Baxandall eq fet buffer needed

[screamersusa]

I'm trying to use the ampeg style baxandall 2x 1meg pot eq
for a clean channel pedal.
I built op amp buffers but they sound muffled. I guess I just dont like
opamps in my guitar pres only post.
Can someone point me to a ready to build variable gain bs170 sho w mpf102 follower
that will drive low impedence?
I'm also having issues with interfacing a formula5, riot, bor, into eq sections in the same box.
I think such a buffer would solve my issue while still maintaining a non muffled tone.
The pre Im using runs direct. I dont have a nice tube amp anymore nor can I make that kind
of noise here.
Cheers

[brett]

Hi
I'm no expert.
How low is this impedance that you want to drive?
Typically, an MPF102 source follower will have a couple of kilohms of output impedance. Not much lower than the super hard on, so there's not much advantage in adding it.
On the other hand the JFET has the same effective input impedance as the SHO (virtually unlimited).  So you might consider just using a variable J201 gain stage to do all of what you want (e.g. Stratoblaster).
cheers

[R.G.]

I'm trying to use the ampeg style baxandall 2x 1meg pot eq
for a clean channel pedal.
I built op amp buffers but they sound muffled. I guess I just dont like
opamps in my guitar pres only post.

This is not a comment about you or your preferences, but if an opamp buffer sounds muffled, you built it wrong in some way.

I would guess that if you're after very high input impedance, you picked the wrong opamp for a buffer. There are JFET input opamps like the TL07x series with quite-large input impedances. Bipolar-input opamps will have much lower input impedances and could possibly sound muffled in some situations. There are CMOS rail-to-rail opamps that have input impedances as high as the highest discrete MOS devices, because they DO have MOS devices for inputs.

With a JFET or MOS-input opamp, the input impedance is primarily the input bias network. With bootstrapping, it can be even higher than that. A bootstrapped *bipolar* input can get to an effective value over 100M.

Can someone point me to a ready to build variable gain bs170 sho w mpf102 follower
that will drive low impedence?

The MPF102 follower will have significantly less drive current available than either a bipolar or MOSFET. For high input impedance in a discrete design, you divide and conquer. A MOS device like the BS170 will get you high input impedance OK, as long as your input bias network doesn't lower it too much, but once you've buffered the input, a MOS device can easily drive a bipolar output follower for plenty of output current. The JFET follower after a MOSFET gives away a lot of performance you could get.

[screamersusa]

RG.. no offense taken at all.

I used to repair and mod mixers etc with +-15v supplies. Im rather fond of the sound of
soundtracs stuff such that I have a few M and solo module eq's floating around as utility
eqs.  I seem to be having issues with single polarity op amp stuff.
I use tl072 a lot.
I'm very into what you hear is what you get. I.E. what my preamp puts out is what the
PA will get via direct. Stage, I was using cabs and poweramps adusted to match the tone
the pa was getting or self powered 15 cabs. Paranoid vocalist

For some reason I like the sound of the fet mosfet stuff at lower gains cascaded.
The muffled sound I refer to is more of a loss in feel and probably high end.
I tend to like higher cutoff frequencies with steeper rolloffs.
I like Eminence 4x12 cabs or Eminence/Celestion mixed.
I dont have the luxury of owning a real Marshall or Carvin anymore nor the ability
to use one around the house. Have a randall RX I cant even use here.
I'm stuck in direct out speaker sim DIY land with headphones most of the time.
A little frustrating when you are my age...lol

[R.G.]

... I seem to be having issues with single polarity op amp stuff.
I use tl072 a lot.
... For some reason I like the sound of the fet mosfet stuff at lower gains cascaded.
The muffled sound I refer to is more of a loss in feel and probably high end.

OK, that helps. FETs and MOSFETs have in common with triodes, but to a lesser degree the introduction of soft asymmetrical distortion, just a bit. Opamps don't do that. The single ended stuff tends to make things sound just a touch sweeter than it really is. The silly opamps just put out what they're given, at least until they're overdriven even slightly.

What you say makes absolute sense. I suspect that you're missing the slight sweetening of a FET single ended stage, or even several cascaded. Loss of feel is a very accurate way to describe it.

[brett]

Hi again
RE:

The muffled sound I refer to is more of a loss in feel and probably high end.

Getting your lost feel back is a tough ask.
It is interesting to try to work out why you might have lost top end with an op-amp.
As RG suggests, most opamps have enough input impedance for guitar pickups and other circuits. Some bipolar input opamps may be low (esp. the popular NE5432/4), but they are the exception.
1M input impedance, 100kHz roll-off, gain of 100, low noise, and output impedance of 1k is easy to get with op-amps like the TL072.
Maybe it's just an academic exercise, but it would be interesting to know what opamp and what opamp circuit failed you.
cheers

[brett]

Interesting how RG and I posted almost opposite ideas about whether 'feel' is a simple thing to interpret (RG), or not (me).
Totally agree that overdriven op-amps can be a really bad sound. I was thinking more narrowly - clean, not overdriven.

[screamersusa]

I think Im trying to get my head around two different issues:
1) getting tones and feel I like before introducing op amps into the equation in
eq stages etc. So the basic feel/tone is already there before op amp processing.
Preamp needs to reflect what the fingers are doing.

2) dc blocking and impedence issues when adding the op amp circuits to the fets.

Examples:
  Formula 5 and bor feed zvvex bs170 inventobox eq fine, but drop dead on the sans amp tri ac eq or
an op amp driven eq. My clean first stage j201 drives the opamps just fine.

I like the riot sound but it's a little under a blanket. Same with the sans amp stuff.
I'm building the engl mod 3 (bs170) as my last ditch med to hi gainer build before giving up
and sticking with the GNX3000 as my hi gain (MFX supermodel Diezel and engl sim) and the
Jfet splitter as clean pristene, driving the bor at low gain sparkle (marshal clean), and the formula 5 as my clean
channel through a deltalab box through the aux in on the gnx. (All selectable of course).
Im trying to put my tones in one box (no fx) as a 4 channel preamp that I can run into anyting.
Carvin super clean or Marshall grit clean, Rock (bor)or blues (form 5) OD channel, Rock diezel ch,
Heavy diezel engl channel (something between the two, diezel ch3 and savage compression.)
Two inserts (clean/rock) and one master eq. Most controls as trimpots under cover.

[R.G.]

Interesting how RG and I posted almost opposite ideas about whether 'feel' is a simple thing to interpret (RG), or not (me).
Totally agree that overdriven op-amps can be a really bad sound. I was thinking more narrowly - clean, not overdriven.

Well, "feel" in general is almost impossible. However, when guitarists start talking about the difference in feel between single ended stages and opamps, we've seen the question before on a previous exam. The guitarist's ear is usually tuned in to the slight sweetening of especially a triode, but also a JFET or MOSFET stage with little feedback, so the bend in the  transfer curve gives just a bit of soft distortion.

It's not that the question is easy, we've just seen it before. I agree, the more general question is much harder to answer.

[screamersusa]

I would think that "feel" may also relate to how the pre amp responds to transients and dynamics.
Kind of like air filled bicycle tires versus solid filled plastic ones.  They both work fine and serve the purpose.
When you take turns on the air filled tires, the whole bike has a "spongy" feel and you can use that flexibility
to "hang on" to a turn with slight balance and directional changes. On filled plastic wheels you have to "fight"
to keep the bike from slipping out from under you. 
I often hear the complaint "I have to fight to get the notes out" from guitar players using high end digital gear.
I sometimes feel that way about op amp preamp circuits. I've been using a simple unity gain fet preamp (no controls)
to make my gnx "feel" a little nicer. Especially on clean tones. I have a straight line level tube buffer as well but prefer the fet.
Give a Juliard graduate reading sheet music a digital rig and he is probably happy.
Us untrained hacks need a little "feel" like some people prefer a wood handled hammer versus a metal one.
The tool becomes an extension of ourselves, requiring more control from within the person using it,resulting in
a better product reflective of the artist, not just the training.

[R.G.]

I would think that "feel" may also relate to how the pre amp responds to transients and dynamics.

I would too, but probably not in the way you're meaning it.

Let's take transients - things that happen, then they're over. Generally these are taken to be FAST signal content.

Both modern opamp circuits and discretes can respond faster than the ear can hear in most cases.

In a earlier generation, opamps could not follow high speed, high amplitude waves quickly. Modern ones can. The modest TL07x family can follow the fastest slew rate of a 20kHz sine wave. There are those that will argue that they can hear the difference between that and a circuit that can respond in the hundreds of MHz. Maybe. It could happen. I have a friend who used to hear the ultrasonic "silent" dog whistles in his youth. I tried one on him when he wasn't looking. But I think that's the exception.

It's fashionable in hifi tweako circles to talk knowingly about "difficult waveforms" as though the amplifier was trying to ride a roller-coaster of the signal voltage. It doesn't really happen that way. Amplifiers don't have a memory of where the last signal voltage was. They only know what the voltage is ...right...now... and they can either follow it to the next instant because they can slew fast enough to get to the next quantum-time signal or not.

So an amplifier is either fast enough to follow a signal or it's not. There are probably degrees of not-ness, but if it's fast enough, even just barely, it can follow the signal, and its output can't be told from a circuit that is capable of following a signal 100 or 1000 times as fast.

As for dynamics, I'm guessing that these are situations where a circuit both objectively and subjectively can't follow the incoming signal, and to the guitarist, that's a good thing. Call it a signal-level-dependent gain, where the size of the signal affects the gain of the circuit. In tube amps, big signals make the power supply sag, lowering the gain of preamp sections and the output power of the power stage, at the same time mixing in a serving of power rectifier ripple.

Taken together, this probably relates to the description of opamp circuits as "sterile". And to a certain extent, they are. They largely don't contribute the idiosyncrasies of some discrete circuits. If the oddities are what you're looking for, not seeing them might be described as either "accurate" or "sterile", both meaning the same thing, but depending on one's mood when the words are used.

[screamersusa]

Wonderfully put and thoroughly understood