{"id":4321,"date":"2015-01-17T12:37:20","date_gmt":"2015-01-17T12:37:20","guid":{"rendered":"http:\/\/www.bartola.co.uk\/valves\/?page_id=4321"},"modified":"2017-07-08T07:51:28","modified_gmt":"2017-07-08T06:51:28","slug":"01a-preamp-gen2","status":"publish","type":"page","link":"https:\/\/www.bartola.co.uk\/valves\/dht-pre-amplifier\/01a-preamp-gen2\/","title":{"rendered":"01a Preamp (Gen2)"},"content":{"rendered":"

Introduction<\/h1>\n

\"IMG_1962\"<\/a>The 01a preamp has a long history in my system. I played a lot with many DHTs in the past\u00a0from 26 to 71a<\/a>, 30sp, 112a<\/a>, 46, 4P1L and 10<\/a> amongst others as preamp valves. I particularly found the CX301a<\/a> 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<\/a> if you are interested.<\/p>\n

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<\/a>. I’m utterly happy with my 4P1L stage, but\u00a0adding back\u00a0the particular tone of the 01a to my system seemed the right thing to do.<\/p>\n

Design<\/h1>\n

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.<\/p>\n

This circuit has been covered many times and in particular I wrote about this here<\/a>.<\/p>\n

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.<\/p>\n

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<\/a> which provides a \u00a0235V 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<\/a>\/2SK170<\/a> and LND150<\/a> pairs.<\/p>\n

\"01a<\/a><\/p>\n

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:<\/p>\n

\"SX-201a<\/a><\/p>\n

 <\/p>\n

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.<\/p>\n

I use NOS Dale\u00a0wire wound\u00a0resistors rated at 20W I have in stock. You definitely don’t need that power rating capability in this circuit as less of 1W is\u00a0dissipated\u00a0across this resistor. A good quality wire-wound 3-5W should do well here.<\/p>\n

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<\/a> which still needs further tweaking. \u00a0Instead of the cathode follower, I’d suggest you opt for a MOSFET source follower instead. Much simpler indeed, and it sounds good.<\/p>\n

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\u00a0recommendations\u00a0for 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:<\/p>\n

    \n
  1. A 13V split bobbin 100VA transformer (100VA)<\/li>\n
  2. Schottky bridge and 100n\/47\u03a9 snubber network in the secondary<\/li>\n
  3. 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.<\/li>\n
  4. 43mH CMRR choke and a final 22mF EPCOS cap stage<\/li>\n<\/ol>\n

    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.<\/p>\n

    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.<\/p>\n

    Build<\/h1>\n

    \"IMG_1951\"<\/a>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 \u00a0here<\/a>. 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\u00a0immediately\u00a0want to blame the design for it.<\/p>\n

    \"IMG_1959\"<\/p>\n

    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:<\/p>\n

      \n
    1. Build part 1<\/a><\/li>\n
    2. Build part 2<\/a><\/li>\n
    3. Build part 3<\/a><\/li>\n
    4. Build part 4<\/a><\/li>\n
    5. Build part 5<\/a><\/li>\n
    6. Build finished<\/a><\/li>\n<\/ol>\n

      If you’re interested in getting a PCB for the gyrator, see here<\/a>.\u00a0<\/a><\/p>\n

      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.<\/p>\n

      \"IMG_1962\"<\/a><\/p>\n

      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.<\/p>\n

      \"IMG_1995\"<\/a>
      LT supply<\/figcaption><\/figure>\n

      Here is a view of the back top plate with the supply connectors and output RCA sockets:<\/p>\n

      \"01a<\/a><\/p>\n

      \u00a0Performance and Listening impressions<\/h1>\n

      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 100\u03bcV (-80dB) shown below:<\/p>\n

      \"01a<\/a><\/p>\n

      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:<\/p>\n

        \n
      1. Gain: 18.55dB (or 8.4)<\/li>\n
      2. THD < 0.02% for Vin < 400mV<\/li>\n
      3. Output noise lower than 100\u03bcV @ 50Hz<\/li>\n
      4. Bandwidth: 10Hz – 100kHz<\/li>\n<\/ol>\n

        Bandwidth should be slightly better that what I managed to measured with my soundcard below:<\/p>\n

        \"01a<\/a><\/p>\n

         <\/p>\n

        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:<\/p>\n