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Old 14th June 2017, 04:28 PM   #1
DIYBras is offline DIYBras  Brazil
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Default A little study about two old germanium projects

Hello friends of DIYAudio,

In last times I am making a lot of measurements, and someimes I like to dissect some projects, like this I show here: one Germanium amplifier, and a Germanium preamplifier. Both more or less "classics". The amplifier in one using AD161/AD162 output transistors, and the preamp is like one published in Discrete design: 2-transistor RIAA preamp (the Linsdale MM, with some minor alterations).
In next posts I will show the results (and I need to draw the schematics ) normally I build things WITHOUT schematics
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Old 14th June 2017, 04:36 PM   #2
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The MM Dinsdale is modified to flat response.
Folows the result for OLG in 2 different supplies, and OLG vs CLG.
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File Type: png PGG 6.png (50.8 KB, 233 views)
File Type: png PGG 5.png (51.3 KB, 232 views)
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Old 14th June 2017, 04:38 PM   #3
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Results for maximum-minimum gain structure from test circuit (min-max pot settings), and resulting output noise
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File Type: png PGG 2.png (44.6 KB, 219 views)
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Old 14th June 2017, 04:42 PM   #4
DIYBras is offline DIYBras  Brazil
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FF result for 500mV output with 5k load, and HD sweep for almost same level and same load, 20 to 30000Hz, showing individual harmonics (but 6H+ is summed)

I don't have this pre anymore; this are old measurements from last year (I've made this pre for a guitar player).
The schematic is at home, but tomorrow I post it.

EDIT: I used japanese Ge transistors instead of OC originals.
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File Type: png PGG 1.png (49.0 KB, 20 views)
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Old 14th June 2017, 05:08 PM   #5
DIYBras is offline DIYBras  Brazil
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Default Now time for power amplifier...

...for it I have draw the schema now!
Is a conventional amplifier for AD162/2 usage, but I used Ge even for input (a low noise 2SB486), and bias differ from originals. The missing values for input bias I don't remember now but is only to match the quiescent output voltage.

Some results for (please see the figure notes, at lower left): measured open loop vs actual use closed loop, FFT for open loop condition, HD sweep for open loop condition, FFT for low output bias (~<20mA) and high bias (approx. 200mA).

First time I have built this amplifier is in 1996, when I found some Telefunken AD161/162 from a old car radio, and tried a classic circuit model, with success (in mono). Since then, I have made some betterments (from simulations, 2008; from measurements, in 2009), stereo version (2012) and a cabinet for it. I have fused the original Telefunken's in 2011 . Is far from being my better or preferred amp, but is like a timeline for me, since I have it from my DIY beginings (1996)... In fact, is the second amplifier I've made in my first DIY year.
Attached Images
File Type: jpg 20170614_135150.jpg (190.0 KB, 136 views)
File Type: png GeAD9.png (26.9 KB, 124 views)
File Type: png GeAD 19.png (51.4 KB, 27 views)
File Type: png GeAD 99.png (47.8 KB, 23 views)
File Type: png GeAD 9.png (49.9 KB, 21 views)
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Old 14th June 2017, 05:22 PM   #6
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Result for 1W/8R/20Vcc/250mAiq for this amp.
Is a fair result for a such simple amplifier, with early transistors, all alloy type (low Ft).
I like the ground connection for the "second stage" ("VAS"), is less prone to suplly ripple, but... maybe the bootstrap have less PSRR than the stage gain, if stage is the "classic" Vcc based? In 2009 I simulated this and preferred the VAS in ground instead Vcc.
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File Type: png GeAD 1W 8R canal 1.png (49.6 KB, 22 views)
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Old 14th June 2017, 10:50 PM   #7
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I have a portable radio that uses GE transistors that belonged to my dad from the 1960's. I remember listening to it, & it still works & sounds nice as it did. Not HiFi, but a nice sound & has bass too None of that awful tinny sound many had !

All the best with your adventures
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Old 15th June 2017, 02:31 PM   #8
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Default What low noise transistor?!

Here is the preamp schema.
Contrary to "common sense" when one don't have full knowledge about electronics like me, is that the lowest noise transistor need to be employed at input. Wrong. The second transistor/stage have higher gain in this design than the input, so I used the low noise 2SB486 in second stage, and more modest noise figure 2SB187 in input! Result: at least 10dB less noise than if one use the lowest .noise transistor in input and higher noise at second stage (output)...
This is measured in practice. What is going here in theory? Is about the stage gain I mentioned? Or stage impedance?
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Old 15th June 2017, 02:46 PM   #9
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In 2011, I add a knowed trick in this amp output: a output current feedback, to lowering damping factor. With moderate Zi (like 2R), allied with the "finite" distortion, sounds like some tube amplifiers, but with irrestrict bass. (note the 0R12 and two 4R7 resistors network from load to feedback negative input)
This Ge amplifier here really "don't have the right to sound good as it sounds in reallity", ie. is a good sounding one.
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File Type: jpg 20170615_111927.jpg (152.8 KB, 82 views)
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Old 15th June 2017, 02:50 PM   #10
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Quote:
Originally Posted by DIYBras View Post
Here is the preamp schema.
Contrary to "common sense" when one don't have full knowledge about electronics like me, is that the lowest noise transistor need to be employed at input. Wrong. The second transistor/stage have higher gain in this design than the input, so I used the low noise 2SB486 in second stage, and more modest noise figure 2SB187 in input! Result: at least 10dB less noise than if one use the lowest .noise transistor in input and higher noise at second stage (output)...
This is measured in practice. What is going here in theory? Is about the stage gain I mentioned? Or stage impedance?
The corrected schema, sorry...
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File Type: jpg Ge 01.jpg (58.4 KB, 71 views)
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