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Old 1st May 2010, 08:14 AM   #3031
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Default What do you recommend

I havent played with FETs from a design aspect properly for donkeys

I like to start in miniature and work my way up

Rather than leap up to doing some stonking great amp - I'm thinking of perhaps trying a FET based Pre-amp with RIAA - Line and other inputs + A FET H'phone out at low rail PD

This way I don't have my eyebrows removed again and end up looking like my avatar

My preference in BJTs at signal level are BC547,549,550,560,557,638, 2N3816,2N3838, - 2SC2705, 2SA1145, ZTX650,750....etc...

Now I have absolutely no idea what are the best small signal FETs would be in such an application...anyone any ideas?
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Old 1st May 2010, 11:33 AM   #3032
godfrey is offline godfrey  South Africa
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Originally Posted by PhaseLockLoopy View Post
I like to start in miniature and work my way up
[snip]
This way I don't have my eyebrows removed again and end up looking like my avatar
20/10 for attitude, Loopy!

...as opposed to clueless noobs wanting to build a 1000W amp as a first project.

Sorry, can't help with the actual question. Never did quite get the hang of them new-fangled fet things myself.
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Old 6th September 2010, 07:50 PM   #3033
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Here is #1, varying Vds
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Originally Posted by Nelson Pass View Post
Here is #2, varying Ids
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Originally Posted by Nelson Pass View Post
Here is #3, varying generator impedance
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Here is #4, varying the load from 8 to 4 ohms
Where did the pictures go?
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Old 9th September 2015, 12:25 PM   #3034
RNMarsh is offline RNMarsh  United States
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Originally Posted by john curl View Post
I used to make MY coils from 14Ga oxygen free copper. So there!
IMO -- or FWIW: 18 awg or 1600 circ.mils is a very good compromise number.... this gives a suitably high skin depth starting at 17KHz and suitable current for high peak current levels. But, more importantly to myself is the size of the field radiated from the PA output coil and cross-talk. So care in placement of this coil is needed. As would be the left-right speaker leads and power leads and pcb layout.

Outside the chassis, L-R speaker leads in close proxy to one another and power (ac) and signal leads needs to be examined for cross-talk under two channel load conditions or even better is measured in-place. Most all ac wiring to and signal in/out of preamps/power amps/sources are a rats nest all near one another.

THx-RNMarsh

Last edited by RNMarsh; 9th September 2015 at 12:32 PM.
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Old 9th September 2015, 04:58 PM   #3035
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Originally Posted by RNMarsh View Post
IMO -- or FWIW: 18 awg or 1600 circ.mils is a very good compromise number.... this gives a suitably high skin depth starting at 17KHz and suitable current for high peak current levels. But, more importantly to myself is the size of the field radiated from the PA output coil and cross-talk. So care in placement of this coil is needed. As would be the left-right speaker leads and power leads and pcb layout.

Outside the chassis, L-R speaker leads in close proxy to one another and power (ac) and signal leads needs to be examined for cross-talk under two channel load conditions or even better is measured in-place. Most all ac wiring to and signal in/out of preamps/power amps/sources are a rats nest all near one another.

THx-RNMarsh
I agree, #18 is a good compromise, including regarding skin depth and the size of the radiated field.

A 2-layer #18 coil in a radial orientation may be smaller and have a smaller magnetic footprint, but this approach is probably not necessary if the coil value is only 1uH.

Coil diameter of 1/2 inch is good.

Cheers,
Bob
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Old 10th September 2015, 02:09 AM   #3036
Bonsai is offline Bonsai  England
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The best low noise, cheap JFET available now is the BF862 from NXP. This was confirmed independently by Dimitri who measured a number of different devices types. The BF862 came out at c. 0.8 nV rt/Hz and had a low 1/f corner.

You can get them from Mouser or DK for really good prices

There is no P complement. However, single ended designs are still excellent performers (see Self's single ended MC preamp and Syn08 BF862 for some SOT examples).
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Old 10th September 2015, 04:54 AM   #3037
ljm_ljm is offline ljm_ljm  China
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In fact I like most is the complementary amplifier. NPN + NPN
They include Britain's typical. QUAD405 606 707, MF X-A50, NAIM, LM3875, LM4766.
Also the most expensive FM - 115
They all have one thing in common. Nice voice.
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Old 10th September 2015, 06:05 AM   #3038
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Quote:
Originally Posted by Bonsai View Post
The best low noise, cheap JFET available now is the BF862 from NXP. This was confirmed independently by Dimitri who measured a number of different devices types. The BF862 came out at c. 0.8 nV rt/Hz and had a low 1/f corner.

You can get them from Mouser or DK for really good prices

There is no P complement. However, single ended designs are still excellent performers (see Self's single ended MC preamp and Syn08 BF862 for some SOT examples).
Are these devices available in TO92 package? Just looked at Mouser/ DK and they seem to be all SMD.
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Old 10th September 2015, 12:09 PM   #3039
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Originally Posted by Bonsai View Post
The best low noise, cheap JFET available now is the BF862 from NXP. This was confirmed independently by Dimitri who measured a number of different devices types. The BF862 came out at c. 0.8 nV rt/Hz and had a low 1/f corner.

You can get them from Mouser or DK for really good prices

There is no P complement. However, single ended designs are still excellent performers (see Self's single ended MC preamp and Syn08 BF862 for some SOT examples).
Although not cheap, the JFETs from Linear Systems are quite good in the noise department. The LSK389 dual monolithic is of particular interest. I used these in my VinylTrak preamp with very good results. The single version is the LSK170. They also now have the LSJ74 single complement.

For a lower-transconductance, lower capacitance dual, the LSK489 is also quite a good part with very low noise given its lower transconductance.

Cheers,
Bob
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Old 12th September 2015, 09:01 AM   #3040
jsdyson is offline jsdyson  United States
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Default Fairly low noise FET alternatives.

The BF862 does look very good. Also, onsemi has an interestiing line of JFETS.
The CPH3910 has a narrow (20-40mA, and a high gm(40mS typical) -- fairly low noise. The 2sk932 has a narrow Idss ranges(7.3-12,10-17,14.5-24ma) and a very high gm (50mS typical.) The 2sk3557 is speced with low LF NF, good gm (35mS, and has versions with 10-20mA Idss and 16-32mA Idss) They have several different JFETS with several different optimized characteristics. the noise spec for the 2sk3557 looks especially good -- but is specified in terms of NF instead of en/in. Even so, the NF of 3dB at (10Hz, 1kOhm, Id=1mA) and 2dB at (rin=100 Ohms,1kHz.id=1mA) seems pretty good.
Havent tried them yet -- until a few weeks ago, they had a TO92 version, but seems to have disappeared. Price of the JFETs is at the jellybean level.

John Dyson
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