulation.. Note that there is plenty here as well about modeling tube circuitry in LTSpice.

Twenty four years ago when I designed my first phono stage early versions of spice were only available at few universities, but I managed to design and build a passively equalized phono pre-amp that required only very minimal tweaking to one resistor value. I admit at the time I was aiming for accuracy of about +/-0.5dB as I was not really able to measure to a much greater accuracy level, and getting tight precision parts before the internet was difficult.

Ultimately no matter how you arrive at a (good) design the passive components are going to be the let down. Careful selection and trimming may be required not because of a design issue but because of the parts themselves.
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Old 6th June 2012, 01:49 AM   #15
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I've just modified the schematic as below based on your recommendations/comments except for the value of R17 which I chose at 78K falling in the range of 100K (78K+22K) in order to get the values of other RIAA components close to standard values. Is it acceptable?
Click the image to open in full size.

BTW, is it ok if I use a 100K volume pot at the position of R17 (keeping the other RIAA components' values remained) and adjust the volume pot 'till getting the best result?

Thanks and regards
Andersen

Last edited by Andersen; 6th June 2012 at 01:59 AM.
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Old 6th June 2012, 04:17 AM   #16
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I would not use the volume pot as you will not have anything like the required resolution, in addition at these low signal levels it is likely to add noise and distortion, measurement and small incremental resistor trims are the best way to go.
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Old 6th June 2012, 04:34 AM   #17
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Kevin, in order to get the values of other RIAA components by using the calculator I posted, the value of R1 should be the value of (R17+Zo)//R25, is it correct?

Taking my schematic as an example, the value of R1 should be (78K + 22K)//470K equal to 82K456?
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Old 6th June 2012, 04:51 AM   #18
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Quote:
Originally Posted by Andersen View Post
Kevin, in order to get the values of other RIAA components by using the calculator I posted, the value of R1 should be the value of (R17+Zo)//R25, is it correct?

Taking my schematic as an example, the value of R1 should be (78K + 22K)//470K equal to 82K456?
No, moving R25 ahead of the network removes it from the network equation - I would purchase 76.8K plus 499, 1.21K, and 1.69K. The 76.8K is permanently installed, start with 1.21K in series with it, if too bright change to 1.69K, if too dull 499 - this is basic trimming.. Range could be potentially greater than my suggested values. Best to measure if you have the means. You should not trust your ears for this task.

Do you have a meter, scope and generator? Good soundcard in your PC?
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Old 6th June 2012, 05:17 AM   #19
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Quote:
Originally Posted by kevinkr View Post
No, moving R25 ahead of the network removes it from the network equation - I would purchase 76.8K plus 499, 1.21K, and 1.69K. The 76.8K is permanently installed, start with 1.21K in series with it, if too bright change to 1.69K, if too dull 499 - this is basic trimming.. Range could be potentially greater than my suggested values. Best to measure if you have the means. You should not trust your ears for this task.

Do you have a meter, scope and generator? Good soundcard in your PC?
Ok, I see. So my statement is only right if R25 stays in its original position, right? What types of resistors and capacitors would you recommend me?

I don't have those equipments/tools but will find a store to have it measured properly.

Thanks and regards
Andersen
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Old 6th June 2012, 04:32 PM   #20
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I'd recommend good 1% tolerance 1/2W (RN60) metal films like Dale or PRP (Precision Resistive Products), for the eq caps polystyrene film and foil caps like the REL RT series. For coupling and bypass capacitors Russian FT-3 series comes to mind. Output cap probably a good polypropylene like the Clarity SA/ESA series or something from Janszen.. There are many choices, these are just some that have worked for me in the past, none are too expensive.

Alternately you could order parts from Michael Percy, and have them shipped to Vietnam by mail.. I've bought small quantities of parts from him for more than 24yrs now - highly recommended. Here: Michael Percy Audio Ordering Information Not sure about duty and the like however.
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