Hello,
I was trying to post this topic in the "Circuit Block" section but I didn't find the "new topic" button there...so I'm writing here.
I'm asking to everyone who knows how to help me understanding a particular case, which I've seen it's quite common in many, many preamp designs.
It's been a long time that I started to try to figure out how to calculate the actual bright boost involved when there is a capacitor with a resistor across it, and a potentiometer (ie a "gain", or "preamp volume" control...) to ground just after it.
I was reading this page, which is about the first stage of a Soldano SLO preamp:
http://www.ampbooks.com/mobile/classic- ... -preamp-1/
.......if you look at the schematic, there's a 0.002 uF cap with a 470k resistor across it, and a 500k to ground potentiometer after those.
If you scroll the page, in the "Bright boost" section, it explains this "bright boost" which should consist of about 6dB of bass cut at 82Hz.
BUT, if I enter the values in this calculator: http://www.ampbooks.com/mobile/amplifie ... alculator/
with 49k output impedance of the previous stage, 2000 pf (the bypass capacitor), 470k (the resistor), and 0 percent (the volume setting), which should be the correct value when a capacitor bypasses the entire value of the resistor, I obtain a MUCH bigger bright boost.
So, I wondered if the 500k potentiometer limits somehow the bright boost which results from this calculator.
If so, could you explain this, please? And how could I calculate the correct bright boost involved?
Thank you VERY much in advance, and sorry for my English...!!
Help understanding bright boost circuit just before a pot
-
phatt
- Transistor Tuner
It won't do what you ask because you are asking for a situation where there is a fixed series resistor with cap bypass.
The calculator is giving the answer for a cap across the Volume Pot Not across a fixed resistor.
Calculator gives amount of treble boost increase as the volume is reduced. Less volume = more treble.
Phil.
The calculator is giving the answer for a cap across the Volume Pot Not across a fixed resistor.
Calculator gives amount of treble boost increase as the volume is reduced. Less volume = more treble.
Phil.
Thank you Phil!
I thought that a fixed resistor like that was the same than using a volume pot with 0 percent of setting, so that the cap bypasses the entire value of the volume pot, and so that it gives the full on bright boost...I thought that, in this last case only, I could use that calculator even if it's designed to calculate the response of a cap and a volume pot.....Hope that makes sense....You are saying that still, even with 0 percent of setting, it isn't the same situation, right? Why? ...And if so, how could I find the right calculator for a cap that bypasses a fixed resistor, then?
Thank you very much again!
I thought that a fixed resistor like that was the same than using a volume pot with 0 percent of setting, so that the cap bypasses the entire value of the volume pot, and so that it gives the full on bright boost...I thought that, in this last case only, I could use that calculator even if it's designed to calculate the response of a cap and a volume pot.....Hope that makes sense....You are saying that still, even with 0 percent of setting, it isn't the same situation, right? Why? ...And if so, how could I find the right calculator for a cap that bypasses a fixed resistor, then?
Thank you very much again!
-
phatt
- Transistor Tuner
I don't know of any calculators on line but why not just cheat like I do.
http://my.ece.ucsb.edu/York/Bobsclass/2 ... _maker.htm
There are likely far better (more complex) simulations around but for my purpose this was easy to understand and use.
Wether you use paper and pen or sims to get a number for a circuit just be aware that a high pass idea that works in one amplifier may not produce the same result in another amp design.
In the end you may end up needing to tweak values because although schematics might look similar the speakers might be very different,, Output transformers may have marked tonal differences. The tone you end up hearing is a culmination of many factors.
pickups, speakers, pedals,, the list goes on. even the room you play in can alter the perceived sound.
And don't forget mother nature can play tricks on you,,, summer and winter,, humidity can play a part.
Amplifier design is like a big rubber band,, alter one part and something is lost somewhere else in the circuit.
In other words don't go looking for an exact cap resistor match and assume it will be perfect.
Phil.
http://my.ece.ucsb.edu/York/Bobsclass/2 ... _maker.htm
There are likely far better (more complex) simulations around but for my purpose this was easy to understand and use.
Wether you use paper and pen or sims to get a number for a circuit just be aware that a high pass idea that works in one amplifier may not produce the same result in another amp design.
In the end you may end up needing to tweak values because although schematics might look similar the speakers might be very different,, Output transformers may have marked tonal differences. The tone you end up hearing is a culmination of many factors.
pickups, speakers, pedals,, the list goes on. even the room you play in can alter the perceived sound.
And don't forget mother nature can play tricks on you,,, summer and winter,, humidity can play a part.
Amplifier design is like a big rubber band,, alter one part and something is lost somewhere else in the circuit.
In other words don't go looking for an exact cap resistor match and assume it will be perfect.
Phil.
-
howmuch
- Breadboard Brother
You could use this calculator to look at the high-pass shelving filter you're referring to (aka treble peaker)
http://www.ax84.com/contrib/filtercalc. ... rc3=Submit
http://www.ax84.com/contrib/filtercalc. ... rc3=Submit
-
phatt
- Transistor Tuner
About 10dB up over 3 octaves.J-fish wrote:Thank you very much! I'm just trying to understand that particular case
Be aware Actual outcome at speaker won't look like this which is why I suggest simulation as you will learn heaps faster if you apply yourself to learn how to use that.
A picture is worth a thousand words and will allow you to grasp the whole Amp not just snippets like this which can be rather misleading.
Phil.
- Attachments
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Thank you very much Phil, honestly I had some problems with the installation of the software...but your picture helped me a lot still, really helpful, thanks!
..So, it seems that the filter is a shelving high pass of a few dBs...not that big bright boost thing that I thought before.
I was wondering, from an inexpert point of view, if that filter should be considered being not really effective in the circuit because it's also connected to ground, via the 500k pot just after it...
So, from one side, it follows the signal path, via the 500k pot to the next tube stage, but, from the other side, it goes to ground, via the same 500k pot...In this case, wouldn't that slight high pass filter be nullified after all...?....Hope that you can understand what is my doubt....
Thank you a lot again!
..So, it seems that the filter is a shelving high pass of a few dBs...not that big bright boost thing that I thought before.
I was wondering, from an inexpert point of view, if that filter should be considered being not really effective in the circuit because it's also connected to ground, via the 500k pot just after it...
So, from one side, it follows the signal path, via the 500k pot to the next tube stage, but, from the other side, it goes to ground, via the same 500k pot...In this case, wouldn't that slight high pass filter be nullified after all...?....Hope that you can understand what is my doubt....
Thank you a lot again!
-
phatt
- Transistor Tuner
Hi Jfish,
Oh bum,, yes it's older software which may glitch with later systems.
You can right click properties on most win versions and change what version the EXE will run as. Does not always work but worth a try.
Anyway I have no doubt there will be other Free software that will do the same job if you hunt around.
Meantime the volume pot is just a voltage divider and straight resistive so has little effect on frequency when driven from a low Z output.
In the case of Valves where the output is often taken from the plate there is hi frequency loss across large value resistance but not so much a problem with opamp circuits and like where the output is mostly low z and following pot is low value resistance.
Get a sim setup of some sort and just load the given circuits and press mouse buttons until something makes sense.
Heck I knew very little about how to use a scope let alone software stuff but it did not take long to grasp the idea and return some meaningful results.
Circuitmaker comes with a bunch of circuits ready to simulate,, one being a whole Valve power stage. It put me light years ahead in a very short time.
Reading books and building stuff is great but being able to simulate saves a whole lot of land fill and time and will really speedup the learning curve.
When you consider how much bench space and gear you would need to test a concept then the cost time factor is a no brainer.
As this is really only a hobby for me simulation is just brilliant.
Long before I heat the iron I've already got an idea of what goes on inside the circuit.
Phil.
Oh bum,, yes it's older software which may glitch with later systems.
You can right click properties on most win versions and change what version the EXE will run as. Does not always work but worth a try.
Anyway I have no doubt there will be other Free software that will do the same job if you hunt around.
Meantime the volume pot is just a voltage divider and straight resistive so has little effect on frequency when driven from a low Z output.
In the case of Valves where the output is often taken from the plate there is hi frequency loss across large value resistance but not so much a problem with opamp circuits and like where the output is mostly low z and following pot is low value resistance.
Get a sim setup of some sort and just load the given circuits and press mouse buttons until something makes sense.
Heck I knew very little about how to use a scope let alone software stuff but it did not take long to grasp the idea and return some meaningful results.
Circuitmaker comes with a bunch of circuits ready to simulate,, one being a whole Valve power stage. It put me light years ahead in a very short time.
Reading books and building stuff is great but being able to simulate saves a whole lot of land fill and time and will really speedup the learning curve.
When you consider how much bench space and gear you would need to test a concept then the cost time factor is a no brainer.
As this is really only a hobby for me simulation is just brilliant.
Long before I heat the iron I've already got an idea of what goes on inside the circuit.
Phil.
