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RE: DIY Jadis preamp

Posted by Norbert2nd (A) on August 7, 2009 at 06:20:30

In Reply to: DIY Jadis preamp posted by DAVID on August 6, 2009 at 22:54:09:

Hi David,

I have successfully built a complete JP-200 "deluxe" kit (with some alterations, see below). Analogmetric as a company are easy to deal with but delivery is not exactly the fastest. They respond quick to emails.

If you are a beginner as a kit builder I have to warn you about lack of building support. The information on the analogmetric web site and forum is very basic and all you get. There is no overall wiring diagram or step by step instructions to lead you. So you have to know what goes where.

I have no idea if that preamp is even close to the original Jadis JP-200. I guess not because the original is a 4-chassis design including phono preamp stages.
The sound of this line stage is very much depending on the tubes you use. If you put in Mullards you get that typical (brown) Mullard sound which is not my cup of tea.

With the right tubes you can build a very transparent and detailled preamp featuring a huge soundstage. Before I have used a Silk-based TVC from DIYHifiSupply which tends more to the lean side. Compared to the TVC the JP-200 clone has more timbre, body and weight. Amount of detail is on the same (high) level.
Over the years I have come to the conclusion that a preamp is really a necessary component in a decent sounding system. Not to add voltage gain but to add those properties mentioned above. For me it is a deliberate departure from a "neutral" amplifier system. But sins of omission are not tolerated!

The rest of my system is a slightly moddified SlimDevices Transporter as source and Hypex-based Class-D poweramp feeding my Tannoy IIILZ Monitor Mk II speakers.

I have made the following alterations to the kit:

Power supply:

- Use the optionally available 10H choke
- Use 6x4 as rectifier tube (7-pin socket is not included!)
- Replace C1 and C3 with a single Mundorf tube cap of 10uF, 1000V
- Replace C7 with a TAD Oil Cap of 0,25uF, 600V

Amplifier board:

- Replace all resistors with Takman REY 0,5W metall film types
- Change R7A and B to 1kOhm
- Change R10A and B to 1,5 kOhm
- Change R14A and B to 100 Ohm
- Add C9A an B (which are not supplied with the kit) two TAD Oil Caps 0,25 uF / 600 V
- Replace C1A and B with Ampohm Aluminium Foil, Paper in Oil Cap 0,047 uF / 630 V
- Replace C2A and B with Ampohm Aluminium Foil, Paper in Oil Cap 0,1 uF / 630 V
- Replace C5A and B with Ampohm Aluminium Foil, Paper in Oil Cap 1,0 uF / 630 V (depending on the imput inpedance of your poweramp you might need a bigger value)
- Replace C4A and B with silver mica 8 pF / 500 V and place a silver mica 22 pF / 500 V capacitor between grid and cathode of T1A and B to fight self oscillation
- Use ECC81 / 12 AT7 for the cathode follower position (T3A and B) to lower output impedance

Lowering gain from 20 to 12dB:

- Change R5A and B to 33 kOhm
- Change R3A and B to 1 kOhm

General:

Be warned about the input selector and output delay protection kit which are included in the "deluxe" version of the kit: Both have ground planes which are connected to the local power supply ground. There is no dedicated signal ground plane! If you use the Analogmetric tube tube power supply there is only one low voltage supply you can use to feed both modules which is the unregulated part of the heater supply. Bad thing is that the heaters are connected to pin 4 and 5 of the ECC83s and pin 9 is grounded. As a result the ground plane of both modules is at -6,3V to ground! This converts ground planes to perfect anntenas.

- Add two ceramic 10 nF / 500 V capacitors to HF-wise shunt the "ground planes" of the input and output modules to signal ground
- The delivered Shuguang 12AX7B are not too bad for chinese tubes but I would recommend to find out which NOS tubes you like

Best regards,

Norbert

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