I know that my question has already been addressed in post #5566 but with so many posts in a single subject it becomes difficult to find the right information...
I have turned the Bourns 3296 pot clockwise (with pins 1 and 2 shorted) to get 0 ohms but the minimum value I get is 0.4ohm.
This may due to the fact that I have not soldered the pins 1&2 (I just used a poor crocodile tool, so there might be an additional resistance) but I would like to know what is the average value for a proper bias setting.
I know that you have to start with 0 ohms and increase progressively, but if the value is far higher than 0.4ohm, then there is no risk for me to solder the Bourns in this position to start the bias adjustment.
Thanks !
I have turned the Bourns 3296 pot clockwise (with pins 1 and 2 shorted) to get 0 ohms but the minimum value I get is 0.4ohm.
This may due to the fact that I have not soldered the pins 1&2 (I just used a poor crocodile tool, so there might be an additional resistance) but I would like to know what is the average value for a proper bias setting.
I know that you have to start with 0 ohms and increase progressively, but if the value is far higher than 0.4ohm, then there is no risk for me to solder the Bourns in this position to start the bias adjustment.
Thanks !
There is no 'average' setting because the bias is dependent on the functions of the driver transistors and the output transistors.
Set the pots to the min value.
Solder them in place.
Use 3 meters to set the bias - borrow some or buy cheap meters, but 3 makes the process simple.
Set one on the output, one across R11 and the other across R12
Start turning the post up a turn or two at a time until you see readings across the resistors. Soon, you will find that one of the pots appears to control the bias, and the other pot appears to control the offset.
Turn the pots until you have approx .55 volt across the resistors with 0 offset.
Let the amp sit this way for 1/2 hour to get warmed up, then adjust the bias to .6v and 0 offset
Put the top cover on the amp and let sit for another 1/2 hour. Check the bias and offset one last time, and make final adjustments
Set the pots to the min value.
Solder them in place.
Use 3 meters to set the bias - borrow some or buy cheap meters, but 3 makes the process simple.
Set one on the output, one across R11 and the other across R12
Start turning the post up a turn or two at a time until you see readings across the resistors. Soon, you will find that one of the pots appears to control the bias, and the other pot appears to control the offset.
Turn the pots until you have approx .55 volt across the resistors with 0 offset.
Let the amp sit this way for 1/2 hour to get warmed up, then adjust the bias to .6v and 0 offset
Put the top cover on the amp and let sit for another 1/2 hour. Check the bias and offset one last time, and make final adjustments
1. You probably haven't zeroed out your ohm meter. Your ohm meter likely reads 0.2 - 0.4R when you just touch the meter leads together. (I'm assuming you're not using the 4-wire technique for measuring resistance.)
2. 0.4R is close enough to zero.
3. When you zero the pot, it's not the reading across the pot's wiper and one leg you need to zero. It's the reading across R4 and R3 which you want to see zero (or very close to). You see, you may zero the pot as measured by the wiper and one leg, but that then assumes a particular orientation of the pot in the circuit. Install it the other orientation and you've not achieved 0R across the associated resistor, R3 or R4. So, instead install the two pots and R3 and R4 into the circuit, then adjust the pots so that the resistance across R3 and R4 is zero (or close to). Also, note which way you will eventually turn the pots to later increase the resistance.
Dan
Hello,
I know that the subject of speaker protection was raised, but i have some questions as I'm considering a relay based dc-protection circuit.
1. What are the chances of getting dc on the outputs?
2. Would relay contacts degrade the sound?
I have expensive and discontinued speakers and don't want to take chances with them.
Thanks
I know that the subject of speaker protection was raised, but i have some questions as I'm considering a relay based dc-protection circuit.
1. What are the chances of getting dc on the outputs?
2. Would relay contacts degrade the sound?
I have expensive and discontinued speakers and don't want to take chances with them.
Thanks
Hi AndrewT,
Yes, to be more precisely, i am running two channels at 1,25A Bias from one single transformer.
I know that is undersized, but like i said, is not a final situation.
I take temperature after 4 hours running and external temperature is 42ºC with 22ºC ambient temperature in my room.
Because it is a prototype assembly, Tranformer is out of the box, and the central hole is open.
I think is not bad. According to RS, temperature rise is 60ºC. 0225P1-2-018 Nuvotem | Toroidal transformer,225VA 2x18V o/p | 223-8162 | Welcome to RS Online
Like you said, I do not know what is the temperature inside.
An externally hosted image should be here but it was not working when we last tested it.
Can you explain how do you calculate the 83% ?
From my calculations, i get 41.5% of 225VA.
Maybe I'm doing wrong calculating
Novotem Talema 225VA data sheet http://www.talema.net/en/products/pdf/0225P1_1001.pdf
I have the second transformer ready to run
An externally hosted image should be here but it was not working when we last tested it.
Tanks AndrewT
Regards
VA = Vac * Iac when driving a resistor to maximum power rating.
When you attach a capacitor input filter, the manufacturer tells us that you must apply a de-rating factor. Somewhere around 0.7 for DF is about the usual. This takes account of the extra heating effect of I^2 R when the current is not sinusoidal nor smooth DC.
The crest factor ofr peak current to average current is at least 5 and possibly as high as 20, when very good capacitors are used for smoothing.
The total maximum continuous output power of the transformer must be ~70% of the VA rating when feeding a capacitor input filter.
Rectify and smooth the output from your transformer.
The maximum continuous DC current must be ~ 50% of the Iac rating.
The total maximum power drawn from the transformer is ~0.7 * Vac * Iac = Vdc * Idc
re-arrange the equation to find maximum continuous DC output current and you get:
Idc = 0.7 * Vac * Iac / Vdc = 0.7 * Vac * Iac / sqrt920 / Vac (using Vac here takes account of the power absorbed by the rectifier. ClassA bridge rectifier need to be cooled).
Idc max = 0.7/sqrt(2) * Iac ~= 50% of Iac rating.
BTW,
I have an almost identical 225VA (25+25Vac) RS transformer. I use it to power a chipamp.
When you attach a capacitor input filter, the manufacturer tells us that you must apply a de-rating factor. Somewhere around 0.7 for DF is about the usual. This takes account of the extra heating effect of I^2 R when the current is not sinusoidal nor smooth DC.
The crest factor ofr peak current to average current is at least 5 and possibly as high as 20, when very good capacitors are used for smoothing.
The total maximum continuous output power of the transformer must be ~70% of the VA rating when feeding a capacitor input filter.
Rectify and smooth the output from your transformer.
The maximum continuous DC current must be ~ 50% of the Iac rating.
The total maximum power drawn from the transformer is ~0.7 * Vac * Iac = Vdc * Idc
re-arrange the equation to find maximum continuous DC output current and you get:
Idc = 0.7 * Vac * Iac / Vdc = 0.7 * Vac * Iac / sqrt920 / Vac (using Vac here takes account of the power absorbed by the rectifier. ClassA bridge rectifier need to be cooled).
Idc max = 0.7/sqrt(2) * Iac ~= 50% of Iac rating.
BTW,
I have an almost identical 225VA (25+25Vac) RS transformer. I use it to power a chipamp.
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The design of the F5 is elegant, the parts count is low, and the performance is superb...why would you have expected anything less?
