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Old 11th July 2009, 10:38 PM   #4481
pro is offline pro  Italy
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I gave him a twin brother
Now I must give them an House $$$$$

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And now, They are playing in stereo

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I have only one word to define the sound. Natural.

I have made some new measures.
The clipping, without load is simmetrical at +22.45 -22.75.
Under 8 hom load it is not +18.72 -21.2
The voltage drop at +/- 1.25V is 63mV so OI is 0.42 and DF is 18.8.
At +/-3.45V is 310 mV so OI is 0.8 and DF is 10
At clipping on the minus VD is 1.57 on the + is 3.74 and I don't know which one to use.
Offset rises as the volume goes up, reaching about 800 mV just before clipping. I don't know if this is normal or not.
Maybe swapping the 140N with a 240 could improve the performance. I have read somewhere that this pair is well matched. Have you any advice on this question?
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Old 12th July 2009, 07:54 AM   #4482
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Don't you fear, with a 350ma bias, to be a little bit far from the "sweet spot" Nelson is speaking about.
I am inquiring , as i plan to bias one of mines at about 500ma.( treble operation).
I fear to loose sound quality, doing this.
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Old 12th July 2009, 07:57 AM   #4483
EUVL is offline EUVL  Europe
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The lower the bias, the higher the distortion.
That is why you build Class A in the first place.


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Old 12th July 2009, 08:32 AM   #4484
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Quote:
Originally posted by pro


...
Under 8 hom load it is not +18.72 -21.2
The voltage drop at +/- 1.25V is 63mV so OI is 0.42 and DF is 18.8.
...


My F5 has bias of about 0.95A, compared with the original of 1.3A.

At 2W and 6W output powers (100Hz frequancy) into 8 ohm load, I find DF of about 50 which is lower than 60 of the original F5.


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Old 12th July 2009, 08:35 AM   #4485
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Quote:
The lower the bias, the higher the distortion.
The question is: at which bias is that distortion audible.
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Old 12th July 2009, 09:24 AM   #4486
AndrewT is offline AndrewT  Scotland
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Hi Pro,
no wonder you have had to strangle the F5 by biasing it to 350mA.
That sink is not big enough for one channel and you have stuck two on it!

If you really want to run a pair of FETs in ClassAB mode then go to another thread, not the F5 one.
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Old 12th July 2009, 01:48 PM   #4487
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Bobo, I don't know how distorsions rises lowering the bias, but for shure, just listening, these distorsions don't seem to annoy the ear. The ampli has an his imprint, and I don't think it will change so much increasing the bias. At most it can increase the depth of the soundstage a little.
At the measures, probably the thing will be different.

Andrew, I know perfectly that heatsink is not enough for one channel, but at least I am listening at a F5 having spent 45 for the PS, and having everything else on the shelf, so now I can decide if to spend other money for box and heatsink, or not, and since it seems that it is worthwhile, I am going to order this onee for 185 out of the door.
Do you think i'ts enough?


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Old 12th July 2009, 02:46 PM   #4488
udailey is offline udailey  United States
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If I recall at modushop they do tell you the rating of the heatsink. I think that its the 5U case that has enough heatsink but dont quote me on that. Check the ratings of those sinks.
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Old 12th July 2009, 03:09 PM   #4489
massimo is offline massimo  Italy
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Dissipante Pesante 4U x 300 mm should be ok for F5
4U x 400 mm is ok for Aleph J or similar. Each side can dissipate 100 W
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Old 12th July 2009, 03:14 PM   #4490
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Hi Massimo, I think i'll go for 4U 400, so I'll have a bit more room for the PS that is a bit overkill (ingombrante dir poco)
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