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Old 22nd February 2015, 08:53 PM   #1
Cortez is online now Cortez  Hungary
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Default Heatsink vs OS stability

Hi,

I am just finishing a classic class AB amp with a sensitive
triple-darlington OS with 2 pairs of transistors at the end
and it has some oscillation and I found out by chance that
when I touch the heatsink with all of my palm then the
oscillation dissapears completely at all output level...
Just touching it with 1 finger is not enough.

My 1st question: why does this "human touch" solve the oscillation?
What could be the schematic here to model this phenomenon?

2nd: how should I simulate myself in an electric way to solve this? :-)

(Connecting the heatsink directly to the PS GND or to the 230VAC protecting earth just make the oscillation even worse...)

Thx!
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Old 22nd February 2015, 08:59 PM   #2
hahfran is offline hahfran  Germany
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Huh? The heatsink should be connected to PSU ground. Just to avoid oscillation.
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Old 22nd February 2015, 09:00 PM   #3
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Quote:
Originally Posted by Cortez View Post
Hi,

I am just finishing a classic class AB amp with a sensitive
triple-darlington OS with 2 pairs of transistors at the end
and it has some oscillation and I found out by chance that
when I touch the heatsink with all of my palm then the
oscillation dissapears completely at all output level...
Just touching it with 1 finger is not enough.

My 1st question: why does this "human touch" solve the oscillation?
What could be the schematic here to model this phenomenon?

2nd: how should I simulate myself in an electric way to solve this? :-)

(Connecting the heatsink directly to the PS GND or to the 230VAC protecting earth just make the oscillation even worse...)

Thx!
Try a heavy ground wire from the heat sink to the chassis ground. Also make sure the tab on your transistors aren't conducting to the heat sink.

Last edited by jwilhelm; 22nd February 2015 at 09:02 PM.
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Old 22nd February 2015, 09:35 PM   #4
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Your body is like a capacitor.
My class A/B design would oscillate too. I wound up adding a 500pf cap across the network that performs the bias current function. I also know Doug self always puts a small cap 10pf? from collector to base on the negative side transistor of his class A/B complimentary designs. He admitted never figuring out why only that it always seemed to be necessary.
Oscillation is usually indicative of an AC transfer function whereby the gain of the circuit increases at some high frequency. This is because there will always be some phase shift at some high frequency at that point where the transistors begin to drop off in frequency response. Note that in a feedback circuit the phase shift is ideally 180 degrees (totally negative). Your circuit may not have a global feedback by design, however, even things like a 0.1 ohm resistor in the emitter leg of an output transistor is an example of (local) negative feed back. Since there's no such thing as a transistor that has infinite frequency response one has to make sure to roll off the circuit gain well before you hit that frequency point. There's a general rule of thumb that goes like this:
At that frequency in the response of the circuit whereby the phase shift is greater than 40 degrees (away from the ideal 180), the closed loop gain of the circuit at that frequency point needs to be less than one. This is because if we continue to phase shift to say 180 + 40 equals 220 degrees (due to lack of transistor speed) we begin to approach 360 degrees. When this happens your negative feedback becomes positive and guess how one makes an oscilator - with positive feedback.
There will always be some equivalent phase shift where the response starts to roll off. One just has to make sure the entire closed loop gain is less than one at that point.
Easy - add a capacitor to attenuate the gain well before that point. That pont may be 1meg hz for example so who cares if you roll off the gain at 100Khz
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Old 22nd February 2015, 09:40 PM   #5
Cortez is online now Cortez  Hungary
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None of the transistors are conducting I checked it!
Connecting the heatsink directly to the GND / Earth just does not stop the oscillation.

New experiments:
- The oscillation dissapears when I touch the heatsink AND the input wires at the same time.
- If I touch the signal generator's + OUT then the ripple on the square increases...
- The oscillation is the biggest when the input potentiometer is in the middle.
At "0" and at max level there is no oscillation at. The pot is 10k.

I guess this should be something capacitive...?!
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Old 22nd February 2015, 09:41 PM   #6
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Post the OPS design .
While grounding is good , the OPS should still be rock solid
with a floating heatsink.

OS
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Old 22nd February 2015, 09:55 PM   #7
Cortez is online now Cortez  Hungary
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campsquire: thanks for the detailed reply but if it is possible I would like not to change in the OS if the oscillation can be eliminated with just a simple "touch".
It would be great to: 1) understand how my touch works and 2) create a similar solution without me... :-)

OS is simple triple darlington with 2 pairs of transistors.
Grouding: basically star like and the input GND goes to the pot and then further to the AMP and from there to the common PS GND.

New results: oscillation disappears when I touch the HS with my left hand and just "shield" the pot around with my right hand about 3-4 cm (without touching it...)
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Old 22nd February 2015, 10:01 PM   #8
rayma is offline rayma  United States
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Quote:
Originally Posted by Cortez View Post
Connecting the heatsink directly to the GND / Earth just does not stop the oscillation.The oscillation dissapears when I touch the heatsink AND the input wires at the same time.
First, make sure that the heat sink is actually grounded with an Ohmeter.
Connect a 0.1uF/100V low inductance film capacitor to the input socket common, and then going directly to the chassis ground, with short lead lengths.

If this doesn't work, try connecting the capacitor from the input ground to the heat sink (using a sheet metal screw), with the wire only as long as necessary.
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Old 22nd February 2015, 10:13 PM   #9
Cortez is online now Cortez  Hungary
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There is no chassis just the HS right now.
I already tried to connect the input RCA GND to the HS (directly, via 100n, via R) but that doesnt help...
I'll draw a schematic maybe it helps...
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Old 22nd February 2015, 10:17 PM   #10
rayma is offline rayma  United States
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Quote:
Originally Posted by Cortez View Post
There is no chassis just the HS right now.
I already tried to connect the input RCA GND to the HS (directly, via 100n, via R) but that doesnt help.
You likely have more problems than just the oscillation. Build up the circuit properly before any more testing, and rethink the "star" grounding,
which often causes more problems than it solves. Is there a pcb or hf compensation?
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