Germanium diodes with bias. - Videokarma.org TV - Video

catman · #1 01-06-2014, 02:43 PM

G'day all, a circuit observation more than anything. Most will know of my passion for Hi Fi AM and low distortion AM detectors. Running diodes with applied and preferably adjustable bias is a good approach, however I normally use schottky diodes like the 1N5711 and I love their very clean sound when bias is used.

Last night though I tried a experiment with using bias with conventional gernmanium diodes (I used an old GEX34 germanium diode), and the results were interesting.

Germanium diodes almost don't require bias due to their inherently excellent 'sensitivity', but the addition of bias (when correct bias is applied), reduces overall audio distortion and the germanium diodes have a nice 'rounded' sound which is quite pleasant to the ears.

Interestingly enough, even when full bias is applied germanium diodes react a little differently to schottkys. Germaniums produce a more graceful sounding 'thump' as detection stops, whilst schottkys produce a more nasty sounding 'clunk,' if that makes any sense.

Overall, I think that I prefer the softer/easier/more rounded sound of germanium diodes with applied bias. Any comments or thoughts? Regards, Felix (vk4fuq) aka catman.

Username1 · #2 01-06-2014, 04:44 PM

Well that is pretty interesting..... Interesting in that someone would undertake such an experiment.....

So do you force a forward current all the time, or bring it up to just under conduction...?

Since yer into that, have you thought about using high performance operational amplifiers as detectors, which should require no forward current. Kinda like a tube I guess....

What effects do you think the impedance loading effects of your bias circuit has on your audio quality....? Do you attribute all of the audio differences to the diode, or some to the biasing circuitry....?

catman · #3 01-06-2014, 05:04 PM

G'day mate, well the loading effect of the diode does broaden the audio bandwidth slightly which is nice, feeding from a crystal set type of front end, and as to the bias, I just adjust it for the clearest audio quality, nothing too scientific.

Yes I have tried precision rectifiers based on op amps and I found them nothing particularly special, in all honesty. Regards, Felix (vk4fuq) aka catman.

JBL GUY · #4 01-06-2014, 06:30 PM

Greetings Felix

As always, interesting information.

Username1 · #5 01-06-2014, 06:57 PM

Ah... I didn't think about what type of front end you might be using..... I thought you were intercepting the signal off the last stage of a regular 455 radio, and sending it through a different detector, and then back into the same radio's audio stage.... Just looking at the effects of a different detector on an already assembled full blown radio....

When I was in school we had a crystal set diving a small tube amp and a giant old speaker, WOW! That was the best sounding am radio I ever heard.... Possibly because it had such a wide audio bandwidth on the front end, now that I think of it....

I bet your crystal set, driving a good quality audio amp, has quite the good sound quality.... Inherent in crystal sets...

Electronic M · #6 01-06-2014, 10:13 PM

One thing I've found cool about crystal sets is how some folks will build two crystal sets and use one tuned to a local high power station as a power supply for an amplification stage hooked to another set so as to increase the sensitivity of the other one.

catman · #7 01-07-2014, 04:14 AM

G'day all, an interesting one this afternoon, I was listening to my crystal set and biased diode detector feeding my studio amp and speakers when things suddenly sounded rather distorted.

The reason: My (occasionally) poor soldering caused the ground connection of my 100pf RF bypass capacitor to come adrift and my audio preamp reacted badly to the unfiltered RF being fed into the preamp, causing audio distortion. Regards, Felix (vk4fuq) aka catman.

wa2ise · #8 03-07-2014, 03:18 PM

Quote:

Originally Posted by catman

...

Last night though I tried a experiment with using bias with conventional gernmanium diodes (I used an old GEX34 germanium diode), and the results were interesting.

Germanium diodes almost don't require bias due to their inherently excellent 'sensitivity', but the addition of bias (when correct bias is applied), reduces overall audio distortion and the germanium diodes have a nice 'rounded' sound which is quite pleasant to the ears.

...

I did something similar on an AM/FM/police band radio.

I had to make the biasing switchable, as the radio used the AM detector circuit as a bias circuit for an FM IF amp transistor. I adjusted my R1 to counteract the bias circuit and to get the diode in a nice detection range for weak stations.

Username1 · #9 03-07-2014, 06:35 PM

Like those PNP's do ya-.....

I remember in tv school when we went from tubes to transistors, most of us didn't like them... And lots hated the thought of PNP's.... You know because of the polarity...

Wa2ise; This is more of the type of biasing I was imagining when first thinking about what catman was doing....

Is this from an existing radio, or did you make it from scratch..?

Now that this is a topic, I wonder if one of you guys will do an experiment like this on a store bought radio, and compare sound of the original design, with your mods.....?

Well, good luck, and keep us posted....