cx-112a DHT preamp

Introduction

This was one of my first DHT preamps. I found a quartet of NOS CX-112a Cunningham (globe) back in Buenos Aires many years ago and built one of my first DHT preamps. I loved it. I played with it before I moved into the 26 and then started the long exploratory journey with DHTs. 

The CX-112a can be easily fit in an existing 01a preamp. Take a look at what Thomas Mayer recently blogged about this valve, worth reading it. 

Well, you can get more current drive than 01a (nearly double) but no thoriated tungsten filaments. Anyhow, the gain is slightly higher but is very easy to adapt to my gyrator-based circuit, that I couldn’t resist to take the quartet out of my valve stash and make them sing again after so many years.

The circuit

Again, an easy drop in without modifications. If you want to learn more about this circuit then I suggest you read the 01a blog posts. In summary, here it is:

I used my existing 01a, however I tried newer PCB boards with the IXTP08N100D top MOSFET as well as slight variation of the components as shown int the diagram. I like the fact I can drive it at 8mA, better Slew Rate and deeper bass as the BF862 transconductance is higher. 

Performance

This is an outstanding preamp. I have to admit that I was even impressed with the bass, closer to the VT-25. It has good dynamics but probably the level of detail not as close as the 01a. I think it’s really good and is worth listening for some time. I will leave it running as I enjoy it very much. 

The measured performance is great. Let’s have a look at the frequency response:

Flat response from 3Hz up to 700kHz. High-bandwidth, clear treble and strong bass. A real contender amongst the DHTs

Distortion can be as low as 0.005% for 1V output to 0.11% for 10V swing. The harmonics haven’t  got the best cadence when at full tilt, however, at normal levels they are great:

THD @ 4Vmrs
THD @ 10Vmrs

I don’t know how easy are the CX-112a to get these days, but certainly they are worth using. The sound of the preamp stage is great. Detailed and with strong bass and clear treble. I feel at this early stage is a good contender to the 01a as well as the VT-25.

i will play it for a while. Boards and preamp overall needs some break-in time. 

Cheers

Ale

 

 

7 Replies to “cx-112a DHT preamp”

  1. Hi Ale,

    Can this circuit use an output transformer to replace the Output Capacitor(220nf/600v)?

    Gary

    1. Hi Gary, why do you want to do that? If you want to use an OPT this isn’t the circuit to use. You can use it in parafeed mode if you want to connect a low impedance load like headphones. Search the blog and you will find an example. You will still need the capacitor though. Unless you complicate the power supply to get the mu output close to 0V. You will need a servo, not worth the added complexity in my view

  2. Hi Ale, Are You now finding that the IXTP08N100D(which I have used in other projects) used as top Mosfet to be a better choice than the DN2450?

    1. Hi Barry, yes I have. I tried many combinations over the past months:
      1) Standard DN2540
      2) DN2540 with bias multiplier
      3) DN2540 with battery bias
      4) Enhancement MOSFET with bias from CCS (STP3NK60ZFP) a la Gary Pimm. This requires a lot of hacking on the PCB, but works very well
      5) IXTP08N100D

      All these options increases the HF and overall bandwidth so it makes it more challenging in a breadboard to ensure you don’t get the stage to oscillate (4P1L due to its higher transconductance). They are all very good choices. I think the use of Gary’s design isn’t justifiable these days as you have devices like IXTP08N100D. However, the choice of a lower JFET like BF862 or the enhancement BSH111BK makes a big difference.
      I personally have settled for the IXTP08N100D/BSH111BK pair for stages with current above 10mA. I’m currently swapping every week the CX112a with the VT-25 and CX-301a amps to listen and compare.
      It’s all good fun!
      cheers
      Ale

      1. Forgot to clarify that options 2-5 are seeking to increase the VDS of the lower cascode device (either BF862 or BSH111BK in my case) to ensure the reverse capacitance (Crss) is minimal. With the depletion mode devices, IXT08N100D has nearly double VGS (th) than the DN2540 which takes the lower FET to a couple of volts. Options 3 and 4 can dial the voltage difference to 10V, but in practice haven’t notice any improvement compared to the IXTP08N100D.

  3. Just to clarify I am using the BSH111BK in my 4P1L Pre-amp, so that I can get the plate current higher, due to a BF862 failing when drawing to much current after drifting! I must admit soldering this is not at all easy, but can be done, I used blue tac to hold it in place on one side & checked it with Microscope before soldering one pin, then remove the blue tac & solder the other 2 pins, I didn’t use paste, but You have to be quick at this!!

    1. Yes indeed. The other (easier) way of doing it is to buy the SOT-23-P to DIP adaptor PCBs and solder the BSH111BK on to them. Depending on the pin arrangement of the SOT23P adaptor, you can use the J310 or the 2SK170 pins to mount the daughter board with the BSH111BK on top of them and with 3 small bare solid core pins you can solder it. I have also tried this and works perfectly fine, no penalty on frequency response or whatever. I personally prefer soldering the BSH111BK to the board myself as I have the gear to do so. I may offer in the future pre soldered BSH111BK daughter boards with the gyrator PCBs if there is interest for the ones who aren’t used to solder SMD, etc.

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