{"id":8650,"date":"2020-07-15T20:00:10","date_gmt":"2020-07-15T19:00:10","guid":{"rendered":"https:\/\/www.bartola.co.uk\/valves\/?p=8650"},"modified":"2020-07-15T20:00:10","modified_gmt":"2020-07-15T19:00:10","slug":"300b-se-amp-46-driver-part-ii","status":"publish","type":"post","link":"https:\/\/www.bartola.co.uk\/valves\/2020\/07\/15\/300b-se-amp-46-driver-part-ii\/","title":{"rendered":"300B SE Amp: 46 Driver (Part II)"},"content":{"rendered":"

The 300B amplifier journey continues as planned. A bit lower than expected, however small steps being made. Recently I mentioned about the 46 driver<\/a>.<\/p>\n

I made some progress on the LL7903<\/a> step-up transformer which is wired on 1:8.<\/p>\n

With 1:8 wiring, don’t expect same performance level as in 1:2. However, I carefully trimmed the Zobel network in the secondary to reduce the resonant peak. The suggested network obviously isn’t fit for purpose in this mode and even tuning the resistor with the suggested 400pF isn’t good enough to tame the peak. I resourced my box of Russian NOS mica capacitors and pulled out a 4,700pF one. By playing with the frequency response test y tuned it manually with a potentiometer and found that 4K7 was right resistor:<\/p>\n

\"\"<\/a>
Zobel network in the LL7903 secondary<\/figcaption><\/figure>\n

You will see on the picture below, the driver will have 2 boards. One mounted on top of the other. The lower PCB holds the LL7903 pair and there is further room for a couple of fixed bias regulators. When I get to experiment again with pentode drivers, I will use this space to place the screen regulator:<\/p>\n

\"\"<\/a>
This is the lower PCB of the 46 driver<\/figcaption><\/figure>\n

Here is the frequency response for the LL7903 in 1:8 with the zobel network. The secondary is unloaded as it goes straight into the 46 grid. The source impedance in this case is 50\u03a9. Shouldn’t be massive difference in response with the DAC’s impedance which is lower. I guess you can’t expect the 70kHz response of the 1:2 wiring and the LF pole will be dominated by the 46 hybrid mu-follower. The 27kHz is good enough for me:<\/p>\n

\"\"<\/a>
LL7903 1:8 frequency response with Zobel network<\/figcaption><\/figure>\n

The driver circuit remains the same. I will play with the operating point so Rk may change. As am running the 300B from a single supply, I may run the 46 cooler somewhere between 20 and 30mA depending on the current balance needed with the output stage:<\/p>\n

\"\"<\/a>
46 driver<\/figcaption><\/figure>\n

With a mu of 5.6 and 1:8 SUT we should get a gain close to 44 (32.8dB). With high efficiency speakers it will do fine with a single stage for the 300B. Promising. As said before, my previous 46 driver incarnation<\/a> was a delight and sounded fantastic. Yet, I will never implement the crazy idea again of filament bias with these bias voltage and current levels needed!<\/p>\n","protected":false},"excerpt":{"rendered":"

The 300B amplifier journey continues as planned. A bit lower than expected, however small steps being made. Recently I mentioned about the 46 driver. I made some progress on the LL7903 step-up transformer which is wired on 1:8.<\/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":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[673,5],"tags":[610,836,1323,394,272,864,44,875,538,78],"class_list":["post-8650","post","type-post","status-publish","format-standard","hentry","category-300b-amp","category-valves","tag-300b","tag-300b-amplifier","tag-300b-se-amplifier","tag-46-dht","tag-46-driver","tag-bartola-gyrator","tag-gyrator","tag-gyrator-pcb","tag-ll7903","tag-rod-coleman-regulator"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p2r2tK-2fw","jetpack_likes_enabled":true,"jetpack-related-posts":[],"_links":{"self":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/8650","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=8650"}],"version-history":[{"count":4,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/8650\/revisions"}],"predecessor-version":[{"id":8658,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/8650\/revisions\/8658"}],"wp:attachment":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/media?parent=8650"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/categories?post=8650"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/tags?post=8650"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}