P3 is the single biggest thing you can have to shape the sound of your amp -- it makes more difference than you would believe... The catch is that you need some specialized tools to set it. (A signal generator and computer, with good FFT software and a cheap an interface probe at the minimum)
I would set it to the midpoint and install it, and perhaps in the future you will have an opportunity to try adjusting it.
I would set it to the midpoint and install it, and perhaps in the future you will have an opportunity to try adjusting it.
I'm also doing a plain vanilla build. From what I read, P3 is optional. Nelson Pass stated that in his opinion, it was impossible to reliably adjust by ear for lowest distortion, you need an analyser. That said, maybe you can adjust for most pleasing distortion by ear. I think I'll plug it in and leave it halfway for the time being.
My FET's are on their way from Asia, only thing different from the standard build is that I'll be using IRFP9140 in stead of the recommended IRFP9240 to pair with the 240, looks good on paper and my supplier had these in stock. And I'm thinking of omitting the thermistors.
My FET's are on their way from Asia, only thing different from the standard build is that I'll be using IRFP9140 in stead of the recommended IRFP9240 to pair with the 240, looks good on paper and my supplier had these in stock. And I'm thinking of omitting the thermistors.
I built mine without thermistors and I wouldn't do it again. The amp probably sounded marginally better without (as my subjective memory goes), but in summer the sinks ran ten degrees hotter and the resistors were cooking.
No problems with the output devices because I had a wide thermal margin on them, the resistors were another story (source and feedback).
Anyway if I biased it down in summer it would run pretty cold in winter, and not sound so great. Those things are there for a reason
IRFP9140 is a closer match to IRFP240 if the rail voltage is not a concern.
You could probably adjust P3 by ear for one channel but without an analyser the second channel will have a different distortion profile and stereo imaging would suffer quite a bit. I would imagine setting P3 for best symmetry in output currents between N/P channel devices would result in lowest H2, so you can probably (and I'm not so sure about this) use this reading as a proxy for distortion.
I'm actually struggling with this problem myself and maybe I need to get a jig so I can hook up my measurement setup to the amplifier.
No problems with the output devices because I had a wide thermal margin on them, the resistors were another story (source and feedback).
Anyway if I biased it down in summer it would run pretty cold in winter, and not sound so great. Those things are there for a reason
IRFP9140 is a closer match to IRFP240 if the rail voltage is not a concern.
You could probably adjust P3 by ear for one channel but without an analyser the second channel will have a different distortion profile and stereo imaging would suffer quite a bit. I would imagine setting P3 for best symmetry in output currents between N/P channel devices would result in lowest H2, so you can probably (and I'm not so sure about this) use this reading as a proxy for distortion.
I'm actually struggling with this problem myself and maybe I need to get a jig so I can hook up my measurement setup to the amplifier.
OK, so I guess the thermistors are in. I take it that their thermal dissipation factor is not critical for this application (have a choice between various NTC at 4.7k R but some dissipate a lot more/faster than others)?
BTW: 6L6 and AndrewT: Thanks for your guides and all the help you give to newbies like me who don't really know what they're doing (yet), much appreciated.
BTW: 6L6 and AndrewT: Thanks for your guides and all the help you give to newbies like me who don't really know what they're doing (yet), much appreciated.
current through drain situated NTCs is small , so dissipation isn't an issue
use small ones (4 to 6mm dia) , place them as Papa does - see hi-res pics on 6moon
do not lean them on mosfet screw washer (some boyz experienced problem with NTC outer isolation) but lean them on mosfet ceramic part of case
small blob of thermal goo in-between is welcome , blob of RTV silicone across is welcome overkill
use small ones (4 to 6mm dia) , place them as Papa does - see hi-res pics on 6moon
do not lean them on mosfet screw washer (some boyz experienced problem with NTC outer isolation) but lean them on mosfet ceramic part of case
small blob of thermal goo in-between is welcome , blob of RTV silicone across is welcome overkill
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Another newby question to set my mind at rest.
Can someone please confirm that this is the correct circuit for the current boards in the site shop.
http://www.diyaudio.com/forums/imag...n/P-F5-1V20/P-F5-1V20-schematic-revision1.pdf
Can someone please confirm that this is the correct circuit for the current boards in the site shop.
http://www.diyaudio.com/forums/imag...n/P-F5-1V20/P-F5-1V20-schematic-revision1.pdf
If the P3 is omitted in the circuit does it need a replacement part as in fixed resistor?
I have variable 200ohm resistors on its way and can add them later as the PCB's still lack a few components. If I add them I will measure min/max and set pot to 50%. If P3 variabel pot is not to be used will a fixed 100R do?
The original F5 schematic had 10 ohm source resistors for the JFET and no P3, and sounded mighty fine and biased up quite well indeed.
It is not a necessity but if you do not populate it and add it later you should back the bias down to zero - and start setup procedure from scratch, because Id of the JFET will increase (= Vgs on the output MOSFET and thus output bias) as Rs goes down which it will when you add P3.
It is not a necessity but if you do not populate it and add it later you should back the bias down to zero - and start setup procedure from scratch, because Id of the JFET will increase (= Vgs on the output MOSFET and thus output bias) as Rs goes down which it will when you add P3.
Hi all,
My F5 was working great for over a year now but now the fuse burns few seconds after turn on (as soon as the LED is turned on)
Could the problem be the 8 power suplly 3W 0.47 ohm resistors? it seems like their color is changed a bit?
Is there anything elso to look for?
Thanks,
IK
My F5 was working great for over a year now but now the fuse burns few seconds after turn on (as soon as the LED is turned on)
Could the problem be the 8 power suplly 3W 0.47 ohm resistors? it seems like their color is changed a bit?
Is there anything elso to look for?
Thanks,
IK
Hi
What is the rating of the fuse and how many channels is the fuse connected to?
If the bias drifts upwards with time and change in climatic conditions it may cause close rated fuses to blow.
Do you not have current limiting implemented as per the official F5 schematic? If so the addition of a fuse is not totally essential unless you are in the habit of short-circuiting amp outputs for fun. The circuit works adequately for momentary shorts without being too intrusive under normal operation.
I would check all parts for shorts and burns, and all of this without load connected. As a last resort, start setup from zero base (light bulb + zero bias) and go step by step. Remember the light bulb should not be used while setting bias.
What is the rating of the fuse and how many channels is the fuse connected to?
If the bias drifts upwards with time and change in climatic conditions it may cause close rated fuses to blow.
Do you not have current limiting implemented as per the official F5 schematic? If so the addition of a fuse is not totally essential unless you are in the habit of short-circuiting amp outputs for fun. The circuit works adequately for momentary shorts without being too intrusive under normal operation.
I would check all parts for shorts and burns, and all of this without load connected. As a last resort, start setup from zero base (light bulb + zero bias) and go step by step. Remember the light bulb should not be used while setting bias.
Just an update regarding my F5 build that everyone has been so helpful with. I ordered the matched JFETs from Spencer per Patrick's recommendation (~$40). Not $20, but better than ordering the LSs w/no guarantee of two closely matched pairs.
In the meantime I've finished the chassis, so hopefully after the JFETs arrive I'll be able to pop them into the boards and set the bias without too much drama. After that, I'll post some pretty pics.
Like I said earlier, this amp is for my son. I got a Bottlehead Smash preamp kit for Christmas, which I finished last night. That'll drive the F5 until I build my Amp Camp monoblocks, at which point he'll have to go back to using is AMC hybrid preamp, or build his own pre.
OK, one teaser photo, attached
In the meantime I've finished the chassis, so hopefully after the JFETs arrive I'll be able to pop them into the boards and set the bias without too much drama. After that, I'll post some pretty pics.
Like I said earlier, this amp is for my son. I got a Bottlehead Smash preamp kit for Christmas, which I finished last night. That'll drive the F5 until I build my Amp Camp monoblocks, at which point he'll have to go back to using is AMC hybrid preamp, or build his own pre.
OK, one teaser photo, attached
Attachments
regarding smash
if you're having decent source (ball$ to drive) , toss that attenuator and balance drek , treat yourself with pair of Slagle's AVC , pick your stacking and enjoy
just as info : http://www.zenmod.in.rs/my-people/avc-temporary-case/
if you're having decent source (ball$ to drive) , toss that attenuator and balance drek , treat yourself with pair of Slagle's AVC , pick your stacking and enjoy
just as info : http://www.zenmod.in.rs/my-people/avc-temporary-case/