[Hammarlund] Bob Camp-HQ180

[Bob Camp]

Hi

The audio transformer on the HQ180 acts as the main load on the audio 
output tube. If you run the audio output into a high impedance load 
then the transformer presents a high impedance load to the tube. This 
is not a good idea. The easy fix is to get a resistor rated for a 
couple watts with a value in the three or four ohm range. 3.2 ohms 
would be ideal but your local parts bin may or may not have any.

The resistor goes in the same place as the speaker normally would. That 
way you replicate the load the radio was designed to run into.

The audio output stage on the HQ180 is part of an active filter. When 
you change the gain inside the loop of the filter you change it's 
performance. By increasing the load impedance on the output tube you 
increase the gain of the stage. This moves around the shape of the 
audio pass band. That's not to say it makes it worse or better. Best 
bet is that it makes it worse though ...

Often you will find that when running a radio with only speaker output 
into an external amplifier you have to turn the audio way down on the 
radio. This also is probably not a very good idea. Some kind of 
attenuator between the radio and the amplifier is needed in this 
situation. A 10K ohm resistor and a 100 ohm resistor normally do an 
adequate job. One side of 10K to radio output hot side, Other side of 
10 K to amp input and 100 ohm resistor.  One side 100 ohms to amp input 
and other side of 10 K ohm resistor. Other side of 100 ohms to speaker 
cold side on radio. Ground (chassis) on radio hooks to ground on 
amplifier.

If you really want to get good audio out of this or any other radio the 
best thing to do is to modify the radio. Depending on your approach to 
radios this may or may not be something you even want to think about. 
The closer to the detector you can grab the audio the better the 
fidelity will be. You can feed the audio into either a mono tube amp or 
*gasp* a mosfet based solid state amp. Either way it's going to sound 
better than the output stage in most radios.

	Enjoy!

		Bob Camp
		KB8TQ



On Apr 19, 2005, at 9:53 PM, Mike Taylor wrote:

> Robert, are you suggesting that I put a 3.2 ohm resistor in series 
> between
> the HQ-180 and the stereo pre-amp input? Or perhaps in parralell? If 
> there's
> anyone else on the list that has experience with listening to a
> communications receiver through hi-fi equipment, let's hear your
> experiences. Maybe I'm pushing an envelope, but technically, nothing's
> impossible. Robert's not thrilled with the audio from his HQ-180 and 
> neither
> am I. Over the last three years, several others have stated the same 
> thing.
> There must be a way to make this very nice receiver's audio sweeten or 
> get
> brighter. Waiting now on the Hammarlund Digest Geniuses to chime in. 
> Hang on
> Robert, we may and hopefully be in for some audio education. 73's, Mike
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