Tag: dht

46 DHT driver final tests

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Having built the 4P1L filament bias driver stage in a breadboard, I now have the sufficient voltage swing to drive the 46 to maximum sweep. In my 4-65a SE amp, a maximum of 200Vpp is required to drive the amp into class A2.

The following tests conditions were used:

  • 4P1L first stage:
    • DN2540 gyrator in mu follower output
    • 220nF/450V Capacitor coupled into 46 driver
    • Filament bias: 15 ohms, Vgk=-10V
    • Vsupply=355V and Va0=210V
    • Output set to about 30-32Vpp to drive 46 at 200Vpp
  • 46 driver stage:
    • IXYS 01N100 gyrator in mu follower output
    • Load impedance is 100K (Pete Millett’s interface)
    • Filament bias: 10 ohm / 100W Vgk=-17V
    • Vsupply=355V and Va0=204-208V
    • Output set to 200Vpp

I tested 28 valves. Just a few of my lot are NOS. The average THD was about 0.4-0.5% but a good selection of 8 valves (mainly Sylvania NOS) provided a consistent 0.18% THD:

4P1L into 46 driver test2Happy now with the initial tests and selection of 46 pairs for the amplifier, I can now continue with the build…

4-65a SE Amp: refining the 46 driver

 

I did some tests today and looked at minimising distortion of this 46 driver in filament bias and found that Va=230V (instead of 184V) to provide best performance:20130101-134450.jpgFilament bias resistor array is now laid out horizontally to improve the dissipation of heat.

Here is the performance (0.05% at 17Vrms) at maximum drive input from my audio test set:

46 Driver Test2 17Vrms

 

4-65a SE Amp: 46 Driver Gyrator

 A day of PCB etching

20121230-194358.jpgAfter a lot of work today in designing many PCBs, I finally got a pair of mu-follower MOSFET gyrators for the 46 driver stage.  The driver has to provide very low impedance to operate the 4-65a output valve in class A2. The gyrator in mu-follower configuration will enable the right bias point as the amplifier is DC coupled as well as maximum signal (and current in A2) with minimum distortion.

Many don’t like sand at all in their amplifiers. I have a lot of experience with gyrators and CCS loads in pre-amps and drivers as well. I have to say that with MOSFETs gyrators the sound is really nice. For an A2 driver, not many options are available and the gyrator is a great choice for this job.

I built two PCBs (one per channel) and the circuit is the classic depletion-mode MOSFET gyrator based on the high-voltage IXTP01N100D. I guess that a DN2540 should work as well here but I’ve been saving the IXYS for this occasion. The reference voltage for the anode bias point is provided by the CCS formed by M1 (LND150) which provides a higher impedance in AC improving the frequency response of the gyrator.

4-64-65a driverThe 46 is operating in triode-mode and filament bias with a Rod Coleman filament regulator. R6 is approximately 1/gm and output voltage is set by P1 to achieve the 4-65a bias point as the amplifier has stacked power supplies given coupling is DC, so no capacitors in the path to the grid.

Next: some tests on these gyrators and the filament boards…

 

 

 

Testing a CX226 pair

A nice pair of CX226 in the new preamp. They are in very good shape, very low distortion measured in the workshop time ago and no hum induced, so great for this preamp.

Sound is definitely warmer, without loosing any of the bass and bright treble from this preamp. A great preamp so far, happy like a bunny!

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26 Preamp Gen2 completed

Finally, here it is.

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Improved sound compared to LL1660 version. This OT (LL2745/8mA)is better suited for the 26 in this configuration. Sacrificed output impedance a tad to get more gain. Filaments starved at 760mA. Distortion is as low as 0.02% @ 3Vrms output.

First listening session with Mingus, exceeded my expectations. Rounder bass and more space. Like this sound!
Will do some proper measurements but for now will enjoy and listen to it…