A full write up of this preamp design and build can be found here
Tag: Cx301a
01a Preamp Gen2: Build (Part 1)
The first step in the building process of this preamp has been to get the active load boards ready. A reasonably quick assembly process this afternoon:
01a preamp (revisited 2013)
Looking at improving the CX-301a preamp with cathode follower output I modified the gyrator load by replacing the DN2540 by a LND150 and 2SK170 which have a lower capacitance and will improve the performance. Likewise, the tail CCS now has low-noise audio transistors such as the KSC1845 and KSC3503. Bias point is roughly the same, a bit lower than before. Overall distortion in the simulation is great: THD<0.005% @ Vo=16Vpp and loading 100K and 330pF (cable load representation).
U2 could be 6Z52P, D3a or PC86. The latter will require and adjustment as the filament is 3.8V and not 6.3V.
I think is time for building and listening to a potential great preamp…
Improving the CX301a DHT pre-amp
An idea discussed with Rod Coleman, Andy Evans and others in DYIAudio forum which still is evolving. The purpose is to increase the output driving capability of this preamp using a cathode-follower stage. A bonus is to provide the filament bias using the cathode-follower valve filaments:
A 26 DHT pre-amp from Albert
Albert sent me recently some great pictures and feedback of his 26 pre-amplifier using gyrator as the anode load similar to my design on the 26/01a.
Here are Albert’s notes on his 26 preamp:
“I have my 326 with separated power supply, 5U4G – 8uf PIO – 20H – 50 ASC – 30H – 50 ASC + 18uf pp feeding the Salas’ HV Shunt V2 Regulator, then to 2 ( left and right ) cascoded DN2540 gyrators. Rod’s filament supply coupled with filament bias @ 850mA with anode @135v. I don’t have anything to check the distortion, but this is the most good sound preamp that I have finished to date. It pleases my ears so much that I have the intention to turn it on once I’m in the house. It sounds gorgerous with very good image seperation. Sound stages are wide, deep and absolute quiet. The highs are sparkling smooth with lots of details. Mids are lush and warm with admirable vocals. I’ve found it even better at the lows if compare with 2x 156c. It goes deeper with clear notes.”
01a DHT preamp updated
Updated this superb preamp. See here for details
Enjoy
Ale
26 preamp (gen2) article
For those who are keen on the 26 preamp in filament bias and OT, here is my latest incarnation of this great pre-amp.
A preamp design with the LL2745
Lundahl LL2745 based OT DHT pre-amplifier
Before breadboarding a pre-amp with these nice OT provided by Thomas Mayer, I decided to simulate some options in LT Spice to see what results I got.
CX301a DHT pre-amp
First one is my preferred CX301a thoriated-tungsten DHT. I wired the LL2745 in 5.6:1 step-down configuration. This should provide a low output impedance which is what we want in this configuration where we need to drive the cables to the amplifier with sufficient capability:
Looks very promising. Of course gain will be much lower than a gyrator-based pre-amp, the 01a anode load is optimised providing very low distortion: 0.015% based on my SPICE model with curves taken from real CX301a. Gain is low at 3.1dB, but we don’t want loads of gain in this pre-amp. Sound is what we are after…
26 DHT pre-amp
Now it’s time for the revered 26. I used Dmitry’s model based on the RCA manual curves. I’d like to simulate this again using a model based on starved filament curves with a real 26.
Originally planned to bias the 26 in a different operating point based on feedback from Andy Evans, however after playing a bit with the OP I found that a more linear point was around Ia=5mA and Va=114V @ Vgk=-6.85V
26 looks more interesting in principle as the output impedance with this model is lower than the CX301a thanks to having a lower Ra (7KΩ against 11KΩ) so LF response will be slightly better in a side by side comparison.
Will be trying these two with filament bias and Rod Coleman’s filament regulators. The HT will be provided through a Salas HV shunt regulator.
Stay tuned…
New Output Transformer for the 01a / 26 pre-amps
New Lundahl LL2745 OT pair arrived today in the post. Very excited, Thomas Mayer kindly send me this pair for trial. These are specifically designed for the low current and high anode resistance DHT used in pre-amps (e.g. 26, 01a, CX112, etc.). Wiring and primary details are similar to the LL1689. Hope we can get a final datasheet in September 🙂
These are gapped at 8mA. So the primary inductance is about 200H in theory (160-180H in practice according to Thomas). The labels are incorrect. This is 2×2.8:4×1, providing the option of wiring it as 5.6:1 if both primaries windings are in series and all secondary windings are in parallel.
Will rebuild the 26 or 01a preamps and do some test.
I would like to build a quick breadboard with the 01a to do a comparison against the gyrator loaded preamp I’m using at the moment.
THD benchmark
I’m still in the process of testing valves, here is how the ranking is coming up so far. This is a mix of driver and output valves. All tested at Vo=+22.22dBu:
Looking at the chart above a couple of interesting points to highlight:
- 4P1L is the most linear valve I’ve found so far.
- 6e5P and 6C45 are expected to be on the top five anyway.
- 12P17L despite of having similar characteristics as 4P1L is not that linear
- 6N6P and 6N6P-I disappointed me. I thought the would be more linear..
- 46 and 47 in triode mode are superb drivers
Have so many other ones to test, but limited time….
Expect this chart to be updated in the future, so stay tuned 🙂