{"id":3113,"date":"2014-03-13T20:00:18","date_gmt":"2014-03-13T20:00:18","guid":{"rendered":"http:\/\/www.bartola.co.uk\/valves\/?p=3113"},"modified":"2014-03-19T20:29:53","modified_gmt":"2014-03-19T20:29:53","slug":"fixed-bias-regulator","status":"publish","type":"post","link":"https:\/\/www.bartola.co.uk\/valves\/2014\/03\/13\/fixed-bias-regulator\/","title":{"rendered":"Fixed bias regulator"},"content":{"rendered":"

\u00a0Testing the Rod Coleman Fixed Regulator<\/h1>\n

As part of my 300B SE project design, I looked into various fixed bias arrangements and regulators. Rod Coleman has developed another fantastic circuit after the success of his DHT filament regulator <\/a>which is now the\u00a0preferred\u00a0filament supply kit within the DIY audio community. After many years of refining the DC filament regulator, Rod came up with a clever design for fixed bias using the same concept: a gyrator and a temperature-compensated CCS. Instead of feeding a current through the DHT filaments, in this case the current is used to generate a clean bias voltage across a “bias resistor”. The bias resistor is bypassed by a capacitor as the high impedance loop formed by the regulator and the bias resistor is\u00a0sensitive to pick up HF noise which could be amplified by our system.<\/p>\n

The kit is of the same quality you would expect from Rod’s boards and very easy to build. It takes less than an hour to build the boards:<\/p>\n

\"20140209-185856.jpg\"<\/a><\/p>\n

I used the following voltage doubler to test the boards. A cheap 15VA toroids can be used to generate +100V of clean raw supply. You can use alternative components subject to what you have at hand. Although with just one 470uF filtering capacitor is fine (C3) I added an extra which was a spare in my stock. R1 and R2 not just help smoothing the supply, but also act as a fuse as they will limit the maximum current:<\/p>\n

\"20140209-190017.jpg\"<\/a><\/p>\n

N.B.<\/strong> The fixed bias resistor between grid and VBIAS node is missing in the above diagram.<\/p>\n

The regulator is rock solid. I checked with my oscilloscope and didn’t pick up any HF noise. Good for a prototype on the bench. I used a bulky 20uF\/250V Russian PIO for decoupling the bias resistor which ended up being a 4K7\/5W\u00a0wire-wound\u00a0as I didn’t have a 3K3 at hand. I set the regulator at 20mA so bias was about 95V. My raw supply was 135V with one board connected to the voltage doubler.<\/p>\n

\"Board <\/a>
Board ready for tests <\/div> <\/div> <\/div>
\"Measuring <\/a>
Measuring inband noise <\/div> <\/div> <\/div> <\/div> <\/div>\n

The bias current can be adjusted with the trimpot between 20 to 25mA which gives a nice voltage range for bias purposes. You need about +20 to +40V headroom for the board FETs to operate at their best. This regulator is suitable for low voltages to high bias voltages without any problems. Just a careful raw power supply design and build is needed.<\/p>\n

The raw supply is really quiet. I added a second 470uF\/160V at the end instead of one 470uF capacitor. After my initial tests I was expecting a higher rejection of the raw supply ripple (e.g>70dB) for this regulator. However, my workbench was too noise for me to measure this level of noise rejection. \u00a0To give you an idea, we are talking about that my workbench noise level (with supply disconnected) is about 90uV (see next diagram), then the 120.4uV of inband noise is very low! Just 30uV from the overall noise in my workbench! Yes, if you shield all this thing around, you will probably get better results. \u00a0For a high-impedance node is really good in my view! I had the board with alligator clips and point to point raw supply connected next to it, so no shielding here.<\/p>\n

\"regulator<\/a><\/p>\n

This is the inband noise level of my workshop when I measure the noise across the 4K7 bias resistor with the power supply turned off:<\/p>\n

\"regulator<\/a><\/p>\n

First tests were\u00a0successful. I just need to build the 300B stage to try this and listen to the regulator. The proof is in the\u00a0pudding\u00a0as they say.<\/p>\n

I was thinking of using one of the 4-wire Kelvin film capacitors to bypass the bias resistor as it will be very useful to improve the HF rejection given its low leakage inductance.<\/p>\n

The regulator proved to be very effective to provide a stable and reliable fixed voltage whilst isolating the grid from the mains AC noise thanks to design of this regulator. A cheap low power toroid transformer is good enough to provide effective raw supply and its small size given low power requirements keep the parasitic coupling capacitances at low levels which are not impacting the circuit (note this is not the case in the DHT regulator due to power levels the EI transformer with split bobins is\u00a0preferred).<\/p>\n

Hope Rod releases this new board shortly as it’s a very promising design which I’m sure it will take fixed bias designs to the next level.<\/p>\n","protected":false},"excerpt":{"rendered":"

\u00a0Testing the Rod Coleman Fixed Regulator As part of my 300B SE project design, I looked into various fixed bias arrangements and regulators. Rod Coleman has developed another fantastic circuit after the success of his DHT filament regulator which is now the\u00a0preferred\u00a0filament supply kit within the DIY audio community. After many years of refining the … Continue reading “Fixed bias regulator”<\/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":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[530,12,157,158,5],"tags":[544,545,401,543,275,78],"class_list":["post-3113","post","type-post","status-publish","format-standard","hentry","category-components","category-dht-valves","category-push-pull-designs","category-se-designs","category-valves","tag-300b-fixed-bias","tag-300b-single-ended","tag-fixed-bias","tag-fixed-bias-regulator","tag-rod-coleman","tag-rod-coleman-regulator"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p2r2tK-Od","jetpack_likes_enabled":true,"jetpack-related-posts":[],"_links":{"self":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/3113","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=3113"}],"version-history":[{"count":9,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/3113\/revisions"}],"predecessor-version":[{"id":3208,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/3113\/revisions\/3208"}],"wp:attachment":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/media?parent=3113"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/categories?post=3113"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/tags?post=3113"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}