Further to the JE Labs Linestage built in 2011…
A quick Frequency Response plot with 3V pk-pk sine input and 0.4V out (-33dB volume attenuation). Response from 20Hz – 20kHz is -1.2dB, -0.1dB, ref 1kHz. Not bad, but low frequency response could still be improved.
Square wave response shows minor HF overshoot and some ringing. (The signal generator used here, is also not the best.) Note: 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. (later re-installed, see update.)
- 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.
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!