Need some good advice

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Hello everyone ,

I made some measurement on my F6 and the result are not what they should be .

using an 4.1 ohms non inductive 10 w load ,loaded at 5 w from 1.0 v input the frequency response fall after 10 khz .

the gain is as follow :

1 khz is 4.22

10khz is 4.12

15 khz is 3.52

20 khz is 2.97

in terms of power it's 55 % less from 1khz to 20 khz !!

what could i have made wrong do have such horrible result ?

PSU output volts is stable all the way .

ripple noise is 5/6 mv per rail.

Thanks to those who like to answer :)



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using an 4.1 ohms non inductive 10 w load ,loaded at 5 w from 1.0 v input the frequency
response fall after 10 khz.

First, make sure that the input voltage remains at 1V for all the measurements.
Are you using an accurately calibrated scope probe/DVM? What happens at 8R loading?
Which input transformers are you using? The First Watt spec is -3dB at 50kHz.
 
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Hi Rayma , i use an Fluke 287 with fluke probes .

input voltage is stable all the way , i did not try 8 ohms , but I will tomorrow :)

input trafos are the JT123-FLPCH.

i forgot to say , the amp have PCF , Rsens at 1.2 ohm , but measurement were made from the start point 0v .

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Exact* schematic and detailed pictures of your build would help.

*exact means what you actually did (especially concerning your implementation of PCF), not what you think you did - for example, changing gate resistor in JFET buffer from 1k to 100k (such mistakes happened already to others) will have similar effect to what you are experiencing.
Make sure that values of capacitors and resistors you used are as in schematic.
 
I have made all the measurement today , with 8 ohm and without PCF .

with 8 ohm the rool off is a bit less , but still there.

without PCF the rool off is very much less almost "normal" , and the gain very close to 5 .

my PCF setup was exactly like the picture , Rsens at 1.2 ohm

diyAudio


it seems that PCF it's not a simple "toy" to play with, if you dont have great knowledge in electronic , and even there you may have some special skills .

I should know it , stuff that seems easy are usually not :rolleyes:


i took it off my amp , and after 3 week of "PCF" listening , i found the sound better without , something like less "fatigue" , and more "wide stage" , it's hard to describe with word but that's it.

thanks to you all for your help ;)


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I got my "stock" F6 back now , and what's sure is that PCF looks more like an illusion ,dont ask me how, why , and so on .

be care in doing it , you'll be first amazed .....

but soon or later , "reality" will call you back ...

failing is learning ....


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The one and only
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It's quite simple. Pcf will exacerbate any bandwidth limitations in an
amplifier, either on the top or bottom.

One reason that pcf did not become popular back in the 50's was the
bandwidth limitations of the amplifiers of that era - they had output
output transformers. The F6, having a transformer in the front end, has
similar limitations.

The phase shift at the bottom and top of the bandpass of a transformer
creates a situation where the "positive" starts becoming "negative".
 
... my PCF setup was exactly like the picture , Rsens at 1.2 ohm
diyAudio

Your implementation of PCF has lowpass filter that sets the fc (-3 dB point) at 15.9 kHz. I would omit that 10n capacitor since the signal transformer in F6 makes strong filter already. Also, 1k resistors (including R9 since it becomes part of PCF) used in your PCF are way too small of a value.

Taking additionally into account effects explained by Mr. Pass, it's no wonder that you experienced undesirable results.
 
the RC filter is made by the first résistor and the cap , in my case (1.1 kohm /4700p ) , setting the roll off at 31 khz ?.
So it was not the exact schematic what you have shown here.. Anyway, you have to think about filters in terms of phase shift too, not only frequency response - read carefully what Mr Pass wrote.


and in any case it filters the feedback signal , not the main ? .
If you think that "main" signal is not influenced by feedback, then why all the trouble ? Try to understand how amplifier works and what's the role of feedback...
 
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