{"id":4830,"date":"2015-08-19T20:30:05","date_gmt":"2015-08-19T19:30:05","guid":{"rendered":"http:\/\/www.bartola.co.uk\/valves\/?p=4830"},"modified":"2015-08-19T20:30:05","modified_gmt":"2015-08-19T19:30:05","slug":"a-simple-line-stage","status":"publish","type":"post","link":"https:\/\/www.bartola.co.uk\/valves\/2015\/08\/19\/a-simple-line-stage\/","title":{"rendered":"A simple line stage"},"content":{"rendered":"

Driving your amp<\/h1>\n

A typical challenge we may all face is how to drive effectively our amp via a stepped attenuator or an AVC<\/a>. I have a 4P1L preamp<\/a> which drives very well my AVC, however, I have now an LME amp<\/a> which has a wimpy input impedance of less than 7K.<\/p>\n

How do we deal with this? A simple line stage which is capable of driving the low impedance of the amp is what we need in this case. Several options are available, however I settled down for a simple cathode follower.<\/p>\n

Why? Because I love valves, and I wanted to play around a cathode follower design here.<\/p>\n

\"vinilo\"<\/a>\u00a0A heavy load for your preamp or music source may increase distortion and\u00a0we don’t want that.<\/p>\n

I set myself the challenge to design a simple linestage with a minimum number of power supplies. I could have gone for a MOSFET follower, but hey: I wanted some hollow state stuff in there! Ok, if we look into a cathode follower as the core design, this means that we need at least an HT supply and a filament supply. If we could leverage a bucket converter, we could provide the HT from a LT transformer, probably best to look into two windings to separate the filament supply from an HT one. There are cheap ready build step-up converters for peanuts, and this is what tempted me to explore this solution.<\/p>\n

I tested recently some step-down bucket converters and was encouraged by the noise levels and the FR.<\/p>\n

The first design, getting us started<\/h2>\n

<\/p>\n

Well, a cathode follower is not rocket science. Still, there are some subtle aspect of the follower to bear in mind during our design.<\/p>\n

A high-mu valve can help us in maximising the performance of the follower. Yet, we want a high transconductance one as well to keep the output impedance low. Remember that the output impedance of the follower can be approximated to 1\/gm.<\/p>\n

Not many high-mu valves have high-gm, and we typically settle for high-gm\/mu triode-strapped pentodes (e.g. D3a, E280F, 6J51\/52P, etc.) or triodes like 6C45P.<\/p>\n

I will play here with 6C45P or 6J52P simply because I have many around, and I have created some Spice models which are quite accurate.<\/p>\n

I picked up the 6J52P first and looked at the curves to chose a good operating point. Not too much anode current but also away from potential grid current which will introduce distortion. I settled for Vak =125-130V and Vgk=-1.45V to target 17 – 18mA of anode current. This could be done with a 150V HT supply. Probably a cascoded DN2540 CCS feeding a VR to improve PSSR, or Morgan Jones’ statistical regulator.<\/p>\n

To avoid a negative supply for the tail CCS, I looked into the self-biased follower. This allow us to have a tail CCS referenced to ground and biasing the valve with a bootstrapped resistor (R2) as shown below:<\/p>\n

\"LME49830<\/a><\/p>\n

 <\/p>\n

The tail CCS is not rocket science. A pair of NPN transistors will help improving the output impedance of the CCS. Probably Q1 could be downgraded to a BC550C and benefit its higher hFE.<\/p>\n

As we are forced to have an output capacitor (ouch) why not bother to have an input one, albeit small. C1 can be kept to very low value thanks to the bootstrapped R2 which is effectively in parallel with R1 which provides a path to ground for C1.<\/p>\n

R3 helps us setting the operating point of the valve. The CCS current is set by R5 and the LED array current is set by R4.<\/p>\n

C3 unfortunately is driven by the low value of the output load. With the 7K load we need nearly 5uF to get the LF corner below 5Hz. An oil cap will do nicely here.<\/p>\n

Distortion simulations are encouraging. THD is less than 0.007% for output signal below 2Vrms, which is more than I need!<\/p>\n

This needs refinement, of course.<\/p>\n

Time to play a record and drink some wine. Bye for now \ud83d\ude42<\/p>\n

 <\/p>\n

 <\/p>\n","protected":false},"excerpt":{"rendered":"

Driving your amp A typical challenge we may all face is how to drive effectively our amp via a stepped attenuator or an AVC. I have a 4P1L preamp which drives very well my AVC, however, I have now an LME amp which has a wimpy input impedance of less than 7K. How do we … Continue reading “A simple line stage”<\/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_feature_clip_id":0,"_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":[707,706,156,162,67,5],"tags":[795,772,794,791,294,793,792,709],"class_list":["post-4830","post","type-post","status-publish","format-standard","hentry","category-class-a-solid-state","category-lme49830","category-pre-amplifier-valves","category-source","category-thd","category-valves","tag-6c45p","tag-6j52p","tag-bc550c","tag-cathode-follower","tag-d3a","tag-ksc1845","tag-line-stage","tag-lme49830"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p2r2tK-1fU","jetpack_likes_enabled":true,"jetpack-related-posts":[],"_links":{"self":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/4830","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=4830"}],"version-history":[{"count":3,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/4830\/revisions"}],"predecessor-version":[{"id":4834,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/4830\/revisions\/4834"}],"wp:attachment":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/media?parent=4830"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/categories?post=4830"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/tags?post=4830"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}