F5 power amplifier

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#5,602
Thanks for the reply!
That's exactly the post I was talking about. I guess my question was more on the lines of:
Do I need to build that two transistor contraption or just two resistors would have been enough?
Although I don't see why not.
Thanks anyway, probably gonna just hook up the two transistors and see what happens..
 
#5,603
I have tried this arrangement (resitors only and transistors) with different amplifiers.
I noticed that different resistors values are necessary to obtain equal voltages on the two halves when loaded.(by two modules when stereo).
Equal resistors produce equal voltages at powering on but a slow drift occurs untill an equilibrium.
I found easier to equilibrate using a low power potentiometer and transistors.
 
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#5,604
My torody transformer just got in, just have to build an inrush protection now.

The transformer is 500va 230volt pre and dual 18v secondaries.

I looked at some of the ntcs needed, and I found a shop that has these:

Display Electronics - Siemens B57364S509M - Siemens NTC 5 Ohm - 5 watt

These should be rated at 8.5 amp. 5ohm resistance (when hot?)

My transformer not having a centertap on the primaries, can I just use two of those thermistors on both the ac taps, and thats it? Or should I put them in series of eachother on 1 ac tap?

I cannot upload a picture right now, so hope you understand.😱
 
#5,605
or buy a single B57364S100M at Conrad for €1,95 ?
=> 10 Ohm, 7.5A, 5.1W [CL60 is 10 Ohm, 5A, 4.5W]

Eeh, if your transformer is a 230V type you only have 2 primaries, no ?
Watt-The-Fuse does it matter where you put them, they'll always be in series. The idea is that you have 2 times 5 Ohm in series COLD, aka 10 Ohm, to limit the inrush current.
 
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#5,606
if the pair of series connected primaries are ~1r0 and the two series connected Thermistors are 10r, then that is probably not sufficient resistance to prevent fuse blowing at start up on a 220/240Vac supply, using T2A as the mains fuse.

I think the inductance of the de-energised transformer is virtually the same as an air cored inductor with those turns on it.
That will have a very low impedance at 50Hz.
The peak start up current <220V*1.414/11r <28Apk under worst case conditions.
I think you should be aiming to half that peak current (double the cold series resistance to 22r) to get the transformer to start on a T2A fuse.
 
#5,607
As I refine my F5. . .

Could someone that has an F5 simulation running tell me what will happen if one rail of the power supply goes open?



My thought it that nothing bad will happen, but before I test this with working parts I am hoping that someone can just turn off one of the rails in a simulator.

I am planning on putting in rail fuses and I want to make sure that if one of them blows it won't take out the other side of the circuit. If so then I will come up with something different.

Thanks.
 
#5,610
AndrewT said:
if the pair of series connected primaries are ~1r0 and the two series connected Thermistors are 10r, then that is probably not sufficient resistance to prevent fuse blowing at start up on a 220/240Vac supply, using T2A as the mains fuse.

I think the inductance of the de-energised transformer is virtually the same as an air cored inductor with those turns on it.
That will have a very low impedance at 50Hz.
The peak start up current <220V*1.414/11r <28Apk under worst case conditions.
I think you should be aiming to half that peak current (double the cold series resistance to 22r) to get the transformer to start on a T2A fuse.
Click to expand...

Thanks for the info, I will use 2x 10ohms then to get to around 22r
Should be under 15A so well possible. 230*1.44/22= 14.6A
They are rated for 7.5 amp (epcos that Jacco advised few posts back) , so it should be very close.

Oh and Jacco, you are right. It doesnt matter where I put them... ups.. 😉

Still I wonder what the rest of the world does with such a transformer... Can`t possibly line up 3 thermistors just to get this beast started? Sigh.
 
#5,612
If you line your thermistors of 10R up in series you will get 20R, yes. But you wont get 15A you will still have 7.5A. If you put them parallel to each other you will have 15A and 5R. I am sure of the resistance part and not 100% sure about the current part but thats the way it makes sense to me.
If you put thermistors on the primaries then they clamp on the current from your house voltage. If you put them on the secondaries they clamp on the current from the stepped down voltage.
An example: If you have 3A at 240V coming into your transformer then you have 24V secondaries you can assume something around 30A on the secondaries. It matters where you put them if you only have a certain value. It doesnt matter so much if you can pick any thermistor you want.
 
#5,615
Hi Uriah,

I have used a sinlge 5 Ohm thermistor in conjunction with resistive/relay "soft start" circuit for 2kVa,

Recently i have been using a single thermistor for 500VA or less

and once again a soft start / thermistor combo for 600-700VA

The softstart circuits available at Ampslab have worked well for me.
There is alot of different options around.

I do understand your frustration but if you use a combo of a thermistor in the primary circuit of the power transformer and a soft start (resistive either power resistor or thermistor) i think you will find your problem disappears.

-Dan
 
#5,616
Finaaaallly got my F5 working tonight!!!! I just need to say thanks to all the contributers here and especially Nelson for sharing your hard work and advice. I've never built an amp and am not an electrical engineer but all the info on these forums made it possible for me to finish without any (major) hiccups. I

I had to give up on biasing last night but got it down nicely tonight (<2mv DC on each output @ .600v bias)... It's definitely easier to bias if you have at least 2 multimeters .. and with hook probes so you can just hang em and turn the pots. I had to do it one channel at a time and with just one multimeter and it took longer than I had hoped since I had to wait for both channels to get hot before I could get a setting I liked.

After hooking up some crappy test speakers and quadruple checking (it's my first amp .. never can check enough 😛) It's warming up again now and my first listen on my RBR's just sounds fantastic. All the glowing reviews have said it far better than I can but I'm extremely pleased. It's very neutral, the highs sound clean and airy, the bass is clean and deep and the owner is happy 🙂

I was worried it wouldn't be able to play loud enough (83db sensitivity speakers) but there's definitely enough volume from what I've heard so far. I haven't pushed it yet but just a quarter turn on my volume knob is plenty loud for casual listening.

Now to build a preamp .. using my old yamaha mid-fi receiver just doesn't do justice to the rest of my setup.
 
#5,619
tinitus said:
Nice
Consider yourself lucky
A real good amp is worth a lot, and hard to find

Please try some of your less good recordings
I bet they too sound better than usual
Please report, I would like to hear of this 🙄

Managed to order my outputs today, and hope to join you soon🙂
Click to expand...

I've been lucky enough to have frequent access to a friend's truly amazing reference system in the past that puts pretty much anything to shame. To put it simply, I thought hearing that setup had spoiled me for life.. and it's been hard for me to cope with mediocre audio ever since. I built the F5 in hopes of getting close to the feeling I got when I first heard that system. I had rest my hopes pretty high and expected to be somewhat pleased (it would certainly be better than what I had) but was a little afraid i would be let down when I compared it to that reference setup.

The F5 is the first component I've heard in a long, long time that gave me the same tingles I got when I first heard my friends system. It's definitely the star of the show in my humble setup. It excels at everything I've thrown at it so far, even the lesser recordings. The soundstage is huge and accurately seperates instruments and singers. There's so much more detail than I'm accustomed to that I find myself frozen listening to music I've heard many, many times before. I hear things that simply weren't there before and it's got me captivated to re-discover some of my favorite music. The tonal balance is perfect and uncolored. It just sounds like music.

:cloud9: I must say I had pretty high expectations going into this build thanks to all the rave reviews and successful stories from this forum. To avoid any possible let-down I bought some pretty pricey components since the parts count was so low. I used Fairchild output devices, Cardas wiring and binding posts, all the .25-.5w resistors are Tx2575 'nude' bulk foil resistors from texas components, a 600va Antek xformer and 90,000uf worth of nichicon kg super through caps for the supply. Total cost for the build was still <$400 because I was able to re-use an old chassis/heatsinks I had layin around.

This is the first time I've ever had a component in my system that I haven't found anything I'd like it to do better than it does. It's the complete package. It's truly world-class. 😎
 
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