F5 Turbo Builders Thread

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#1,861
oiphy said:
yep..... and it saturates the trafo so it starts humming..
there's an easy fix with two diodes and two caps I believe... edit : AndrewT posted a link to it a while ago...


edit: http://www.federalpacific.com/literature/drytrans/10transformernoise.pdf maybe some help here
Click to expand...

Thanks, I also found a thread on DIYAudio:
http://www.diyaudio.com/forums/solid-state/2080-dc-filter.html

I'm in electronics for many years, but never heard of this, but hey, never needed transformers over 500 VA. 😀
 
#1,866
bridge snubbers

WalterW said:
Making some progress...

PSU is build and tested, it's modular, it can be removed as a whole from the amplifier.
All the nasty and noisy rectifiers are shielded under the transformers...
ZM told me the rectifiers would be warm 😀, so I made a U-profile for extra cooling and at the same time shielding for the large 625 VA transformers. Also the thick rubber is his idea...Yes I do listen sometimes😎
All PCB holes in the sinks are drilled and tapped. All transistor's are matched and selected, just like the resistors.
So now the nice part of populating the PCB's starts..."bestücken" yeah..

My last pic with the green terminalblock...what is better? Tinned leads, or just with the bare copper leads ???
What gives the lowest contact resistance? Also in long term?

Or should I not use them at all, and solder the leads directly on the PCB's?
But then my PSU is not so modular anymore😕

Walter
Click to expand...

Hi Walter,
I saw your finished amp in the "pictures of your diy pass amplifier" thread. Very nice job.🙂
I see you used snubbers on your bridge diodes. Can you comment on the values you used?
Also, you used 0.22R mosfet source resistors rather than the 0.5R in the schematic. From my understanding, the lower value will result in a smaller jump in bias when the diodes conduct. Is this why you chose the smaller value?
 
#1,871
alazira said:
Hi Walter,
I saw your finished amp in the "pictures of your diy pass amplifier" thread. Very nice job.🙂
I see you used snubbers on your bridge diodes. Can you comment on the values you used?
Also, you used 0.22R mosfet source resistors rather than the 0.5R in the schematic. From my understanding, the lower value will result in a smaller jump in bias when the diodes conduct. Is this why you chose the smaller value?
Click to expand...

Hi Alazira,

I used 100 Ohm and 100nF/63V for the snubbers.

The 0.22 Ohm is appropriate for my 2SJ201/2SK1530 output devices, cause they handle 'only' 12A max. I saw it in the F5X thread.
An additional advantage is that with 1.25 A there is idle only around 0.3 V over the resistors and diodes, which is a value Papa named to be safe...
 
#1,873
vdi_nenna said:
36v secondaries and 400va won't work with 100w class A?
Click to expand...

My rule of thumb: You can't have too much transformer VA rating! Chokes are good too, so leave some room!
At the point it is too heavy to move, then it is just right.
If you are using two transformers, has anyone thought about using one transformer per rail? It would give the rail a chance to recover before the next transient.

Rush
 
#1,874
WalterW said:
Hi Alazira,

I used 100 Ohm and 100nF/63V for the snubbers.

The 0.22 Ohm is appropriate for my 2SJ201/2SK1530 output devices, cause they handle 'only' 12A max. I saw it in the F5X thread.
An additional advantage is that with 1.25 A there is idle only around 0.3 V over the resistors and diodes, which is a value Papa named to be safe...
Click to expand...
Hi Walter,
I was re-reading the F5Turbo document and noticed this part:
We see that these devices will slowly start conducting at voltages just above
the idle voltage across the 0.5 ohm Source resistors. It will be necessary to
heat sink these diodes, remembering that their case is electrically connected.
The point at which the diodes conduct is temperature dependent, so you will
want to set the bias so that it makes a nice transition above the bias point and
doesn't run away when the amplifier gets hot.
If you are competent, fearless and also own a fire extinguisher, you can find
this point. Just run the amplifier into a reasonably low impedance until it gets
good and hot – as hot as you plan to let it get - ever. Then adjust the bias to
a point below where the idle current starts to really take off. You should find
that this point is around 0.4 volts across the 1 ohm resistors. If you are a
fraidy-cat, then just set it at 0.3 volts, and conservatively fuse the AC line.
Click to expand...
As you can see, NP biases at only 0.8A (0.4V/0.5R) standard and 0.6A "fraidy-cat" for the fairchild devices, while you have biased at 1.25A for the lower Amp rated Toshiba's... I hope there is no problem.
 
#1,875
ITs not about the bias amount, but the V drop across the source resistors that cause the diodes to turn on. For instance, 2A across .2R source resistors, would cause a Vdrop of .4V across the diode, keeping it within the SOA as written above. In this way you can manipulate just how much bias you have through each fet simply by changing the Rs value. Of course you have to consider heat dissiaption and other such issues, but that is another converstation
 
#1,876
I still believe the diodes would do better if they only turned fully on, when the output transient current exceeds the ClassA current.
That requires the Vrs to be <=300mV.

The result of this is that the ClassA performance of the F5T is not affected while the current stays below the ClassA limit.
The diodes pass when excess current passes to the load, i.e. the amp has transitioned into ClassAB and Iout > 2*Ibias
A consequence of this is that the diode does not turn on often and when on it is generally only for the short period of the transient.
This makes for near cold diodes. It may be that when set up like this that the diodes will not require heatsinks. This will become evident when playing your choice in music.
 
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