F5 Turbo v2 Stereo Amp Illustrated Build (my first amp build)

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OK. This has been a long time coming. I joined in 2013 and planned to get on to building shortly thereafter. But then I moved around the world a couple of times, a few different cities, and even more houses within the cities. Throw in a couple of kids and a couple of renovations and 2020 rolls around. Even then, it took an international pandemic to give me time to do the build.


Firstly, I want to thank everyone on this forum for the questions, answers and photos. I have read extensively, learned a lot and there;s still more to learn - and build. Thanks to everyone who has answered my questions as well.



In particular, I want to thank 6L6 for taking the time to talk to me (I think that was in 2014 when I was in NYC) and also for your illustrated build guides. With this guide I am following your lead and hope that this can give back to some other new diyers. So thanks Jim.



Also many thanks to Nelson Pass for providing the designs and articles for the amps, and being an avid supporter of diy. It is very much appreciated.


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This thread is a work in progress. I will add photos and words and edit them over time. Today (23 August 2020) is the "first draft" if you will. I will try to bring in what I have learned from lots of different threads and add notes on things I found useful especially from someone who had no electronic/lab equipment at all.


On to the build....(see next post)
 
Setting Up Shop


Hardware:



I didn't have anything to build this amp with. So I had to setup shop. I hate not having the right tools so was happy to buy a bunch of stuff. This for me was the hardest since I really didn't know essential items. Anyway, here's what I bought.


1. HakkoFX-888D soldering iron. and Hakko chisel tip 2.4mm x 14.5mm

2. Kester 24-6337-0027 63/37 solder for the PSU

2a. Cardas soldering wire quad eutectic silver solder with rosin flux for the gain and output stages - will it make a difference? I don't know but it couldn't hurt and we all want the best.

3. Variac 130V 5A from circuit specialists. I wanted the 250V but this was cheaper. I found this an easier solution than the light bulb test and I knew that I was going to do more builds so I just bought it.

5. A good Digital Multi Meter (DMM).I spent about $90 on this. Cant remember which brand. It'll be in the photos.

4. I researched standoff sizes and hex head bold, philips head bolts and basically spent far too long doing that. I ended up just ordering from Amazon an assortment of everything: M3 bolts, nuts and washes. I also got some nylon washers, lock washes, M3 fender washes (you need these for the fets on the heat sinks, M3 nylon lock nuts (not used on anything that gets hot).

5. The standoffs I got have 6mm thread and 6mm standoff. That's something I wasn't sure of - the depth of the holes in the heat sinks. 6mm seems to work.

6. Precision screw drivers and spanners/wrenches.
7. A silicon soldering mat.
8. Panavise vice
9. Quad Hands Flex Pus
10. Solder dispense reel
11. Some small parts storage case from Aldi for the components.
12. Two row Terminal Barrier blocks with 4 position, not the euro block as I learned. The black barrier ones are heavier and better for high current applications. I changed the euroblock ones i had out (you'll see the change in the photos).
13. An bunch of PCB mount, plug-in, screw in, 2 and 3 pole terminal connectors.
14. An assortment of insulated interconnects (ring, spade, fork) and different sizes.
15. A good wire stripper and crimping tool.



That's about it. I spent far too long researching this stuff, so just get the assortment.



Hookup wire


Well well well, this also took up far too much of my time as well. Stranded vs solid; gauge size; material; coating, how does the signal travel in solid vs stranded, and the list goes on. This gave me a massive headache. Since I have built the amp here are my thoughts:


1. I used solid 14 AWG for the PSU because I had it. It was hard to work with. Next time I will use stranded but the same gauge.
2. I got some 18 AWG (I think) silver coated copper stranded cable covered in PTFE from Apex (or is it Aphex) for the gain stage and output stage. This turned out to be too thick to fit in the holes of the red PCBs so next time I will buy 20 or 22 AWG (whichever one fits).
3. I will only use stranded from now on for the entire amp since it is easier to work with. I doubt I'll hear the difference.



Components


Another massive research project. This is worth doing to understand the different types of resistors (metal film, metal oxide, carbon ....) and capacitors. If you look in the BOM for the F5Turbo it has the part numbers so just get them.



1. I bought the vishay-dale metal film resistors for the amplification. After all the research I am still not sure how each of the parts affects the signal except for the transistors. But I spoke to 6L6 and he informed me (correct me if I am wrong here 6L6 if you read this) that the resistors and capacitors in the F5Turbo are not the most significant component and that I didn't need audiophile resistors that cost an arm and a leg.

2. I bought the matched JFETS from the diyaudio store way back when - the B-grade ones.
3. Thanks to some members here I got some matched FQA12P20 and FQA19N20 (I need to double check that)

4. But essentially, for the F5 Turbo v2, get the active parts from there now that they have them.

5. Get the pre-drilled chassis from the store. So much easier.

6. The back parts I got these from PartsExpress (the store was sold out) and they fit the back panel perfectly (dont forget the Shurter power entry module (see BOM) and 2 x 2.5A slow blow fuse):

- 1 x Neutrik NF2D-B-2 RCA Jack D-Series Red/Black (Part # 092-232)
- 1 x Neutrik NF2D-B-9 RCA Jack D-Series White/Black (Part # 092-239)
2 x Dayton Audio BPFI-G Fully Insulated Binding Post Banana Jack Pair Gold (Part # 091-630)


ok now on to the build...
 
I'm not sure why but my pictures aren't attaching and I can't put them inline and comment on them. I'll keep trying.
 

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So it looks like I can only upload one file at a time. I have 70+ photos which will take a significant period of time to upload one at a time, so I will add them over time.



Here is a link to an album to make viewing the pics easier:


Microsoft OneDrive - Access files anywhere. Create docs with free Office Online.



The problem is I am unable to easily comment on what was going on in the pictures.


Moderators, please let me know if there are any issues with posting an album and I will take the link down.


If anyone knows a better way to attach pictures here please let me know, especially putting them inline so I can comment.


Thanks and stay tuned.
 
Here are some more PSU pictures.


I used the Rectifier bridges, no the individual diodes shown in the first pictures
 

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more PSU.
 

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I have more PSU pics but uploading them is such a punish I'll just skip to the amplifier stage. I have just finished soldering the connections for everything except the input and output signals.


The PSU powers up well and nothing burns up. I'm getting +/- 36 VDC.


Here are some pics:
 

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So last night I finished the amp and set the bias with the help of 6L6.



I have added the final photos to the album so have a look at that. The link is here:


Microsoft OneDrive - Access files anywhere. Create docs with free Office Online.


The photos here show the final bias ~350mV and 0V DC offset across the speakers.


Thanks again to everyone and if anyone (especially new members with limited knowledge) have any questions please ask away and I'll do my best to answer. I am more than happy to talk about process itself if not the specifics of the circuit.
 

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Things I learned (for the next build)


The main takeaways for me:


- make life easy for the first build and buy pre-drilled sinks et al. from the DIY Store. In fact buy everything from there.

- Stuffing and soldering was actually pretty quick and painless.
- Solder (or connect) the wire to the gain stage board before attaching to the heat sink!! It gets pretty busy in there and becomes increasingly difficult to add wire. I would actually attach all the required wire to that and then secure it to the heat sink and then from there attach the other end of the wire to wherever it needs to go.
- Get three DMMs for setting the bias and DC offset. You need three!

- Less reading, more building.



That's about it.
 
Thanks.


Well I have to test the sound out today.


I thought I had a pre-out on the amp upstairs with the turntable but, last night when I checked, there was none. So I have to get it going down here with the home theater amp that has pre-out.


I'll report back when I've got it done.
 
Bias Procedure for F5 Turbo Version 2

Bias Procedure for F5 Turbo Version 2

After you have completed your build you need to bias the output (measured in mV) in order to generate enough power to activate the MOSFETS. You need to do this while keeping the DC Offset (the potential across the speaker terminals) at 0V. This is achieved by increasing gradually, in an alternating pattern, the resistance of P1 and P2. You are not directly measuring the resistance but the voltage across P1 and P2.

Things you’ll need:

3 x DMM
1x Variac
1 x completed F5 Turbo v2

Set P1 and P2 to 0Ω
Setting these to 0 prevents current flow through the FE stage while you power up the amplifier. To make sure you are starting at zero bias at the beginning.

- Attach leads to TP1 and TP2, and TP3 and TP4 on the front end board.
- Turn P1 and P2 whichever way is required to set the resistance across the test points to 0.
- Make an arrow on each pot to indicate what way to turn the pot to increase resistance. Clockwise may not be “up”.


Powerup the amp, check the PSU voltages.

- Make sure the Variac and the power entry module (PEM) (the Schurter switch) are correctly and properly fused.
- Connect the PEM to a Variac or light bulb tester with appropriately current rated power cord (AWG12 and above should suffice).
- Turn Variac to 0V.
- Switch Variac and amp on.
- Check voltage between positive and negative (neutral in the U.S.) with the DMM (set to check VAC) across the barrier block connection. Ensure it reads 0V.
- With the leads still connected to the terminals, slowly increase the voltage on the Variac. If smoke appears turn off the power and troubleshoot the problem. If the fuse blows, then there is a short in the system. Locate and troubleshoot the problem.
- If there is no smoke, continue slowly increasing until you reach mains voltage.
- Once at mains voltage, you can check the step down AV voltage across the secondaries to ensure it matches the value of the transformer.
- Switch DMM to measure DC volts.
- Check the voltage out of the rectifier and before the capacitor bank to ensure it is DC.
- Finally, check the DC voltage at the terminals of the PSU. This is the power that will go to the Gain Stage and Output boards.
- Connect the positive lead of the DMM to the V+ and the negative lead to GND. You should have a positive reading.
- Now connect the positive lead from the DMM to GND and the negative to V-. You should have the same potential but a negative signal.
- If you don’t you need to check your wiring.
- If you do then proceed to setting the bias.



Setting the bias

- Make sure the mains are at max voltage. Remove a light bulb tester if you are using one.
- Set your three DMMs to check DC voltage. If the DMM is manual, set to 2V or 20V.
- Using alligator clips, attach one DMM to TP2 and TP3 on the output boards: One on the P-Channel and one on the N-Channel. Ensure it reads 0V. These will measure the bias.
- Attach the third DMM across the speaker output. Ensure it reads 0V. This will measure the DC offset.
- Grab an insulated screw driver and turn P1 (or P2 – it doesn’t matter) 360deg. There may be no change on the DMM. This is normal.
- Turn the other potentiometer 360deg. Again, there may be no change in the voltage. This is normal.
- Continue alternating increasing P1 and P2 with 360deg turns until you start to read a voltage across the potentiometers.
- While increasing the resistance, keep a watch on the DC offset. It will increase or decrease depending on which potentiometers you turn. It should remain around 0V.
- Continue alternating turning P1 and P2 and bring them up to 300 mV (0.3V). Ensure that the DC offset remains at 0V.
- As you get closer to 300mV, you may have to adjust the turns to a smaller amount since a full turn can have a dramatic change in voltage.
- The heat sinks should be warming up.
- Once you get the bias on each output board to around 300mV (there may be a slight difference) in order to keep the DC offset at 0V. It is ok if the bias voltage difference is within 50mV, and the offset zeroed.
- Set the lid on and let the amp rest (or ‘cook’) for 20 minutes.
- Keep a watch on the DMMs. As the thermistors heat up they will adjust to keep the DC offset at 0V.
- The bias may change as the MOSFETs heat up. This is normal and should settle after about 20 minutes.
- After 20 minutes, and if the bias readings have settled, and there is no smoke, increase the bias to 350mV by alternating turning P1 and P2.
- You may not get exactly 350mV bias on both boards at the same time as keeping the DC offset at 0V. This is normal. Get the bias between P1 and P2 as close as you can to each other within the 350mV range all the while with DC offset at 0V.
- Let the amp cook for another 20 minutes.
- The bias should not change too much, if at all, since the heat sinks have already warmed up.
- If this is a stereo amp, repeat this process for the other channel.
- Once the other channel is done, recheck the bias and DC offset for the first channel, and make any adjustments to the bias to bring it up/down to around 350mV and 0V DC offset.
- That’s it! Congratulations, you have just biased your amp.
- Turn off the amp and let it cool a little.
- Attach your test speakers (if you have them) and turn on the power. If nothing happens then you should be able to integrate the amplifier to your system.
- If you don’t have test speakers, and you attach them to your system speakers, you run the risk of damaging them. So proceed down this path at your own risk.
 
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chuckd:

Nicely done! What do you think of it?

Regards,
Scott


OK. So I was hesitant to run MP3s through it but I couldn't wait. So I used the preout on the Yamaha AVR from the home theater downstairs. I have Audioquest Type 4 cable which I got for free and the source is my music on Amazon Music on Fire TV.



It took a little bit for it to warm up or run in or whatever but then I just spent the next 3 hours listening to my favourite tunes at a very very loud volume. I measured -80db on a phone app. It was awesome. Really awesome. I had ringing in my ears and everything.



Even the Yamaha AVR switched itself off during the Beastie Boys and the Plump DJs. It couldn't keep up. I need a more worthy preamp.



I'm finally getting out of the speakers what they can give. If I had this amp 10 years ago, I wouldn't have needed a subwoofer (I still would have bought one though, but just probably not needed it). The mids and highs are very crisp and the base is, well, like nothing I've heard from these speakers. And that's from MP3s. It seems really fast - I'm not sure if that's a real thing in amps and speakers, or if it was just the fast music or perhaps my heart racing since I'm playing fast break beats so loud that the cutting boards upstairs fell off the bench.



I also don't need tone control. Normally, when I used Pure Direct on the amp, I'd miss the treble and base. But with this, I turn on Pure Direct and it's got everything it needs.


The basement smells like a burnt in amplifier. It's just great.


Anyway, I think I need better, more sensitive, speakers, and two F5Turbo v3 monoblocks. I've got one (pretty much) done already so it should be easy to build another. My wife will be so happy to see two of them. Who doesn't like symmetry...
 
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