CX301a DHT pre-amp<\/h2>\n
First one is my preferred CX301a thoriated-tungsten DHT. I wired the LL2745 in 5.6:1 step-down configuration. \u00a0This should provide a low output impedance which is what we want in this configuration where we need to drive the cables to the amplifier with sufficient capability:<\/p>\n Looks very promising. Of course gain will be much lower than a gyrator-based pre-amp, the 01a anode load is optimised providing very low distortion: 0.015% based on my SPICE model with curves taken from real CX301a. Gain is low at 3.1dB, but we don’t want loads of gain in this pre-amp. Sound is what we are after…<\/p>\n Now it’s time for the revered 26. I used Dmitry’s model based on the RCA manual curves. I’d like to simulate this again using a model based on starved filament curves with a real 26.<\/p>\n Originally planned to bias the 26 in a different operating point based on feedback from Andy Evans, however after playing a bit with the OP I found that a more linear point was around Ia=5mA and Va=114V @ Vgk=-6.85V<\/p>\n 26 looks more interesting in principle as the output impedance with this model is lower than the CX301a thanks to having a lower Ra (7K\u03a9 against 11K\u03a9) so LF response will be slightly better in a side by side comparison.<\/p>\n Will be trying these two with filament bias and Rod Coleman’s filament regulators<\/a>. \u00a0The HT will be provided through a Salas HV shunt regulator.<\/p>\n Stay tuned…<\/p>\n <\/p>\n","protected":false},"excerpt":{"rendered":" Lundahl LL2745 based OT DHT pre-amplifier Before breadboarding a pre-amp with these nice OT provided by Thomas Mayer, I decided to simulate some options in LT Spice to see what results I got. CX301a DHT pre-amp First one is my preferred CX301a thoriated-tungsten DHT. I wired the LL2745 in 5.6:1 step-down configuration. \u00a0This should provide … Continue reading “A preamp design with the LL2745”<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_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":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[12,5],"tags":[779,134,59,6,21,47,8,785,132,136,18,9,78,135,74,784],"class_list":["post-422","post","type-post","status-publish","format-standard","hentry","category-dht-valves","category-valves","tag-779","tag-coleman","tag-cx301a","tag-dht","tag-dht-pre-amplifier","tag-dht-preamplifier","tag-hi-fi","tag-ll2745","tag-lundahl","tag-ot-preamp","tag-pre-amp","tag-pre-amplifier","tag-rod-coleman-regulator","tag-salas-hv-regulator","tag-salas-regulator","tag-thd"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p2r2tK-6O","jetpack_likes_enabled":true,"jetpack-related-posts":[],"_links":{"self":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/422","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=422"}],"version-history":[{"count":1,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/422\/revisions"}],"predecessor-version":[{"id":425,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/422\/revisions\/425"}],"wp:attachment":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/media?parent=422"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/categories?post=422"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/tags?post=422"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}![\"\"](\"https:\/\/i0.wp.com\/www.bartola.co.uk\/valves\/wp-content\/uploads\/2012\/07\/CX301a-preamp-based-on-LL2745-1024x709.png?resize=584%2C404\" "\\"CX301a")
<\/a>26 DHT pre-amp<\/h2>\n
![\"\"](\"https:\/\/i0.wp.com\/www.bartola.co.uk\/valves\/wp-content\/uploads\/2012\/07\/26-preamp-based-on-LL2745-1024x720.png?resize=584%2C410\" "\\"26")
<\/a>