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DIY F2 clone
DIY F2 clone
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Old 6th October 2019, 04:34 AM   #601
csample is offline csample  United States
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DIY F2 clone
I tried to take both measurements tonight. I am using the built in sound card on my computer and the generator in REW to create the signal. I am running an 8 ohm dummy load and measuring at the input to the amp.

To get an idea of the frequency response I varied the input frequency to the amp and measured the voltage of the output - I did not see any variation in the output voltage of the amp from 20 Hz to 20,000 Hz, but it did drop off a little at 10Hz. A pretty crude test, but the results seem ok.

I am having some trouble obtaining the FFT measurement, probably due to my inexperience. The questionable measurements I have now show that I need to have a 5 Vrms output on the amplifier to get a second harmonic peak make it above what I think is a -50dBV noise floor. I'll try again tomorrow.
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Old 7th October 2019, 03:15 AM   #602
csample is offline csample  United States
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The images below show what i am measuring for frequency response and the best I can do for the FFT.

The frequency response starts to drop off below 20 Hz, but is flat from there up to 20,000 the highest frequency measured.

The FFT shows a ~47dB difference between the fundamental and the second harmonic. These measurements were taken at 1000Hz and 5.2 Vrms or about 3.4W output. If I am getting the math right, this is about .45% distortion. The First Watt distortion chart for the F2J shows about .6% THD at this power level. So allowing for an increase due to higher order distortion, I think my amp measuring .45% seems like it is in the ball park.

Any recommendations of what to try next? I was considering trying a different C3, maybe making a 12,000 uF bank of Nichicon KZ or Silmic II? Anyone know what type of capacitor was used in the real F2J?

Is it possible that this amp/speaker combo just is not a good pairing?
Attached Images
File Type: png Screen Shot 2019-10-06 at 9.56.29 PM.png (21.1 KB, 129 views)
File Type: jpg F2J FFT.jpg (96.5 KB, 130 views)

Last edited by csample; 7th October 2019 at 03:16 AM. Reason: corrected grammar
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Old 7th October 2019, 04:51 AM   #603
Zen Mod is online now Zen Mod  Serbia
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that is looking good, so nothing to conclude as possible culprit

any other amp as comparison , trying other source ?
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Old 8th October 2019, 12:51 PM   #604
csample is offline csample  United States
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Quote:
Originally Posted by Zen Mod View Post

any other amp as comparison , trying other source ?

I just biased up my BA-3 last week and it sounds great! Not as much bass with the Vulcans as the F2J, but much clearer. It also sounds great with some 4 ohm Vifa/Scanspeak 3 way towers I built years ago, the bass is so clear and precise that it makes me think I never had an amp that could handle the low impedance 10" woofers before.


Eventually I will build the DCG3 preamp that I have parts for, maybe that will help with the 10K input impedance of the F2J. Although I though the B1 would have helped with this too...


Do you think there would be any value in changing the output electrolytics, or is this just a waste of time and money?


Thanks for the input Zen Mod, as always you are very helpful.
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Old 8th October 2019, 01:05 PM   #605
Zen Mod is online now Zen Mod  Serbia
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changing caps can only bring it closer to your taste, but can't cure-it-all

find a reason why you're not satisfied - it's either source side or speaker side

for starters - try feeding it with anything which has headphones output with volume

I remember having some amps in Yore, and I was happiest (then) feeding them ditto from crappy Technics CD headphones out

that will tell some ......
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Old 8th October 2019, 05:30 PM   #606
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I will try to feed it with my Zen-Like headphone amp and see what happens. Thanks
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