In the last post I mentioned a hum issue that I’d been having with the VRX-1 that I bought as a kit from 4SQRP. When I related that it only happens on connecting the antenna, Jason NT7S told me it was probably common-mode hum. For a better description, refer to EMRFD, but my layman’s understanding is that it is caused by signal from the local oscillator being radiated by the antenna, picked up by AC wiring in your house, and re-radiated with the 60Hz component from your AC supply imposed on it (which the receiver then picks up). The solution is to prevent the receiver from radiating, and an RF pre-amp can do this.
The first place I looked for a suitable schematic was Todd VE7BPO’s site and lo and behold, I found one (not – this link no longer works, as the preamp was on Todd’s old site). It uses a J310 JFET in common gate configuration.
I’m feeling the need to make some kind of excuse for the messy look of the circuits I have built Manhattan style. Part of the reason is that I’m not as disciplined with my building technique as are masters of Manhattan like Jim Kortge K8IQY and Chuck Adams K7QO. The other part of it is that I think I need to recalibrate the way I look at my projects and reign in my OCD tendencies. With an experimental project like the VRX-1, it was inevitable that bits would get added on, and it would be a work in progress for a while.
Here’s the completed RF pre-amp board:
And here it is installed in the VRX-1. It cured my common-mode hum problems beautifully, by the way:
If you’d like to hear how it sounds, I created a YouTube account for my ham radio shenanigans, and the video of the VRX-1 is here:
Dave Richards AA7EE 
Dave – wow, it sounds great! Certainly no hum at all. What will you be building for a companion transmitter?
Congrats on solving the hum problem and spiffing up your VRX-1 with some additional circuits! In addition to introducing people to Manhattan construction, one of my other primary goals was to encourage people to experiment with the simple design of the receiver. It’s really gratifying to see you do both with the project.
Happy building!
Jason
I forgot to mention, you are far too critical on your Manhattan building technique. It looks great. It’s certainly better than the controlled chaos on my boards!
Jason
John – I don’t think I’ll be building a transmitter to go along with it – just enjoying it as a receiver. Although I don’t consider my VRX-1 finished I think I’m going to put it aside for now, as I have another receiver project I want to tackle.
Jason – it feels good to have approval from the designer!
John – I don’t think I’ll be building a transmitter to go along with it – just enjoying it as a receiver. Although I don’t consider my VRX-1 finished I think I’m going to put it aside for now, as I have another receiver project I want to tackle.
Jason – it feels good to have approval from the designer. Thank you!
You’re posts are quite enjoyable to read, thanks for posting and I hope you keep it up.
Dave – When you installed the RF preamp. did install it after the vrx-1 double tuned filter or before? does that mean you two double tuned filter?
Before the VRX-1 filter fernan. The first thing a signal sees when coming into the antenna jack is the RF pre-amp.
Yes – 2 double-tuned filters.
Dave any idea how to now get to the schematic for the RF preamplifier ? I guess I could re-draw it from your picture, maybe. Curt
Curt – I don’t have the schematic any longer and, as you probably noticed, the link on this post no longer links to it. However, any simple RF preamp using a JFET in common gate mode should work fine. You are not looking for gain – all you want to do is provide a good degree of reverse isolation between the antenna input and the rest of the circuit.
73,
Dave
AA7EE