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First Watt F7 review
First Watt F7 review
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Old 8th July 2018, 05:18 PM   #1221
Nelson Pass is offline Nelson Pass  United States
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First Watt F7 review
It's very easy to do an F7, probably the simplest FW amp. I think I've
seen enough workable efforts on this forum...
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Old 8th July 2018, 05:37 PM   #1222
oreo382 is offline oreo382  Canada
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I have just finished switching from my M2 to my version of the F7 (thanks to Mr.Pass and other members for the hints).To my ears and speakers the F7 is fantastically balanced top to bottom.Bass is tighter than any of the other amps I have built(AJ,F6,M2).Highs are as good as the F6 in my opinion.Mids are a perfect balance,female vocals are stunning. For those not diy inclined the F7 is probably the best money spent period.With Mr.Pass's version I can imagine you get the best possible sound out of this topology.I myself cannot afford one but if I could it would be a lifetime amp.

Last edited by oreo382; 8th July 2018 at 05:52 PM.
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Old 31st July 2018, 10:50 PM   #1223
analog_sa is online now analog_sa  Europe
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Quote:
Originally Posted by oreo382 View Post
THATS WITH A 0.33 OHM pcf RESISTOR AS WELL.

Arriving very late to this show, but my sims indicate 0.33 to yield a very modest DF. Much higher values seem appropriate, at least in sim. Have you tested the DF of your build?
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Old 1st August 2018, 01:05 PM   #1224
oreo382 is offline oreo382  Canada
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Quote:
Originally Posted by analog_sa View Post
Arriving very late to this show, but my sims indicate 0.33 to yield a very modest DF. Much higher values seem appropriate, at least in sim. Have you tested the DF of your build?
What is the bias and jfet IDSS you have your sim set at?That makes a difference.Also for a given bias different jfet IDSS makes a difference.For eg. my amp with 1.25A bias 0.33 ohms and 13/11ma jfets gives a sim df of 133.Amp with 1.25A bias 0.33 ohms and 8/8ma jfets gives df of 61.No I haven't actually measured my actual value,my ears tell me it is pretty high.
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Old 1st August 2018, 01:52 PM   #1225
analog_sa is online now analog_sa  Europe
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Thank you!

Idss are 11/12mA but the bias is only 0.9A which is probably the reason.
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Old 1st August 2018, 02:32 PM   #1226
analog_sa is online now analog_sa  Europe
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With some patience adjusting the pcf got the desired DF=100 with Rsen=0.33ohm. It can perhaps be achieved at lower bias, lower Idss as well, just did not try hard enough.

Wonder how they achieve repeatable results in production.
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Old 1st August 2018, 02:33 PM   #1227
Zen Mod is online now Zen Mod  Serbia
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Old 1st August 2018, 02:42 PM   #1228
analog_sa is online now analog_sa  Europe
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Exactly
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Old 14th August 2018, 12:11 AM   #1229
analog_sa is online now analog_sa  Europe
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Built a prototype with parts from my drawers. Did not have a good sounding 0.33, so used a 0.25. The measured DF for both channels varies from 10 to 20 @8ohm. Why? Because resistors need to be fine tuned for a specific Df. It is interesting how sensitive the Df is to Rsen. Of course it also depends on the fets Gm.

Here is the simulated output impedance. Vertical axis is in miliohms, Rsen is varied from 0.25 to 0.35ohms.
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File Type: jpg output impedance vs Rsen.JPG (170.8 KB, 349 views)
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Old 14th August 2018, 04:27 AM   #1230
jameshillj is offline jameshillj  Australia
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Pardon my ignorance, but what is the significance of the DF and how does it affect the sound of the amplifier?
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