Differential Pre-amp for DC Receivers

I modified my receiver to use a differential audio pre-amp like the Juma does. My initial thoughts are this represents an improvement over the audio pre-amp stages I was using. I used a NE5532 because the calculations I did suggested it would be 10dB better than a TL072 and 13dB better than a LF353.

If I had a better OpAmp to hand I would have used it though my results suggest this would be unnecessary. My calculations showed the NE5532 was more than adequate and if purchased new represented real value. I was fortunate to have recovered almost 100 NE5532's from a few circuit boards I had lying around. So my cost was ham appropriate.

My subjective testing showed this modified receiver was good enough to reveal previously unknown deficiencies in my HP signal generator. I was getting a high pitched tone from the signal generator though this did not prevent me from copying stations on a DX net with ease. However, when I switched to the DDS board I used in my existing 40m transceiver the high pitched tone disappeared and stations from the other side of the world were armchair copy. And I feel this configuration, despite the lash up approach I used, was even more stable at high volumes.

I rate this as a worthwhile improvement. I'm not sure why it appears better than the previous discrete transistor audio preamp I was using. Part of the explanation I put down to the unknown pedigree of the recovered SMD transistors I was using.

Anyway, here are the modified schematics and I heartily recommend this approach to you.

Regards
Richard VK6TT

Audio pre-amp and Low Pass filter

Band pass filter and Product Detector