{"id":1461,"date":"2013-04-28T10:17:33","date_gmt":"2013-04-28T09:17:33","guid":{"rendered":"http:\/\/www.bartola.co.uk\/valves\/?p=1461"},"modified":"2013-04-28T10:17:59","modified_gmt":"2013-04-28T09:17:59","slug":"push-pull-fascination-part-2","status":"publish","type":"post","link":"https:\/\/www.bartola.co.uk\/valves\/2013\/04\/28\/push-pull-fascination-part-2\/","title":{"rendered":"Push-Pull fascination (Part 2)"},"content":{"rendered":"

Thanks to the great feedback from 45, we found out that I made a mistake in modelling the LL1682 OT in my previous post<\/a>. In a nutshell, I was getting half of the power, doh!<\/p>\n

I should have started from scratch, looking at the push-pull curves and estimating at least the A1 power from a pair of 6C4C in push-pull. So following the B.J. Thomson method plotted the curves in push-pull:<\/p>\n

\"6C4C<\/a><\/p>\n

\u00a0N.B. The bias difference in the above curves is due to the way the DHT was biased to plot these curves in LTSpice. In reality the -66.3V bias point above corresponds to -60V.<\/em><\/address>\n

This quick and dirty analysis doesn’t take into account the OT losses at all, just a theoretical 8K8 anode load from the 8\u03a9 load reflected in the anode. The actual real power will be less of course. \u00a0The anode power will be the triangle area formed below the Zaa\/4 load line. So looking at the maximum power then:<\/p>\n

\"{P}_{a}=\frac{1}{2}<\/p>\n

\"{P}_{a}=\frac{1}{2}\cdot<\/p>\n

In class A1, this output stage should deliver at least 8-8.5W (if we factor in the OT losses and that grid current should kick-in before grid voltage reaches zero) with a driving signal of about 65V peak. So clearly the inductance ratio of my LTSpice model was incorrect. Revising the model I found where the problem was as both secondaries are in parallel looking at the Lundahl specifications:<\/p>\n

\"LL1682PP <\/a>
LL1682PP model <\/div> <\/div> <\/div>
\"Zaa <\/a>
Zaa response <\/div> <\/div> <\/div> <\/div> <\/div>\n

We can see above that the reflected secondary impedance in the anode is about 8K8.<\/p>\n

The revised amplifier then is presented below:<\/p>\n

\"6C4C<\/a>
6C4C Push-Pull Design (revised)<\/figcaption><\/figure>\n
\"Anode <\/a>
Anode and grid currents and voltages <\/div> <\/div> <\/div>
\"THD <\/a>
THD analysis <\/div> <\/div> <\/div> <\/div> <\/div>\n

The fixed-bias is provided by a MOSFET follower which also delivers the grid current required when approaching into A2. No further changes in the design compared to previous post. We can see now that the circuit can deliver about 8W in A1 and with a nice harmonic profile. The H2 component is provided by the 4P1L driver and the H3 and H5 is characteristic of the push-pull topology. It’s nice to see that the THD is about 0.2% @ 8W until A2 grid current kicks in and a sharp increase of odd harmonics arises given that the driver is not suitable for A2 operation (THD is about 1.1% for 9W). Yet, there is grid current growing before reaching to 0V as we would expect. The 4P1L driver imparts a clear harmonic fingerprint in this amp so I’d be keen to enjoy the sweetness of the 4P1L or even some other drivers like 6e5P, D3a that would work quite nicely in this place and provide additional gain in case you don’t have the preamp required to deliver 8V peak at the input to take this amp to maximum power.<\/p>\n

Interesting experiment to continue refining…<\/p>\n

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

Thanks to the great feedback from 45, we found out that I made a mistake in modelling the LL1682 OT in my previous post. In a nutshell, I was getting half of the power, doh! I should have started from scratch, looking at the push-pull curves and estimating at least the A1 power from a … Continue reading “Push-Pull fascination (Part 2)”<\/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":[157,5],"tags":[217,382,383,218,388,385,386],"class_list":["post-1461","post","type-post","status-publish","format-standard","hentry","category-push-pull-designs","category-valves","tag-6c4c","tag-6c4c-pp","tag-6c4c-push-pull","tag-6c4c-thd","tag-ll1660pp","tag-ll1684","tag-ll1684pp"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p2r2tK-nz","jetpack_likes_enabled":true,"jetpack-related-posts":[],"_links":{"self":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/1461","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=1461"}],"version-history":[{"count":22,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/1461\/revisions"}],"predecessor-version":[{"id":1607,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/1461\/revisions\/1607"}],"wp:attachment":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/media?parent=1461"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/categories?post=1461"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/tags?post=1461"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}