“Shiny Eyes” CX-301A Line stage

all side 2

Here is another preamp that is using the amazing 01A tube. It has a very pleasant sound that will not fatigue your senses even when played for long hours.

This is an  DHT preamplifier/ line stage using the tungsten filament Cunningham CX-301A tube.  I has one stage and is built with separate enclosure power supply.

The power supply unit uses Hammond 270BX for main HV in a CLC configuration -47uF(Obbligato Film-Oil)-15H(75mA)-47uF(Obbligato Film-Oil). HV is supplied with a low ripple which is perfectly adequate considering that SSHV will add at least 60-80dB PSSR bringing the ripple at extremely low levels ( I could not measure with my equipment).  The raw power supply for the filament bias are using Hammond167L12, 1N5822 and 10mF-2x1ohms-10mF-0.1uF-220pF. PSU is supplying all voltages (HV and LV) through a 7 wire umbilical cord, which is permanently attached on the main preamp side and can be disconnected on the PSU side. However, it is not recommended to run the PSU without load because of the risk of damaging the preamp when reconnect the umbilical cord.

The main preamp uses one stage CX-301A in filament bias, DC input and transformer coupled output. It uses the Lundahl LL2745/PP that is working great for 3-6mA single ended. The main preamp enclosure incorporates the SSHV and two Coleman filament regulators. The 01A is filament biased at 7V on 30 ohms at 230mA.  The secondary of the LL2745 is using either Alt Q (2.8:1)or Alt R (5.6:1) providing two fixed gain alternatives. A 6 pole two position rotary switch is facilitating this function. The preamp is provided with volume control. I installed individual stepped attenuator by Gold Point ( a very good price quality solution).

Specifications

The following measurements are only to give you a sense  of the approximate performance. I used a computer software and interface and this comes with some artifacts and noise that are not present when the amp is connected in a audio path. One, if owns high quality audio analyzer, can take more precise measurements

  • Gain : Alt R: 3-4dB(x1.5) and Alt Q: 9-10dB(x3)
  • Input impedance: 80kohms, unbalanced
  • Output impedance measured comparing output on Z load (523K and 300pF) and (10k and 300pF)

Alt R: 437 ohms

Alt Q: 1750 ohms

  • Power consumption: about 50W

“Shiny Eyes” 01A Line stage Measurements

  • LL2745 Alt R  6:1       
  • FR 20Hz @ -1dB, 20kHz @-1.5dB,  12Hz-27.5 kHz @ -3dB

ShinyEyes01APre_R_FR

  • THD at 2.9V output (1.92Vrms input) Gain 3.58dB, 2nd @ -75.5dB bellow fundamental, 3rd @ -73dB

ShinyEyes01APre_R_THD

  • LL2745 Alt Q 8:1       
  • THD at 5.83V output (1.92Vrms input) Gain 9.65dB, 2nd @ -73.3 dB bellow fundamental, 3rd @ -72.7dB

ShinyEyes01APre_Q_THD

  • FR 17Hz @ -1dB, 42kHz @+1 dB,  one dip at 11kHz @-1.6 dB

ShinyEyes01APre_Q_FR

Recommended Operating Mode: Alt R or 3dB gain position. Reason: the 3 dB gain has a very good frequency response and low output impedance, consequently providing the best sound. The 9dB gain option was provide in case there is not enough gain in the audio chain and there is no other alternate option.

More pictures:

topPlates topPlate_box wooden_KevinVancouver psu top back main front all side top back2  back 1 top trafo no cover top trafo no cover wiring

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6 thoughts on ““Shiny Eyes” CX-301A Line stage

  1. Dear Radu,

    I started collecting parts for a 4P1L line stage and then I have read Moglia, Evans, and your reports on the 01a and would love to hear what you are hearing.

    What do you think the chance of this driving a FW SIT1 in single stage mode (10K input impedance)? AS an aside, these are limited to 500 hz and above by using appropriate input and output caps.

    I had figured I would have to use output transformers.

    Your opinion would be greatly appreciated.

    Thanks and take care,

  2. Dear Radu,

    Not sure if my question was posted so if there is a repeat, it was not my intention.

    Wanting your opinion as to whether any iteration of the 01a could drive a First Watt SIT 1 amplifier in one stage mode (10k input impedance) If it makes any difference, the input and output caps have been sized to limit response to just a bit below 500 hz.

    Was collecting parts for a 4P1L but after reading about the 01a I would like to hear it.

    Thanks and take care,

    • Hi Rick,
      I measured the output impedance of the mu output and its about 450 ohms. This makes the ratio Zin/Zout=22. Many designers would recommend this ratio of 100 and more. I’ve never tried to drive a 10k input but don’t think that there not possible. When driving 10k there is a loss of gain less than 0.5dB which would not be a problem. If looking at current driving capability then 2Vtms on 10k is about 0.56mApkpk. A 3 mA running anode current will provide just enough signal current without adding slewing distortion

      • However, the 26 and 4p1l will provide more driving capability. And one can tell the difference. Personally, I like the sound of 01A when driving 47k and up.
        Good luck in your quest for the perfect sound…

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