Direct Reactance Drive Variantions

Author: Dmitry Nizhegorodov ( My other projects and articles

1.   Introduction

In this article we explore variations of 2-stage SE topology with choke-loaded driver and direct coupling to the output stage. This topology gained considerable popularity in recent years. The DRD amplifier by Jack Elliano is by far the most popular. We use PSPICE simulation to explore characteristics of this topology. Our results show that this topology can deliver considerable power and low THD due to (1) advantages of direct-coupled choke loading and (2) a variant of THD cancellation (THDC) between the stages. We argue that the former is highly desirable but the latter has both its positive and negative sides because THDC lowers THD by lowering even harmonics and by trading a decrease in low-order harmonics to an increase in higher-order products.

2.   Direct Drive, Choke-Loaded SET Topology

For these experiments we've chosen 6P14P connected in triode mode as the driver tube and 2A3 as power tube. The driver tube is loaded with an inductance (choke). The output tube grid is directly connected to the driver tube plate.

3.   The Power Tower

DC connection between the tubes means that the voltage at the driver tube plate equals the voltage at the output tube grid. To align the DC level of the stages, in our first configuration we use separate sources of plate voltage - one supply for the driver and another for the output tube. The supplies are not ground-connected, but instead are "towered":

Notice the presence of cathode resistor induced current feedback on both stages in this configuration. The input of the amplifier is protected aga ins RF by a RCRCR filter tuned at ~100 KHz. I live near an AM radio station and my audio gear needs good RF interference protection. This filter does not impact audio frequencies, but cuts off RF. The output transformer represents a 3K load when feeding a 8 Ohm driver.

With PSPICE, we obtained the following data for 1 kHz, 5 Vampl Sin wave input: Vout = 5 VRMS, THD = .6:

    NO         (HZ)     COMPONENT    COMPONENT    (DEG)       PHASE (DEG)
     1     1.000E+03    7.062E+00    1.000E+00   -8.032E-01    0.000E+00
     2     2.000E+03    2.563E-02    3.630E-03    9.083E+01    9.243E+01
     3     3.000E+03    3.134E-02    4.438E-03   -4.429E+00   -2.020E+00
     4     4.000E+03    7.746E-03    1.097E-03   -1.008E+02   -9.755E+01
     5     5.000E+03    2.355E-03    3.335E-04    1.718E+02    1.758E+02

1st tube is biased at 17 mA and sees ~ 210 V of cathode-grid voltage. 2nd tube is biased at 62 mA and sees ~ 310 V.

Here the output probe is in the FFT - Fast Fourie Transform - mode:

4.   Overload Behavior Study - Output Stage with no Local NFB

The stage presented above generates lots of high-order harmonics when overloaded, its clipping behavior is not very pleasant. A degree of local negative feedback, introduced by the autobias resistors, may be responsible for this. Such feedback lowes THD in small-signal region but may alter sonic timbre of musical signal when amplitude spikes.

In the next variant we added a bypass capacitor for 2A3 tube:

We also made several less apparent changes. The driver stage receives only 75V on the plate. We also re-biased the input tube: with 11k resistor it run less hot. These changes give considerable THD cancellation effect. With 3.5 Vampl we get 35 mV of 2nd and 3rd harmonics.

With this input, the amp delivers over 4 W RMS:

This configuration is capable of delivering more power with lower THD than standard 6sn7+R, 2a3 topology.

Still, it does not have a very good overloading behavior. For Vampl= 4 we get 5.5 W RMS, and the spectrum is bad:

lots of odd harmonics, definitely audible.

Interestingly, looking at THD, we can only conclude that the result is very good because 5.5 W RMS is much more than 3.5 W RMS or so that the traditional topology can deliver with equal THD. However, distortion of the traditional topology is predominantly 2nd-order. This show that spectrum data is much more revealing than THD.

5.   How about no local feedback

Here is our next variant, where both stages use a bypass capacitor to get rid of current feedback:

we could expect increas of THD at moerate-to-high levels of signal but in fact with 3.1 Vampl as input we get almost 4 W RMS with .8 THD:

Here is the output of the FFT PSPICE analyzer:

NO         (HZ)     COMPONENT    COMPONENT    (DEG)       PHASE (DEG)

 1     1.000E+03    8.159E+00    1.000E+00   -1.739E+00    0.000E+00
 2     2.000E+03    4.534E-02    5.557E-03    1.222E+02    1.257E+02
 3     3.000E+03    4.620E-02    5.662E-03   -1.625E+01   -1.103E+01
 4     4.000E+03    1.641E-02    2.011E-03   -1.157E+02   -1.087E+02
 5     5.000E+03    7.431E-03    9.108E-04    1.550E+02    1.637E+02

This is encouraging! How about overloading? Here we simulate pre- clipping conditions - input is raised to 3.6 Vampl.

NO         (HZ)     COMPONENT    COMPONENT    (DEG)       PHASE (DEG)

 1     1.000E+03    9.345E+00    1.000E+00   -1.772E+00    0.000E+00
 2     2.000E+03    8.499E-02    9.095E-03    1.036E+02    1.072E+02
 3     3.000E+03    1.776E-01    1.900E-02   -1.055E+01   -5.238E+00
 4     4.000E+03    2.708E-02    2.898E-03   -1.242E+02   -1.171E+02
 5     5.000E+03    7.944E-02    8.502E-03    1.686E+02    1.775E+02


We got the same spectrum we saw previously - with lots of odd harmonics. However, this clipping happens due to screen current of 2a3, which is evident if we disconnect the second stage.

6.   Harmonics interplay

Here is a simple example - we compare 2 regimes, 3 Vampl input, with 2 very different B+ values for the driver stage, first run is with 250V, the second one is for 75V. In both cases cathode resistor in the driver stage is 5k.

B+ 250 V  Rc 5k

   NO         (HZ)     COMPONENT    COMPONENT    (DEG)       PHASE (DEG)

    1     1.000E+03    7.539E+00    1.000E+00   -2.185E+00    0.000E+00
    2     2.000E+03    1.895E-01    2.514E-02    8.447E+01    8.884E+01
    3     3.000E+03    6.120E-02    8.118E-03   -1.166E+01   -5.108E+00
    4     4.000E+03    1.179E-02    1.563E-03   -1.089E+02   -1.002E+02
    5     5.000E+03    2.574E-03    3.415E-04    1.643E+02    1.752E+02

B+ 75 V  Rc 5k

   NO         (HZ)     COMPONENT    COMPONENT    (DEG)       PHASE (DEG)

    1     1.000E+03    8.130E+00    1.000E+00   -2.063E+00    0.000E+00
    2     2.000E+03    5.096E-02    6.268E-03    1.001E+02    1.042E+02
    3     3.000E+03    5.681E-02    6.987E-03   -1.544E+01   -9.246E+00
    4     4.000E+03    2.131E-02    2.621E-03   -1.110E+02   -1.027E+02
    5     5.000E+03    9.697E-03    1.193E-03    1.586E+02    1.689E+02

1st harmonic increased 10%, 2nd decreased more than 3-fold, 3rd dropped slightly, 4th is slightly higher and 5th is 3..4 times higher. A tradeoff? How about if we have comparable 1st? With Vin 2.8 Vampl we get

B+ 75 V  Rc 5k

   NO         (HZ)     COMPONENT    COMPONENT    (DEG)       PHASE (DEG)

    1     1.000E+03    7.603E+00    1.000E+00   -2.062E+00    0.000E+00
    2     2.000E+03    3.353E-02    4.411E-03    1.077E+02    1.118E+02
    3     3.000E+03    3.950E-02    5.195E-03   -1.620E+01   -1.001E+01
    4     4.000E+03    1.267E-02    1.666E-03   -1.135E+02   -1.053E+02
    5     5.000E+03    4.928E-03    6.481E-04    1.551E+02    1.654E+02

We traded a 2-fold increase in the 5th to much lower 2nd, somewhat lower 3rd and a notch smaller 4th. This tradeoff is easier. However, 5th is extremely unpleasant to the ear, and humans are very sensitive to it. IS it better to lower 2nd 6 times by rasing the 5th 2x? Something to think about - everyone should make personal conclusion as tastes may differ.

Another interesting observation is how harmonics interplay across the power range. Here is one more notch - .2 Vampl as input:

   NO         (HZ)     COMPONENT    COMPONENT    (DEG)       PHASE (DEG)

    1     1.000E+03    5.437E-01    1.000E+00   -2.027E+00    0.000E+00
    2     2.000E+03    2.212E-04    4.068E-04   -1.795E+02   -1.754E+02
    3     3.000E+03    1.955E-05    3.596E-05   -1.616E+02   -1.555E+02
    4     4.000E+03    4.200E-05    7.725E-05   -1.721E+02   -1.640E+02
    5     5.000E+03    1.816E-05    3.339E-05   -1.731E+02   -1.630E+02

here, with 15 mWRMS of power and .04% THD we still do not see a more "natural" spectrum - as given by a "classical" 3-stage SET, instead the 3rd dips under the 4th and is equal to the 5th.

The following is a plot across output voltage range of .1... 8.2 V ampl:

or the same data but in logarithmic scale:

A "dip at 2nd" pattern at higher power gradually transforms into a "dip at 3rd" pattern as power goes down. Does it introduce changes in timbre that affect listener's perception> If so, how? IS this a desirable variation or not? These questions are still open. It is my opinion that the topologies described here can be implemented in such manner that adjustments can be done in real time, with a knob (a THDC knob?) or 2 separate knobs - for bias and plate volatage - and the future will tell which positions will be most preferable.

7.   Driver tube selection

EL84/6p14p may be a promising driver tube for a THDC-adjustable amplifier because it shows an interesting THDC interplay with 2A3, however most interesting behavior is in a corner area of low cathode current. And while it is low by driver tube standards, it is extremely low for EL84/6P14P, a power tube! Very low currents are not practical because the grid of the output tube consumes current in non-linear fashion and a weak driver simply can not deliver enough current. Another way to say it is that driver must have low enough output impedance, which is usually not found in low current regions.

Non-linear TV tubes such as 6an4, tubes such as 6aq8 and high-mu high-current tubes such as 6c45pi may be promising as potentially allowing THDC at higher currents. 6c45pi maybe be the most promising since 6an4 seems to pre-distort "too much" for 2a3.

8.   Part Two: Randall-2 DRD Ultrapath amplifier.

In this section we use PSPICE to explore some of the properties of Randall-2 amplifier, which is a design based on Direct Reactance Drive and Ultrapath ideas. 40+ of these amplifiers were built with a success by Bay Area tube enthusiasts at a class at the SF Randall Museum.

Click here to go to Part Two

Author: Dmitry Nizhegorodov ( My other projects and articles