Germanium and silicon transistor output stage audio amplifier
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- čas přidán 10. 09. 2024
- This "mixed breed" amplifier was used in a Magnavox console stereo in the late 60's when there were PNP power germanium and NPN power silicon transistors at reasonable cost. I thought it would be interesting to investigate this amplifier circuit and replicate it on the breadboard.
i may have to try this myself one day, this is the first time I've seen silicon and germanium transistors in the same circuit together.
Good info. It was a lot harder for me to figure out amp problems in the 70's
All good info John. I use Pink Floyd for "earoscope" testing, mainly because I've been listening to it for so long (and heard it on many high end systems) I can pick out anything that doesn't sound quite right. Of course it also offers a wildly dynamic range and, in a single track, can offer up everything you need to set up a system.
This topology was VERY common. Heathkit used it in all of their early 30 watt class amplifiers and receivers back in the 60's and early 70's. 2N301 and 2N3055 type transistors were common. Once the MJ2955 Si PNP transistor became available at low cost, the 2N301 was no longer used.
Thanks for the information. I was wondering how widely this was done.
Quite a common output stage around this time were a pair of matched AD161 and AD162 Germanium power transistors.
They had very good gain of around 100,so were easy to drive.
You may be interested to check out the Sinclair Z12 which was a hobby amp using this configuration.
I did own one,and remember it sounded good.
Another great video John, keep them coming.
Probably a product of component procurement problems during a period of silicon taking over from germanium. Getting PNP in silicon was a problem until they solved the doping difficulties. So to keep product production going they mixed the types.
Ty for sharing ✌️😁
Interesting...cheers.
I have the exact same circuit on my magnavox stereo with a leaky germanium PNP. I bought some NKE 219 silicon matched complimentary pair to replace it. How would I adjust the resistors to properly bias the new transistors?
excelente amigo
equivalent ad161 ad162 ?????
Are the transistors matched yes but actually no.
Even though it is from 60's the amp Sounds good to me.
i am Glad there getting along
I think the 250 uF cap on the 1st stage is supplying, below unity gain, +ve feedback to the input, so increasing its input impedance? Be nice to know by how much? Interesting video.
I have an astro sonic from 1965 but can't find a schematic as i'd like to replace the driver transistors
Wondering about the driver and predriver. 94N1 is Si, 99P1 is Ge ?? Is that diode Si? Put a couple of those diodes aside for me if you have them, or let me know where you got it, as that style is used in the .01Tiger and I'd like to find a source.
Those are both Si transistors so I used "modern" ones. The diode is a single Si diode (.6 v forward drop). They are RCA 0135 manufactured in 1968. You are welcome to them (two) if they are what you need. I can send them to you with some green LEDs for your stake light mod or bring them the next time I come up (or you come down here).
No immediate need, but haven't been able to find them.
I hjave an astro sonic recei9ver from 1965, would that have all germanium transistors?
I'm no expert on these old receivers, but given the year, it might be all germanium.
Something pink Floyd..... Yo john yo..
Hey, John. I have an untested design (it should work fine, tho), a simple one. just 3 small signal transistor, 3 medium power transistor, and 1 or 2 pair power transistor. would you like to test it out? it's a free design
John, I have a question for you. How do we stop a amplifier from clipping when the pot is all the way up?
Add a resistor of suitable value to the input of the POT. It is best you don't crank-up the pot all the way up just stop before you start hear the distortion.
add an L-pad on the input. but isn't it easier to just turn down the pot?
Yes of course, but I have people cranking the stereo to 11 and blowing my speakers. Friends suck when they crank a speaker until it blows then tell you they only turned it up a notch. It sucks when you can't trust friends while you're gone for 10 minutes and they don't even offer to even help with repairs. I'm building a Class A~B I'm not that worried with that one because it's not 10% THD it's next to no THD @ 400watts Nice 2.1 Boom Box. Nobody is allowed in the house when I go to the store anymore.
When it clips in that situation: use a voltage divider before the pot or after the pot and align it all to maximum undistorted level with the audio control pot fully up. Example: when you precede the audio control pot with a 2 x 50 K resistor (or 2 x 10 K or 2 x 100 K) and connect the node from that voltage divider to the top of the audio control pot, your signal input is half of its original level. When you use a pot from 100 K or 50 K or 10 K that precedes your audio level potmeter you can find this out in a few minutes. Take that pot out when all works fine, measure its resistances (wiper = reference) and take these resistances to choose fixed value resistors, standard values close to the measured resistances.
radiofun232+ Thanks, I was thinking about taking the pot apart and put a spot of epoxy on the ferrite so the wiper couldn't go any further. Just a thought
So what's about silicon-germanium transistor pair? Didn't get it. Looks like it's good idea, but...
It was all about the economics of the time, and the semiconductor industry's transition to a mostly NPN Si topology from the PNP Ge topology that had dominated the early days of the transistor. NPN Ge and PNP Si transistors were expensive and generally inferior to their compliments.
Was there a date on this schematic? 1960s?
There are 1968 date codes on several parts in the original chassis. Probably designed in 1967.
The first transistors that were produced in the late 1950's and 1960's were almost always Germanium transistors. They had a low barrier voltage (say 0,3 V) and were (very) sensitive on temperature changes. All kinds of measures had to be taken to prevent the so called "thermal runaway", heat collected inside the transistor, the collector-emitter current got higher and higher, finally self destruction due to heat. Silicon transistors (they came later in history: I mean: available for the industry and public, not in the laboratories) did not suffer from these problems. That is the reason why Germanium transistors are now more or less history. Anyway: this combination from 2 transistors with different parameters is quite unique, it shows that they can be combined into a successful audio amplifier. These different properties also show that it is a challenge to combine them succesfull in a power-circuit. In low power applications (milliwatts) this is not a big problem.
Don't get me wrong. I did assemble similar amplifier myself some years ago. Just assembled without much understanding of what's going on inside. It is interesting. What I missed from the video are these small explanations about differences between germanium and silicone transistor. Explanations in a classic John's style.
Such Germanium BJTs are now obsolete though. Please try to do one day an amp using 2sc5200 and 2sa1943.
He DID make an amp with those. Like here: czcams.com/video/TvqNxLJoRQ0/video.html
You can use whatever, tip41&42, tip35&36, 3055&2955; it'll just require some tweaking.
germanium transistors, do they still exist....
NOS, i guess?
S. Kojina
Some secretly making it .
Indian BEL make it .
Indian amplifier still use them in input stages for public class amplifier
Mainly companies like Stranger and Ahuja
ah, nice to know!. but why? aren't they're a lot noisier and low in current gain bandwidth?
oh wait.. i was thinking public class = high power amplifier. i see no problem for something like public announcement amplifier
S. Kojina
Acoustic effect is much better .
They aren't noisy . They sounds better than si on proper circuits.
Those Indian Ge transistor are highly optimized
You can say modern ge transistor with older AC and af name
Quantos watts RMS amigo??
About 9 watts.
william guillen obrigado excelent amplifier .
Can't be. Some one put the wrong thing together