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Old 18th September 2012, 09:56 PM   #1
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Default Trying to understand vintage amp design

This is a 1968 vintage Grundig receiver.
I am trying to understand how the amp part works.
The 1st picture is the original diagram, the 2nd is a simplified drawing by myself.
At first glance it looks like a simple standard complementary power output stage.
But what puzzles me is the way they handle signal ground.
I know that it is completely arbitrary where to define ground, but this particular point between the speaker and the coupling cap I have never seen being used before.
Question #1: What could have been the motivation ?

The original used Germanium transistors AD161/162 which are either unavailable these days, or if so, no longer reliable. These cans were often filled with silicone grease which causes whiskers to grow internally over time and develop shorts to the case. So I don't want to use them for replacement.
Question #2: What if I replaced them with modern Silicon transistors ? Can this be done at all ? And if so what needs to be modified at a minimum ? I guess at least the bias setting components around the thermistor at the base ... anything else to consider ?
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Old 19th September 2012, 12:54 AM   #2
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Yes, change the 25R pots (R107 & R108 I think) to 100R @ 3-4 times the wattage rating of the originals.

Or add 40R in series with them...see if they still adjust in their mid range.

If they need more or less range one way or another change the 40R to 30R or 50R as required.
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Old 19th September 2012, 02:10 AM   #3
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Quote:
Originally Posted by payloadde View Post
Question #1: What could have been the motivation ?
Your sketch resembles the common emitter output championed by Ed Cherry.
See his article in Wireless World July 1997 for detail of the claimed benefits.
It never became popular despite his support.

Best wishes
David

Last edited by Dave Zan; 19th September 2012 at 02:15 AM.
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Old 19th September 2012, 04:51 PM   #4
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If my memory serves me properly the circuit should have the speaker negative connected to the junction of two electrolytics connected between the Voltage rail and ground, the positive end of the speaker is directly connected to the mid point.

I think this was done to prevent turn on thumps.
rcw
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Old 19th September 2012, 07:25 PM   #5
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Quote:
Originally Posted by DUG View Post
Yes, change the 25R pots (R107 & R108 I think) to 100R @ 3-4 times the wattage rating of the originals.

Or add 40R in series with them...see if they still adjust in their mid range.

If they need more or less range one way or another change the 40R to 30R or 50R as required.
R107 /108 are actually thermistors to stabilize germanium transistors thermally.
So replacing them w/ 100R pots and adjust to proper standing current is what you propose.
Or maybe better leave the thermistors and put the pots in series to keep at least some protection against thermal runaway. Is silicon better when it comes to this ?
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