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Old 6th June 2008, 11:17 AM   #51
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Quote:
Originally posted by Mooly
Did that info arrive by the way ?
Yes it did thank you very much.

I will try what you suggest later this weekend.
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Old 12th June 2008, 09:33 PM   #52
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After fixing the fish tank water pump broke down problem the last few days back in business.

The 10X probe arrived and I used it set at 10X across the speaker output with input shorted. Can not measure anything wrong with all kinds of settings.

The only thing I noticed is that since it is still connected to the mains via VariAC

When the DC rail at the amp board is 25V
Q2a/R15 is at 56mV
Q2 /R13 is at 86mV

Q1a/R16 is at 88mV
Q1 /R14 is at 55mV


When the DC rail at the amp board is 29V
Q2a/R15 is at 14mV
Q2 /R13 is at 107mV

Q1a/R16 is at 81mV
Q1 /R14 is at 41mV

When I connect to the mains directly the DC rails are at 30.5V and the load is really tilt to one side the R13 and R14 is taking the 90 % of the load

Could that be because of the driver transistors causing this effect with higher DC rail supply? Or it is still due to the different charactericstics of the 2N3055?

Any other way to measure and see if the amp is stable? I am trying to find a pre-amp to connect to the CD player or can I just use a 25ohm / 3 watts wirewound potentiometer as volume control instead?

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Old 13th June 2008, 06:59 AM   #53
Mooly is online now Mooly  United Kingdom
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Hello Chris,
Easy one first-- 25 ohm pot is way to low to use as a volume control. 10K to 100K is fine, you need a dual gang pot preferably with a logarithmic track but a linear is OK as well. Try to get 10 or 22 k if you can. Do you know how to wire it up ?
These unbalanced currents. You have to understand the way a transistor operates to visualise whats going on. I will just take the lower pair as an example Q1 and Q1 A. Firstly the characteristics of each device can vary wildly and still be "in spec"
The current gain or hfe may be as low as 15 for one and as high as 50 for another. The base emmiter junction will not pass current until a voltage of around 0.6 to 0.7volt is across it at which point the device will begin to conduct and current will flow from collector to emmiter. How much current depends largely on the gain of the device. Temperature also plays a big part, even if you had matched them, if one were hotter than the other the "gain" and other characteristics vary.
The circuit designer should ensure that these variations do not upset the performance of the circuit and that the circuit will work with all devices, whether they are at the top or bottom of the specification.
So back to your circuit, the volt drop across R9 biases the transistors into conduction. If Q1 and Q1A were identical and R14 and 16 are equal the current MUST be shared equally. In practice this will never occur, so what can be done ? This is where R14 and 16 come in. Imagine Q1 begins to conduct a bit more-- it gets hotter-- this increase the hfe-- so it conducts more still. This higher current will increase the volt drop across R14, so the effective base and emmiter voltage is now slightly less. This reduction will (but in your case it's not enough) cause the current to reduce and hopefully share more equally.
This is why I suggested increasing the value of all the emmiter resistors to say 0.33 or 0.47 ohms. This will tend to force the outputs to equalise the currents more equally as the variations in current will cause more of a change in that all important base emmiter voltage. If you can also match the transistors then that helps as well.
Remember the bjt or junction transistor is NOT a "voltage" driven device but rather a cuurent driven one. This is why a tiny change in base emmiter voltage can cause such a large change in the collector current.
If the amp were unstable you would see it on the scope as an unwanted continuos signal probably in the 600khz to 5 Mhz range.
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Old 13th June 2008, 04:44 PM   #54
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Hello Karl,

Am I being silly to suggest that replace those 4 emitter resistors with trimmer resistors then the current sharing can be tuned ?

Thank you for the quick reply

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Old 13th June 2008, 04:56 PM   #55
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Just a minute, fell off the chair, right O.K. erm what can I say.
Give me minute
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Old 13th June 2008, 05:01 PM   #56
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Presets eh. Well, it's a new one on me. If you can get some 0.5ohms pots well why not. Actually though, that set me thinking, its not so strange an idea. Well a bit maybe. Problem is the current Chris, it's to much. Many years ago TV's had such things in them for convergence adjustment etc, but they were big arkward things. No try increasing them first 0.33 or 0.47 and then perhaps you could ADD a parallel resistor across them on a selected one off basis to trim them--- there you are, so it wasn't a strange idea after all.
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Old 13th June 2008, 05:05 PM   #57
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Have you got any low eg 0.47 0.33 0.22 etc 3 or 5 watt resistors spare. The transistor with the lowest voltage (current flow) of each pair is the one to trim by adding a resistor across it's emmiter resistor.
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Old 13th June 2008, 06:13 PM   #58
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Quote:
Originally posted by Mooly
Have you got any low eg 0.47 0.33 0.22 etc 3 or 5 watt resistors spare. The transistor with the lowest voltage (current flow) of each pair is the one to trim by adding a resistor across it's emmiter resistor.

No I do not have them at hand. I have to go and buy them. Please let me know what values trimmer I do need as well.
Shopping here in Canada is not as easy so I try to make the trip worth it. I will have to drive 40km each way and hope they are not out of stock

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Old 13th June 2008, 06:31 PM   #59
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Hi Chris,
I think Mooly is suggesting that you trim with the 0.47 0.33 0.22 etc 3 or 5 watt resistors instead of using a typical trimmer because of the amount of current they will have to sink.

I think that was the gist of it.

Andrew
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Old 14th June 2008, 06:34 AM   #60
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Spot on Awasson .
I still think it best to increase all four to at least 0.33 ohms first. You will never get it 100%, certainly not if the outputs are not all thermally in contact with one another but I would guess that if you can get it within 25% or so imbalance, thats not so bad.
If you are going shopping especially for these I would suggest you get 4 each of 0.47 and 0.33 and 0.22. Don't forget the volume pot 10 or 22K log dual gang.
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