Grounded Grid Plus

This Grounded Grid Plus preamplifier provides enhanced performance out of the original Grounded Grid design. This new design adapted a feature like using beta-elminiated BJT current sources to act as constant current sources to bias the differential pairs of the channels separately, whereas only resistors are found in the original design for the current source, which are noise suspectible and voltage dependent. The power rectifier and regulator circuitry is redesigned to support full-wave rectification which is provided as another kit. A lowpass filter with an inductor option is incorporated after the full-wave rectified voltage. There are two independent output voltages (B+: 200-400V DC/100mA) and one independent filament ouput voltage (0-30V DC/3A); they can be fine trimmed by the variable resistors.

This Grounded Grid Plus preamp will not make any artifacts to the sound, and play 100% originally and naturally, due to the simplicity of circuit design and symmetric layout design without adding spared components. This design is fully verified by the simulations and evaluated with a working prototype. There is clear difference between the original GG and GG plus. The GG plus provides extra quiescent background due to lower noise (decoupling cap of 20000uF for tube filament, two independent B+ supplies with full-wave rectification and LC filter and decoupling cap of 200uF, and constant BJT current source). The human voice becomes much more detail. In addition, for the signal path, the simplicity of the differential amplifying stage configuration and negative feedback provides high and frequency response and flat pass band as reflected by its high dynamicity.

FEATURES

1. Use of WIMA, Japanese Nichicon, Rubycon capactors and Dale resistors.

2. Shuguang 12AU7 x3

3. PCB thickness: 2.5mm, double layer, 2oz copper.

4. Separated power rectifying and regulatoring unit.

5. Dedicated ground and supply voltage between main amplifier block and power supply unit.
6. High dynamic range, output voltage can > 20Vrms.
7. S/N ratio: ~80-85dB
8. Ultra low noise and high dynamic.
9. Flexible: it can be reconfigured to the original Grounded Grid preamplifier.
Also, you can easily to reconfigure it to the original Grounded Grid preamplifier. Fore more detail, please refer to the assembly guide.

Precaution
1. Dont use finger / any body parts to touch the components or board! It is hazardous 400V, since the HV capacitor may not fully discharge.
2. Turn off the main power if any parts of components or the transformer are getting hot and smoke is observed.

Assembly Guide
All photos shown are for Grouded Grid Plus Pre-amplifier. This guide also includes the steps to reconfigure it to original GG.

1. Download the schematic and part list from the "Files Download" section.



2. Solder the resistors according to the part list
For GG plus: all resistors except R27 and R28
For original GG: all resistors except R22, R23 and R26


3. Solder power BJT
For GG Plus: Solder the power BJT (2N3439/2N3440).
For original GG: skip this step


4. Solder the tube sockets, connectors, and then the output capacitors. Note to the polarity of the electrolytic capacitor (big and black in color).


5. Connect the DC power lines 12.6V, +200V, -200V and GND according to the photo.


6. If the components are correctly soldered, the tubes will light up after turn on power.
7. Apply analog signal to the input socket (IN) and obtain the output signal at the output socket (OUT).

Assembly Guide for Variable High Voltage Regulator
1. Following the procedure to assembly the high voltage regulators by the following link:

http://kikitronic.blogspot.com/2009/11/grounded-grid-pre-amplifier.html

2. A Grounded Grid Plus Preamplifier requires two Variable High Voltage Regulator modules to provide +200V and -200V.
3. Adjust the resistor trimmers of the regulators modules so as to provide +/-200V DC for B+ and also 12.6V DC for filament supply.



The Circuit Schematic & Layout

SIMULATION
DC analysis:
The DC operating points are biased so as to maximize the dynamic range of this preamplifier. A beta-elimination current mirror is adopted to get rid of the voltage dependent current source which is found in original GG.


AC Analysis:
The -3dB bandwidth is ~700kHz which is far beyond the maximum audible frequency 20kHz. This reflect high dynamic of this pre-amplifier. Also, there is high flatness and almost no phase shift of the frequency response as seen in the passband. This flat frequency response extends deep to a point where is nearly to the zero hertz.


Transicient Analysis:
1V sinewave signal (green) at 20kHz is applied to the input terminal while the output signal (red) is obtained at the output terminal with ~4V.