F5c V2.0 Cascode Build Documentation

I thought I would post some comments and photos on the construction of an F5 turbo my son and I completed about a month ago. The amp currently is up and running. We had built the F5 previously and had many components remaining, just waiting to be used in a project. When the F5 turbo discussions began, we decided to go for it, mainly to be able to power 4 ohm speaker columns. We certainly are NOT experts, but had greatly appreciated the photos documenting previous F5 builds for our first project, especially the work of 6L6. Thus, we hope these notes that show our mistakes, misconceptions and successes might assist a beginner in building their first amp or, at least, first F5turbo. This record was made as we went along during the build, and has been assembled and edited for this thread.

The complete document is a 7-8 Mb pdf, and can be found at the following url:

https://docs.google.com/file/d/0BzSXoTifUddcNU5TWmpTbm4xYkk/edit?usp=sharing[/url]

The following short url goes to the same location:

is.gd]is.gd - Shortened URL

I've also broken the complete document into 12 smaller pdf files if, for some reason, that is more desirable for some members. I can place ten of them in this posting (which is the limit) and will upload the final two in the next comment if I can. Please be patient with us as this is our first attempt at posting material such as this. We look forward to criticisms and suggestions.

NOTE: The 3rd pdf (of 12) has been removed from this first post and is now found on post #43 of this thread.
The 12th pdf (of 12) has been removed from post #5 of this thread and is also now found on post #43 of this thread.

NOTE: Updated information concerning measurements of the operating amplifier have been added later in this thread, and an updated complete pdf containing this information was uploaded to the above web site on 18 Feb 2013.
 

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Steve and Matt,

You are good craftsman. Nice work! Thanks for taking the time to help others too with the document.

I made an F5c also, and used a very similar power supply, only not cascoded, and 27 v rails. I ended up with too much ripple, and am now rebuilding the psu. My question... did you scope the psu under load? if so maybe those Jensens make a diff, as your amp is biased higher than mine. I used panasonics. Do you have any audible hum, how does the amp sound?

thanks
 
Steve and Matt,

You are good craftsman. Nice work! Thanks for taking the time to help others too with the document.

I made an F5c also, and used a very similar power supply, only not cascoded, and 27 v rails. I ended up with too much ripple, and am now rebuilding the psu. My question... did you scope the psu under load? if so maybe those Jensens make a diff, as your amp is biased higher than mine. I used panasonics. Do you have any audible hum, how does the amp sound?

Hi,

Great point on scoping under load. No, we have not done that and I'll be sure to check rail voltages and waveforms when I get around to checking the bias again in the week or two.

The amp is stone-silent - no hum. Sounds great, but again, we suspect we might be missing a little on the low frequency end compared to the F5, but this may entirely be our imagination. We are going to place some power resistors in series with speakers (for example, 0.5 ohms to 1 ohm) and see if that brings out the base a bit more. But again, the amp sounds great.

Steve
 

6L6

Moderator
Paid Member
2010-10-22 6:43 pm
Denver, Colorado
Fantastic build guide!! Very impressive, I'm sure people who are deciding on the F5, F5C, and F5T will learn much from your work.

Thanks so vey much for making it and posting, this is precisely the type of effort that makes this forum and community so incredible.

This should be an official guide, or at least linked to on the PCB page in the store.
 
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Great post thank you!
I am also building this amp
This post has already saved me
problems and given me insight
as it;s challenging trying to decipher
the cviller BOM and schematic along with the F5T
information.There;s a lot of choices that need to be made.
I know I would have missed those
resistors that you attached to the bottom of the board
I am now using this to help with my build
 
Thanks for the kind words, zoe. I assume you found Nelson's discussion of those resistors in his initial F5 turbo document after looking for them. It may be that the amp works fine without them, but we took the time to install them as they certainly could not hurt. We also increased gain (by increasing the value of feedback resistors) as Nelson had suggested.

We spent quite a bit of time comparing schematics from Nelson's with that of Christian, and then looking at the parts list for Christian's board from the store, etc. As you know, the numbers on various resistors differed between schematics, etc., making this initially confusing. One of the schematics also had a typo on one of the resistors of the voltage divider circuit for the cascoding transistors (was it Nelson's? - can't recall without looking), but that was pretty easy to pick up, as the circuit had to be identical to the base of both transistors. But Christian and others were of great help in quickly responding and answering questions. I honestly have not looked at the parts list and schematic that are posted at the diyAudio store in several months, so things may be a bit different. I'll take a look tonight again out of interest.

Please let us know how your build goes and, if you have time, post some pictures and your experience.

Best regards,

Steve and Matt
 
Coils,

I just looked at the current BOM and schematic posted at the store for the FVc V2.0 board that we used. Still confusing. The schematic shows the voltage dividers for the cascode transistors to be 10 ohms (R101, 102, 103, 104), which is incorrect - this would be 1.7 amps through each rail to ground (34 volts through 20 ohms). I assume they meant to have them be 10 K ohms. And the BOM shows them at 4.7K ohms, which would be fine. As I pointed out in our build document, 4.7K or 10K or something similar is fine, as long as they are all the same and of high enough resistance to limit current to something safe and manageable.

The schematic also shows a thermistor on each MOSFET, rather than just on the center two, as is found on the board.

Steve
 
I am going to build a F5TV1 with the F5C boards here is some pictures of my boards
 

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Zoe,

Looking good. Nice job on the resistors on the bottom side. I see parallel 5 watt 1 Ohm resistors on the sources of the MOSFETs. What are you shooting for with regard to bias, or is that just what you had lying around? That should sure do the job.

Just a note: on the photo of the underside of the board, some of those soldering connections look like they could be cold - but a photograph can be very deceptive and you obviously can see it much better than I.

What are you using for MOSFETs?

Steve