Grounded Grid Pre-amplifier

OVERVIEW

Grounded Grid tube preamplifier DIY module, simply called as GG preamp, circuit designed by famous tube amp designer: Bruce Rozenblit from Transcendent Sound. Bruce has profound power in design tube amplifier, and good at creating special and unique sound effect. His design is received good comments from US media. Grounded Grid preamp will not make any artifacts to the sound, and play 100% originally and naturally




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. Built-in rectifying circuits.

5. Separation PCB design of ground and supply voltage between main circuit block and power supply unit.
6. High dynamic range, output voltage can > 20Vrms.
7. S/N ratio: ~80-85dB


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 the transformer are getting hot and smoke is observed.

Assembly Guide
1. Hook up all the components according to the schematic, part list, and kit photos.



2. Connect the power lines HT, +200V, -200V and GND by flywires on the board.
3. Test the output voltage of the power transformer; output voltage 200V AC and 12-15V AC.
4. Connect the power transformer to the GG main board.
5. Unplug all the tubes before turn on the power supply.
6. Check the voltage HT (12.6V), +200V, and -200V, with reference to the GND, respectively.
7. If the voltages are correct, plug in the tubes and turn on the main power.

FEEDBACK

From Kevin:

Wo~I have just bought a wonderful DIY Grounded Grid Preamp from Analogmetric!! It is absolutely silent in background. The spacial feelings and level of human sound is definitely clear and it is really outstanding. When I first time see this item, the price was very cheap and I suspected its performance by its cheap price. However, I felt it is my best and most reasonable DIY preamp I bought before. Really worth to try, recommended to all.

http://social.msdn.microsoft.com/Forums/en-US/embeddedwindowseefs/thread/3e108da7-c60b-4cc5-afad-c9a9c7b2b432

For many years, Grounded Grid tube preamp has been tested and verified by many worldwide DIYers. This DIY kit adopts the same amplifying stage which is invented by Bruce Rozenblit. The simplicity of this circuit enables the beauty of circuit symmetricity, and also simplify its power supply circuitry. This kit can be supplied only by using one main transformer (toroid main power transformer T30)

The high voltage supply for the tube preamp is come from the secondary coil (200VAC) of T30. This kit corporates with the RC filter to reduce undesired power supply ripple.

The static current consumption of GG tube preamp (2 channels) consumes approximately 8mA, taking the power consumption of ZENER rectifier part into account, the total current consumption is just about 13mA. The power supply of the filament is come from the secondary coil (12.6VAC ) of T30, with the low voltage dropout of the LT1085CT and relative large surface area of the heat sink, the temperature of the heat sink almost remain the same and it is very stable.

Separation of PCB design of power supply unit (PSU) and amplifier core circuitry. They are interconnected by flying wires. Three shuguang 12AU7 vacuum tubes are used for amplification. The voltage gain is 10-15dB. It can be set by adjusting the ratio of R10/R12 and R9/R11 by the following equation.

Voltage gain (log) = 20log(R10/R12) (for right channel)
Voltage gain (log) = 20log(R9/R11) (for left channel)

In our kits, R9 and R10 uses 100Kohm; R11 and R12 uses 20k, the gain of 14dB is obtained

The signal grounding of this small signal preamp is very crital to the overall perfomance, better signal grounding can reduce the noise distortion or addition to the original weak audio signal. In practical experience, either using the method suggested by Bruce Rozenblit: The bypassing the GND of RCA input with a 10ohm (0.5W) or the GND of filtering capacitor via copper strip to the metal chasis, it can greatly reduce the surrounding noise interference. There is no much different of the two methods.

A GND hole for copper strip is designed for this purpose. Just using a copper strip to connect this hole to metal chasis, good shielding and low noise will be resulted.