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F5 Turbo Builders Thread
F5 Turbo Builders Thread
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Old 1st May 2019, 08:46 AM   #5561
Bfpca is offline Bfpca  Canada
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F5 Turbo Builders Thread
Sounds like possible rf oscillation. Running without any input connected is not a normal situation and if there is any tendency to oscillate that is when you will see it. The dc offset you are measuring with open circuit could be the result of rf oscillations which upset the dc operating points and can confuse measuring equipment that is not designed to measure RF.
In my f5ts I used the compensation caps that Nelson recommended and also increse the gate resistors to prevent this from happening.
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Old 1st May 2019, 11:39 AM   #5562
CLiu is offline CLiu
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Default Newly Completed Stereo F5T V2

Hi All,

A friend wanted to have a go at DIYing a Pass design that is reasonably powerful and since I have some experience with F5T and MoFo, I agreed to help him with it. After some weighing on cost, speaker load & physical space requirements etc. we arrived at the decision to build a single chasis stereo F5T V2. We considered a few choices of PCB, including the ones from diyAudio Store, but my friend feels that 3 boards per channel would be too complicated for him, so we looked for a single board per channel version, and found a version made by fellow diyaudio member La Mer who has posted about his boards previously on the forum, which got the permission of Papa to make the boards(he forwarded a copy of the e-mail to me).

His board is slightly altered from the original V2 circuit, in which there are 2 pairs of fast recovery diodes in parallel with the source resistors with 2 pairs of output transistors in the article, while La Mer has changed it to 1 pair of diodes for 2 pairs of output transistors, and he has also used a different fast recovery diode (S10A2CIC by KEC) instead of MUR3020W as in the original design to save space (it is a TO220 package).

Anyway, long story short, it is completed and biased up (after changing R5 & R6 from 1k to 2 x 4.7k in parallel). Voltage across the 1 ohm source resistors come up to 290mV, with the output offset around 15-20mV. I attach some pictures taken while it is breaking in and playing some music paired with B1 buffer.

IMG_20190418_180430.jpg

IMG_20190501_183420.jpg

IMG_20190501_184715.jpg

My friend is a new to DIY world, and is very impressed with the sound of the F5T. on behalf of my friend, I would like to express our gratitude to Papa Pass for sharing the wonderful design. Thanks a million!

Best regards
Liu
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Old 1st May 2019, 12:33 PM   #5563
gbowling is offline gbowling  United States
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Liu, that is a really nice and clean build. Congratulations and Kudos for helping get someone else into the DIY world. I bet it sounds great, very nice work.

gabo
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Old 1st May 2019, 01:35 PM   #5564
CLiu is offline CLiu
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Quote:
Originally Posted by gbowling View Post
Liu, that is a really nice and clean build. Congratulations and Kudos for helping get someone else into the DIY world. I bet it sounds great, very nice work.

gabo
Thanks gabo. I think the sound of the F5T got my friend hooked, and he is considering to start a preamp project e.g. B1 or B1 Nutube soon.
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Old 1st May 2019, 01:36 PM   #5565
6L6 is offline 6L6  United States
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F5 Turbo Builders Thread
B1 and NuTube B1 are polar opposites in terms of sound.
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Old 1st May 2019, 01:42 PM   #5566
CLiu is offline CLiu
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Quote:
Originally Posted by 6L6 View Post
B1 and NuTube B1 are polar opposites in terms of sound.
This friend of mine owns a Line Magnetic 508IA tube amp, so I think he might like the NuTube B1 more. He likes the combination of my B1 and his F5T as it is now though.
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Old 1st May 2019, 03:01 PM   #5567
dewdrop is offline dewdrop
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F5 Turbo Builders Thread
Quote:
Originally Posted by CLiu View Post
Thanks gabo. I think the sound of the F5T got my friend hooked, and he is considering to start a preamp project e.g. B1 or B1 Nutube soon.
The NuTube goes very well with the F5T, I have the same combi!
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Old 1st May 2019, 03:02 PM   #5568
BigE is offline BigE  Canada
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Quote:
Originally Posted by Zen Mod View Post
matchedd outputs is a must

regarding input , it should be irrelevant is there dead short or 1K or whatnot, when everything is ok
Thanks!
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Old 1st May 2019, 03:10 PM   #5569
BigE is offline BigE  Canada
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Quote:
Originally Posted by Bfpca View Post
Sounds like possible rf oscillation. Running without any input connected is not a normal situation and if there is any tendency to oscillate that is when you will see it. The dc offset you are measuring with open circuit could be the result of rf oscillations which upset the dc operating points and can confuse measuring equipment that is not designed to measure RF.
In my f5ts I used the compensation caps that Nelson recommended and also increse the gate resistors to prevent this from happening.
The compensation caps are in place.

My scope does not show any oscillation..... it shows noise in the 20-500 MHz range around 1mV when referencing case as ground....well above the bandwidth of the amp.

I'm going revisit this amp once I get a proper set of outputs installed.
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Old 2nd May 2019, 01:57 AM   #5570
BigE is offline BigE  Canada
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Bfpca,

In the event that my scope is mis-reading the frequencies it reports (entry level Rigol), When taking the outputs off, I noticed that the anti-oscillation caps were not properly soldered to the boards on the bad channel.

It is better to be soldered directly to traces, which is top side. They were soldered on the rear strain relief pads with minimal solder only. There was enough room to get to the top side legs/pads and make the connection directly to the fat traces.

The other option would have been to mount them from the back an solder normally, to get good heat to those big fat traces.

All resistors were re-flowed as well.

Thanks.
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