F-5 Turbo boards

Preparing for two F5t v3 mono blocks and about to order two custom toroids with 800VA and 2x24vac secondaries. I don't really need more power but want to go "optimal".

The cases will use bigger heat sinks with larger and more fins than the 5U.
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After reading original F5 and F5 turbo articles and much of the threads for each, I am considering constructing F5 Turbo, version 2 (4 output transistors) at 28 volt rails, as mono blocks in two (2) Dissipante 4U chassis. The goal is to achieve 50W class A to drive Vandersteen IIce speakers, 8 ohms.

I was planning to use a F5 regular front end board in each chassis, so I could have a limiter. The output boards would be F5 turbo. Two output boards would be used for each mono block: one board with two transistors for positive and one board with two transistors for negative on each heatsink. The MUR diodes would not be used, but all source resistors would be doubled up to increase total wattage rating as per the normal turbo build.

MY first question: does my plan sound doable and practical - in particular the driving of the turbo output boards by the original F5 w/limiter front end board?

My second question: given that I am raising rail voltages only to 28 volts, am I correct that not using cascoding is an acceptable risk?

My third question: how do I calculate the revised resistor values for the limiters, given the higher rail voltage? I read and re-read the original F5 article describing the calculation of the original values but I am not able to suss out the way to determine what amp rating I should be assuming to achieve 50 W Class A into 8 ohm load, with a margin for peaks. Can anyone help?


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1) Not really practical. Very complicated and kludgy for little to no benefit.

2) I wouldn't go to 28v without the cascodes. You might get a way with it, but the Jfets are expensive and cascoding is sonically invisible.

3)Just build a F5Tv2, make the rails around 32V, and cascode. Leave the MUR3020 diodes off the amplifier board. Make sure you build it in the 4U 400mm deep chassis.

As for power, unless you regularly to listen to rock at "neighbors-calling-the-cops-on-you" volume, 50W is more than you'll ever need.
Thank you, 6L6. I am going to adopt your advice: buy turbo boards, use PS = 32V, build with cascode, leave MUR diodes off the board.

I checked and rechecked the DIY Audio Store, but I have not been able to identify the 4U Dissipate or 4U Deluxe with 400 mm deep chassis (I assume that should be the same as the UMS heatsink length). All I see is a 300mm depth 4U chassis (either plain or Deluxe) and a 300 mm UMS. What am I doing incorrectly when browsing the DIY Audio Store? Is the 300 mm deep chassis acceptable for the 32V Turbo V2 using cascade? Plan was to try and bias at 1.3A - a little more than the 1.0A used in the original Turbo V2 article.
6L6 or other mods,
I was gifted a F-5T v2 board set for Christmas. Went to the DIY Store page https://diyaudiostore.com/products/f-5t to pull up the F-5T v2 Bill of Materials under the help links, but found this link broke. I searched other places in the forum for a BOM, but same results. Could this link be fixed, or gently guide me in the correct direction for a F-5T v2 BOM? Thanks for the help

I build the F5T v3 and absolutely love it (review and photo in diyaudio store - the one with the cricket ball) I’m looking at monoblocking it and have purchased another set of boards etc.
Question : what voltage transformer should I use to get 100w + out of each monoblock?
I’m currently running with 30v transformer which gives me about 40v dc….will that do the trick in one block or should I go higher?
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As for power, unless you regularly to listen to rock at "neighbors-calling-the-cops-on-you" volume, 50W is more than you'll ever need.

Reading this thread.

If you run Maggies, you will need 100 watts and PLENTY of current.

After a lot of thought, going back and forth, I've decided to got for A2s instead of F5 Turbo V3s to drive my Maggie 1.7's. My A5 monos are barely enough.

BTW, I tried my fully rebuilt Marantz SR2325... no go! Even though it claims 125 wpc and it weights a ton... Big transformer, it has less ooomph than the A5 monos. I plan on going to F5 v3 sometime later this year... just for kicks, not for needs.