Input buffer question

[GibsonGM]

I had to look at that again and realize that it is just the LPF HANGING OFF OF THE OUTPUT because there is no feedback resistor!  And it doesn't do much anyway.  It needs to be deleted.  It confused me, but not as much as the 'designer' who left it here, LOL 

Sometimes I really dislike opamps ha ha.

[ElectricDruid]

I thought you'd twigged that when you described it as a "legacy chunk"! That's exactly what it is.

[GibsonGM]

Yeah - cuz to prove it to myself, I tacked it onto a buffer in LT Spice, and while it DOES show some sort of "HPF" effect...it's like 1/10th dB per decade, LOL.   

[FSFX]

Yeah - cuz to prove it to myself, I tacked it onto a buffer in LT Spice, and while it DOES show some sort of "HPF" effect...it's like 1/10th dB per decade, LOL.

Using real components rather than ideal ones and using some resistance and inductance for the PCB trace will make LTSpice results interesting.

[Rob Strand]

Yeah - cuz to prove it to myself, I tacked it onto a buffer in LT Spice, and while it DOES show some sort of "HPF" effect...it's like 1/10th dB per decade, LOL.   

Even that looks too much.  I'd be suspecting the opamp output impedance is not modeled correctly in the spice model!

You have to expect things to deviate from ideal in the high frequency region say 20kHz and up, depends on the opamp.

[FSFX]

Yeah - cuz to prove it to myself, I tacked it onto a buffer in LT Spice, and while it DOES show some sort of "HPF" effect...it's like 1/10th dB per decade, LOL.

Did you run three versions of your LTSpice simulation to verify exactly what the effect was and how it differed from some 'reference' configurations.

As well as running the spice model with the 100k and 1uF connected back to Vref, you should also run a model with the 100k and 1uF connected direct to ground as well as running a model with the 100k and 1uF not connected to either.

This does bring up point that I raised recently when discussing the modelling of the Bluesbreaker in LTSpice. That is the use of a resistive divider for Vref. Even if the Vref connection is decoupled with a capacitor like a 10uF or a 47uF, it is not a perfect short circuit to ground to AC. So there is often a signal fed back to Vref from the parts of the circuit that it connects to.

In the case we are looking at here, the output signal from pin 1 of the 1458 op amp gets fed back via the 1uF capacitor and the 100k resistor to the Vref voltage divider. The impedance of the 1uF capacitor and 100k resistor form a voltage divider with the impedance at the Vref supply divider. If you run your LTSpice model with a signal going into the pedal and then look at the AC signal present on the Vref supply, you will see that there is some present. That AC signal will then get fed back into the other op amps via their inputs that are connected to Vref.   

So there is more to this than most people seem to think and it does pose the question of just how clean the Vref is in a lot of designs and whether the AC signal on Vref is in-phase or out-of-phase with the other signals on the inputs to the op amps.
This does mean that in some cases, if it is out-of-phase, it can produce negative feedback and so reduce the overall gain of the circuit. In the other case, where it is in-phase, with very high gain amplifier circuits it can introduce instability and oscillation.

[antonis]

!!! NO GARBAGE IN V_REF !!! NO GARBAGE IN V_REF !!! NO GARBAGE IN V_REF !!!  NO GARBAGE IN V_REF !!!  NO GARBAGE IN V_REF !!!

[FSFX]

If you can't make intelligent comments then please don't make them at all.

[antonis]

If you can't make intelligent comments then please don't make them at all.

Aye Aye, Sir,,!!

No other comment till utmost degree of intelligence reached..

[GibsonGM]

FSFX: No, I didn't spend much time examining 'the circuit', beyond tacking the offending parts to a buffer made with a TL071 model (not 'ideal opamp').   It was enough to note that there wasn't much filtering action going on whatsoever...whereas the buffer worked, and a gain stage (Tube Screamer) previously made with the same model and topology showed the expected HPF action from the negative feedback loop, as expected. 

I therefore surmised that what Antonis had said was Not in fact a fairy story, and that for any filtering action to occur via this topology, a feedback resistor would be required; and that its absence was the cause of 'no filter action'.   And the fact that the 'filter' was set so low, also suggested some major design malfunction, even if it were used in something like a TS      

[FSFX]

FSFX: No, I didn't spend much time examining 'the circuit',

It would seem that I am no longer welcome here in this forum according to Aron, the administrator because I post actual facts which some of the veterans of this forum like Antonis don't agree with.

So it looks like this will be my last post here before Aron kicks me out the forum.

Back then to tutoring all of the more grateful beginners in the Facebook groups.

Bye all.

[GibsonGM]

I don't think anyone is disagreeing with the Facts, FSFX...just that many of us are not EEs, and do this for FUN.  It's nice to be able to access the deep, esoteric info that you and others certainly provide, but for noob's, that is often intimidating, and requires one to almost have to read up a degree on their own at home in order to understand.  A little...too focused maybe, sometimes - it can make understanding seem inaccessible.    So guys like me are here too (hobbyist), who try to relate our own experiences as 'the uninitiated' who are still able to enjoy the hobby, learn some stuff, and make a ton of great-sounding effects that we use.  Even a newcomer can help someone else, and learn stuff at the same time.   It's a balance that has come over time and does seem to work.

Will we all end up with the same library of knowledge people like you have? Nope.   But we'll still gain as much as we wish! Keeping that door open to ppl with little to no background seems important.  Like playing some simple riffs for a new player instead of only playing hot Malmsteen runs, no? 

Cheers....

[antonis]

It would seem that I am no longer welcome here in this forum according to Aron, the administrator because I post actual facts which some of the veterans of this forum like Antonis don't agree with.

In case you have such a judgment about Aron or any other admin/moderator, I'm deeply sorry but walking out of forum indeed is the most advisable action..

[Rob Strand]

It would seem that I am no longer welcome here in this forum according to Aron, the administrator because I post actual facts which some of the veterans of this forum like Antonis don't agree with.

So it looks like this will be my last post here before Aron kicks me out the forum.

I read over the last few posts.  It seems unjustified to get kicked out over such trivial matters.

[Edit: Apparently it's not related to this thread but the one here:
https://www.diystompboxes.com/smfforum/index.php?topic=130044.msg
Don't know what went on there.]


I'm not buying into any specific arguments in this thread but in general you can only present the facts or counter bad information with correct information.   As for people accepting facts, forums are like some of those Jordan Klepper interviews.

[ElectricDruid]

If you can't make intelligent comments then please don't make them at all.

if you'd like to stay, I'm sure we'd all be glad to have you here, but you need to stop being so damn prickly. Antonis' "NO GARBAGE IN Vref!" is a *joke*, to lighten the tone a little. There's no need to get upset with him about that. I realise you two have already managed to rub each other up the wrong way, but we could let the past be and move on?

Up to you.

[bluebunny]

If you can't make intelligent comments then please don't make them at all.

But where would be the fun in that?  The occasional facetious comment does us all good.  Who could possibly resist all five vowels in one word and in the correct order?   

[duck_arse]

what was the original question again? something about an opamp buffer?

[antonis]

Yeaaappp..

But I presume Ranko's query has already been covered by previous participants..

[aron]

> It would seem that I am no longer welcome here in this forum according to Aron, the administrator

FSFX, you know that is not what I said at all. I said if threads start getting argumentative and edits get out of hand, I will have no choice but to stop "the posts".

Do whatever you want. This forum has lasted for over 20 years, people come and go and honestly, I want people that get along, and enjoy each other's company. 

I know you want to help, but if I were you, I would take a serious look at your responses and see if your replies are actually helping people. Not the facts or the posts about the answers, I'm talking about after that.

[Phend]

There are no "useless" components.
Someone made money selling them to (you / us) to put them in there.

The Wright brothers got the plane off the ground without hi tech radar and wind tunnel experimentation. It was a hobby. Just like my case, the mechanical guy building effects with schematic help and help from this forum when I need it. Thanks all !