Portable H2 Generator - It's Not Portable After All (Part 2)

The Jfet transistor just arrived yesterday. It is a long delivery due to many public holiday in my country last week. I have this J112 original from On-Semi and fortunately it is not hard to find the seller for it yet.

The installation to the board is straight forward. It is a super simple circuit that anyone could make it on universal board. I using Amrtans resistors, few Dale and Silmic II caps. The connector is 3.5 mm female jack for testing purpose only.

This is the final layout during testing. The Ipod line out will go to H2 pre-amp, then the H2 output goes to Cmoy amplifier.

At this moment I can make the pre-amp working but not in proper condition. The power supply which is DC step up converter from 12 volt to 30 volt are not give enough watt to crank the Jfet up. The voltage output from LM317 drop during load testing and only about 10 volt. It is drop about 6 volt compare while idling without connecting to the Jfets. It seems that the circuit need so much more power.

Then I switch to 50 Va R-core transformer, the secondary output is 18 volt CT. It is not become portable after all due to miss calculation for the DC step up and power required for the Jfets. I using 2 diodes as the DC converter from the transformer, then directly connect to the input of the LM317 as per schematic on Nelson Pass manual. The CT from transformer act as ground. It is now read 24 volt DC output on the volt meter. Using only 2 diodes is not the best design, I will improve it later on.

The DC output after LM317 is now 15 volt and can be adjust through the potentiometer on the LM317. It range about 14 to18 volt. With this proper power feed to the Jfet, they are now generate heat. Still, no heat sink required.

The sweet spot is 16.5 volt. Lower than 14 volt make the sound distorted and higher than 16 volt makes the gain up but create very audible hum. Later on I found that the linear power supply will do help reducing this hum. Using 4 diodes for full rectifier the AC to DC converter will eliminates this hum.

When idle, H2 pre-amp generate pink noise. It is quite audible when I crank the Cmoy volume up to 10 o'clock but still tolerable when listening the music. By the way, I am using Audio Technica ATH-M40x as sound monitor during this testing.

The others thing about this circuit is just like others Nelson Pass design, is this H2 need time to warm up before operating. It is about 3 minutes to warm it up. It let the Jfets get proper heat then the music can pass through without distortion.

The difference between using and not using this H2 is just like comparing Gainclone amplifier with F5 amplifier. It boost the resolution and dynamic while still maintaining the mid-range become more airy. It is a fun project and I will consider this H2 as the good pre-amp for my next project.

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