[HBR] HBR-11/2000. Comments? -- And GC-HBR progress

[Walt Hutchens]

Jim said:
> "If it was easy, everyone would do it"
> 
> "Experience varies directly as equipment ruined"
> 
> "Engineering is doing for a shilling what any fool could do for a pound"
> 
> "Scientists dream of doing great things
> Engineers do them" (attributed to Wehrner Von Braun)
> 
>  Age here is 54, been messing with electronics since the 
> > 
> > early '60s.

Ah ... so my long-needed sig line is "Ruining for a pound what 
most any damn fool could ruin for a shilling, for going on 68 years."

Well ... rather a lot, isn't it.   But it expresses the truth pretty well.   
Ever since that first alarm clock when I was 6.

I've got the metalwork done for the 1st and 2nd mixer 
subassemblies for the GC-HBR project.   AND -- I think I've finally 
solved the problem of applying voice coil-to-input feedback to the 
audio amplifier when not using a power transformer.   

The issue is that if you don't use a power transformer you can't 
take anything out of the set that isn't isolated from the line by 
some other transformer -- for example, headphones with one side 
connected to B- (= power line neutral) would be 'hot' for AC if you 
happened to plug into a reversed power outlet.   The (now) obvious 
answer is to use the usual sort of output transformer hookup, do 
feedback from the voice coil winding (you can hear the difference 
so this is worth some effort) and then use another transformer or a 
separate winding at chassis ground to feed the headphones.   

An added wrinkle is that if you can't go from the voice coil winding 
then with a push-pull output stage you can't make very effective 
use of feedback -- only from another transformer winding can you 
feed back the effect of both tubes.

Food for thought department:  The most interesting thing I found 
looking at the May '65 Goodman QST article ("Some thoughts on 
home receiver design") is his note that he too dealt with the 
problem of BFO pickup in the IF amp input.  That's a problem 
because you can't get proper S-meter action and if the problem's 
bad enough you lose dynamic range too.  

The way I've been dealing with that in recent designs is by very 
careful layout, sheilding, and filtering, AND using a bridge circuit 
copied from the Tempo-ONE transceiver that feeds some voltage 
from the BFO into the detector with reversed phase to cancel out 
the unwanted stuff.  Goodman's approach is cleaner in some ways  
-- he does another conversion immediately ahead of a no-gain 
detector so the BFO isn't at the IF.   The price is one more stage 
and one more oscillator.   

If all the gain is at one frequency you have to do the filtering, 
sheilding and all that anyway, or you will never get the IF passband 
right.  The tiniest bit of feedback causes it to be terminally screwed 
up.   

Hmmmm ...

Walt
KJ4KV