{"id":21,"date":"2012-05-13T17:58:07","date_gmt":"2012-05-13T17:58:07","guid":{"rendered":"http:\/\/www.bartola.co.uk\/valves\/?p=21"},"modified":"2012-05-13T18:25:01","modified_gmt":"2012-05-13T18:25:01","slug":"first-test-post","status":"publish","type":"post","link":"https:\/\/www.bartola.co.uk\/valves\/2012\/05\/13\/first-test-post\/","title":{"rendered":"Improving the 01a DHT preamplifier stage"},"content":{"rendered":"\n\n\n
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After playing with DHT preamps (26, 71a, 4P1L, 46), I ended up staying with a CX301a version of it. I liked its warm sound and tone so I decided to stay with it. It’s been over 2 months so far that I haven’t returned to my precious 26 preamp (I will probably revisit my 26 preamp based on latest updates from Andy Evans).As I’m working on a version of the 4-65a SE amp by Michael Koster, I started investigating options and ways of improving the 01a as will be the first valve of my amplifier.<\/p>\n

Using curves plot with my curve tracer and Dmitry’s composite model, I got a very well matched spice model for the 01a.<\/p>\n

I modelled in LTSpice two versions of the gyrator loaded preamp with filament bias and Rod Coleman’s superb filament regulator:<\/p>\n

1) Version 1: Using Anatoly’s PNP-FET Gyrator<\/p>\n

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2) Version 2: using classic cascoded-FET gyrator with mu-follower output.<\/p>\n

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Interesting results came out of this first simulation test. I think I haven’t managed to optimise (again) Anatoly’s gyrator despite have managed to keep good VDS (38V) to keep the CoSS low. I get slightly worse THD (0.03% versus 0.016%) compared to the MOSFET version.<\/p>\n

Looking at output impedance, version 1 has 10K @1kHz whereas the version 2 can get as low as 734 ohms but impedance raises significantly at low frequencies (70k version 1 versus 80k version 2). This can be reflected in the frequency response where version 1 performs better than version 2.<\/p>\n

Output impedance (Version 1)<\/strong><\/p>\n

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Output impedance (Version 2)<\/strong><\/p>\n

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At high frequency both preamps perform very well up to 1MHz despite version 2 performing slightly better.<\/p>\n<\/div>\n

Frequency response (version 1)<\/div>\n
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Frequency response (version 2)<\/div>\n
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At this point, it looks like if output capacitor of version 2 is increased in value to improve the LF rolloff, it seams that overall performs better than version 1 as THD and output impedance are better?\u00a0\"\"<\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n","protected":false},"excerpt":{"rendered":"

After playing with DHT preamps (26, 71a, 4P1L, 46), I ended up staying with a CX301a version of it. I liked its warm sound and tone so I decided to stay with it. It’s been over 2 months so far that I haven’t returned to my precious 26 preamp (I will probably revisit my 26 … Continue reading “Improving the 01a DHT preamplifier 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_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},"jetpack_post_was_ever_published":false},"categories":[5],"tags":[10,6,8,9,7],"class_list":["post-21","post","type-post","status-publish","format-standard","hentry","category-valves","tag-01a","tag-dht","tag-hi-fi","tag-pre-amplifier","tag-valves-2"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p2r2tK-l","jetpack_likes_enabled":true,"jetpack-related-posts":[],"_links":{"self":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/21","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=21"}],"version-history":[{"count":6,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/21\/revisions"}],"predecessor-version":[{"id":28,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/21\/revisions\/28"}],"wp:attachment":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/media?parent=21"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/categories?post=21"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/tags?post=21"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}