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).
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
- THD at 2.9V output (1.92Vrms input) Gain 3.58dB, 2nd @ -75.5dB bellow fundamental, 3rd @ -73dB
- 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
- FR 17Hz @ -1dB, 42kHz @+1 dB, one dip at 11kHz @-1.6 dB
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.