F5Turbo Illustrated Build Guide

[Robodog]

Hello all, another new member here.

After following the forum for the past two weeks I think I got finally got enough motivation to begin the initial steps of building the F5Turbo v2. Going to try to source what I can from DIYAudio, but already running into a bit of confusion that I hope someone is willing to clarify for me.

I am going to order the F-5T circuit board kit, but I am not sure if the F5 parts kit is used with these boards or not. From reading I think the F5 parts kits is meant just for the original version F5 board and I will need to source my own components for use on the F-5T boards. Is this correct?

Thanks

 

[Halauhula]

You could buy the F5 parts kit for convenience, but recognize that: (1) some of the parts - resistors in particular - in the F5 kit would not be used in the Turbo version; and (2) there are additional parts you would need to purchase for the Turbo version which are not included in the F5 kit. Also, you should know that the F5 parts kit does NOT include the front end JFETs- you need to buy them separately on the DIY Audio store or source them elsewhere. You would also have to purchase more output MOSFETs for the F5T version.

 

[Dennis Hui]

Is this the F5 parts kit you were looking at? https://diyaudiostore.com/products/f5v3-kit

This one is really for the original F5.

 

[Robodog]

Thanks for the quick responses.

I just wasn't sure if I needed the F5 parts kit or not for my order, but from what you are telling me it seems like I don't. I knew I needed to order the front end JFETS separately and already added them to the cart; Toshiba 2SK170/2SJ74 Quad 6-8mA.

I will need to source most of the PSU components externally anyway, so I will just research and order what I need for the F-5T boards with that order. Is Mouser the recommend vendor? How about Jameco or Digi-key?

 

[Halauhula]

Your preference but I use Mouser and Digikey, aa they are the distributors in US that carry the quality level of audio parts that is used on these forums. They do have deep stock although in these supply chain-stressed times, one or the other may be out of stock. I go to Jameco only for stuff that the Mouser and Digikey do not carry - usually stuff that has been discontinued or no longer used by OEMs.

 

[Robodog]

I prefer to source from as few places as possible, but with shortages every where it seems I may need to be open to the idea of using multiple vendors. I already know I will need to order from DIYAudio, AnTek (transformer), and one of the big three electronic supply houses.

Too bad the V3 Universal Power Supply Circuit Board is currently out of stock, I was thinking of starting with that half of the build and getting the PSU squared away before adding the amplifier components. Do they restock those boards frequently?

 

[elwood625]

Do they restock those boards frequently?

Yes, even the DIYAUDIO Store is not immune to supply issues.

 

[ItsAllInMyHead]

@Robodog - Welcome! Were I in your shoes, I would e-mail the store and ask when they may be in stock. My guess is that they will likely have a rough ETA from the board supplier.

If they don't have an ETA that suits you, you can check out the group buys and/or swap meets to see if there is something that will work well for you. There are a great number of 'standard' rectifier + CRC boards out in the wild. rhthatcher usually has several types. Some of them are very similar to the store's design if you happen to need a build guide to follow. You mention that you're new to the forum, but you don't mention your build experience. Either way, they are very simple to build.

Many of the options from others have provisions for including LEDs, thermistor(s) for soft-start, a nifty way to do the ground lift, and snubbers if any of those things are important to you.

tl;dr - if you 'know' that you'll be using monolithic rectifier bridges vs. discreet, then just getting a set of filter boards from Randy (rhthatcher) would be my best advice. You'd be tossing the rectification section of the boards anyway if you get them from the store, so why buy it if you don't need it.

For different types of options - I'm familiar with two:

Prasi's LT4320-based solutions. I have used them, and I like them a lot. Check the thread or directly with Prasi, and ask him how high you can run your rails. Some of the devices are voltage limited.
https://www.diyaudio.com/community/threads/lt4320-based-active-rectifier.336572/page-59#post-6988053

XRK's SLB is also very popular. I have not tried it (yet), but it is well regarded.
https://www.diyaudio.com/community/...ct-crc-cap-mx-class-a-power-supply-gb.336479/

Edited to add a comment re: asking Prasi about the voltage limit of some of his solutions.

 

[Dennis Hui]

@Robodog: Randy's power supply board has a lot of nice features for a stereo build. It's an easy recommendation, assuming you do not need onboard rectifiers.

 

[Robodog]

@ItsAllInMyHead - Thanks for the informative response. BTW we are almost neighbors, I am across the river from you in WI.

I do have some limited build experience, my earlier years I spent as an electronic tech before moving into IT and becoming a Network Engineer. I still know enough about circuits to often find out I don't know a whole lot.

I was thinking of using the method you mentioned, and I seen a couple others here use, where they utilized both the included rectifier board and then used a monolithic rectifier bridge to raise the ground level a bit off the chassis. I would be open to using any other designs, I just thought the ready made boards would be easier to get going with. I like the idea of this pretty much being a ready to go project rather than having to reinvent the wheel so to speak.

I have many ideas for a few improvements and add-ons already, but I decided early on I first want to build it as clean and true to the basic design as I can so as to get the best sound. Then when I have a base-level to reference I can add improvements and will have a way to determine if they negatively impact the sound or not.

Right now I got the basic setup in my head and believe I have read enough to at least start ordering parts. I guess I would rather start procuring them now before waiting and having something else go out of stock. I am not in a huge rush, but of course once the parts start arriving I would like to start assembling something.

While my initial thought was to build the PSU section first, as I have the most background with that aspect, I could also start on the amplifier boards if I can get those parts first. I do have a small bench setup with some basics, scope, bench PS, etc., so I could build the boards and power them individually in a test setup.

Anyway, got a bit long winded. Thanks for the links and I will check them out.

 

[ItsAllInMyHead]

Happy to help. There's a lot of DIYers on your side of the river in WI.

Just in case... what I was referring to was using monolithic bridges instead of discreet diodes for the AC rectification. If you go that route, and many people do, then you don't need a large portion of the DIYA boards. You could just get some filter boards of your favorite type.

Yes, you are correct, you can also use a monolithic bridge as part of the ground lift.

You may have gotten all of that, but I wanted to be sure.

Have fun with the build!

 

[Robodog]

Thanks for the useful information.

I placed a partial order for the F5 amp, but while browsing parts I also seen the B1 Full Kit was in stock. Realizing I might want a nice pre-amp to drive the F5 I added that to my cart as well.

Figured it might be a bit easier to start with a simpler kit, especially since it has all the parts and help ease myself back into the swing of things. I'll probably hop over to the B1 thread for awhile as I am sure i will have more questions on that project.

 

[forge67]

Hello everyone,

About JFETs for F5T v2, it is recommended to use with Idss 6-8mA. They have been out of stock on the Diyaudio store for a while now.

My question is : can we use the Idss 8-11mA version instead ?

What does a different Idss change in principle ?

 

[ItsAllInMyHead]

Darn. I thought they had some LS N/P quads. Maybe when they got the Toshiba parts they stopped selling the LS parts in N/P quads / octets.

Yes, you can use the higher Idss parts.

Did you happen to also find the page linked below? It may provide some insight. If you have further / more specific questions after giving it a look, ask away.

Enjoy the amp!

https://diyaudiostore.com/pages/idss-choice

 

[forge67]

Thanks !

Yes, I had read the links page, it recommends Idss 6-8mA for v2, hence my original question.

I will be able to buy Fets at same time as the Mosfets then. I also have a question about Keratherm transistor insulators.

Is there an equivalent model from Mouser to those sold on the DiyAudio store ?

I won't hesitate to ask questions, the DiyAudio community is great for that !

 

[ItsAllInMyHead]

If you read it, but don't have the answer you were looking for, the key parts of the page (to me) are the points 1 (cost savings) and 2 (re: source resistance/degeneration). Here are a few other important sentences.

"You can operate the Jfets at lower and higher currents than this figure (the Idss), but it is generally considered that you want an Idss equal to or higher than the intended use"

- So, you're good.

"Much of the time, we will choose the 8-11 mA Idss parts since they can be operated at the lower currents as well as the 6-11 (6-8) mA parts"

- 8-11 may be, but not always are, a bit more versatile if you ever want to repurpose them in another circuit (perhaps like the V3).

Lastly, and of importance (to me)...

"The transconductance of these fets is a function of the operating current, not the Idss, so they are largely identical at any given bias current."

- So, regardless of the Idss, as long as you're operating them within their intended specs (with a bit of margin), they all behave the same in terms of transconductance within the circuit. So, in principle (and in practice), there will be no meaningful change to the performance of your amplifier when using the 8-11mA Idss parts vs. the 6-8mA Idss parts.

------

I'm not aware of similar Keratherm products from Mouser or Digikey already pre-cut. I believe there are sources for uncut sheets. Be sure to get the same formulation for best thermal properties (86/82).

An alternative product, that I really like, is something similar to (or exactly like) these.

https://www.mouser.com/ProductDetail/Aavid/4180G?qs=2v7q0MSBcBMwt5GBytMENg==

Happy building and happy listening!

Edited for clarity and grammar. More coffee, please.

 

[Dennis Hui]

There are other insulators you can use though any of the soft/flexible ones not needing thermal paste will likely be inferior to the Keratherm. That said, many builders use other soft pads. If you are willing to use thermal paste then you should look into mica or aluminum oxide pads.

8-11mA jfets will work fine. In the worst case of 11mA, the jfets will run a bit warm (32V rails -> about 28V across the jfets and 11mA Idss generated by 10 ohm gets you around 9mA current and about 250mW dissipation.) So you might consider cascoding or at the minimum get some small TO92 heatsinks for the jfets.

 

[ItsAllInMyHead]

^ Dennis, you're far more knowledgeable than I. Could you help me out, please? I clearly left out a key consideration re: the heat. I didn't even think about that. It was my understanding that regardless of the Idss of the part, that the heat generated within the part would be a function of the operating current through the part. Does the Idss of the part (let's say when measured at 10V) have a direct and linear correlation to the Ids current in operation? i.e - would one expect a 33% increase in dissipation when going from a 6mA Idss part to a 8mA Idss part? It seems particularly key in this application, since we are not using a pot to set the bias current though the JFETs.

Is that correct, or where else did I go wrong?

As always, thanks! @forge67 - apologies for not thinking that key aspect through all the way.

 

[Dennis Hui]

Hi Patrick,

The relationship doesn't look quite linear, but I think it is a reasonable first approximation.

With Vds reasonably large, we will ignore the effect of the curves from slight changes in Vds. We are trying to see the change in current cause by degenerating with a 10 ohm resistor (ignore the P3 pot also). Basically you can look at the current-vgs graph (please see attached from from the 2sk170 datasheet) and slide along a curve to find a Vgs value 'x' where 10 * current(x) = x. Eyeballing gives about 7.5mA for 10mA Idss, about 6mA for 8mA Idss, etc...

You had brought up the important points with different Idss relevant to the performance of the amp and the thermal aspect is just potentially something to watch out for.

Cheers,
Dennis

 

[ItsAllInMyHead]

Thanks so much as always, Dennis. Greatly appreciated. Still making sure that I am grasping the fundamentals. I have one piece that still hasn't fully clicked re: "self-biasing". Another day with a fresh mind.