JE Labs Linestage – Pt II

Further to the JE Labs Linestage built in 2011…

A quick Frequency Response plot with 3V pk-pk sine input and 0.4V out (-17.5dB volume attenuation).  Response from 20Hz – 20kHz is -1.2dB, -0.1dB, ref 1kHz.  Not bad, but low frequency response could still be improved.JEL-FResp-050314

1kHz square wave - Top input, Bottom output

1kHz square wave – Top input, Bottom output

10kHz square wave -  Top input, Bottom output

10kHz square wave – Top input, Bottom output

Square wave response shows minor HF overshoot and some ringing.  (The signal generator used here, is also not the best.) Schematic 080314Note: This circuit inverts phase & I have inverted polarity at the input connections to the TVC.  1 signal channel only shown.  PSU supplies both channels.

The current schematic incorporates these modifications:

  • 6X5GT rectifier chosen (instead of 5Z3 etc) because the Power Transformer had a 6.3V winding, not 5V.
  • Cathode Bypass Cap (Black Gate NX) removed (from the 76 tube) –  sonically less ‘shouty’, lower distortion. more natural tone.  Gain reduced by 4-6dB.
  • Choke Input Filter – PSU B+ filter changed to LC choke input with 28 Henry input choke (the original C1 10H choke + an additional 18H choke in series).  The original 0.1uF Russian K72 teflon cap was removed and L1 increased from 10H to 28H, as per JEL recommendation.  B+ drops by 10-15V.  Sonically – increased orchestral ‘scale’ & violin ‘power’ (possibly better regulation under load with choke input filter).
  • B+ Dropper Resistor (1k5 Mills wirewound) changed to a 26H L3 choke (later increased to 40H) – smoother, more natural tone, improved top & bottom bandwidth ‘power’, clarity and resolution.
  • C6 PSU Decoupling Cap experimentally changed from 47uF Panasonic TSUP to 70uF Obbligato polypropylene in oil – sonically ‘big scale’ & rich tone.  However, less clear ‘focus’.  ‘The jury is still out’ on this one – to be confirmed.
(Still untidy sorry - decoupling caps should be closer to tubes, choke leads to be shortened...much work to do!)

(Still untidy sorry – Obbligato decoupling caps should be closer to tubes, choke leads need to be shortened…much work to do!)

That’s about up-to-date for now.  I will be listening for any sonic evidence of the slight HF ‘ringing’ seen on the scope.  I will also look at ways to further improve low frequency response.

I want to increase B+ voltage, so that I can have the 76 operating slightly higher on its curves, for lower distortion (as in JEL’s design).  This may mean a new power transformer.

On balance, this is still a very nice preamp.  The 76 tube promotes a clear, delicate, energetic and open sound, with plenty of low level detail and maybe a slightly enhanced sense of ‘presence’ and ‘scale’ – which I  like!

Next time:  Details about parts selection, caps, resistors, tubes and chokes.

Update 6 April 2015: See Magic and Microphony.

Any comments welcome!

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2 thoughts on “JE Labs Linestage – Pt II

  1. Pingback: JE Labs Linestage | D a r k L a n t e r n

  2. Pingback: Magic and microphony | D a r k L a n t e r n

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