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Thursday, 7 June 2018

Taming the Amplifer


Following my recent purchase of an SPE Expert Amplifier I described here, I've been pondering how to keep the amplifier under control. It is currently back at the manufacturer for three issues:
  1. Extremely noisy fans;
  2. Bent front panel;
  3. Attenuation fitting.
I attended Ham Radio 2018 at Friedrichshafen this year and spoke to the owner (Dr. Gianfranco Scasciafratti I0ZY) of SPE while I was there. I tried to explain my concerns about such low drive level requirements and the reliance on ALC and associated risk of failure; the conversation concerned me because he didn't seem to want to listen to a word I said.

SPE have sent a video of my amp under test:

So hopefully it's on its way back to me now.

I've designed a thingamabob to put in-line with the amplifier as recommended by I0ZY to provide attenuation to the input drive level. Gianfranco suggested that the attenuation could be permanently in-line as on receive it "didn't matter". However, I'm not happy with that so I have come up with the following to switch the attenuation in line during TX and remove it during RX:

In reality the build looks like this:

There are quite a few bits in there, there are two coax relays (50 GBP), the connectors and the resistors in the Pi Attenuator (50 GBP) plus the case and other assorted bits and bobs (75 GBP) - so a total of 175 GBP just to tame the amplifier. Of course, I will need two of these if I want to use the amp as designed with two radios. I find it odd that they would recommend such a solution - why not just put sufficient attenuation in the amp to start with?

The gubbins has a good flat response from 0 to 50Mhz and also give us about 6.16dB of attenuation.

Throughout the build construction Florrie Cat has been assisting:

and somehow, you just know it's not going to end well:

Odd situation, don't you think?

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