Potentiometer in feedback loop and offset currents - Page 1
Forum section: electronics / beginners

author: ramonmaruko
author_rank: Contributor
timestamp: May 30, 2017, 04:43:01 pm
content: Good day. I am looking at a schematic for a guitar distortion pedal(BluesBreaker clone). I read that it is not a good idea to have DC flowingthrough a potentiometer, so I am trying to prevent that from happening. The waythe circuit is now, any bias currents from the opamps could flow through thepotentiometer.1. I need pointers on how to do this without audibly changing the frequencyresponse of the circuit. The attempts I've done basically mutes the output(-60dB to -120dB). What I've done is putting a capacitor in series with theinverting input of U1 and the nodes to C2, R2. I also tried putting a capacitorin series with the output of U1 and the nodes to C2, VR1.2. How do I properly simulate the offset currents for the purpose of testing DCflowing through VR1? What I am doing now is by turning off the AC input `in`and putting a DC voltage source to the inverting input or the output pin of U1.I then place current probes at the three pins of VR1. In another simpliercircuit with DC properly blocked, I still get a femtoampere; but I don't thinkthat will damage or cause noises when flowing through the potentiometer. Am Idoing it correctly? from "Small Signal Audio Design" by Douglas Self

author: mdszy
author_rank: Unknown
timestamp: May 30, 2017, 04:47:44 pm
content: Why does the book say it's a bad idea to have DC through a pot? Does it mean high currents, or any current? Low currents are 100% fine through a pot... but you don't want to go putting a few amps through one!

author: Benta
author_rank: Super Contributor
timestamp: May 30, 2017, 04:51:12 pm
content: Quote from: mdszy on May 30, 2017, 04:47:44 pmThis is old knowledge for audio guys. Just a little bit of DC current through, say, a volume or tone pot will give scratching noises (in the audio signal) when moving the slider/wiper. Try it yourself :-)

author: mdszy
author_rank: Unknown
timestamp: May 30, 2017, 04:55:09 pm
content: Quote from: Benta on May 30, 2017, 04:51:12 pmHuh, interesting. I didn't know that! I guess that leaves me confused... how does the pot operate then? On the actual audio signal itself, so it's AC and not DC?

author: Zero999
author_rank: Super Contributor
timestamp: May 30, 2017, 05:09:14 pm
content: Quote from: mdszy on May 30, 2017, 04:55:09 pmThe audio signal is AC of course. Capacitors can be used to block DC and prevent it from flowing through the potentiometer.

author: ramonmaruko
author_rank: Contributor
timestamp: May 30, 2017, 05:15:06 pm
content: Quote from: Benta on May 30, 2017, 04:51:12 pmSomething I've also come across is thisdocumentby ALPS.Quote

author: ramonmaruko
author_rank: Contributor
timestamp: May 30, 2017, 05:52:54 pm
content: I just noticed that I forgot to attach the schematic. I just fixed it now and attached the schematic to the first post.

author: Zero999
author_rank: Super Contributor
timestamp: May 30, 2017, 06:10:39 pm
content: Quote from: ramonmaruko on May 30, 2017, 05:52:54 pmThere will be no DC in the potentiometer, except for the tiny bias currents required for the op-amp, because both sides of the pot. are at a DC virtual earth.

author: Benta
author_rank: Super Contributor
timestamp: May 30, 2017, 10:40:56 pm
content: Quote from: Benta on May 30, 2017, 04:51:12 pmMy apologies for imprecise wording.A DC current running from resistance track through the wiper/slider will cause scratching. If the wiper is AC coupled to the next stage, DC through the track will not cause problems.

author: ramonmaruko
author_rank: Contributor
timestamp: May 31, 2017, 06:03:05 am
content: How do I prevent the bias currents from flowing through the potentiometer? Theop amp I am planning to use is the NJM4580 which has an input bias current of(typ) 100nA. In an example circuit of Self's book (Fig. 11.6), he says:QuoteI don't know if I am allowed to post the mentioned figures in this thread. For Self'scircuit, he used a NE5532 which has a (typ) bias current of 200nA.As mentioned in my first post, I tried following some of the patterns he usedto block the offsets; but all severely changed the frequency response of thecircuit.

author: Zero999
author_rank: Super Contributor
timestamp: May 31, 2017, 10:01:21 am
content: Quote from: ramonmaruko on May 31, 2017, 06:03:05 amThe simplest way to overcome this is to switch to a FET input op-amp, such as the TL072, which will have tiny bias currents, around the same order as the leakage current through a capacitor.

author: StillTrying
author_rank: Super Contributor
timestamp: May 31, 2017, 02:41:27 pm
content: I can't see why you can't just put a 100nF in series with the pot's wiper or left side, and then a 2Meg2 resistor from the opamps output to -ve input to carry the bias current.If you're not very careful a few nA of DC current flowing through a control pot in a guitar distortion pedal can cause all kinds of imaginary problems!

author: ramonmaruko
author_rank: Contributor
timestamp: May 31, 2017, 04:05:29 pm
content: Quote from: StillTrying on May 31, 2017, 02:41:27 pmThank you! How did you come up with 100nF and 2Meg? I tried 1uF and 200K, whichshould have the same frequency cutoff, but that showed a different frequencyresponse.

author: Zero999
author_rank: Super Contributor
timestamp: May 31, 2017, 05:16:46 pm
content: Adding a capacitor will quite likely affect the frequency response.Have you actually built this circuit to see how good/bad it is? This isn't something which can be simulated.Have you considered a FET input op-amp? They really aren't that bad and with the input impedances you have, the noise will be similar to a bipolar op-amp in the same price range.

author: Buriedcode
author_rank: Super Contributor
timestamp: June 01, 2017, 12:59:16 pm
content: Quote from: Hero999 on May 31, 2017, 10:01:21 amThat, along with cost, is why so many guitar pedals use the TL072/TL074. Guitar effects generally don't require >4MHz GBP bandwidth, but require low noise opamps if there's significant gain (ie: distortion pedals).As for modding the original design. I don't think much improvement can be made (in terms of design, not "tone"). But making tweaks to max gain, and/or the tone stack in pedals is a great way to experiment/learn and end up with guitar effects you actually like.

author: Zero999
author_rank: Super Contributor
timestamp: June 01, 2017, 02:00:57 pm
content: I forgot to mention the difference in noise between BJT and FET input op-amps such as the NE5532 and TL072 respectively.With a low input impedance, the NE5532 will have lower noise than the TL072, since it has a lower noise voltage. At higher input impedances, the TL072 will be the least noisy, since it has tiny bias currents and the large bias currents of the NE5532 and their associated noise, will generate a corresponding noise voltage in whatever impedance is at the inputs. From memory the crossover point when the TL072 becomes better than the NE5532 is around 30k of input impedance, which is not far off what this circuit has.Unfortunately I didn't see a specification for the input noise current on the NJM4580, which doesn't inspire confidence with that characteristic. I doubt it's better than the NE5532 and even if it is, then TL072 will still be better because the lower bias current through the pot. is worth it.

author: ramonmaruko
author_rank: Contributor
timestamp: June 02, 2017, 05:43:32 am
content: Quote from: Hero999 on May 31, 2017, 10:01:21 amQuote from: Hero999 on May 31, 2017, 05:16:46 pmQuote from: Hero999 on June 01, 2017, 02:00:57 pmThe input impedance of my guitar amplifier is rated at 1Meg ohms. I'm assumingthat they were expecting the source impedance (guitar) to be at max 100K ohms.Does the circuit that I posted have around 19K ohms input impedance? (R5 ||R1). That really makes using a FET input op amp more correct in this case. Istuck with using the NE5332 since that was what's on the original schematic,but it looks like I'll have to use a FET input op amp to easily avoid theoffset currents that I don't want.Another (unrelated?) question that I have, how do I calculate the currentconsumption of an op amp circuit, for power supply sizing? Would it simply be:((expected max Vo of the op amp) /(max impedance of the feedback network)) +quiescent current of the op amp?

author: Zero999
author_rank: Super Contributor
timestamp: June 02, 2017, 08:04:27 am
content: The input impedance of the circuit you posted is equal to R1 + R5 = 519k, at audio frequencies.I don't know that much about guitar pick-ups. I believe they're inductive so the impedance increases with frequency. If the amplifier has too lower impedance, then it will cut the high frequencies, leaving the low. I don't think a guitar produces much in the way of treble but could be wrong. Hopefully someone else here will provide more insight.In this circuit the static supply current is simply equal to that taken by the op-amp because there's no DC current through the resistors in the feedback loop. With a signal, the peak current through the resistors in the feedback loop adds to the current draw but in this case the resistors have fairly high values so you can probably ignore that too and concentrate on the load.

author: ramonmaruko
author_rank: Contributor
timestamp: June 02, 2017, 09:06:45 am
content: Thank you very much for answering my questions and being patient with me.