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Tube phono preamps
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<blockquote data-quote="tangmonster" data-source="post: 634114" data-attributes="member: 1581"><p>this calculator give your 12ax7 output impedance as 38.46kohm</p><p></p><p>https://www.ampbooks.com/mobile/amplifier-calculators/output-impedance/calculator/</p><p></p><p></p><p>You need to add that 38.46kohm to the 200k when calculating your RIAA circuit</p><p></p><p>So.</p><p></p><p>I get</p><p></p><p>sticking to 200k as you have it.</p><p></p><p>34606ohm (where you have 33k , you might ad 1.5k to be more precise?)</p><p></p><p>3.1nF (where you have 3.3nF which is as good as it gets) </p><p>9.1nF (where you have 10nF, i'm sure you are going to be close enough with 10nf)</p><p></p><p>I messed around with my d3a's riaa but mostly the cathode bias to get a good riaa.</p><p></p><p>In the end i only trusted using an oscilloscope and function generator and scrolling through the frequency. But after i fixed the tubes cathode bias problems i was VERY close to perfect riaa using the calculators.</p><p></p><p>My understanding is for your circuit you need to make R1 = 238000 and do the calculation since you have a 200k resistor in your circuit and the tube output impedance is 38k.</p><p></p><p>http://mh-audio.nl/CalculateRIAA.asp</p></blockquote><p></p>
[QUOTE="tangmonster, post: 634114, member: 1581"] this calculator give your 12ax7 output impedance as 38.46kohm https://www.ampbooks.com/mobile/amplifier-calculators/output-impedance/calculator/ You need to add that 38.46kohm to the 200k when calculating your RIAA circuit So. I get sticking to 200k as you have it. 34606ohm (where you have 33k , you might ad 1.5k to be more precise?) 3.1nF (where you have 3.3nF which is as good as it gets) 9.1nF (where you have 10nF, i'm sure you are going to be close enough with 10nf) I messed around with my d3a's riaa but mostly the cathode bias to get a good riaa. In the end i only trusted using an oscilloscope and function generator and scrolling through the frequency. But after i fixed the tubes cathode bias problems i was VERY close to perfect riaa using the calculators. My understanding is for your circuit you need to make R1 = 238000 and do the calculation since you have a 200k resistor in your circuit and the tube output impedance is 38k. http://mh-audio.nl/CalculateRIAA.asp [/QUOTE]
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