[Drake] Drake 2B audio improvements

[K6JEK]

With the encouragement of Kees, K5BCQ, I tried a few simple 
modifications to the Drake 2B audio and carefully measured the before 
and after.  Summary:  paralleling the caps coupling the detectors to 
the first audio stage, C54 and C58, with .01 mylar caps made a big 
difference in low end response and reduced distortion.

I don't claim these are optimum values, they are just ones I tried and 
measured with my HP-339A distortion measurement set.    Replacing C54 
and C58 should work too.  I just paralleled them for the ease of 
experimentation.  Changing the cap across the output transformer, C71, 
to a .001 did little if anything.  Paralleling the coupling cap from 
the first audio stage to the second, C53, with a .01 further improved 
response below 100 Hz but not dramatically like the other changes.

Here's the data:

Drake 2-B Audio Measurements				K6JEK, 8/21/2005

Audio in at audio pot wiper .17V.   Audio out at spkr terminals, 4 ohm 
non-inductive load
All measurements with HP-339A Distortion Measurement Set				

		Original 			Add .01 across C54, C58	
	
Hz		V out	THD %	V out 	THD %
			
40		0.05		*1		0.34		*1			
50		0.09		"		0.48		"			
60		0.14		"		0.60		"			
70		0.22		"		0.68		"			
80		0.25		"		0.75		"			
90		0.39		"		0.80		"			
100		0.47		18.0		0.84		3.0			
110		0.60		16.0		0.86		2.9			
120		0.65		14.0		0.88		2.8			
130		0.70		12.5		0.89		2.8			
140		0.72		11.0		0.90		2.6			
150		0.75		10.0		0.90		2.5			
160		0.78		9.5		0.91		2.4			
170		0.80		8.7		0.92		2.3			
180		0.81		8.0		0.92		2.2			
190		0.82		7.5		0.92		2.1			
200		0.83		7.0		0.92		2.1			
300		0.87		4.0		0.93		1.6			
400		0.88		3.0		0.94		1.3			
500		0.88		2.4		0.94		1.1			
600		0.88		2.0		0.94		1.0			
700		0.88		1.7		0.94		1.0			
800		0.88		1.5		0.95		0.9			
900		0.89		1.4		0.95		0.9			
1000	0.89		0.9		0.95		0.9			
2000	0.88		0.8		0.93		0.8			
3000	0.86		0.7		0.91		0.7			
4000	0.84		0.6		0.88		0.7			
5000	0.81		0.6		0.85		0.7			
6000	0.79		0.6		0.81		0.7			
7000	0.76		0.6		0.78		0.8			
8000	0.74		0.5		0.75		0.8			
9000	0.71		0.5		0.72		0.8			
10000	0.69		0.6		0.70		0.9			
20000	0.54		0.5		0.55		0.9			
30000	0.30		0.5		0.31		1.0			
40000	0.12		0.9		0.12		2.0			

*1 HP 339A distortion measurements flakey  below 100 Hz