Output impedance of tl072 input buffer

[mladenu]

Hello friends,

I want to modify tone stack from preamp, and need output impedance value from marked point to simulate new one (tone stack) through LTSpice.. How to do that?

Thanks in advance!

[Roly]

To a first approximation it's zero.

It's actually about 200 ohms, but it's so low compared to the values in your tonestack as to make no never mind.

On the datasheet the equiv cct shows two 64 ohm emitter resistors into a 128 ohm series resistor to the output, so it's at least 128 and more like 200-odd.

In LTSpice you are going to use an op-amp model, and this will have the internal source resistance accounted for.  You can find a TL07x model at Yahoo LTSpice group, a great resource.

Or you could measure it.   8|

An op-amp provides almost perfect voltage source drive from very low impedance which is just what this tonestack wants.  It's the load on the tonestack output that is quite important, generally the higher impedance the better.

[mladenu]

Thank you! Now it is much clearer.

My intentions is to add origin Fender tone stack, with mid pot, like that on first attachment and from TSC... Current tone stack have similar frequency response like Fender TS, i am aware of that situation after simulated it via LTSpice, but i want control of middle frequencies.. And what confusing me... Zsrc of Fender TS is 38K... Do I have to achieve this value, as with, let say 200ohm of Zsrc, i get a totally different frequency graph in LTSpice?

I was comparing in LTSpice this two tone stack, but with Zsrc of 38K, and were very similar, almost identical graphs ..

[Loudthud]

The 38K in TSC is a rough approximation of the output impedance of a 12AX7 with a 100K plate resistor.

[mladenu]

I understand that, but what the impact will be on the response curves when I add a Fender tone stack instead of the existing one, when i have only a few hundred ohms of output impendance after inbut buffer, but before tone stack.. I ask this, because simulation of that situation drasticaly changes response curves..

[J M Fahey]

In fact it does not drastically change *shape*  which is what's real important, what gives it "flavour".

What it changes basically is attenuation, but in a more or less even way.

I don't use LTSpice but simulate with excellent Duncan's TSC and which has memory of different curves, so simulate the same with different generator impedance, say 200r , 1k , 4k7 , 22k and 38k .

You'll see they are roughly parallel.

As of the effect of the mid control, all it does is set the depth of the filter notch, so yes, in a way, it "controls mids" .

Don't expect much from a simple resistive, non frequency selective control.

[phatt]

Hi mladenu ,,
Um  unless I'm mistaken the drawing is from a Rockette amp which was discussed here not long ago.
So might help if you mention What amplifier are we talking about? :tu:

This is not easy to explain but I'll give it my best shot;
As has been said already forget the output Z of the opamp, the response curves set by the components wrapped around (and in series with) the first stage will alter the result far more than output Z.

Many of those famous Valve amps have identical tone stacks yet sound very different.
In other words, just inserting the perfect tone stack inside an existing circuit will not magically make mojo.

In fact with 2 identical circuits just changing one Cap value before or after the preamp and tone circuit can reap 2 very different results.

Tone circuits are just there for tone trim by the user they Do Not (on there own) dictate the Tone, as That is the sum of the whole circuit not just knobs on the panel.

Now as to the circuit snippet you have given;
There is a Cap with no value? So that will alter the amount of top end that passes.

If this preamp is from a Rockette 30 or similar then it's highly likely it will be very bright. I did some research for member *Jungle Jim* Posting here; http://www.ssguitar.com/index.php?topic=3647.0
Jim  found some of the Caps with no value were not on his circuit and this dramatically changes the end result. Without them the amp will most likely sound very harsh.
A simulation of his circuit reveals the tone is very bright, I'd research the whole circuit before a major renovation. 8|

Phil.

[mladenu]

Phatt, You are totally right, the rockette 30 is the subject of topic. I read all posts pointing to Jungle Jim`s hiss problem, and also share that issue with him... BUT, i found some interesting sentences, maybe problem related, on some other forum:

Quote:
I 'warmed up' my Rockette quite a bit by replacing a resistor and a capacitor in the input-circuit just before the first op-amp (allowing more high frequencies to flow to ground before getting amplified). I'm guessing Stewart Ward does a similar thing when he talks about taming the treble of the amp. Very simple and effective mod that will only cost you half a pound in parts. But you have to be able to read a schematic (they are on the Award-Session website) and locate the parts on the print, de-solder the old parts and solder the new parts in. It didn't take me more then 30 minutes though and I am very pleased with the result: a fuller tone and no more overly-bright trebles. But this has nothing to do with the RetroTone mod, as that concerns the power-amp and negative feedback circuit, 'current feedback' Stewart calls it.

And another one quote:
Look at the first op-amp after the input. There's a 100k resistor and a 10nF cap in series. I changed those to 47k and 220nF and liked the result a lot! (Got this from a Sessionette 75 user.) After that mod you can cleary hear that now it's the reverb that adds some hi-freq 'hiss' or 'sizzle' and not anymore the pre-amp.

End of quotes.

I have no problem with hiss, nor with tone, but think the desired fender tone stack could improve this little amp, which have very good response, transparent sound, etc... I understand that wrong or bad topology with perfect tone stack mean nothing...

[Roly]

LTSpice sim of Fender tonestack with source impedance swept 0 to 50k x5k steps attached.

If we think of the situation as a voltage divider, the source impedance Zsrc being the top impedance and the entire tonestack being the lower element; and that the source impedance is actually resistive and not frequency selective; then it follows that simply changing the source impedance will only change the absolute amplitude of the frequency selective contour imposed by the tonestack, not the shape, and that this will be true for all settings of the tonestack controls.

[Enzo]

Tone circuits are just there for tone trim by the user they Do Not (on there own) dictate the Tone, as That is the sum of the whole circuit not just knobs on the panel.

YES.
This very  central fact is so often overlooked by people wanting to alter the sound of their amp by focusing only on the tone stack.

[mladenu]

All your advice helped me a lot and a lot of them I learned .. As it was obviously that i not enough professional use LTspice ...  :loco

Thanks Roly for that observation...

Let me explain you what is my aim... Session Rockette 30 amp which i have is tonally very similar to fender amps.. Only reason for why i want to add original fender tone stack is because i want to controll mid range... I could instead 6K8 resistor put 10k potentiometre... But while modify TS circuitry, let's do a complete mod..

[Roly]

I could instead 6K8 resistor put 10k potentiometre

A bit of a fiddle in Tone Stack Calculator suggests that if you want some real control over the Fender mid-scoop that you use a 50k Linear pot for your Middle control.

[phatt]

You could always do what I did!
Build an external tone box,, and just leave the existing one alone.  8|

Fender and similar tone stacks work like a big lump of jelly,,If you alter one R or C value it then tugs at another freq and although the tone calculator is a great help it's still only one part of the whole system and as such may or may not work in your amp. As mentioned before and noted by ENZO who is way more qualified than myself,, (thanks for the comment Enzo :tu:)
The whole system has to be taken into account.
An option;
Dave Reeves Hiwatt tone control is by far the best at mid control. :tu:

I've researched A LOT of tone stacks and the Hiwatt concept is about as good as it gets. I've built quite a few tone circuits based on Hiwatt idea in stand alone boxes as modifying the internals of amps is always a pain and likely to reduce the resale value if you bore to many extra holes in the front panel. 
An external tone unit also gives you two tone control options and I've found that to be a big asset.

I had a chap come over with a brand new Fender Deville 212 and after I added some castors for him (they are very heavy) we plugged in for a quick jam.
He was impressed with the tone of my humble setup and asked How, what and why??? Needless to say I just plugged my simple tone box between his guitar and Amp and the difference was night and day.

Having said that It's no magic bullet and likely won't work well if the amp is already very harsh and bright.

The mod you mentioned, the leg from Neg input of U1 with the 220n and 47k leg to ground looks good on sims. That returns all the bottom end, though the gain does go UP quite a bit so that may cause issues if circuit noise is already a problem.

If you want my take on Dave Reeves HiWatt tone circuit it's here somewhere,, just search for PhAbbTone Ver 3.5
I may have an updated schematic coming soon as some noise still gets in as these hi z circuits are all prone to picking up noise.

With some gear it may work better after gain stages, like in the FX loop.
Every bit of gear is always slightly different so you have to experiment, especially if you want to hack an existing rig.
Phil.

[Roly]

Parametrics mate.   <3) :dbtu:

[phatt]

Parametrics mate.   <3) :dbtu:

Yes Para EQ is brilliant but most of the guitar players I meet would struggle to spell Parametric,, let alone know how to use one,  :lmao:

I was going to mention the parametric type deep notch the Hiwatt produces.

while testing I did indeed test the PhAbbTone against my Alesis Para EQ and the effect is close. Of course a full parametric EQ would be a lot harder to build using hand drawn ink pen on the copper.
:duh
When I built most of the early Ptone boxes I did not have a laser printer to make iron-on PCB's but now I can make them a lot smaller.

As for the Ptone response, With bass and treble at full and mid off the notch is around 30 dB deep (from top off bass curve).

The Alesis has a claimed +/- 15 dB boost/cut which should do a similar job of tone shaping but the PhAbbtone even with it's fixed freq just seems to work better for guitar. <3)
In most of the fender tone circuits you are lucky to get ~10dB of mid dip and the deep notch is never apparent.

I also tested the fabled Dumble tone setup with tone shift switching. IMO, not worth the effort. xP
Phil.