Home Stereo Amplifier

6922- E88CC Tube Pre-Amplifier

2009-11-19T19:57:00.000-08:00

The pre-amplifier design is a single gain stage triode tube with cathode follower. However there is an important difference, a 1k "stopper" resistor is added at each grid input. This 1k resistor is required to avoid parasitic oscillation. Without it, the 6922 would oscillate easily.

Please note that you should NOT use 6DJ8 in this design, as the heater-cathode maximum voltage limitation is different with 6922. Besides 6922, the E88CC or E188CC tubes can be used.

Pre-Amplifier Schematic

Pre-Amplifier Gain Formula
The gain for this amp can be calculated by this formula :

Gain =

(Tube Gain x Plate load resistor)/(plate load resistor + plate resistance + ((tube gain +1) * cathode resistor))

With 6922, the plate resistance = 3k, tube gain = 33. Using these value in the above formula, the calculated gain is 22.

Pre-Amplifier Impedance
The output impedance of the 1st gain stage is equal to the plate load resistor paralleled with the plate resistance, so in my case it is about 2.7k, which is a bit high. So a cathode follower is needed to reduce the output impedance. With the cathode follower, the output impedance would roughly equal to 1/(tube's transconductance).

For 6922, its transconductance is about 12000 micromhos. So the cathode follower's output impedance would be at about 85 ohms, which is a good low figure.

Pre-Amplifier Power Supply
It is very important that the regulated power supply can provide a clean B+ and heater voltage supply. For this design, any ripple in the B+ line will appear on the output signal. Noise or ripple in the heater supply will also be coupled to the output.

The 6922/E88CC has low distortion, high output current, and has high transconductance which make a cathode follower with low output impedance of 85 ohms. All these properties make the 6922 a good choice as a preamp tube.

In fact, among the 3 tube preamps I created, this 6922 preamp sounds the best, it has the most resolution details and having excellent dynamics. I like it so much that I searched for the best 6922/E88CC tubes to use in it.

Source: 6922 / E88CC tube pre-amp

100W 2x LM3886 Audio Power Amplifier

2009-11-19T10:46:00.000-08:00

This audio amplifier designed uses two LM3886 per channel, in parallel circuit, based on the PA100 parallel amplifier detailed in National Semiconductor's application note - AN1192. This amplifier can deliver about 50W into a 8-ohm speaker and 100W into a 4-ohm speaker. This is a stereo amplifier and therefore 4 LM3886s are used.

The LM3886 circuit is in a non-inverted configuration, so the input impedance is determined by the input resistor R1, i.e. 47k. The 680 ohm and 470pF resistor capacitor filter network is used to filter out the high frequency noise at the RCA input. The 220pF C4 and C8 capacitors are used to shot out the high frequency noise at the LM3886 input pins.

I used high quality audio grade capacitors at several locations: 1uF Auricap at the input for DC blocking, 100uF Blackgate for C2 and C6, and 1000uF Blackgate at the supply filter.

Power Amplifier Schematic

The PCB is designed in a way that the power ground is separated from the signal ground, as you can see from the below layout. The signal ground is located in the middle and surrounded by the power ground. There is a thin trace near C5 connecting them. The PCB layout is done by using PADS PowerPCB 5.0. I think it is a powerful layout software.

Amplifier Printed Circuit Board (PCB)

Amplifier Power Supply
The power supply used is a regulated power supply. I used 10000uF per rail before the LT1083 regulator. After the regulator, I have 100uF on the regulator board. The advantage of using regulator is that the power supply ripple voltage is removed. If power regulation is not used, I can hear very little 50/100Hz hum from the speaker.

The high current MUR860 diode is used to ensure high current flow. The voltage regulator used is LT1083, it can provide about 8A of current. Transformer used here is a 500VA 2x 25V. The power supply is then regulated by 2 LT1083, after the regulation, the voltage is 30V.

I did some DC measurement and the result is quite good, I got 7 mV of DC offset at the speaker terminal. The voltage difference between the output of the 2 chips is less then 1 mV.

The sound of this amplifier is similar to my LM3875 amplifier, which is very clean and detail. It has no hum, no hiss and no noise. Compared to the LM3875 Gainclone, this amp can deliver twice the power to my 4-ohm speaker, and it improves the dynamics and bass punch a lot.

Source: PA100 GC - 100W per channel, 2x LM3886 in parallel power amplifier

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2009-11-19T05:38:00.000-08:00

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