| 5150ed |
What would happen if I remove R3 and .1mf cap and connect pin 7 to ground directly?
Is there any harm? |
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| Greg Erskine |
hi 5150ed,
I've made a few IGC, and all have NI input connected directly to ground. So I wouldn't expect you to have any significant problems.
I believe R3 is an attempt to reduce DC offset, I could be wrong. Why don't you try both and see what happens?
regards |
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| jackinnj |
yes, there are some microAmps flowing out of the opamps input pins --
that design uses unusually large resistance -- about 1/4 the magnitude would be better. |
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| janneman |
Yes, the input bias currents are flowing through the two balanced 220k resistors (the 10k on the inv input isn't there for DC) so unbalancing by deleting the one 220k will surely give gross DC output offset.
Cutting down the values on the 220k and 10k series R would be much better, also from a noise standpoint, but that would lower the Zin and you would need a lower vol control than 100k.
This is a major problem in inverting apps and also the reason why most would avoid it.
Jan Didden |
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| 5150ed |
Thank you so much.
But I am very new here.
Can you Janneman can you explain in easier words?
I should keep 10K and reduce 220K to how many K?
Is this also reduce gain of my amp? [I am so confused. I am using OPA541AP chip, some said I should keep the gain at only 10, some said between 10 to 20 is ok. Do u have any idea?]
I only have 100K step, can I adjust it to 50K by parallel 100K resistor with the step output? |
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| Nuuk |
If you are new to all this, make things simple for yourself. Don't try to modifiy circuits that you come across with different chips! :att'n:
Have a look at the circuits on my web site. There is a very basic IGC there and a fairly comprehensive FAQ section. ;) |
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| 5150ed |
Thank Nuuk.
Can you give me your web site's address? |
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| Nuuk |
| Click on the www at the bottom of my posts. ;) |
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| Christer |
| quote: | Originally posted by Nuuk
Click on the www at the bottom of my posts. ;) |
Hm, I can't see any URL there. But he can click on your name "Nuuk" instead, and find the link there. ;) |
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| Nuuk |
| There is a button with www on it. In the same place there is a button with email on it at the bottom of your post! :att'n: |
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| Christer |
| quote: | Originally posted by Nuuk
There is a button with www on it. In the same place there is a button with email on it at the bottom of your post! :att'n: |
Oh, sorry. I had never noticed it said "www" instead of "email" in your case. :ashamed: |
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| Nuuk |
| quote: | | Oh, sorry. I had never noticed it said "www" instead of "email" in your case. |
No worries. ;) |
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| Greg Erskine |
| quote: | Originally posted by janneman
Yes, the input bias currents are flowing through the two balanced 220k resistors (the 10k on the inv input isn't there for DC) so unbalancing by deleting the one 220k will surely give gross DC output offset. |
Hi Jan,
I don't remember any of my IGC having "gross" DC output offset. (i.e. all were under 100mV, I think). DC output offset seems to vary quite a bit depending on the chip. If I was overly concerned, I'd recommend putting a pot in so DC output offset could be adjusted depending on chip variance.
Hi 5150ed,
My comments were regarding the LM3875 as per schematic, I have no experience with the OPA541AP chip.
As a matter of routine, you should always check DC output offset before connecting an amp to your speakers.
regards |
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| 5150ed |
| Anyway, I still question that is there any way to make 100K step to 50K ? |
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| Nuuk |
| 100K what? :confused: |
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| 5150ed |
| I mean can I adjust 100K Pot to 50K by adding resistors, if yes how? |
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| Nuuk |
| quote: | | I mean can I adjust 100K Pot to 50K by adding resistors, if yes how? |
NO! If you are building an IGC then you really want a 10K pot anyway! :att'n: Have you read the FAQ at Decibel Dungeon? ;) |
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| 5150ed |
Ho **** I need to buy a new 10K step?
Is that why I feel that the high frequency of my amp distored?
Thank U so much Nuuk |
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| 5150ed |
"The solution is to use a linear pot which has better tracking than it's equivalent log type. Then, we modify it to fake the curve that the log pot produces. The modification involves soldering a resistor between the out and ground pins of the pot, and this is called 'Law faking'. The value of the resistor used is between one tenth and one twentieth of the value of the pot. So, for a 100K pot the law faking resistor value is 5K and 10K, for a 50K pot 2K4 and 5K1, and for a 200K pot, 20K and 39K."
I quoted from your website, Nuuk.
Can that Law faking apply to the stepped attenuators as well?
Ho god I do not want to buy another real expensive stepped attenuators just because its impendence isn't match :bawling: . |
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| Nuuk |
| quote: | | Can that Law faking apply to the stepped attenuators as well? |
I don't think so but you mis-understand. Law-faking is not used to chaged the impedance of the pot.
What stepped-attenuator do you have there? |
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| janneman |
| quote: | Originally posted by Greg Erskine
Hi Jan,
I don't remember any of my IGC having "gross" DC output offset. (i.e. all were under 100mV, I think). DC output offset seems to vary quite a bit depending on the chip. If I was overly concerned, I'd recommend putting a pot in so DC output offset could be adjusted depending on chip variance.[snip] |
Hi Greg,
I was looking at the engineering side of it, anything over 10mV for me is gross offset ;) as it is unnecessary. Surely anything approaching 100mV is bad implementation! The fact that it varies quite a bit from chip to chip is bad implementation; the point of good design is that the amps specs do NOT vary from chip to chip (at least not appreciably).
Jan Didden |
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| 5150ed |
100K TKD stepped-attenuator I bought it for my last tube int' amp project.
Real expensive step but worth very $ of its price :bawling: but I do not want to pay it again. |
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| Greg Erskine |
| quote: | Originally posted by janneman
I was looking at the engineering side of it, anything over 10mV for me is gross offset ;) as it is unnecessary. Surely anything approaching 100mV is bad implementation! The fact that it varies quite a bit from chip to chip is bad implementation; the point of good design is that the amps specs do NOT vary from chip to chip (at least not appreciably). |
Hi Jan,
Sort of shows everything is relative. :D I agree 100%, if I was a professional designing for the "real" market, I'd definitely make sure things measure properly. In the DIY world though, I haven't noticed in my medicore system an audible difference between 70mV or 10 mV DC offset with these gainclones thingies.
regards |
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| Nuuk |
| quote: | | I was looking at the engineering side of it, anything over 10mV for me is gross offset as it is unnecessary. |
For me anything under (or over) 144 mV isn't gross ! :D
5150ed - if you want to make use of your attenuator, build a non-inverting GC!
:att'n: |
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| janneman |
Well, yes, gross is in the eyes of the beholder. But where do you draw the line?
And it so easy to make it less offset, more predictable, less variable and less temperature dependent. Why not use something like the attached?
Jan Didden |
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| janneman |
| quote: | Originally posted by Nuuk
For me anything under (or over) 144 mV isn't gross ! :D
[snip] |
Haha, I like that!
Jan Didden |
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| Nuuk |
Jan, when I started out with GC's, I built inverting and played around with various methods of reducing the DC offset.
After much testing, I clearly preferred the sound when using a wire from non-inverting to ground, even though of course that produced a higher DC offset.
Depending on the chip, it is still possible to get DC offsets of well under 50 mV. Perhaps not up to professional standards but the bottom line for the DIYer is if it sounds better, then it is better! ;) |
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| janneman |
Hi Nuuk,
Yes, I can imagine that largish resistors like the 220k do in some way impact the output signal, and that it becomes audible. I never am comfortable with those large values, gut feeling I admit.
But things should be much better with 10k or even 5k, like the attached. I don't know how far you can push this, haven't tried it, but if someone still hears a difference with 5k in series with the input, I would think we enter the realm of perception rather than actual differences :D
Anyway, if the original poster had a 10k level switch instead of 100k it would even be better. No more excuses for more than 10mV or so of offset!
Jan Didden |
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| 5150ed |
I am using OPA541AM chips, can I use them with non-inverted schematic?
Is there anyproblem with it gain? |
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| Nuuk |
| quote: | | I am using OPA541AM chips, can I use them with non-inverted schematic? |
The OPA541 circuit I posted for you is non-inverting. Just build it exactly like it is, use your attenuator and you should be fine except for the lower gain.
If you want higher gain, buy some LM chips and build the (NI) circuit on my site or one of the many others posted around. ;) |
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| janneman |
You can download the data sheet, that will tell you the min gain for stability. But a gain of 20 will not be a problem, mostly the limit is 5 or 10 times, depending on the type of chip.
Anyway, the data sheet will give you also basic connectivity diagrams and component values.
It beats asking on the internet, getting several conflicting responses and trying to figure out who the **** is the one that talks nonsense ;) .
Jan Didden |
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| janneman |
Added the numbers to the diagram - Ohms law, basically.
Jan Didden |
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| 5150ed |
Thank u Nuuk.
Anyway, The OPA chip has lower gain than LM chip??? |
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| Nuuk |
| quote: | | nyway, The OPA chip has lower gain than LM chip??? |
The chips don't have gain until you set it with the input and feedback resistors. The different chips do have different characteristics that mean they operate best at different levels of gain. :att'n: |
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| 5150ed |
I see. Thank U so much Nuuk.
I will try your schematic as soon as I can get .1 R Resistor.
Last night I try to reduce the gain of my amp from 22x to 120x, it sounds much better but so lack of gain.
Anyway, noninverted have a lot higher gain 33X compair to 22X of inverted, will have the same distortion in high frequency problem as I have with the 22X inverted???:confused: |
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| Nuuk |
| quote: | | Anyway, noninverted have a lot higher gain 33X compair to 22X of inverted, will have the same distortion in high frequency problem as I have with the 22X inverted??? |
NO, NO, NO NO, NO! :dead: Don't keep thinking that a certain type of chip, or circuit has a set gain - they don't!
YOU set the gain as you want but you must set it in a range that makes the chip work optimally (and safely) and you can find that range on the datasheet for each chip. ;) |
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