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Old 13th November 2006, 01:39 PM   #1
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Default Threshold FET 2

Over the weekend i came across an old FET 2 preamp in slight need of attention (blown MC input, malfunctioning muting relay).

This was hardly a statement product in its heyday but the quality of case, pcb and layout really impressed me. The sound also exceeded my expectations.

Fwiw, i think that it would make an excellent simple project for anyone wishing to build a nice sounding discrete preamp. I am fairly certain that its phono section is better than any of the el-cheapo, single opamp designs.

So, this morning i attempted drawing the circuit. Although trying to be careful, i may have made an error or two, so if anyone spots anything funny please let me know.

The preamp has a separate MC gain stage which, being a common gate circuit is rather unusual. It uses a parallel 2N5566 and i found that a 2SK389 works quite well. The trimpot is used to set 0v at the input which seems to be quite reliable and stable.


Predictaby, this is the worst sounding part of the unit. Most probably because of the PS - remember this pre is built for a limited budget. Entire pre is supplied from a nice but tiny 30VA 2x20v toroid followed by a bridge, 2x2000uF Mallories and separate 7824,7924 for each channel. No local regulation, just single caps per rail near the line amp (no markings, must be around 100uF)

IME a MC gain stage requires the absolute best PS with batteries or dedicated Jung type regulator to sound acceptable.
Coupling cap to MM stage is a 1uF polyester-looking Electrocube.

R1 is selectable for 3 values - 50, 100, 200 ohm.
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Old 13th November 2006, 01:50 PM   #2
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After getting disappointed with the MC performance i plugged a Tribute cobalt amorphous toroid into the MM input and continued listening. What a change! Dynamics, bass, separation, detail - all the audio cliches fit

So, the MM stage must be rather good then. Not that i can see how a two stage circuit can have sufficient gain for correction plus some extra nfb but it seems to work.

There is a bank of switchable caps at input while the output feeds a balance and volume pots (25k and 50k ?)

Output and feedback decoupling caps are 10uF Sprague polyesters which surprisingly don't sound too bad.

The unobtanium 3N172 can apparently be replaced by a zvp3310 (available even in SA )
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Old 13th November 2006, 01:51 PM   #3
Zen Mod is offline Zen Mod  Serbia
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next part ,please
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Old 13th November 2006, 02:01 PM   #4
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And finally there is the line amp. Exact same topology as the phono with a green 10uF ERO MKT polycarbonate cap (Threshold signature cap?) at output.

As i don't use active line amps at present i only compared this to my TVC and an ALPS pot. What can i say? I am probably not a line amp type of guy

But seriously, FET2 sounds quite decent, possibly because it's so simple. I wish i had these circuits in 1985...
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Old 13th November 2006, 02:08 PM   #5
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The diode-looking thingies are, of course CCS; couldn't find a more convincing-looking symbol in Protel. J505 - 1mA, J507 - 1.8mA, J511 - 4.7mA.

There is a muting relay in parallel to the output. It has a 10s delay on power-on and a connection to the power switch to instantly mute at power-off.
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Old 13th November 2006, 02:15 PM   #6
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And a pic
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Old 13th November 2006, 02:28 PM   #7
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I had one of these about 20 years ago and it was my main preamp for a while until I got heavily into DIY and started replacing it with homebuilt equipment. I think my brother-in-law may still be using it.

My recollection was that the line stage was the best part of the preamp. I had a hard time bettering it with my homebrew line stages. The MM stage seemed easily bettered by my homebrew tube units but I never used it with MC cartridges and good quality step ups. Back in those days I believed that the Shure V15 was the best cartridge out there.

Thanks for drawing out the schematics. I may need to get a hold of this unit and see if it is suspeptible to some simple tweaks to get up to snuff.

---Gary
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Old 13th November 2006, 02:39 PM   #8
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Quote:
Originally posted by GaryB

Back in those days I believed that the Shure V15 was the best cartridge out there.
What
Isn't Shure the best I can get for my record player?

----------------------------

I like those small Keep It Simple circuits attached here.
Nice basic starting pre amplifier circuit setups
to try to modify and improve, yourself.

Thanks, analog_sa


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Old 14th November 2006, 06:05 PM   #9
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Thanks for doing this analog_sa!

FYI, my FET 2 (Series II) uses the Sprague 431P caps for the line stage as well as the phono stages.

I've been thinking about doing some mods to it. I have already replaced most of the electrolytic caps. I still need to change the 2000uF PS caps. (Large axial caps are hard to find.) I plan to switch out the Sprague 431Ps too.

Bill
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Old 14th November 2006, 06:20 PM   #10
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Quote:
And finally there is the line amp.
Where would you put the pot on this circuit on the input or output?
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