Preamp Pedal

[GuyB]

I was wondering if any members have built this interesting preamp pedal, it looks interesting.
http://www.instructables.com/id/Portable-Guitar-Preamp/?ALLSTEPS
Thanks
GuyB

[Roly]

Generally
This "Blackface" preamp has come up before, and somebody has to say that <redcircuits> doesn't enjoy much of a reputation around here, and <runoffgrove> rather less so (but at least it doesn't have a brace of trimpots in this one  ).

This is a preamp, but it's no way "an exact replica" of a Blackface preamp, the actual gain of the 12AX7 stages is more like 50 than "80", and the transfer functions of triode and FET's aren't the same (so they sound a bit different).

The killer here is that the dominant tonality of the classic amps mentioned does not come from their tonestacks, it comes from the output transformer in the valve (tube) output stage, so if you are hoping that this will turn your Peavy into a Fender Blackface, or whatever, it won't.

Specifically
Normally when you lay out a front panel you allow an absolute minimum of 1.5" between controls, and more like 2-2.5", even when using the smallest possible knobs.  In the instructables build there is practically no room at all between the controls, they are almost touching, making them quite difficult to adjust.  Since this is normally done on stage and perhaps while playing you need a good finger space between knobs.

"Instructable" implies that the author has some experience in what they are offering, but this guy is a beginner.  Do you really want to follow a beginner who doesn't understand how the circuits he has copied works?  Better to do your own research I would think.  Just a thought.

[Loudthud]

As noted above the input stage has too much gain. But it's not the gain that is important, it is the clipping level refered to the input. Because the preamp runs off of a low voltage, the gain needs to be about 5 if operating off of a 30V rail, even less if the rail is lower. Once the signal passes through a Volume control, gain is not so critical, you can just turn the signal down. The only problem you might run into is the volume may be hard to adjust if the position of the volume control is too close to zero.

As described in the webpage linked above, the input starts to distort when the signal is about 150mV or 200mV RMS. A 12AX7 can handle about 1V RMS before it starts to distort. A resistor in the Source of Q1 will reduce the gain if the input stage to a level that will allow a bigger signal before distortion. Note that the polarity of C1 needs to be reversed. In the original circuit C1 and C7 would have zero volts across them if the JFETs were all matched. The circuit calls for 2N3819 JFETs, the MPF102 is almost an exact match except for the pinout. It won't sound exactly like a Blackface amp, but it's a start.

[phatt]

Yep Red circuits are a bit iffy and there are better ways but maybe The inventor is a novice like Me and still learning the hidden gotchas in circuit design.

If it helps,, my own (PhAbbTone) design posted on this site went through several (make that many) design stages including fet circuits not unlike the above.

Even with 37VDC supply the signal falls apart badly on big signals and was prone to noise issues. I opted for a single dual opamp setup and I have built ~ 20 of these circuits for many local players with no complaints in 4 years.

ALL these Hi Z tone circuits are prone to picking up noise and that includes mine but I did A/B test the *ROG Tonemender* circuit up against the *Phabbtone* and it was no contest,, the PhAbbTone was the winner hands down in both lower noise and useable tone shaping.
Far better minds here can explain this better but I've found that gobbs of gain After a Hi Z tone circuit is just asking for noise issues.

Even with NO High Z buffer frontend my passive input setup is still lower noise. because all the gain is done at much reduced Z. All done just by moving the buffer.

Takes years for us hobby types to work these things out so unless you are seriously savvy about circuit design stick to opamps until you gain experience.

I second Roly's comments about these circuits not being capable of delivering the magical triode clipping rattle. And also Loudthud is right the front end will square wave and sound nasty but if all fuzz is what you desire it will do that.

But if all you want is good tone control with bare minimum of parts and able to be built cheap then the PhAbbTone might be safer bet.

Oh and as it may not be obvious,, you can insert most of those tone tree circuits into this idea. I like the HiWatt tone shape as it has better mid notch ability.
Phil.

[Roly]

Phil does (or should) know what he's talking about, at least when it comes to guitar tonestacks - it's his thang.

But to keep the party going, there is Rod's Project27, a nice little preamp (and it comes with a PCB and support).

The one that grew out of my rant; which can be treated as a bunch or basic building blocks; power supply, and gain stage(s), tonestacks ad lib - experimental, but that's what this was for.  The core is the gain-settable hybrid FET/BJT gain cell, similar to but better than a 12AX7 stage.



Now some observations on this;


What we have here is another hybrid where the Drain load resistor has been replaced by a constant-current source.  So in fact only two devices, the lower FET's, are really needed for amplification.

Using a current source load has applications, and one is where you are trying to get MAG - Maximum Available Gain.  In theory this can only occur when the Drain resistor is infinite (so the supply voltage has to be also).  A constant-current source or sink approximates this condition until the real supply rail is reached (then The best-laid schemes o’ mice an’ men, Gang aft agley,)

But with a limited voltage headroom the last thing we want when a guitar can produce a volt peak, is MAG!  So this first stage must not have a gain higher than the supply voltage or it will clip; 18VSUP, 1 volt in, gain of x18 just clipping.

We must always consider how our gain is distributed around tonestack losses and amplitude controls like front end Gain and back end Master volume controls, and look for unintended interstage clipping - in a word "headroom".

If this pre was billed as "fully-cranked" or something other than "Blackface" (or Marshall or Vox or whatever) I might not mind so much; if the creator had just had the guts to say "here's what I've done - this is mine".  Then he could own his poor engineering.

AND - I don't like circuits that only have identifiers and no values.  Lose the table and the circuit is useless.  Or maybe that's the retentive idea.  {Poetic justice really - it will just decay away into digital dross.}


You can read all sorts of stuff on the net, but really, what you have to do is buy a handful of components, build a "breadboard" test rig, and start building and auditioning stages, modules, sections; get LTSpice (free) and learn how to build circuits in it, and Tone Stack Calc is fun and useful.

[Loudthud]

I just slapped the first stage together on a solderless breadboard. The gain measured about 35dB into the 100K input impedance of my analyzer. I suggest using lower value pots in the tone stack, 25K/25K/1K and 100K for the Volume. The circuit works quite well assuming the JFETs you use are reasonably matched, 5% or 10% should be close enough. The gain changes slightly as the power supply varies. Run it from a decoupled power amp supply rail to get some of the tube amp compression and touch sensitivity.

The circuit clips softly and generates mostly 2nd and 3rd order distortion until it starts to clip really hard, much like a 12AX7. I like to look at the X-Y transfer curve to set the gain, see the attachments below. The big resistor in the source makes the curve slightly more linear than a 12AX7 and I didn't try to tweek the DC operating point by adjusting R4/R5. Reducing them overall would also lower the gain but preserve some of the non-linearity. The source resistor can be bypassed to peak the treble, but install a resistor in series with the cap to limit the peak. Start with 3.3K.

[J M Fahey]

Thanks again for building and scoping stuff.
Invaluable.
Doubly so in this hype prone Musical world of ours. 

[Roly]

It's really great to see some actual test rig numbers, thanks!   

[Loudthud]

I built up a preamp, made some changes and ran into an interesting problem. As near as I can figure the input impedance of the JFET with Soruce grounded is not as high as you would think. It causes the frequency response to slowly dribble down about 2dB from 100Hz to 10KHz before the dominate pole really kicks in. I didn't notice until I actually plotted the response. The MPF102 was the worst, a J201 gave the flattest response but less overall gain and bandwidth. I tried several JFETs that I just had laying around.

Sound is warm and somewhat tube like. I installed the preamp in an old amp I built some years ago with +/- 20V rails for the power amp. The preamp worked fine with no adjustments, I had used a 30V rail to get it running.There are better sounding preamps, but they are more complicated.

[J M Fahey]

Very interesting and thanks for experimenting and posting.
I think one reason for the behaviour you noticed is not that the intput impedance "is not that high" , (by the way you don't show any input impedance graph or table), but the *output*  impedance is way too high.
The output impedance of a classic 12AX7 is around 40K , coming from a 100K load resistor in parallel with the tube's internal Rp, around 68K .
While here you have the *very* high load impedance provided by a constant current source (which in theory should be infinite, go figure), in parallel with the internal output impedance of a FET, much more similar to a pentode than a triode.
So gain is much more dependent on the complex impedance presented by the tone stack.
Which for a reason I don't understand you downscaled 10X in impedance.
I wonder why, since :
a) it still drives a 1M input impedance recovery stage
b) it's driven by a source impedance at least as high as that of the original triode stage, and probably much higher.
I'd retest with standard Fender values and see/hear what happens.
I'd also retest using TL072 gain stages, around 10X to 20X gain each, and standard Fender values, *or*  in this case, with 10X reduced impedance one if you wish, since the 1st stage will easily drive that and then some.

I have used that for ages with excellent results.
"Tubey"?
Dunno, define "tubey"

[phatt]

If you are still with us GuyB?

I'll assume (like me a few years back) eyes glazed over and wondering what it's all about. (lots of headscratching)

If you simply wish to build your own audio circuits for guitar stuff,,,,

Can I suggest you get a breadboard, a 12volt plugpak/wallwart and throw a few circuits together and just have fun making circuits and test them for yourself.
Yes it will be slow and frustrating to start with but with opamps it's hard to blow stuff up and you stand a better chance of success over discrete circuits.

while doing this learn to use simulation software, many are free.
The combination of the above 2 put me way ahead in a very short time. 

It is by far the fastest way to learn for those wishing to make simple analog circuits.
I'm hopeless at a lot of the maths and still do not fully understand what these very talented folk speak about at times but I'm able keep up in general because I did the above.

Oh I should say I read a few books as well.

I use "Circuitmaker 6.2" which is free up to 50 parts but it was enough to help me build the many ideas I've posted here.
I have no special equipment except for a couple of DMM's a soldering iron and lots of electronic components half of which is sourced from trashed electronic equipment.
Don't be afraid of sims,, hey you can simulate an opamp at 200 volts if you want lol
It matters little what you do wrong as no parts blow up like in the real world.

Just load one the many circuits that come with the program and then start to branch out into other circuits once you become familiar with how it works.
The link still up, http://my.ece.ucsb.edu/York/Bobsclass/2C/Simulation/circuit_maker.htm

Meantime here is an old pic while testing the PhAbbtone up against a fancy Parametric EQ to give you an idea of how easy it is to start making your own test circuits.
Phil.

[Roly]

 


LTSpice simulation software
http://www.linear.com/designtools/software/

[phatt]

Ha good one Roly.
Yeah mate I know you are probably right but my brain is fried enough from trying to come to
terms with new software and technology in general.
I struggle to use our mobile phone an I'll be damned if I'm going to become a one thumb typewriter.
My friends Have ipads, ipods, seems everything is i something.

Looks like I'll just remain an "i dunno?"

I will say that for a novice I found circuitmaker 6 quite easy to grasp.
Some of the others sims are obviously way more powerful but for the basic stuff I mess with it
works.

I did try Cmaker 2000 but found it rather useless as you can't overlay plots whereas Cmaker 6
can shoot multiple plots and you can save them and recall them, even overlay them on a
different schematic.
I found this extremely helpful in finding out which tone circuits worked and which one where duds.

in this example shown I saved the first plot (original HiWatt values) then simply changed the circuit
re sampled the new changed circuit then recalled the other plot, Bingo instant comparison.

You then go do it on a real circuit to make sure you can actually hear the diff as sometimes Sims are not perfect but being able to see on screen then hear what effect it has in the real world is a big asset. Beats the outlay of sig gens, scopes and all the other fancy stuff you teck chaps use. Hey it's really just a hobby so why make life hard if there is no real benefit?

Maybe when I get resettled I'll have a good look at the other LTspice and see if I can get something to happen.

Oh yeah,, God willing I'll be living in Bairnsdale Victoria by mid March. (Hopefully the fires have burnt out by then)
I've got a very good mate down Lakes Entrance way and He tells me there is plenty of music gigs around. He has already got work lined up for me so I'm looking forward to the move.
Phil.

[Loudthud]

Very interesting and thanks for experimenting and posting.
I think one reason for the behaviour you noticed is not that the intput impedance "is not that high" , (by the way you don't show any input impedance graph or table), but the *output*  impedance is way too high.
The output impedance of a classic 12AX7 is around 40K , coming from a 100K load resistor in parallel with the tube's internal Rp, around 68K .
While here you have the *very* high load impedance provided by a constant current source (which in theory should be infinite, go figure), in parallel with the internal output impedance of a FET, much more similar to a pentode than a triode.
So gain is much more dependent on the complex impedance presented by the tone stack.
Which for a reason I don't understand you downscaled 10X in impedance.
I wonder why, since :
a) it still drives a 1M input impedance recovery stage
b) it's driven by a source impedance at least as high as that of the original triode stage, and probably much higher.
I'd retest with standard Fender values and see/hear what happens.
I'd also retest using TL072 gain stages, around 10X to 20X gain each, and standard Fender values, *or*  in this case, with 10X reduced impedance one if you wish, since the 1st stage will easily drive that and then some.

I have used that for ages with excellent results.
"Tubey"?
Dunno, define "tubey"

All the graphs from the Audio Precision System One were done with the tone stack disconnected before the preamp was installed in the amp chassis. The input impedance of the AP is about 100K ohms set to "unbalanced floating". The amp has the tweedish volume/tone setup shown on the schematic. It's a little different from the traditional setup so you can still get some treble boost when the volume is all the way up and there is very little interaction between Volume and Tone.

The output impedance of this configuration is set to a large degree by the voltage divider. The second stage still has the 100K voltage divider resistors. The output of that stage drops 1.2dB with 100K resistive load. For the first stage I lowered those resistors quite a bit to control the gain. This allowed a lowering of the Source resistor which increased low level even harmonic distortion. This is a big contributor to the tubey sound. You hear it at the lowest levels before the amp clips. It gives the tone a warm jangley quality that you just don't get with opamps. It's subtle and you have to train your ear to recognize it.

Another component of tube sound is compression and touch sensitivity. The primitive nature of this circuit, no feedback (almost), unregulated single rail supply and soft clipping make it FEEL more like tubes than opamps and clipping diodes. The first stage has very little compression but the second stage has plenty. I powered the preamp from one rail (decoupled) of a descrete power amp with a small power transformer. If you power the preamp with a regulated supply, the effect will be much less.

I don't have any way to actually look at the complex input impedance vs frequency. I was thinking at the time of my previous post that a non-linear Miller capacitance was the cause of the slow rolloff, but inserting a common gate JFET between the lower and upper JFETs eliminated that. Monitoring the gate with an oscilloscope confirms that the signal is flat at the input. Swapping Drain and Source terminals didn't make any difference. Small amounts (-3 to -12dB) of Source degeneration reduced the effect, but did not eliminate it.

At this point I have no explanation for why the frequency response falls off about 2dB between 100Hz and 10KHz when using MPF102's. I'd be interested to see what simulation shows.

[phatt]

I built up a preamp, made some changes and ran into an interesting problem. As near as I can figure the input impedance of the JFET with Soruce grounded is not as high as you would think. It causes the frequency response to slowly dribble down about 2dB from 100Hz to 10KHz before the dominate pole really kicks in. I didn't notice until I actually plotted the response. The MPF102 was the worst, a J201 gave the flattest response but less overall gain and bandwidth. I tried several JFETs that I just had laying around.

Sound is warm and somewhat tube like. I installed the preamp in an old amp I built some years ago with +/- 20V rails for the power amp. The preamp worked fine with no adjustments, I had used a 30V rail to get it running.There are better sounding preamps, but they are more complicated.

Hey Loudthud,
                   That roll off you noted is likely due to the voltage divider values and C1 value combination.

Well that is what I found when trying out all this some years back. The larger the R values the smaller the C value needs to be to keep it flat.
It is very sensitive to value changes in that regard.

I built an OD pedal on the basic concept and it worked,,, but yeah no Cigar.
too darn fiddly to setup.
Phil.