6BQ5pp DC Power amp.

Differential push-pull 6BQ5 (EL84) DC power amplifier

This is modification from my 6BQ5 differential pp amp, which consists of 1st stage with 6DJ8 differential drive circuit and final with 6BQ5 differential push-pull circuit. This circuit was hinted from 'All stage differential amplifier' introduced by Tetsu Kimura. Each 6BQ5 swings under class A operation and in principle constant current flows from power supply. No AC current goes through filter capacitors. I believe this manner contributes to its clean sound.

Fig.1 6BQ5 differential push-pull power amp schematic (before modification).

The idea here for the improvement is to eliminate coupling capacitor between the stages. By introducing just one pair of PNP transistors, this was possible to realize 'direct-coupling'. I expected this DC-amplifier like circuit would improve the sound quality from this instrument to drive my loudspeakers. The most concern with this configuration is DC drift, but I tried this anyway.
The modified circuit, shown below, is made with DC-amp circuit, which directly couple 6DJ8 and 6BQ5 with 2SA606. This PNP transistors act as voltage change transmission and bias generation for the 6BQ5. The voltage gain is intentionally lowered by its emitter resistance (1.2k). The bias voltage (across 1.5k) is approx. 8.5V. VR2 (200ohm) is for adjusting these bias voltage. Additionally, CRD for first stage differential amp is changed to discrete circuit with 2SC1400.

Fig.2 6BQ5 differential pp DC power amp schematic

The feasibility of the modified circuit was tested by LT-Spice simulation as below.




Two 2SA606 are coupled with small heat sink metal so that both temperature is to be same.
(2SA606 is obsolete, of course, but still be able to buy at high price in Tokyo. Mines were used ones bought in far less price in Singapore)


Constant current discrete circuit with 2SC1400 for first stage. The 2SC1400 is very hard to get obsolete. This is also used one sold at on-line auction site.


Checking bias voltage of 6BQ5.


Oscilloscope record of bias voltage while 1kHz sine signal is fed to input.


Confirming stability of the bias. Approx 8.5V is maintained for several hours.

How is the sound?
It is much better than previous circuit and quite satisfactory because operation is so stable. Sound is also 'Rock'. This is a kind of precise sound of DC-amp. The sound quality was greatly improved by changing the transistor of constant current circuit. 2SC1400 is the best as widely known. 2SC1775A is not so good as 2SC1400, but gives average. This has been also getting hard-to-get item nowadays.

I analysed that the key to success of this circuit were:
1. Capacitor coupling was substituted by simple transistor circuit.
2. The transistor circuit was used with low gain setting and this lead to stable operation.
3. 2SC1400 is great to use here (constant current source for first stage diff amp) because this device used here affects to sound quality much.