eq as pre-amp.

[JAMESJ]

I'm new here and hope I can contribute as well as learn, Any how I have a question,

I was looking at the 6 band eq design on "General Guitar Gadgets" website http://www.generalguitargadgets.com/index.php?option=com_content&task=view&id=46&Itemid=26 and wanted to use it as a pre amp on my 100w amp build instead of the standard pre-amp. It is powerd and has a gain control so I think it will work, but what do you experinced builders think?
Thanks,
J

[phatt]

Experienced builder?   Na! I'm just a mad hobby geek

A Graphic EQ is usefull but prolly not at the frontend.
Depends A LOT on what type of music you are shooting for AND TONE in general is all rather subjective..  Your idea of great guitar tone and mine may well be worlds apart.

That said if you want the classic Rock tones and you only have a SState Amp then here is one way to do such things. [see schematic]
This gives almost Zero distortion but does indeed nail the the tone shape needed for rock. [I've found that distortin is better at the end anyway.]
Though it is not common practice the old valve type tone stacks don't actually need an active front end, [ie, meaning a valve or transistor] They may well work more true to tone without the loading of a previous active stage,,, that of course is open for debate and I'm not about to argue with those who don't agree....
It just works great and I have built about ten of these in the last 2 years and sold everyone I have made. [cept for my first one]

I have come to realise that the "Shape" and "Lossy nature" of those old tone stacks used in tube Amps is part of the magic ingrediant  and inserted in front of most SState guitar amps will improve the realisuim sometimes greatly so.  [Works on some valve amps also]

My Alesis Micro EQ [active parametric EQ] cannot pull this tone shape and believe me I've Tried hard to find fault with my circuit.

Pros, improves the big mid notch dip almost non existant with some SS Amp tone circuits.
Cons, oh bugga there is always a down side.
Because of the "passive high imp input" this will amplifiy resistor Noise if you overdo the gain. so Metal resistors and low noise opamps are needed.
I did try it with an Active front end but noise is still present at high gain ,,somehow it sounds better as shown,,, feel free to improve it if you have the knowledge that I do not.
Hey it's only a hobby for me.

I've opted to use the classic HiWatt tone stack as it is by far the best of them but you can use any of those old tone stacks if you wish.
Fiddle around with different values by all means to refine it to your particular needs.
For those that wish to use the original HiWatt Values be very aware that the Bass pot is 500k Log and will deliver a tiny bit more bass below 100HZ But it MUST be A TRUE LOG ,,not the modern wannabe fake log type.
I hardly see the point as the bottom string on guitar is 82HZ so you won't notice any loss.

If you want huge midrange honk like say british valve amps then it may not deliver what you are after but still this will cover a lot of tonal ground that most tone circuits can't.

Be very aware that the overall end result of any tone is the culmination of many tone shapes all slightly and sometimes dramatically changing as the signal passes through each stage of the amplification system.
At nearly every point in these circuits there is tone shaping happening [intentionally or not] It's not just the tone stack.
This schematic is basic and assumes you know how to develop a bias voltage and no bypass switching is shown.
Use the lowest noise spec opamp you can obtain for improvement but I use the TL072

So in answer to your subject,, EQ as a pre-amp,,, Well I think this would qualify
I use mine in front of a rather complex setup and Believe me I know when it is bypassed
out of the signal path.
Hope you find it useful. Phil Nambour Australia.

[armstrom]

I also wouldn't suggest that circuit as a preamp by itself. While it does have adjustable front-end gain it isn't much. At the low end of the adjustment it's actually attenuating the signal (gain of 0.10x so your output signal would be 1/10th of the input) at max gain the amplification is only 2.6x. This can be easily changed  by altering the component values though. The bigger issue is the input impedance. Having an inverting op-amp as the input stage means that your input impedance is dictated by the inverting input resistor. That means an impedance of only 100K. You want at least a 1M input impedance if you're directly plugging your guitar into this circuit.

As I'm sure you're aware this type of circuit will not be good for getting any distortion sounds. So if you're just after a clean preamp then why not just roll your own circuit? If it were me I would just use a non-inverted input buffer stage with a 1M input impedance. Then follow that with an active baxandall tone control. You could pull the entire thing off with one dual opamp and a handfull of resistors, capacitors and pots. You make all sorts of variations on this design. Change the tone control to a passive FMV and add a recovery stage, add clipping diodes in the gain stage(s), etc...

Edit: Here's an example of what I'm talking about.
http://www.runoffgroove.com/tonemender.html

That has a non-inverting input buffer with unity gain (easily converted to have positive gain), a passive FMV tone stack with some adjustments to shift the response around and then a recovery stage with additional gain.

If it were me I would probably convert the input buffer to a gain stage with adjustable gain and change the recovery gain stage to be fixed gain close to the average insertion loss of the tone stack (maybe 10 - 15dB or so) then just add a master volume control after the whole shebang. I believe it's better to do your amplification up front rather than try to recover too much volume from a depleted signal. Having your gain up front where the signal is strongest and most "clean" (from the guitar) should reduce noise.

[phatt]

Quote from "armstrom"   
"Having an inverting op-amp as the input stage means that your input impedance
is dictated by the inverting input resistor."

No I think you will find it is not that simple,,Heck Electronics would be easy if it was!
ROG seems to like hanging 1meg resistors off just about everything.
If you search it out the 1 Meg is Also in parallel with The internal impeadence of the opamp ,,,   [from what I've read, some are dismal]
the volume pot on the guitar the PU and probably the cable as well.
Now the maths does not look so easy ay?
Someone better qualified than myself may wish to add comment on that.

Yes you get bigger signal with 1Meg more bandwidth and then spend forever trying
to get rid of noise issues and working out how to get rid of high freq hash.
Oh yes I also used to subcribe to that thinking but I've learnt that
kind of design is plauged with noise issues.

Just using the 1k per volt rule of thumb tells me that an opamp with 10volts supply
only needs about 10k across it's input. I know it's far more complicated than that,,,
but Think about it?
a Thermionic valve is a high voltage, high impeadance, low current device
While a transistor is a low voltage, low impeadence, high current device. humm?
The amount of opamp circuits that just use valve values as a rule of thumb
is showing how limited some makers are, they obviously don't do much research.
Opamps are not Valves. That is why if you notice 10<100k on opamps and 100k<1meg on valve circuits.

As I said I did try my circuit in many ways and a buffer frontend IMHO sounded worse.
Putting the buffer "After" tone allows the next Gain section to run at a much lower imp.
Thereby reducing the colletive noise. [Check out Mesa Vtwin opamp section to find
a lower noise technique and BTW Yes I've built a Vtwin circuit]

My circuit develops a 30 to 35Db notch cut at 400HZ!!! [give or take a few db]
now try to get that from tonemender circuit and you will be waiting a long time.
I took the time to sim tonemender for you and you are lucky to get 10Db cut
at 200 HZ with midrange at zero.
BTW the 1meg after treble does nothing as U1b's positive terminal
is already getting DC Ref via the pot chain.

The 500k gain pot Alone would make the tonemender a noise prone circuit.
So If that is what my circuit has to compete with then I am not worried.

Heck it is simple,,,   if you want tone control build one,,
if you want a gain stage?    build one of those as well
but jezz don't try and do it all in one stage.
To much gain at the front will likely stuff up the tone control section anyway.

Gain alone sounds crap by itself is unlikely to produce good tone,
Just sim some famous tube circuits and you might get a fright when
you see how little distortion is in the preamp stages of those Marshall Amps,,,
as most of it is happening in the output stage. Sadly not easy to do with all SState circuit.
The voltage gain is built up over stages one being a LOSSY tone section.

The whole point of this was to add a Tone Stack THAT ACTUALLY WORKED
and did not raise the signal level much past what went in,,
ie 1 to 1, So only the tone is altered. The reason I built this was to get around
the often pathetic tone circuits used in SState Guitar Amps. [and ROG just copies them]

The famous [and much copied] TS9 circuit has a DC rolloff cap connected
to a pot for some basic and rather wonky tone control, Trying desperatly to put tone
and OD all around one opamp. The TS9 circuit has been done to death and no one has woken up to the simple fact that it's not enough by itself.
Try my circuit before your fuzz boxes, you might get a big suprise at how much more control over tone and your OD you will have.

You're obviously more into the teck maths stuff than I am, and good on you for picking
up on some of the details. Hey it's just a hobby for me and I'm not promoting anything.
I still can't believe this circuit and how well it works,,, Again It just does.
I have ten very happy musicians who are smiling knowing that you can't buy this tone at a music shop.
I only mentioned the noise issue because Most learners would likely just stick in a 500kpot  [To much ROG] thinking Gain, gain and more gain and then complain about the noise. THEN Blame my circuit.       OBVIOUSLY you have not built my circuit.
I'm not that good with maths and I do not propose to know ALL. It's taken me 2 years
to even build up the courage to post this and make comment.
I'm quite aware that some folks are way better with teck stuff and will laugh at my circuit and I have no doubt that like ALL electrical circuits there will be limits/flaws somewhere.
But everthing it gets plugged into is greatly improved.

Keep in mind with ANY of these passive tone circuits,
They do not produce gain it is only by creating large loss at different freq
that any real tonal advantage can be had. [And it is uniquely different if you get it right]
The steeper the mid notch the better it will sound as big shallow scoops don't work.

All is well,, Phil.

[armstrom]

I should probably clarify, I was talking about the circuit posted by James when he started this thread, not the circuit you posted as a reply. The input stage of that EQ circuit (the one posted by James) is an inverted op amp gain stage. I'm not quite sure what you're talking about things being more complicated than that.

From here:
http://www.electronics-radio.com/articles/analogue_circuits/operational-amplifier-op-amp/inverting-amplifier.php

"It is very simple to determine the input impedance of an inverting operational amplifier circuit. It is simply the value of the input resistor R1. This is because the inverting input is at earth potential (i.e. a virtual earth) and this means that the resistor is connected between the input and earth."

Which I believe is what I said. The circuit James first posted has a series resistor between the signal source and the inverting input of the op amp. I'm not sure what other resistor is in parallel with it if you assume the inverting input to be at ground.

Now, if we were talking about the non-inverting input then you're quite correct. You have to take into account the resistance of the bias reference resistor (assuming a single supply is being used) in parallel with the internal resistance of the op amp. Now while its true that the internal resistance can vary greatly between types of op amps most modern JFET-input devices (like the TL07x series) have very high input impedances. According to the data sheet the internal impedance of the TL072 is 10^12 Ohms which is 10 TERRA ohms. That huge value in parallel with any resistor in the 1M - 3M range will have pretty much no impact.

[phatt]

Thanks for that armstrom,
Will pay me to read first ay?  :duh
I now see what you meant.
Hopefully James will still get something useful from it all.  8| 
Phil.

[armstrom]

Not a problem. It happens all the time I should have been more clear which circuit I was commenting on.

As for your circuit, Have you tried removing the first op amp buffer? (immediately after the tone stack). Since it provides no gain I don't see how it would affect the sound of the circuit. Since the final gain stage is non-inverting it already has a high input impedance (Assuming you moved the 2M2 R5 to replace the 47K R6). It would be interesting to move the U1a buffer stage in front of the tone stack to isolate it from the pickups in case a guitar is plugged in directly. This would  help prevent loading of the pickups.  You mentioned that you tried adding gain in front of the tone stack but like this configuration better... did you try just a buffer with unity gain?

-Matt

[phatt]

Hi armstrom
Yes I just moved the buffer to the front,, 
It's a 50/50 thing,,,does not seem to make any difference.

Yes I hear what you say one wonders if the buffer needs to be there at all.
That can be deleted also but I figured that Messa know more than me and wouldn't
go to so much trouble unless they had good reason.
I did have the volume wired between buffers just like the Vtwin
but in the end the trim pot does just fine.
Folks like you may have ability and equipment to test which is best.

Also I'm running from battery power and you can chew up power real easy by adding to much.
What confounds me is that with or without an Active front end it seems to work the same.
Which raises the Q. What friggin impeadance does a guitar input actually need to be
able to work in a reasonable fashion?
I get the distinct impression that a lower impeadance may actually be of benifit.
[Especially with hard driven Amps]

I think it is now gaining more exceptence that lots of distortion at full bandwidth
sounds absolutely aweful and some modern tube amps can sound pretty harsh as well.

It was once said that the low input on a lot of the old fenders works better
if you want to run the Amp really loud as it keeps everything together, tighter OD.
68k is ten times smaller that the high 1meg input, Yet the Amp still pumps out close
to the same wattage. [just food for thought]
I personally use the low input on my tube amps more than the high.

I also have a OD circuit that I built from a reverse engineered Nobels Sound studio 1
[a little headphone crunch box from the late seventies.]
It has a non-inverting buffer input with just a 10k series and 68k in parallel.
[so the input imp is probably under 100k]
This buffer drives the main input [a 10k pot] which then goes into a selector
of 4 modes, clean 1/2,  OD and Dist.

While rebuilding I noticed that changing the 68k to the manditory 1meg just makes
the noise and bandwidth increase. This of course makes it a little crunchier
but at the expense of a lot more noise which adds all the harshness,, Yuk!
But my slightly modded rebuild is very pleasing,,lets say I've heard a lot worse.

Back to my tone box,
I know there is the Signal to Noise Ratio issue with my circuit
that being the incoming signal is reduced by 9 to 10 times THEN amplified.
It's open for debate as to which way is better.
If you wack all your gain up front the SNR is better BUT then you've probably
got a whole new set of problems to overcome.

My Vtwin build with a triode frontend was good but triode distortioin was not for me.
It was in fact the powertube stage compression effect that sounded so much better to me.
[it's much richer in harmonic content and I'm glad I went to the extra effort
to work that out].
It was of course while messing with the Vtwin circuit that
I stumbled upon this idea of connecting directly into the tone stack.

So I'm just waving a flag to say: Hey look it works this way also and it's 
a lot less hassle than building a dozen tone circuits like the one you mentioned
and works heaps better.
By FAR The most important lesson I learnt from reading really $expensive$ text books
is that no matter which circuit tophography you choose "there is always something lost
with something gained" and it finally dawns on you that most of the time your just picking
the best of a bad lot.
Once again Murphys law is spot on,,
For every problem you solve you just create another. [There is no Holy Grail].
Phil.

[teemuk]

It was once said that the low input on a lot of the old fenders works better
if you want to run the Amp really loud as it keeps everything together, tighter OD.

I don't know about how accurate the claim is because this would cut out treble and I personally prefer the sound of overdrive when bass is reduced before it and treble is preserved to maintain some "attack" and "crispness". This type of [bass-cutting] voicing is also what people driving amps with things like treble boosters and OD pedals set to low gain actually have and many modern amps also tend to introduce voicing that does this. They may even start to roll off the bass response from about 1kHz or above.

Now if we talk about that "muddy" Eric Clapton "woman tone" that you get when you roll off all the tone from your guitar, then I might agree. But then again, that type of tone just isn't my type of thing. Don't know if it's any "tighter" either. I regard tightness as reduced bass response pre-OD (see above) and cutting the treble results into pretty much the opposite.

I think it is now gaining more exceptence that lots of distortion at full bandwidth
sounds absolutely aweful and some modern tube amps can sound pretty harsh as well.

I think that treble reduce before OD usually only sounds muddy – but it is indeed beneficial after the OD because it cuts out some of the harshest overtones.

Of course, people have different flavours of distorted tones. I tend to prefer those thin black metal type tones, which most people find abhorring so my taste might not correlate with most people.

[phatt]

Hi Teemu,
Just pointing out that using the low input does subdue the hiss,
Use of the low input on older type Valve Amps also Stops the signal from going
to big to fast.
This forces the preamp to run clean/cleaner than it otherwise would and now you have
to turn the volume up higher than before,,, So the "power tubes" are now able to do their magic.
[I'm talking pre Master volume Tube Amps of course.]
The Bass was often pretty fat on old Amps and often turned into mud if you cranked them high,
so this was one way to gently pull back the preamp gain.

I think you will find the response curve [on valves at least] is the same shape just down by xxDb.
Of course I'm only using sims.

As your obviously after bright sound then with the addition of one cap right in front of the 68k/68k voltage division, Those old low input sockets can be turned into an Ultra bright Channel.
Yes this trick IS Also Passive. If your interested? And in case your wondering,,
No sadly it does not seem to work on SState Amps.

No I can't see how use of the low input will give you that mud woman tone you speak of,,
If that's what you mean?  Maybe you had a bad input socket?
As for your taste in tone,, well we are all different and we would all be a boring lot
if we all liked the same thing.   
If your after metal you still need to establish the fundermental part
of your sound which to my mind is rock tone with a scooped out mid section.
I have no doubt that there are other ways to get there that maybe better.

With modern tube rigs it's all about multi gain stages of triode fizz and IMO
it's not quite as real/dynamic as the old Amps which needed to be pushed into poweramp dist
to sound good but that's probably why some of those old tube amps are so famous.

There is a whole section on opamp noise in "Art of Electronics" but I have loaned that book out.
Generally I think the story was, spreading your gain over two or more stages will reap better results for noise issues.
Sadly my brain is getting to darn old to absorb and store a lot of complex info.
Cheers, Phil Nambour Australia.