Building a simple preamp from scrap parts.
Noticed the input capacitor is 47nf non-polar. was going to use an orange ceramic 473J cap.
But, i thought, the point of input, and output caps, is to block the DC current from going to the output device, or input source?
Should i use a polar cap of same value?
i don't want 12V dc going through my guitar circuit, doesn't seem, both for me, and the guitar.
Noticed the input capacitor is 47nf non-polar. was going to use an orange ceramic 473J cap.
But, i thought, the point of input, and output caps, is to block the DC current from going to the output device, or input source?
Should i use a polar cap of same value?
i don't want 12V dc going through my guitar circuit, doesn't seem, both for me, and the guitar.
Hello,
First -47 nano farads is way too small for that position. Most likely 47 micro farads.
Second - Bipolar caps give less distortion than polar ones when flowing the signal thru them. If polar ones are large enough not likely to matter. 100 uf would be plenty. Better yet 100 uf bipolar.
Distortion also goes down some more when using electrolytic caps with a higher voltage rating. So, if 15 volts rating is adequate, 25 volt rating helps a little bit.
A 47uf bipolar cap would be plenty good enough.
Regards,
Greg
First -47 nano farads is way too small for that position. Most likely 47 micro farads.
Second - Bipolar caps give less distortion than polar ones when flowing the signal thru them. If polar ones are large enough not likely to matter. 100 uf would be plenty. Better yet 100 uf bipolar.
Distortion also goes down some more when using electrolytic caps with a higher voltage rating. So, if 15 volts rating is adequate, 25 volt rating helps a little bit.
A 47uf bipolar cap would be plenty good enough.
Regards,
Greg
The difference between a polar cap and non polar cap is a non polar cap can receive DC from any direction. A polar cap will fail or explode if you applied DC in the wrong direction. Non polar is always better but they are generally much more expensive and much bigger in size for same value. I have a 470uF polproplene cap that is about the size of a coke can. 470uf polar I smaller than my thumb.
Oon
Oon
There are much better design of discrete two transistor preamp.
On this circuit I don't know how to set the gain.
Opamp Alternatives
2.0 - Two Transistor Feedback Pair
This one will work better, and every component is explained.
On this circuit I don't know how to set the gain.
Opamp Alternatives
2.0 - Two Transistor Feedback Pair
This one will work better, and every component is explained.
> i have 47uf bipolar, can throw that in
I need 2 pounds of sugar for a big cake. I have a ton (20000lbs) of sugar, can I throw that in?
47u is 1,000X bigger than 47n.
I agree that "about 50nFd" is a good size. As Oon says, the 150k is bootstrapped. There's also 4k7 * hFE. The impedance looking toward the Base is hundreds of kOhms.
I need 2 pounds of sugar for a big cake. I have a ton (20000lbs) of sugar, can I throw that in?
47u is 1,000X bigger than 47n.
I agree that "about 50nFd" is a good size. As Oon says, the 150k is bootstrapped. There's also 4k7 * hFE. The impedance looking toward the Base is hundreds of kOhms.
That transistor stage has about 1M input impedance; definitely not lower than 470K (assuming transistor Hfe around 100) and considering that, even 47nF is more than large enough.
A Guitar preamp might use a 10X smaller one (4.7nF) , go figure.
Much better designs?
Sure.
But this one is good enough and may be quite didactic building (and then analyzing
) a classic bipolar transistor preamp such as this one.
Don´t be tempted to increase gain: 4X is about as much as it will stand a hot pickup with meager 12V supply.
But as is, it´s an interesting experiment.
Enjoy
A Guitar preamp might use a 10X smaller one (4.7nF) , go figure.
Much better designs?
Sure.
But this one is good enough and may be quite didactic building (and then analyzing
) a classic bipolar transistor preamp such as this one.Don´t be tempted to increase gain: 4X is about as much as it will stand a hot pickup with meager 12V supply.
But as is, it´s an interesting experiment.
Enjoy
All capacitors block DC.
Polar caps are less versatile as they mustn't be reverse charged.
Polar caps are only used because polar electrolytics are the most economic for large values of capacitance. If you need a capacitor to handle charging both ways you either use a non-electrolytic, a non-polar electrolytic (or equivalently two electrolytics back-to-back).
For small values of capacitance like 47nF there's never any need for electrolytics, and they aren't made.
Ferroelectric is not another name for piezoelectric, although some ceramics have both properties.
Some ceramic caps can act like a crystal microphone - quite different from capacitor microphone. Then there is the electret microphone, which is a type of capacitor microphone with a built-in bias.
Some ceramic caps can act like a crystal microphone - quite different from capacitor microphone. Then there is the electret microphone, which is a type of capacitor microphone with a built-in bias.
Whoops, yes, all ferroelectrics are strongly piezoelectric, but piezoelectric materials are not all ferroelectric (quartz being a good example). Many ferroelectrics are based on the barium titanate family.
Ok going slightly off original topic (preamp) I have a few questions re DC blocking on power amplifier. Specifically the Rotel series 980 etc .
The main cap is a 4.7uf. electrolytic with a 33.3K resistor accross.on the official spec sheet.
I noticed on one of the recommended mods to improve sonic performance that a PP of 3.3 to 4.0 would be a better bet. I think 3.9 is being used now. Historically I found reference to being able to use lower values for PP that electrolytics. Plus an explanation Ive forgotten.
Is their also an argument for altering the 33k resistor upwards to maintain the same results?....
Personally I have always removed this blocking caps on most of the many amplifiers for over 55 years to get improved sonics. .
Second question relates to Humble HIFI tests capacitors. This seems to be totally for crossover use? Do such opinions count for this DC blocking use?...
The main cap is a 4.7uf. electrolytic with a 33.3K resistor accross.on the official spec sheet.
I noticed on one of the recommended mods to improve sonic performance that a PP of 3.3 to 4.0 would be a better bet. I think 3.9 is being used now. Historically I found reference to being able to use lower values for PP that electrolytics. Plus an explanation Ive forgotten.
Is their also an argument for altering the 33k resistor upwards to maintain the same results?....
Personally I have always removed this blocking caps on most of the many amplifiers for over 55 years to get improved sonics. .
Second question relates to Humble HIFI tests capacitors. This seems to be totally for crossover use? Do such opinions count for this DC blocking use?...
As a blocking capacitor the key issue is the AC voltage across the capacitor (not the DC offset). Often the AC voltage across is tiny as the impedance of the cap is low compared to the loading impedance.
If this is the case the distortion the cap can produce is very small, even if electrolytic, because the bulk of the signal voltage is across the load, not the coupling cap.
Increasing the value of the capacitor reduces its impedance and thus further reduces any distortions. Often this is more economic than replacing a large value electrolytic with a film cap.
If this is the case the distortion the cap can produce is very small, even if electrolytic, because the bulk of the signal voltage is across the load, not the coupling cap.
Increasing the value of the capacitor reduces its impedance and thus further reduces any distortions. Often this is more economic than replacing a large value electrolytic with a film cap.
An electrolytic coupling cap will often be oversized in order to make distortion negligible. This is fine because electrolytics are easily available in large values yet small physically.
Swapping it for a film cap often means that you don't need such a large value. This is good because film caps can be physically large and so pick up hum and interference. You need to decide what LF rolloff is needed at this point in the circuit.
Coupling caps are there for a reason. Removing them is unlikely to improve the sound, but it may make the equipment less reliable.
Swapping it for a film cap often means that you don't need such a large value. This is good because film caps can be physically large and so pick up hum and interference. You need to decide what LF rolloff is needed at this point in the circuit.
Coupling caps are there for a reason. Removing them is unlikely to improve the sound, but it may make the equipment less reliable.
For commercial units they need to have coupling caps at the input because they can never be sure of the source before it. If it is faulty with DC content etc. However if you are confident that the previous stage already has one. Then it would be okay.
Another reason is if your preamp is single supply, so there is a high positive bias. In which case the cap in the previous stage might be biased the wrong way and can lead to damage.
Oon
Another reason is if your preamp is single supply, so there is a high positive bias. In which case the cap in the previous stage might be biased the wrong way and can lead to damage.
Oon
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