TAC Scorpion Overhaul Odyssey

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Potato Cakes

Well-known member
Jul 1, 2014
Nashville, TN
Hello friends,

A couple of years ago on tour I managed to pick up a TAC Scorpion 24x16 for silly cheap. Idea was to do all the updates for fun then sell it and move on to the next project. However it has sat in my garage for some time under a blanket since then. This year I am redoing my studio and the thing I have been wanting figure out is how to pay lots of money for a new shiny console. Struggling with that decision, it occurred that I should look at the Scorpion one more time. Turns out it will do pretty much what I would need a console to do. So once again I will start looking at what helpful and unnecessary mods can be made. It is to be understood that a new power supply with more than enough amperage to accommodate any and all mods will be acquired for this project.

First is the input section. I have a mountain of preamps of all different types of flavors, so I really only need this to be a line level mixer. The Langley line cards that Colin sells seem to be a great option, but a DOA circuit like Eli-Audio has is also attractive. This seems to just be a preference decision, but I would be curious if anyone here who has worked on these channels has any opinion.

Second is the EQ. There are four bands on the channels with HF/LF being selectable shelves and fixed three band on the busses. This circuit seems to be a weakness for this board from what have read from the infallible and all-knowing internet peoples. I had thought about changing out with another EQ board and some how strapping it to the channel PCB. But after comparing schematics, it seems that Mid 1 and 2 sections from the W492 and the HPF/LPF from Harrison console overlay pretty close to the EQ schematic on the Scorpion. I believe with some minor adjustments, I can have both mid sections from the W492 with slightly wider frequency ranges and the Harrison filter sections without the need for crudely attaching extra boards. For the busses it would be nice to do some sort of 553 style EQ, or even a Studer 169 with a fixed mid band. If I can do all of this without buying additional boards that would save me a pile of cash.

I had seem that changing some resistor values for the master fader would change to the SSL style of all the way up is unity as that is to reduce noise. I do not know if it would be wise to do this for all of the buss faders.

The last thing I guess would be what do regarding the busses. The first thing that comes to mind is making the stereo master clean and then having all or some of the busses having character options. I haven't really looked at the bus sections to see if any of that is remotely possible.

The good news is that when I am on tour I have several days at a time when I am off, so once I verify that one channel modded how I am describing works, then I can just take a number of them with me at a time and have something besides sleep to do during my down time.

A new shiny console would be nice, but I don't see it being a financial option at this point. Plus, I feel that the Scorpion has a lot of potential that hasn't fully been explored. Starting next month I hope to begin the arduous process of cleaning and recapping while simultaneously testing my ideas.

In the meantime, I am looking for the schematics for the bus and master section of the TAC Scorpion (I) if anyone has them handy.


Oh boy, I´m through this, let me give you a few thoughts.

If you don´t like what you hear or if you´re missing features then get rid of the desk and get something else. These days large FoH desks are dirt cheap. They are good enough for summing and EQing, some of them are even better than many studio desks. On top of that throwing a lot of new parts at your Scorpion won´t change it into a Neve or Studer or whatever. It will still be a (hopefully better sounding) Scorpion.

If you think of modding your desk then look at the amount of money you´re throwing at it. I´ve spent 1500€ in parts to refurbish and modify my 30/8/8/2 Scorpion. The result was great but the same money would have bought me a desk with more features in a better state. But I still regret that I had to sell it since it did sound exactly the way I like it because I modded it to behave like this. That´s the real advantage of modding.

I´ve modified the stock micpre in my Scorpion. The guy who later on bought it from me sold all his API outboard pres a week later. And he´s got talented ears and a very good monitoring system. This leads to:
Don´t believe internet hearsay about what is good or bad about this desk. Use your ears. There are more wannabes out there in this business than real pros, esp. posting at gearslutz. You never know who´s bashing something and under what circumstances he came to his opinion.

With a bit of help the Scorpion EQ sounds stellar. There´s no serious weakness in this design. Not at all. A few things can be improved but that´s not a design problem. It origins from the target market it has been designed for. After all TAC was the budget line of Amek, so they had to cut a few corners compared to the Amek products (and a lot less than other competitors, btw.)

Your plans to add additional PCBs will turn into a mechanical nightmare. I cannot imagine anyone seriously wanting to do that. I don´t want to discourage you but having done stuff like that I can tell you that it´s a serious lot of more work than you´re expecting.

If you want to improve the desk then you should look at parameters you want to improve not at flashy brand names that promise sonic nirvana. API inputs or Neumann EQs will not necessarily improve any of the real weaknesses of the desk. Some examples:
The micpre chip is not happy driving loads lower than 10k. With EQ disengaged it will have to drive the insert. Connect your Pultec or other 600 Ohm gear and you´ll have a distortion generator.
With EQ engaged the last chip in the EQ circuit will drive the insert. That´s a TL072 which is ok for loads >2k. Same problem here as above.
You´ll need a load of adapters for inputs and outputs because mostly everything is unbalanced and sending/recieving signals at +4 and -10 at a time.

You´re right the desk has a lot of potential but it has to be done with some thought.
Totally agree with everything you've stated. I already have to open it up and clean/recap this guy, so while I have it apart I was going to see if more could be done. I realize that it won't be super cheap and I will incur some headaches, but for I think it will be worth it for my own education and for the benefit of others here. And anyone who reads any interview on how a multitude of iconic records were cut with less than sexy microphones and gear will understand that proper engineering techniques are more valuable than expensive, fancy equipment, so I don't believe all the hype on the internet and I am definitely not wanting this to be GS style thread. I would like to use this as an opportunity to learn a little bit more about design and hopefully the end of this learning process will yield a very high quality sounding console with options that are not found in any other commercially available desk, making working at my place unique to the sea of other studios that are here in Nashville.

During the month I am out, I am going to be working on some drawings and would like to post them here for criticism. I am still trying to find schematics for the bus and master sections, so if I someone has them I would be grateful if they were send my way. And any other tips from other people who have modded theirs would be welcomed.


The mike preamps in the TAC Scorpion use the SSM2015 chip, as does the Symetrix SX202 mike amp. A while back, I worked out some useful mods for that chip, most importantly, new compensation cap values that optimize the gain structure of the SSM2015 and push the knee at which distortion rises at 6dB/octave from 3kHz in some versions of the SX202 to over 40kHz, making it sound much more open and clean.

I checked the schematics, and from the values used in the Scorpion,  it seems that there is some meaningful improvement to be had by changing these three compensation caps. The present values in the Scorpion are: C5: 47pF, C6: 33pF, and C7: 10pF, and these should be changed to C5: 56pF, C6: 7.5pF, and C7: 7.5pF. I used small SMD C0G caps, and to make the 7.5pF, I used a pair of 15pF caps in series. The Scorpion mike amp bias resistor, R17, is 33kΩ stock, and that, along with the ±17.5V supplies, generates the same bias current as the 27K4Ω and ±15V supplies in the SX202, and should thus be left alone as it is already optimal. But, this probably means that these SX202 mods should work equally well in the Scorpion, since both circuits run the SSM2015 at the same internal bandwidth.

I have a web page that details all of my SX202 mods, but this page outlines the compensation cap changes: http://montemcguire.com/sx202-compcaps.html  I think this could be a very simple, low cost way to improve the stock SSM2015 mike amps, making them quite nice if you want a clean mike amp. Of course, you can use the Scorpion with outboard preamps, but rather than carving a potentially good mike amp out of the console, or adding an entirely new mike amp module, you can easily modify it and make it far cleaner than it used to be. I feel that an SSM2015 properly compensated is far nicer than the SSM2017 or similar modern single chip mike amps, mainly because you can get rid of that annoying rising distortion with frequency characteristic. It's one of my pet peeves of many single chip mike amps, and I think it sounds brittle and cheap. Three caps are all you need to make it go away.
Potato Cakes said:
I am still trying to find schematics for the bus and master sections, so if I someone has them I would be grateful if they were send my way. And any other tips from other people who have modded theirs would be welcomed.

The Technical Documents section here has a nearly cmplete set of Scorpion schematics, and hopefully they match your desk: https://groupdiy.com/index.php?topic=60199.0
I´m totally with Monte. There are two ceramic PSU bypass caps close to the micamp. Replace these with 2x 100uF electrolytics and solder the original ceramics underneath the PCB parallel to them. That way you have a local PSU bypass close to the chip. That will help with the transient response. Furthermore increase all coupling caps at least x2 or even more if space permits. And feel free to use 0,01 - 0,1uF bypasscaps parallel to each electrolytic.
I think for the time being I am going to proceed as this being a line input only, and since there a number of options that are available, I am going to circle back and address that issue later.

I've started doing some sketches on paper for the HPF/LPF sections. One of things I want to do repurpose the stock frequency switches on the HF/LF shelves to be a bypass for the filters. For the HF in the schematic, would be best jump together R19 and R42 or R21 and R23? For the LF section, would it be best to jump R46 and C24?

Attached is the EQ section schematic for reference.

I'll get some crude diagrams of what I've come up with posted when I get some days off next week for critique and review.





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Wouldn´t it be more practical to build some outboard HP/LP filters and use them on the inserts? You´d not have to give up some of the functionality of the desk. I found myself using the LF/HF frequency switches a lot, while I never found a need for a single filter bypass. YMMV of course.
The only reason I brought up the filter bypass is that if I was going to do the HPF/LPF for the high/low shelves, then I was trying to figure out some use for those stock frequency select buttons. I could build filters externally like you said, but I figured if I changed the range of the two mid bands then that would cover 95% of what I would do with an EQ as far as boost/cut needs and the filters would take care of the rest. I have a number of other external EQs that I could if something different was needed.  If I was smart enough I would use those switches to change the slope/octave of the filters, but just doing a bypass seems to be more what I can understand doing without someone doing all the work for me.

I regret not bringing a channel with me to do physical measurements and get a better idea what challenges I am unnecessarily imposing on myself.

I do appreciate all the input and thanks for chiming in. This is going to be the last major project I take on for a while and I'm hoping it will turn out like I am envisioning.


Managed to finish up a drawing for implementing a 553 style EQ for the bus channels. The goal is to be able to do this without the need for add on boards as I just started messing around with AutoDesk so I'm a long ways from designing any usable PCB. Also take in mind that I do not have any of the channel strips with me to check my work with the actual circuit layout. The opamp pins need to be relabeled for a DOA to DIP8 adapter to work, or I could just redo the traces for the DIP8 adapter. Maybe I could get away with leaving a dual opamp in there and short out the unused channel like on the Harrison Ford PCBs.

Pending doing actual testing with buss EQ changes, I'm going to move on to the channel. Still doing some debating here. I could get the W492 boards from Gustav and attach them to the main input channel board, using switchable fixed values for the high and low shelves as they pertain to the stock frequency labels. I still like the idea of having the LPF and HPF on board, and I have made some rough drawings how to do that by replacing stock values and adding jumpers. What it may come down to is putting together a schematic of the desired circuit and then hiring someone to do the board layout and provide me the gerber file. I've caught myself making this way more expensive than it needs to be after reminding myself that I would have to buy whatever fancy components I would like to use 24 times. I have a couple of days off so I'm going to do see if anything more simple than what I currently have will come to mind.




  • Scorpion Bus API EQ redraw.pdf
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I found a SSL4000 equalizer schematic and upon first glance it seems that aspects of this design and what is on the Scorpion overlap quite a bit. The only thing is that I would need to figure out a fixed value for the Q.



The stock eq schematic looks similar to what I ended up with when doing some trident style eqs with each individual band separated.  There is an old s800 eq schematic floating around here which could be useful.

With the topology as is you end up with a proportional q type.  It's pretty useful, so I would lean towards minor tweaks over a major overhaul.

john12ax7 said:
The stock eq schematic looks similar to what I ended up with when doing some trident style eqs with each individual band separated.  There is an old s800 eq schematic floating around here which could be useful.

With the topology as is you end up with a proportional q type.  It's pretty useful, so I would lean towards minor tweaks over a major overhaul.

That is mostly what I am hoping to do. The other wonderful EQ's that I have built are all smooth when doing any high mid boosts which make them stand out from plugins and budget analog designs. I don't know exactly what they technically have (or don't have) that makes them different, but my ears immediately notice that something that sets them above the typical small studio options. I try to avoid describing things subjectively here, but in this case I'm not sure how to explain otherwise.

I'll search around for the S800 schematic as I do like the Trident designs. I appreciate the tip.


Using an S800 schematic I have been able to a get a rough drawing started but I'm a little unsure on how a couple of things are implemented. The S800 uses 4 op amps and the S1000 EQ uses 6 (3x TL072). To keep it the same as possible with the S800 schematic, I could use 2 single and one double channel and still use the same IC sockets. That would require some manipulating of the copper traces on the board so the single channel op amps would work properly. This also means I am losing the buffers for the HF and LF bands, and I'm not sure how much that will affect the gain as it leaves the EQ, even with applying the same values and placements of the S800 to the S1000 board.

As john12ax7 suggested, probably some minor tweaks are all that are in order. But with having any real design experience with EQs and knowing what makes things good, I won't be able to determine what that is (opamps, component values, etc) until I have the console in front of me and I'm listening to it. Tomorrow I will post my crude drawing of my attempt to conform the S800 to the Scorpion channel EQ. It seems that I can implement the S800 schematic as is to the stock channel PCB minus the input/output stages, but would like some input from EQ designers to point out the mistakes, as there will be a handful.


I'm with the other guys here. I really wouldn't mess with the EQ to the extreme like your suggesting, putting in new PCB's, discrete op amps...It's a lot of work for very questionable returns.. The suggestion of finding a stage/live mixer with the features you want is the best idea I would say.

But if you really want to get your hands dirty, I would start with that supply. You could  make a really nice one out of some power one modules (picture below). Ton's of current, and quiet.
Then a re-cap of course with the largest values and poly bypass like jensenmann suggested.
Clean the pots, switches, faders.
Maybe change some of the TL072's for NE5532's. I've had good luck in some circuits doing that with lower noise. You will also have 600 ohm drive capability on the inserts with the NE5532's

Plug the console up to some converters and do a mix, see what you think. Maybe dick around with the TL072/NE5532 substitutions till things get as quiet as they can.

So after all that, which kind of has to be done anyways, you still have the need to rip up traces and install mini pcb and stuff, go for it. But I bet you will either like the way it sounds, and leave it like it is, or hate it and sell it all in tact and go hunting for that stage mixer.

But hey if you want to just dive in Tasmanian devil style, go for it!



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The PSU is not a problem. Amek did use the same PSU for much larger desks, so don´t worry about that. I had my Scorpion stuffed with current hungry chips and the PSU worked flawlessly, in fact it still works upto today. The only thing I did to the PSU was recapping, adding bypasscaps, renewing the thermal grease and adding a silent fan.
If you draw high current out of any PSU the problem will be the heat. Adding a fan will take care of that.

2x I had other TAC PSUs on the bench with defective fuse holders. In that case it make sense to install new fuse holders with higher current rating. I did use 20A types as replacement.
jensenmann said:
The PSU is not a problem. Amek did use the same PSU for much larger desks, so don´t worry about that. I had my Scorpion stuffed with current hungry chips and the PSU worked flawlessly, in fact it still works upto today.

Awesome!, sorry I'm not that familiar with the TAC stuff. I just assumed the worst for a mid level mixer like that.

Well that makes it even easier! re-cap, cleaning, couple op amp swaps, good for a test spin!

Finished up the drawing the S800 on top of the S1000 schematic. For the time being, I left the stock capacitor values in place for the frequency selections. Also, R44/R26 and R48/R33 seem to be swapped as they relate to their corresponding op amps compared to the S800 schematic. I don't know if that makes much of a difference.

So while doing this drawing, a couple of questions came to mind. Does losing two channels of opamps gain anything in terms of noise or audio performance? If I kept all of the op amp buffers as they are used in S1000 and just changed the surrounding component values to match the S800 schematic, would I get nearly the same results? Again, I am only doing stuff on paper at this moment for the month I am on the road.

I think I will keep the SSM2015's as suggested and swap the components affecting it and see how that compares. My overall plan is to mod a couple of channels differently and do some blind tests to see if the audio quality deviations between the various approaches will be worth the cost.




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Less opamps can be cleaner,  how many series tl071 (072) can you tolerate before it matters? But if you go to a "better" opamp the problem diminishes.

The main issue with the s800 is that when the mid bands share an opamp their response becomes interactive, adjust the low mids and you affect the high mids, and vice versa. Is this cool or annoying?

I would say keep the current topology.  Instead focus on the other details.  Maybe different caps,  maybe different opamps, adjust resistors to get the Q you want, lower resistance when possible to reduce noise, add psu decoupling, etc.