At various times during the development of the QRP2001, we have tried two different 'add-on' boards. In both cases they were supplied in kit form by Hands Electronics, and in both cases they offer some degree of pre-mixer filtering and amplification. Whilst the filtering can be very useful if you have a large antenna or suffer from a lot of out-of-band QRM, we are less convinced about the merits of pre-mixer amplification. The QRP2001 is inherently pretty sensitive, so unless you are limited to a very inefficient antenna, front-end gain is really only going to reduce the strong-signal performance of the receiver.
If you think that your receiver would benefit from front-end filtering or more gain then we can confirm that both of these boards integrate very nicely with the QRP2001. In particular, the filter selection signals from our control board exactly match the requirements of these products.
The band-pass filter board which we used with one of our prototype receivers has now been superseded, but is extremely similar to the new PRA9 which is available from Hands Electronics. The PRA9 is supplied as a full kit with detailed assembly instructions.
The circuit consists of a set of switchable band-pass filters followed by a receive pre-amp which uses four FETs in parallel for good strong-signal handling. There are two toroidal inductors in each filter which need to be wound by hand, which needs a bit (but not too much) digital dexterity. The filters are quite sharp, which is great from a performance viewpoint, but can also make them rather fiddly to set up. This is particularly true for the wider bands where you may need to compromise on coverage of the whole band. However, none of these issues should represent too much of a challenge for most home constructors.
This board is a rather neat general-purpose transceiver building block. It includes a small (6 - 8 watt) HF amplifier, receive pre-amp, and a set of six relay-selected low pass filters which are used on both receive and transmit. It also provides the RX/TX changeover relay and other changeover switching logic. The RX pre-amp uses four FETs in parallel (like the PRA9), and the PA is based around a push-pull design for low harmonic distortion. If you use the PRA9 board you can order a version for the QRP2001 which is supplied with out the pre-amp parts.
The filters and receive pre-amp are still in use in one of our prototypes, and work really well. It is currently intended that the QRP2001 transmit modulator will be suitable to drive the PA of the AMP-GQ directly.