Another Amek Tac Scorpion overhaul!

[Eliani]

If you want a true line input you should either use a 312 type PCB but with a 10K:10K input transformer.

I'm thinking more in the line of using the 325 as a mic pre as noted in the 325 documents. So differential input without transformer and keep the switch as is for mic or line selection. just need to figure out the gain setting.

Meanwhile I've "painted" a couple of images for my documentation.

 

[Eliani]

I was able to measure the pin layout and board dimensions and drew it up for future use...

edit: removed picture because it was wrong!

 

[Eliani]

And then the PCB design.

 

[mitsos]

cool, what is the grid spacing on the paper showing the pin layout and PCB shape?  My circuit was similar, except there was a spot for a mic input trafo, and a separate differencial line input.  BTW, I still think you should use the other schematic, the 325 just takes the lo signal to ground, so it's like a 312 without the transformer.  Depending on your line levels you'll need a pad going into this board, with a differential you can use the amp to give you less than unity gain. 

 

[Eliani]

I've managed to put the 325 circuit in the Scorpion today! I built it on a 0.1 inch grid perfboard which I carefully made to fit similar to the Langley board. Grid spacing on that former drawing was 0.05 inch by the way Mitsos! But be aware, I made a mistake and reversed the voltages, that's why I blew up a cap today...  funny, but always scares the hell out of me!

But I made it work! I've got a 10k log pot for gain but it's got a strange curve! Slow gain, sudden drop, and then steep upwards. Maybe rev log is better?

As for the 312 vs 325, I didn't mean this one to be a mic preamp, just line level. the way it's hooked up you can either use the xlr input, or the line-input. I would like a differential input though, wouldn't that mean a second opamp is needed, that would have to be a DOA too,,

 

[mitsos]

yes, you need a rev log pot. 

differential is like the last schemo in the API paper I posted.  One opamp is all you need, though you lose variable gain (you could theoretically put it on a DPDT switch maybe).

Look at 2.1 here:
http://en.wikipedia.org/wiki/Operational_amplifier_applications

 

[Eliani]

A balanced/differential input for the Scorpion would look something like this!?

JP7 should be connected to COM, but as the you can see on the board, both JP6 and JP7 will be connected to ground on the main PCB, this way Audio-ground and power ground get separated on the sub-pcb, don't know if that should matter!?

 

[mitsos]

Yes, except RF should be 10K for unity gain. All R's should be the same, and you should match them as close as possible for best CMRR. 

 

[Eliani]

So I managed to finish the design for a simple API 2520 based differential unity gain line-level input sub PCB for the S1000 channelstrip.

I found a few LME49720HA leftover opamps from my 200B project in my opamp-box the other day! Following Jim Williams advice on swapping the master mix-bus opamp (U12) and the S1000 fader booster opamp (U5) with the LME49720's I sence a high frequency presence that was not there before.  Clearly a simple and straightforward opamp switch improved the high transient response. I'll need to test if removing coupling caps before and after will further improve the sonic transparancy, and if I need to add those little .1uF caps from 4 and 8 to ground near the opamps. (Guess I'll need to read up on that one!)

I've got to say I learned a lot about opams, differential circuits, and caps since I started this project. At least it hasn't been a total loss so far!

Compared to the Langley pre's the 325 input, I so professionally built on my little perfboard, sound a bit more forward in the mids and a lot smoother in the low-mids around 400Hz. (Sound samples should follow sometime soon I guess).

 

[gar381]

Yes, except RF should be 10K for unity gain. All R's should be the same, and you should match them as close as possible for best CMRR.

To add to mitsos..

Use .1% 10k Rs for R1,4,5 and a 25k Multi turn trimmer for RF then you can
really fine tune  CMRR.

Best

GARY

 

[mitsos]

To add to mitsos..

Use .1% 10k Rs for R1,4,5 and a 25k Multi turn trimmer for RF then you can
really fine tune  CMRR.

Best

GARY

hmm, I know there is a trimmer in one of jeff's modules adjusted for CMRR, is that how it's set up?

 

[gar381]

To add to mitsos..

Use .1% 10k Rs for R1,4,5 and a 25k Multi turn trimmer for RF then you can
really fine tune  CMRR.

Best

GARY

hmm, I know there is a trimmer in one of jeff's modules adjusted for CMRR, is that how it's set up?

All of Jeff's modules that have a Balanced receiver input have a trimmer for CMRR adjustment .
Jeff's procedure for CMRR adjustment will work fine with these pcbs as well.

GARY

 

[Eliani]

All of Jeff's modules that have a Balanced receiver input have a trimmer for CMRR adjustment .
Jeff's procedure for CMRR adjustment will work fine with these pcbs as well.

GARY

http://www.classicapi.com/catalog/images/gallery/LC53A/Build-docs/VC528-ST2-CMRR-Adjustment.pdf

 

[Eliani]

As I'm going for a daily update here, this is what I've got for you today!

As I'm going through the console, modding channels a few at a time, I'm going to visually set those channels apart from each other. I  really don't like the look of those original knobs!
I've got four channels modded with the Langley mic pre PCB, and for those channels I bought some Davies 1510 style knobs (cheap knockoff guitar pedal knobs!) and they do bring this poor looking critter up a bit. I chose broken white and brown, but might replace the brown ones with a bit more vibrant color later, or just black because I like them better in black and white!

I received my d.o.tec Andiamo back from a demo, so I took a quick measurement using the Rightmark application, these are the results, anyone care to say if that's good? I went through the Langley modded S1000 channels, all the way out the master outputs...

 

[mitsos]

test looks OK, can't say what crosstalk should be though.  You should also do a test of the AD/DA alone, to compare.

BTW, can I have your old knobs? I'm missing quite a few on mine...

cheers!

 

[Eliani]

test looks OK, can't say what crosstalk should be though.  You should also do a test of the AD/DA alone, to compare.

BTW, can I have your old knobs? I'm missing quite a few on mine...

cheers!

The Converter scores -113dB, let me know what knobs you are missing because my gain knobs were not original either...

 

[mitsos]

I meant I don't know what these consoles normally got for crosstalk.  As for the AD/DA test, you should run a full test.

I'll count the knobs again and let you know (console is in pieces atm...), I have some new ones and some that have seen better days..  I know for sure I need some EQ knobs.  Will let you know soon, thanks!

 

[Eliani]

I meant I don't know what these consoles normally got for crosstalk.  As for the AD/DA test, you should run a full test.

Just sold the Andiamo (with some regrets!) so I can't run a full test, but I'm in the market for a new converter. I'm looking nito the Ferrofish A32!

 

[Eliani]

And we continue with another preamp design, this isn't nothing new though. A basic That1512 based preamp for the scorpion sub-pcb. I was designing a preamp on this chip a few years back, then discovered the ThatMic from Mikkel C. Simonsen and bought a few of his instead. I never really used the preamps a lot since I built my 51x rack, and Volkers OV560's which I still hate myself for for selling them! (I'll need to get some more!) But I remember liking the clean sound off these little amps. So this is the schematics, based on the original design paper, and Mikkel' preamp.

And as if this would not count as a daily update we finished the PCB design as well! We've ordered the differential 2520 style board with PCBWAY a few days ago. can't say they offer bad service... so if all turns out well we'll add this little board to the order list!

Before the kids go to sleep I managed to finish the API 312 input too! What to do with free time on your hands, and parts in transit!! So the AE2622 trannie, might replace that one with Lundahl/Cinemag footprints. Now all I need to do incorporate the line-level input, but I've got a feeling I'll manage that even with the original switch in place!

 

[Eliani]

I don't seem to get enough of this today!!

Here is my API 312 Mic-pre sub PCB design with LL1528 transformer for the S1000/S1100 channels! This design takes signal from the microphone input to the excellent and "low profile" Lundahl LL1528 microphone transformer (1:5). There's no other way for line-level signal to go through yet!