The 01a preamp has a long history in my system. I played a lot with many DHTs in the past from 26 to 71a, 30sp, 112a, 46, 4P1L and 10 amongst others as preamp valves. I particularly found the CX301a to be a special one. Disregarded by some as claimed to be microphonic, I found it to be much better behaved than the famous “dancing lady” 4P1L. As a preamp valve it has its limitations. It’s definitely not a 4P1L in terms of current driving capabilities, however, it provides a delicate and warm sound that is unique in my perspective. I simply love the sound of it. I made some further impressions here if you are interested.
After so many DHT preamp/line stage builds, I decided to go back to the CX301a given I found the perfect excuse. I needed a tad of extra gain for the Starlight CD Discrete DAC player. I’m utterly happy with my 4P1L stage, but adding back the particular tone of the 01a to my system seemed the right thing to do.
I tried in the past all sorts of circuit variants for the DHT preamps. I really like two in particular: the transformer coupled and the gyrator-loaded mu-follower. The latter provides maximum gain as well as low output impedance despite the output coupling capacitor. Since my load is simple the 4P1L preamp, I decided to implement the gyrator-loaded mu-follower stage.
This circuit has been covered many times and in particular I wrote about this here.
I will explain again the circuit for those out there that they are just getting to grips with this topology. The 01a is configured as a mu-follower with an active load that is not another valve but instead is a FET. In this case is not one FET, is a cascoded pair formed by a FET and a depletion FET. The DN2540 undertakes the heavy task of the high-voltage in the circuit, whereas the 2SK170 not. The latter has lower capacitance so it provides a better HF response overall. The gyrator has a frequency response similar to an inductor, but is not one. The particular benefit is that it will set the operating voltage to be constant therefore some call it a voltage controlled CCS. The voltage is set by the CCS formed by M1 and M2 and R4. The cascoded depletion FET pair sets a precise current set by P1 which will create a stable reference voltage through R4. The pair R6 & C1 set the low-frequency response of the gyrator. The lower C1 the better so we increase the value of R6, which also helps with the HF response. D1 is there to protect J1. Trust me, you will need it in case you short the anode (as I did in my test rig). R7 is the mu-follower resistor which typically is set to 1/gm for optimal value.
The great benefit of this circuit is its PSRR and the ability to accommodate different power supplies. I use a tap from my 4P1L Siberian preamp which provides a 235V rail. You can get away with a 150V / 10mA supply very easily. I leave it to you to design that supply. It won’t need a big filtering stage thanks to the cascoded DN2540/2SK170 and LND150 pairs.
Filament bias has been extensively discussed in the past before. I will simply stress here that the 01a has its filaments starved. This reduces microphonic noise as well as improve linearity. After several tests, I settled for the following operating point which proves to be quite good:
Starving the filaments to 200mA also brings additional benefits. The Rod Coleman boards don’t need a big heat-sink and the raw supply is easier to implement.
I use NOS Dale wire wound resistors rated at 20W I have in stock. You definitely don’t need that power rating capability in this circuit as less of 1W is dissipated across this resistor. A good quality wire-wound 3-5W should do well here.
If you need the gain that this circuit provides in your system and you also need driving capability (e.g. due to the load presented by the amplifier input) then you may want to consider the addition of a cathode follower (or source follower) to this circuit. Here is an early draft which still needs further tweaking. Instead of the cathode follower, I’d suggest you opt for a MOSFET source follower instead. Much simpler indeed, and it sounds good.
The design of the filament supply is crucial for low noise and performance. I implemented the typical filament raw power supply I published before here which is based on some of Rod Coleman’s recommendations for his regulators. In this case I used all sorts of components I had at hand so the supply is not worth following but here is the description in case you want to use this as a guideline:
- A 13V split bobbin 100VA transformer (100VA)
- Schottky bridge and 100n/47Ω snubber network in the secondary
- Choke input supply based on a 300mH choke and 22mF cap. A tuning cap was added prior the choke to set voltage to desired level.
- 43mH CMRR choke and a final 22mF EPCOS cap stage
Regarding the HT supply, you can get away with a very simple and tiny one. This stage only requires a 150V @ 10mA raw supply, you don’t need the 235V shown in the diagram above. As said earlier, I’m sharing an HT supply with my 4P1L stage. Filtering is not crucial as the cascoded stage of FETs provides at least 100dB of PSR @ 100Hz.
As you can see, I’ve made several compromises here in this design to use parts I had in stock. I think I bought just a few parts for this preamp.
The most important aspect in my view of the DHT preamp build is the correct wiring of the ground and power scheme to avoid loops and interference issues leading the hum. I’ve highlighted this before many times with some tips in addition of what you can find out there. There is a very good article around grounding and interconnection schemes here. I suggest strongly you take the time to read this to understand what could be happening in your system when you hear the annoying hum noise and immediately want to blame the design for it.
I created several blog entries during the design and construction of this preamplifier. I suggest you take the time to read through these if you are interested in some of the details and experience gained through this build:
If you’re interested in getting a PCB for the gyrator, see here.
The valves have a set of silent blocks to help with the microphonic noise, however I found this preamp to be extremely quiet. I can tap the copper top plate and only hear a dim feedback, compared to the 4P1L which is more microphonic by miles.
The LT and HT supplies are separated by 1m or more. The long cables are not an issue. In particular I use a shielded 4-wire cable for the LT raw supply connection.
Here is a view of the back top plate with the supply connectors and output RCA sockets:
Performance and Listening impressions
The preamp measures really well. I still need to take my measurement equipment down to the listening room to do full tests as the measurement below is using my noise bench power supply so 50Hz noise is actually lower than the 100μV (-80dB) shown below:
As you can see distortion is really low (typically 0.01%-0.02% depending the input level) and is consistent across the audio bandwidth. The main harmonic is even H2 and odds are very low. The key measured specs are:
- Gain: 18.55dB (or 8.4)
- THD < 0.02% for Vin < 400mV
- Output noise lower than 100μV @ 50Hz
- Bandwidth: 10Hz – 100kHz
Bandwidth should be slightly better that what I managed to measured with my soundcard below:
So the preamp measures as good as we expected to be, but how does it sounds? Oh, what a question. Never liked writing about the sound as is so personal and subject to your end to end system and listening conditions. However let me share some thoughts after some weeks of listening to this stage:
- The unique sound of the 01a is there. The fingerprint of the thoriated-tungsten filaments in this valve is beautiful.
- The dynamic response is very good as well as the detail that the DHT brings
- The bass and overall frequency response is great. I can notice a slightly improvement in the bass and the cymbals, brass and even top strings sound very nice.
- I was afraid that this pre wasn’t good for rock music, but I was wrong. It does have the stones to make my 4P1L-46-816 SE amp shake without impacting the sound quality.
- Classical or jazz and female vocals come at their best. The beauty of the 01a stage takes the Starlight Discrete DAC player to the next level.
Well, enough of preamps for me for now. I hope this article encourage some out there to build the preamp. Be aware that the NOS 01a valves are expensive and not easy to find and you may be disappointed to buy some of the stuff that is out there in the web. Just do the proper research before embarking on this project 🙂