This week I went to my friend’s (Tony) place to listen to his 01a with the additional follower discussed previously. Tony complained about that the addition of the follower changed the sound of the 01a. Something was lost, detail on the treble, airiness. I brought up again my test gear and surprisingly we found some H3 level which wasn’t there on the previous preamp. Odd, could that be the FET follower? In the end after several tests, I checked the operating point (you should never assume they should be as expected) and found that the follower wasn’t operating at the optimal level, source current was low and also the 01a was running out of place. I recalibrated the gyrator and voila. The sound of the 01a, came back!<\/p>\n
Unfortunately we ran out of time and I did not take any measurements again after re-calibration.<\/p>\n
The H3 bug<\/h1>\n
<\/p>\n
At home, I couldn’t stop thinking about the FET sonic impact. In the past I have used the FET as a follower to drive output stages, and it sounded really nice. The benefit introduced out-weighted any sonic signature, unfortunately I never had the chance to look at the harmonic plot of my implementations.<\/p>\n
So, as usual, I couldn’t help myself and ended up building the following test rig:<\/p>\n
I ran the tests at 2Vrms which is the maximum level I get on my sound card. I will re-run with the tests with the function oscillator to make some further comparisons if I get the time tomorrow. Distortion is really low (<90dB) @ 10kHz. H3 level is 5dB lower than H2:<\/p>\n Now, if we look at the performance of the follower at maximum frequency (i.e. 20kHz) we can see:<\/p>\n <\/p>\n So the distortion here is higher at 0.01% (H2 @ -73.6dB). What happens if we reduce the CCS current down to 3mA (as the 01a will work on):<\/p>\n the distortion crept up to nearly 0.04% (H2=-62.6dB). Yes, here is the slew rate in full action!<\/p>\n Now, if we crank up the CCS current up to 20mA here is what you get:<\/p>\n The distortion goes down to 0.005% (H2=-80dB).<\/p>\n Running the follower at 10mA is ok, 20mA better if you can afford it!<\/p>\n I need to re-run these tests with 10Vpp again to see the H3 behaviour. Hopefully I find the time to do so as well as a comparison with other FETs<\/p>\n <\/p>\n","protected":false},"excerpt":{"rendered":" Introduction Are you writing again about the “Slew” thing? There is a lot to be said about the slew rate still. In fact, this post is about some real life measurements and experiences had in the past few days. This week I went to my friend’s (Tony) place to listen to his 01a with the … Continue reading “Slew Rate (Part III)”<\/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":[153,5],"tags":[653,846,847,833,834],"class_list":["post-5182","post","type-post","status-publish","format-standard","hentry","category-driver","category-valves","tag-01a-preamp-gen2","tag-fet-follower","tag-mosfet-follower","tag-slew-rate","tag-slew-rate-in-valve-amps"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p2r2tK-1lA","jetpack_likes_enabled":true,"jetpack-related-posts":[],"_links":{"self":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/5182","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=5182"}],"version-history":[{"count":2,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/5182\/revisions"}],"predecessor-version":[{"id":5189,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/5182\/revisions\/5189"}],"wp:attachment":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/media?parent=5182"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/categories?post=5182"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/tags?post=5182"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}![\"MOSFET](\"https:\/\/i0.wp.com\/www.bartola.co.uk\/valves\/wp-content\/uploads\/2016\/01\/MOSFET-Follower-Test-Rig.png?resize=840%2C637\")
<\/a>Well, I didn’t use the same follower FET as Tony (which is one of my favourite enhancement mode ones I lately used), and instead I used DN2540. I took me 10min to wire this up on the bench and ran some interesting tests.<\/p>\nTesting the follower<\/h1>\n
![\"Test](\"https:\/\/i0.wp.com\/www.bartola.co.uk\/valves\/wp-content\/uploads\/2016\/01\/DN2540-follower-test-01-10k-1024x380.png?resize=584%2C217\")
<\/a>![\"Test](\"https:\/\/i0.wp.com\/www.bartola.co.uk\/valves\/wp-content\/uploads\/2016\/01\/DN2540-follower-test-02-20k-10mA-1024x379.png?resize=584%2C216\")
<\/a>![\"Test](\"https:\/\/i0.wp.com\/www.bartola.co.uk\/valves\/wp-content\/uploads\/2016\/01\/DN2540-follower-test-02-20k-1024x379.png?resize=584%2C216\")
<\/a>![\"Test](\"https:\/\/i0.wp.com\/www.bartola.co.uk\/valves\/wp-content\/uploads\/2016\/01\/DN2540-follower-test-02-20k-20mA-1024x378.png?resize=584%2C216\")
<\/a>