DC offset in my preamplifier design?

Discussion in 'Analog & Mixed-Signal Design' started by Sulkyoptimism, Oct 7, 2018 at 8:45 PM.

  1. Sulkyoptimism

    Thread Starter New Member

    Sunday
    20
    0
    Okay yeah i will spring for the OPA134s I cant seem to find any through mount packages or sim models but i will i suppose.
    OKay i understand what you mean about the caps and res values, but i've altered what you've said in my sim and there is no difference in the frequency response at all, also having all the coupling caps on the main line in all 5 stages of the parametric as 10uF it causes a hipass at like 160Hz, I understand the maths behind what you are suggesting but i cant seem to make it apply to the simulation?
    Sorry this seems to just get worse at every turn here aha.

    As always very thankful for the help.
     
  2. Sulkyoptimism

    Thread Starter New Member

    Sunday
    20
    0
    I whipped this up quickly, tell me if this still applies because i cannot get what you suggest to play ball here.
    Sorry to keep at it so much.
     
  3. Veracohr

    Well-Known Member

    Jan 3, 2011
    652
    100
    You need to do a transient simulation to see the offset, so if it didn’t show you anything useful I don’t know why. What DID it show?
     
    Sulkyoptimism likes this.
  4. Audioguru

    Expert

    Dec 20, 2007
    10,466
    1,154
    Your posted "Full Para Circuit" has its parts extremely small and very far apart. I zoomed part of it only 2 times nut the details are a fuzzy mess.
    You saved it as a fuzzy Jpeg file type instead of as a very clear PNG file type. Here is what I see:
     
    Sulkyoptimism likes this.
  5. Audioguru

    Expert

    Dec 20, 2007
    10,466
    1,154
    You have the filters in series which is not correct. The outputs of the filters should feed an adder opamp.
    Please post a link to where on the ESP site the copy of the Urei filter is.
     
    Sulkyoptimism likes this.
  6. Sulkyoptimism

    Thread Starter New Member

    Sunday
    20
    0
    I just showed the voltage through the capacitor not a DC offset? Am i reading this wrong?

    Sorry yeah i know its fuzzy but its just a series of para filters, heres the Urei/ESP link http://sound.whsites.net/articles/state-variable.htm
    Wait so how would I cascade them? Within the circuit shown?
     
  7. Veracohr

    Well-Known Member

    Jan 3, 2011
    652
    100
    Just click on the output of the opamp to see the voltage signal coming from it.

    Screen Shot 2018-10-11 at 4.44.20 PM.png

    If you disconnect the signal source you'll just see a DC level. Or you can run a DC operating point simulation (.op) to see it. You just need to know the name of the node, so it's useful to put a label there like "output".
     
    Sulkyoptimism likes this.
  8. Sulkyoptimism

    Thread Starter New Member

    Sunday
    20
    0
    You saviour, thats done it yeah, getting roughly between 60uV-5mV :S Thats far too much isnt it?
    I suppose it will be a case of better opamps then!
    You're idea for balancing them is pretty interesting though so im going to experiment a little.
    Thank you so much!!
     
  9. Veracohr

    Well-Known Member

    Jan 3, 2011
    652
    100
    No, I don't think 5mV is too much for inter-stage offset unless the next stage has a lot of gain. 60uV is definitely nothing to worry about.
     
  10. Sulkyoptimism

    Thread Starter New Member

    Sunday
    20
    0
    Well they are mostly variable gain output stages. So i'd be a little worried but i'll try find ways to minimize it also, this will require finding both an opamp that suits the project and a model.

    Although i am super interested to hear if/how i cascaded the filters wrong because i think thats maybe the issue? Not sure, any ideas Veracohr?
    Thanks for all your help so far though, seriously even giving me the tools to work it out im grateful for.
     
  11. Audioguru

    Expert

    Dec 20, 2007
    10,466
    1,154
    You need to make as many of these state-variable filters as you want, then add their outputs together in an adder circuit like this:
     
    Sulkyoptimism likes this.
  12. Sulkyoptimism

    Thread Starter New Member

    Sunday
    20
    0
    Okay so the signal line can just split into 5 para's? The would probably fix the issues.
    Lemme change the opamps, Try DC analysis and then change the diagram to be parallel rather than series.

    Thanks so much guys! I'll see what i can come up with before the end of the weekend and probably update you.
     
  13. Sulkyoptimism

    Thread Starter New Member

    Sunday
    20
    0
    So i've tried it and yeah it works a lot better the low end is mostly preserved and in line with your calculations, but the cut function of the parametric is severly reduced by the 100k adding resistors, I've tried altering it but lowering the value gives too much signal through the summing amp. Not really sure how i'd get it to work in this situation. DC offsets seem okay now too!
     
  14. Audioguru

    Expert

    Dec 20, 2007
    10,466
    1,154
    Rod Elliott says in his article that the input signal line to the state-variable filter must be an extremely low impedance.
    I found an original Urei schematic of their equalizer and it has an adjustable low cut circuit feeding an opamp that has the extremely low output impedance and it does not use a coupling capacitor to feed a state-variable filter.

    The Urei circuit uses a quad 741 opamp with a fairly low input resistance (50 years old!) then it needs the very low value 5.1k resistors for low noise (a 741 opamp is noisy anyway).
     
  15. Sulkyoptimism

    Thread Starter New Member

    Sunday
    20
    0
    Is that just a input buffer? I have a similar thing between the previous EQ and this one.
    Does this suffice?
     
  16. Audioguru

    Expert

    Dec 20, 2007
    10,466
    1,154
    If you replace C148 with a piece of wire then it will be the same as the original Urei equalizer circuit.
    Why do you have a previous EQ?
    Will you add the separate low cut circuit?
     
  17. Sulkyoptimism

    Thread Starter New Member

    Sunday
    20
    0
    Aha, cool thank you, and honestly i know it makes less sense but its for fun more than anything right now, its a preamp that will do both, 5 band crossover EQ and 5 band Parametric EQ, its just an idea for now but its been a while i've been wanting to try build it. So thats why there are so many EQs. Do i need to add this buffer after each of the Parametric circuits? Before the addition resistors?
     
  18. Audioguru

    Expert

    Dec 20, 2007
    10,466
    1,154
    In my post #31 I show the state-variable filter from Rod Elliott's site with an INPUT. That same input must feed all your state-variable filters but one opamp as an input buffer to feed all of the filters will be overloaded. Then use two or three opamps as buffers.
    Urei did not use a coupling capacitor to feed the filters but Rod Elliott shows a 10uF capacitor that probably is not wanted.

    I keep thinking that this circuit is very old like me. I guess most analog circuits are old, aren't they? Everything is digital today.
     
  19. Sulkyoptimism

    Thread Starter New Member

    Sunday
    20
    0
    So since i have 5 sections i should use 5 buffers all fed from the signal input?
    Yeah i want to avoid caps, although with the set up you have suggested the caps are way less intrusive.
    Haha well maybe i'm starting at the wrong end but i am just wanting to learn and i feel like i have to discover everything by myself or from helpful people such as yourself. Are there any analogue circuits you would suggest aha?

    I'm going to start learning how to do DSP next year but after looking at some of the maths involved i'm just going to take it slower i think.
    Just like to learn and try stuff out, hoping to build at least one unit this year. This circuit is so big i would have to use Shark and i hear thats just for big boys, so unsure how well i'd do on it.
     
  20. Sulkyoptimism

    Thread Starter New Member

    Sunday
    20
    0
    Okay so i used 5 op amps as a buffer in series on the main input line, 5 op amps as single buffers on each input line per filter, adn also tried th emore complex buffer i showed above, none of them helped with the issue, i still cant get the gain/cut/boost to be in a similar region to the previous results, -12dB per filter. Its just stuck at a maximum of -2.5dB cut.
    I'll fiddle with it some more but i cant see how to work this one.
     
Loading...