{"id":4746,"date":"2015-07-04T09:37:18","date_gmt":"2015-07-04T08:37:18","guid":{"rendered":"http:\/\/www.bartola.co.uk\/valves\/?p=4746"},"modified":"2015-07-04T09:46:22","modified_gmt":"2015-07-04T08:46:22","slug":"lcr-phono-design-notes-part-ii","status":"publish","type":"post","link":"https:\/\/www.bartola.co.uk\/valves\/2015\/07\/04\/lcr-phono-design-notes-part-ii\/","title":{"rendered":"LCR Phono: design notes (Part II)"},"content":{"rendered":"

Introduction<\/h1>\n
This is a continuation of my previous blog post<\/a>. I will try to share my experience through the design process of this RIAA stage through these individual posts with an attempt to spark some interest in others and in return to get some valuable input from the experience and knowledge of others. Hope this works!<\/div>\n
<\/div>\n

First stage<\/b><\/h1>\n
\"LCR-phono-test5\"<\/a><\/div>\n
Here is the initial design version for analysis. I\u2019m working through this step by step and refining the circuit in every iteration. The initial circuit is very simple. The first stage is key. We want to achieve as much amplification as possible from this stage before we hit the LCR network. The choice of the 6S17K-V<\/a> valve may appear as a surprise to the ones not familiarised with this valve. \u00a0Here are some notes from Wavebourn around this valve:<\/div>\n
<\/div>\n

<\/p>\n

\n

\u201c[\u2026]the\u00a0tube\u00a0has\u00a0an\u00a0amplification\u00a0factor\u00a0of\u00a0135\u00a0(higher\u00a0than\u00a012AX7),\u00a0<\/em><\/p>\n

transconductance\u00a0of\u00a014\u00a0milliamperes\u00a0\/\u00a0volt\u00a0(8\u00a0times\u00a0more\u00a0than\u00a012AX7);<\/em><\/p>\n

that\u00a0means\u00a0higher\u00a0amplification\u00a0factor\u00a0with\u00a0lower\u00a0noises\u00a0and\u00a0output\u00a0resistance.<\/em><\/p>\n

And\u00a0lower\u00a0capacitances,\u00a0because\u00a0the\u00a0tube\u00a0was\u00a0made\u00a0to\u00a0work\u00a0on\u00a0frequencies\u00a0up\u00a0to\u00a0microwaves.\u00a0[…]<\/em><\/p>\n

It’s\u00a0planar\u00a0frame\u00a0grid\u00a0is\u00a0made\u00a0of\u00a0microscopic\u00a0wires\u00a08\u00a0microns\u00a0thin,\u00a0with\u00a0distance\u00a0between\u00a0wires\u00a018\u00a0micron.<\/em>
\n Cathode\u00a0emission\u00a0is\u00a0strong,\u00a0and\u00a0the\u00a0grid\u00a0is\u00a0close\u00a0to\u00a0the\u00a0cathode,\u00a0so\u00a0grid\u00a0current\u00a0is\u00a0very\u00a0high\u00a0compared\u00a0to\u00a012AX7!<\/em>
\n 100 K\u03a9 grid\u00a0leak\u00a0resistor\u00a0would\u00a0cause\u00a0the\u00a0tube\u00a0to\u00a0cutoff!<\/em>
\n Also,\u00a0cathode\u00a0is\u00a0connected\u00a0to\u00a0one\u00a0end\u00a0of\u00a0filament.\u00a0That\u00a0means,\u00a0it\u00a0needs\u00a0a\u00a0different\u00a0bias scheme.<\/em>
\n Cathode\u00a0should\u00a0be\u00a0completely\u00a0grounded,\u00a0no\u00a0cathode\u00a0bias\u00a0resistor\u00a0is\u00a0needed.<\/em>
\n It is\u00a0good\u00a0because\u00a0it\u00a0eliminates\u00a0hum\u00a0caused\u00a0by\u00a0leakage\u00a0of\u00a0filament\u00a0voltage\u00a0on\u00a0cathode.<\/em>
\n Also,\u00a0it\u00a0eliminates\u00a0the\u00a0need\u00a0to\u00a0shunt\u00a0cathode\u00a0bias\u00a0resistor\u00a0by\u00a0a\u00a0capacitor\u00a0that\u00a0can’t\u00a0be\u00a0ideal.<\/em>
\n How\u00a0to\u00a0bias\u00a0the\u00a0tube\u00a0if\u00a0it’s\u00a0cathode\u00a0is\u00a0grounded?<\/em>
\n It\u00a0can\u00a0bias\u00a0itself\u00a0due\u00a0to\u00a0input\u00a0current\u00a0that\u00a0causes\u00a0<\/em>voltage\u00a0drop<\/em><\/p>\n

\u00a0in\u00a0secondary\u00a0winding\u00a0of\u00a0an\u00a0input\u00a0transformer\u00a0and\u00a0a\u00a0grid\u00a0stopper\u00a0resistor.<\/em>
\n Grid\u00a0stopper\u00a0resistor\u00a0can\u00a0be\u00a0used\u00a0to\u00a0adjust\u00a0bias\u00a0hence\u00a0idle\u00a0current.\u00a0This\u00a0resistor\u00a0can be\u00a0in k\u03a9\u00a0value,\u00a0that\u00a0means\u00a0very\u00a0low\u00a0added\u00a0by\u00a0it\u00a0noises.<\/em>
\n Compare\u00a0to\u00a010M\u03a9 needed\u00a0to\u00a0self-bias\u00a0a\u00a0venerable\u00a0EF86! [\u2026]”<\/em><\/p>\n<\/div>\n

<\/div>\n
I traced this valve some years ago and produced an initial Spice model. A 40dB stage gain can be achieved with this planar triode easily. In my circuit, Rg is the grid stopper resistor which provides the bias to the valve. R1 should be the recommended load resistance for the cartridge. In my case should be a 200\u03a9 resistor or more. Since this initial design doesn\u2019t provide enough gain for the DL103, an input step-up transformer is missing. In which case, R1 would be the required secondary load resistance suitable for the transformer. I will come back to this point later on (probably in a different post). The gain of this first stage is established by the anode resistance (R2), the anode resistance (Ra) and the load resistance (R9) in addition to the mu factor. We want to use a low noise resistor (e.g. wire-wound) for R2. The LF response will be dominated by the pole formed by C6 and R9 in addition to the output impedance of the valve, so we want to keep R9 as high as possible. There is a limitation in terms of the acceptable grid leakage resistor value. The higher the value of R9, the lower the value of C6 (the smaller the cap the better it is), gain is maximised and distortion is minimised. Surely the initial values used here can be optimised. We will come back to this later.<\/div>\n
<\/div>\n
\"6S17K-V<\/a><\/div>\n
\n
In the above plot we can see that at 57V anode voltage and 5.3mA anode current the gain of this triode is approximately 192 (Note that this is higher than the \u03bc reported by Wavebourn above but looking at the datasheet we can see a significant variance in this\u00a0parameter so we should expect a real challenge in matching a pair of these tubes at least). Transconductance is high at 14mA\/V and anode resistance is 13K\u03a9. The gain of this stage is therefore:<\/div>\n
<\/div>\n
Av=\u03bc * RL \/ (RL+Ra) whereas RL = 27K\u03a9 ||100K\u03a9 = 21,260\u03a9 so Av=118.85 or 41.5dB.<\/div>\n
<\/div>\n
Ideally with an CCS load we could achieve at least a gain of 170 (44.6dB). However, I suspect the noise level with the CCS would be a real challenge in practice.<\/div>\n
<\/div>\n
<\/div>\n
The cathode follower<\/b><\/div>\n
The cathode follower is formed by the famous 6C45P-E. \u00a0A tricky fellow which has a reputation to oscillate widely given it\u2019s characteristics. \u00a0This chap has a gm of about 45mA\/V which will provide an output impedance of 1\/gm in parallel with the cathode resistor. About 20\u03a9 in our case. So this means the additional resistor needed to match the LCR impedance has to take into account this intrinsic output impedance of the cathode follower which is the driver of the LCR network. The value of R8 is actually 6K\u03a9 minus the 20\u03a9 of the follower.<\/div>\n
<\/div>\n
An interesting point to add here is that although a 600\u03a9 load is not impossible for a 6C45P-E cathode follower, it\u2019s definitely a heavy one. And this will be reflected in the distortion. I\u2019ve done some simulations to compare the distortion impact of an 6K\u03a9 versus 600\u03a9 LCR and it seems on paper that there is a penalty of 0.01% in distortion. Not trivial so it\u2019s worth looking at an LCR with 6K\u03a9 or more.<\/div>\n
<\/div>\n
The final amplification stage<\/b><\/div>\n
The final stage needs to provide the additional gain required considering the 20dB insertion loss of the LCR network. The additional challenge for this stage is to achieve very low distortion at the desired output levels. I decided to use the 6J52P<\/a> which is similar to the D3a simply because I have many of them, they do sound good and also traced the curves in triode mode not far ago:<\/div>\n
<\/div>\n<\/div>\n
\"6J52p<\/a><\/div>\n
<\/div>\n
\n
With an CCS load, a gain closely to mu can be achieved subject to the output RL. Using a pair of SiC diodes for bias and a low anode current of about 15mA, the gain of the stage with 100K\u03a9 load will be around 34.5dB. The anode resistance of the 6J52P is about 2K\u03a9 so we need a bigger output capacitor which isn\u2019t great. We can improve this (and we will do this later).<\/div>\n
<\/div>\n
A simple coupling option for the LCR into the 6J52p is a capacitor. Unfortunately again, this capacitor has to be of a high value considering the 6K\u03a9 LCR network and the grid leakage resistor (RL in the circuit above). There are several option to consider, like a grid choke to reduce the load impedance to the LCR network or DC coupling. Both option will be explored later.<\/div>\n
<\/div>\n
I ran out of time so will leave more than 10 iterations of this design for later…<\/div>\n<\/div>\n
<\/div>\n","protected":false},"excerpt":{"rendered":"

Introduction This is a continuation of my previous blog post. I will try to share my experience through the design process of this RIAA stage through these individual posts with an attempt to spark some interest in others and in return to get some valuable input from the experience and knowledge of others. Hope this … Continue reading “LCR Phono: design notes (Part II)”<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false},"categories":[449,448],"tags":[772,771,559,768,767,773,770],"class_list":["post-4746","post","type-post","status-publish","format-standard","hentry","category-riaa","category-riaa-stages","tag-6j52p","tag-6s17k-v","tag-lcr","tag-lcr-network","tag-lcr-phono","tag-mc-tube-phono-stage","tag-tube-lcr-phono"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p2r2tK-1ey","jetpack_likes_enabled":true,"jetpack-related-posts":[],"_links":{"self":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/4746","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/comments?post=4746"}],"version-history":[{"count":7,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/4746\/revisions"}],"predecessor-version":[{"id":4756,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/4746\/revisions\/4756"}],"wp:attachment":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/media?parent=4746"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/categories?post=4746"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/tags?post=4746"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}