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OTA-Overdrive-2 (None-Opamps)

Status: Proposal
May 17, 2016

This project is presented as a really new and stand-alone partition, because it comes almost not as a possible modification to the older units from a few years ago (published and printed in Elektor 03/2015), but this "Version II" is completely new featured, including newly routed pcb´s and I hope the website-moderators will accept this and not cancelling this innovation as a repetition to the older units, although if some corrections to that older versions are of course mentioned and the basic principle by using a current source as active element stays the same.

All recommended parts are worldwide available, nothing exotic and almost all parts are not very expensive, so this project is meant as another invitation for all diy´ers and guitar-players, to grab again their soldering-irons for a really new device and concept and getting once more much playground for experiments...

As the project is emphasized in the headline as a circuit "without any opamps" this is only possible based on the facts that "Texas Instruments" has reintroduced the "LM3046-IC´s" again into series production, where these parts have been discontinued some years ago by formerly manufacturing predecessor "National Semiconductor". By the way these chips carry five very closely matched low noise transistors, coming as a versatile transistor-array and this is quite interesting to perform opamp-like constructions and strictly avoiding the usage of higher integrated chips. 

These IC´s are identical and a pin to pin replacement to the older and wellknown "CA3046´s" formerly developed by "RCA/Intersil". Those parts were also published in the very first "Elektor Magazine No.1" back in 1970, so therefore these arrays are coming as true vintage devices, but lately they are produced up today and available only in smd-packages. 

The parts as DIP14-IC´s are almost still available and my personally most interest is intended on electronics performed with such parts, as they are much more useful for any trials, if additional sockets are prepared for a first startup. 

I present the new version of the OTA-Overdrive (also an enhanced Rack-Version is in progress as a following contribution) again as a really unique device, as some major features have been changed and all "children´s illnesses" from the first ota-overdrives have been mostly deleted. 

Strictly, upon the input some heavy critics, this unit comes without the possibility for switching any diode-pairs, so either building the unit as a Germanium- or as a Silicon-Version is from hereup conclusive. Therefore this device is realized with also a smaller and more valuable PCB and only using one footswitch, again reducing costs. Also the pot for the "Soft/Hard-Control" was cancelled and this position changed into a very useful "Volume-Control" at the output, where several trials with different settings and guitar-pickups on the first units have leaded to this very important and surely needed feature.

As this unit has once more a "real touch of the 70´ies", also for all people who insist for implementing all parts "as much vintage as possible" with using some "Low Noise-JFET´s" (Japanese 2SK30A or 2SK170BL) and two IC´s, this unit drives very low power consumption with a little less current than about 10 mA, except for a possible control-led, which should get into account for using the thing properly as "onstage device". 

Here you may download two PCB-Versions, with and without SMD-Semiconductors, if you are able to get the DIP-Versions for the IC´s (still almost very good available on ebay and similar auction platforms, or by contacting: jo2030 ahet, I have left a lot of LM3046´s in DIP-Packages for the moment). 

Another possibility for purchasing quite good LM3046-Derivates/-Clones, is to buy them as the "Polish Unitra - UL1111" from Bulgarian vendors, where those parts are highly recommendable as found out in several trials and sometimes maybe better than the originals!

No inactive chip-sections
Another interesting feature of the unit - and cost reduction - is the usage of the whole LM13700-IC´s, where also the on-chip darlington-buffers are completely accessed. Very important in that case is to mention: This will not work with any LM13600 or NJM13600´s due to the different chip architectures for the buffer-stages.

As also featured in the headline and on expieriences with the formerly version series this unit spares now all trimpots, where possible small mismatches on respectively the OTA´s offset errors are quite easy to cancel out when setting the symmetry pot a little "off-center" and what gets almost quite audible in the sound. 

On a careful cost estimation - for low-budget diy´ers - on possible local varying parts-prices and when using new parts - this unit will come about a maximum of 35-40 $/€ at all, if one would (buy expensive offers and) order commercially manufactured pcb´s in small numbers and by using herefore the presented Eagle-BRD-Files.

How the circuit works
Again adressing the possibility of widely varying guitar-pickups or other possible signal sources, the first stage at the input of the device is a pre-amplifier realized with a JFET on an amplification-factor about Av=10, optimized for standard electric guitars over the needed frequency range. 
This stage is followed by a high-ohmic voltage-follower performed with the first internal darlington stage of the LM13700, whereas the signal is then applied to the OTA´s inputs. 

Additionally, if you drive the unit with a guitar-signal above 250 mVss, this buffer-stage is able to drive the OTA-Inputs very softly into somekind of a "pre-distortion". When modifying R16 & R18 (+/- 10-22 kOhms) you may select this effect for more or less - maybe interesting additionally. 
The main-stage is arranged with the two OTAs as a "symmetrical class-a floating amplifier" onto the clipping diodes without any ac-coupling capacitors and this is then directly followed by the "tone-control" which respectly also acts as an "electrical load" to the used diodes, contemplating them as voltage generators. 

This main section of the schematic, with the diodes, shows additionally two jumpers where you may use again several possibilities to control the sound-characteristics, dependent of what kind of diodes are preferred. 
The first one is the formerly known "Soft/Hard-Control" by a serial resistor directly to the diodes (150 Ohm to 2 kOhms), of course also possible as a potentiometer and the second one is an additional resistor connected in parallel to the diodes with a useful/selectable value around 5 kOhms to 27 kOhms and this also possible as a potentiometer. 

Both described resistors affect the diodes in conjunction and with respect to their leakage-currents and other individual parameters, as also maybe the "picofarads" of the GE-chips and here to say with better words if you use HF-Diodes, for instance.

Of course alltogether with this section completely implemented as variable potentiometers you´ll get a unit with really much controls placed around the diodes and also maybe in the beginning not easy to understand, but with these parts you will get - in conjunction with the choosen OTA-Drive-Current - the best possibilities to individually adjust any sounds with the widely varying characteristics of any germaniums, especially also when they begin to drift with temperatures and this can get a huge impact also on GE´s of the same type. 

Because of these facts one may lastly not say, that any specific GE-Diode will sound "like this, or like that" and these facts are resulting in a large experimental area for trying on different sound-possibilities! 
Therefore those jumper-controls are very important and the resistor´s values should get determined "as audible" and directly at "built-up" of the device, with respect to the finally choosen diode-pair, of course depending on user´s personal choice. 

To get a useful, not too complicated unit, it is recommended to select here for most ease on possible two external SPDT-Switches, with an available middle position (On-Off-On), finally having six possible positions and this can get enough for not getting too much confused with that thing and finally quite quickly getting into a preferred sound back again... 

The LM3046 used instead of an Opamp
The section before the output-buffer of the device is again a little of a special thing and took me some longer time to find that finally very easy solution. It looks almost in some parts as a discrete performed OTA again, where the main goal of this kind of arrangement, is to change the symmetrical signal referred to ground potential again and hereby also getting enough signal-amplification (1-2 Vss on some diy-standard values for all used parts) to boost any following external amp´s inputs and also hereby not being too much affected of variations in the supply voltage. 
The result is a differential amplifier which is controlled/biased by a wilson-current-mirror, where herefore all needed transistors are performed with the use of the LM3046 and as a finally decoupling output-stage there follows the second available darlington-buffer of the LM13700. 

The useful possible supply-voltage may therefore vary from 7 to 16 Volts, without any need for changes on values, especailly for this section. So you may drive your 9 V-Batteries to really "the lowest point of usage"!

Proper control for not exceeding the OTA´s maximum current
A quite critical parameter can get the OTA´s maximum driving current (Iabc), which is generated with a JFET-Current-Source, where for both amplifiers this produced current parameter should not exceed 3.5-3.7 mA, when setting the potentiometer for OTA-Drive fully clockwise. Herefore to control this value, JP3 is additionally implemented, where you may connect a DVM and measuring the current indirect, as a voltage-drop over resistor R48 (100 Ohms), giving a measured value of 100 mV for each milliampere. 
An adequate value would be about 350 mV, to stay inside of the save opertating area. The final value, in that end-position of the pot, is determined by resistor R19, where this part´s value should not get selected too small and then destroying the OTA´s current mirrors. For start-up it would be best, to control this without any OTA inserted and connecting the pin-postions 1 & 16 to ground potential.

Additional Information on the LM3046 Transistor-Arrays
These parts are currently needed in large volume by the automotive industry and in clothes washing machines, because of those well-matched transistors, but also it is just a question of time when those parts will maybe get obsolete again and a "last time life-buying" - just for the moment - is highly recommended, if you want to build that effect-unit, which is presented here with the project with respect to a planning for possible service work in maybe some years later. 

Another information upon this item is, that also the CA3086-IC´s (if you have some in stock) represent the same array architecture and may get herefore useful, with only the difference of not having that guaranteed, closed matching data of the two transistors, which are connected as differential pair; respectively for the matching of Vbe´s and Hfe´s.

Again some thoughts on the basic Overdrive-/Fuzz-/Distortion-Principles
Some backgrounds to this ota-units are based on the analysis of several other concepts, as also Mr. Harry Baggen has mentioned in the publication-text to the first OTA-Overdrive very well, where these other concepts sometimes can cause hundrets, or even thousands of dollars for people to pay for, and what I think of course gets a little crazy...  

If one looks at such circuits, they show mostly some matched (and charge-coupled) diodes in direct conjunction with an adapted tone-control as maybe in the "Fender-Blender" or also to be seen in the "Shin-Ei/Univox Superfuzz-Units" - also in the "ProCo Rat-Distortion", that´s the same, where finally the optimization of the diode´s data to the driving amp-stage and the tone section in the end will make the deal. 

Hereby the most brands of commercial units are known for good, better and best ones and this is almost the mystery to those devices, why and how, but quite often these series units will sound a little different to each other, dependent on some data spreading of the used electronic parts. 
Needed to say, that this does not too much append to SI-Diodes, because they do not offer such a wide possible data spread as the Germaniums, due to the much easier to handle silicon semiconductor at manufacturing. 
In the OTA-Overdrive - it is almost more a distortion than a overdriven stage - this gets handled with the "OTA-Drive-Current" and the "Diode-Damping-Control" (and additionally with the "Soft-/Hard-Regulation" where the most other known and commercial units do not have these individual control abilities, - therefore again having a really unique device. 

Have much fun with this unit, and a nice day!

...Complete folders are in progress and will follow as updates...
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