Another Amek Tac Scorpion overhaul!

[critterkllr]

But as we speak I'm working on something far more important for the little Sorpion! Something to do with inductors!!

Keeping fingers crossed for a time machine!

 

[bluezplaya]

Mmmmmmmmm.....! Inductor EQs! So how much would the 1512 boards be? And I'll bet they'll sound way better than the SSM2015s.

 

[Eliani]

Mmmmmmmmm.....! Inductor EQs! So how much would the 1512 boards be? And I'll bet they'll sound way better than the SSM2015s.

Let's say around €3 each for test-boards only!! No parts, no shipping included... max a couple test boards per person and for a limited time only. Pin-setting is the same as the API2520 boards, Just the gain-pot will have to be wired, and probably swapped too... but I would need to look into that again, as it's been a while since I built myself a 1512 pre-amp...

 

[bluezplaya]

I finally got around to building four of the GAR 1731 opamps and tested the first one out on the S1000. So, it works, but at a very low signal level and the gain does not work. I was not aware, so rereading through the thread I learned that I will need to rewire or replace the gain pot. Can you tell me what I will need to do that?
I listened to some mp3s through the modded channel next to a stock channel and the EQ really thumps! Overall it does have a slightly more pronounced and beefier midrange.

 

[critterkllr]

I finally got around to building four of the GAR 1731 opamps and tested the first one out on the S1000. So, it works, but at a very low signal level and the gain does not work. I was not aware, so rereading through the thread I learned that I will need to rewire or replace the gain pot. Can you tell me what I will need to do that?
I listened to some mp3s through the modded channel next to a stock channel and the EQ really thumps! Overall it does have a slightly more pronounced and beefier midrange.

That doesn't sound right. I had a similar issue with my first channel and the opamp was messed up. It should be the same level as your other channels when the gain knob is at noon.

 

[Eliani]

I finally got around to building four of the GAR 1731 opamps and tested the first one out on the S1000. So, it works, but at a very low signal level and the gain does not work. I was not aware, so rereading through the thread I learned that I will need to rewire or replace the gain pot. Can you tell me what I will need to do that?
I listened to some mp3s through the modded channel next to a stock channel and the EQ really thumps! Overall it does have a slightly more pronounced and beefier midrange.

The Diff.Rev00x boards are not meant to be used with any gain pot in place, they are Unity Gain Line receivers. If there's to little signal in your track, use the XLR input instead of the Jack-input, or remove and bridge the PAD resistors!!

 

[jensenmann]

I don´t want to rain on your parade, Eliani. But simply throwing API into a circuit doesn´t make everything necessarily better. I once refurbished and modded a Scorpion for a friend. I left the bad and ugly SSM2015 in and modified the circuit around it to make it perform better. The result was that the owner sold his API preamps because they found no more use.
Your circuit suffers of mediocre CMRR. If that´s of no significance then go ahead. If it matters then the stock SSM performs better. In certain surroundings technical parameters do matter because they translate into percieved sonic quality. Not everything can be justified with "if it sounds better then..."
Just my 2ct

 

[bluezplaya]

What exactly did you do to mod the circuit around the  SSM2015?

 

[bluezplaya]

And I'd be interested in trying the THAT 1512 board. It would be a cheaper and easier build than the 2520.

 

[Eliani]

I don´t want to rain on your parade, Eliani. But simply throwing API into a circuit doesn´t make everything necessarily better. I once refurbished and modded a Scorpion for a friend. I left the bad and ugly SSM2015 in and modified the circuit around it to make it perform better. The result was that the owner sold his API preamps because they found no more use.
Your circuit suffers of mediocre CMRR. If that´s of no significance then go ahead. If it matters then the stock SSM performs better. In certain surroundings technical parameters do matter because they translate into percieved sonic quality. Not everything can be justified with "if it sounds better then..."
Just my 2ct

I know it's not always as simple, but this one works. I know common mode rejection ratio is not perfect, but that's not what I'm looking for really...

 

[critterkllr]

I don´t want to rain on your parade, Eliani. But simply throwing API into a circuit doesn´t make everything necessarily better. I once refurbished and modded a Scorpion for a friend. I left the bad and ugly SSM2015 in and modified the circuit around it to make it perform better. The result was that the owner sold his API preamps because they found no more use.
Your circuit suffers of mediocre CMRR. If that´s of no significance then go ahead. If it matters then the stock SSM performs better. In certain surroundings technical parameters do matter because they translate into percieved sonic quality. Not everything can be justified with "if it sounds better then..."
Just my 2ct

I'm curious about something; So, you would say that the absolute best sounding gear specifically relates to it having the highest CMRR?

It's great that a previous customer enjoyed your modded preamps so much they decided to get rid of their API preamps. That is really what we're all after. Whatever sounds the best to the end user is the best.

As for the argument that the API line input circuit is going to sound worse that modding the stock circuit, I'm not sure how to go about that without getting into subjective territory. Perhaps you could elaborate on your mod and possibly how to improve on the API circuit? Have you tried the API circuit mod?

 

[jensenmann]

I don´t want to rain on your parade, Eliani. But simply throwing API into a circuit doesn´t make everything necessarily better. I once refurbished and modded a Scorpion for a friend. I left the bad and ugly SSM2015 in and modified the circuit around it to make it perform better. The result was that the owner sold his API preamps because they found no more use.
Your circuit suffers of mediocre CMRR. If that´s of no significance then go ahead. If it matters then the stock SSM performs better. In certain surroundings technical parameters do matter because they translate into percieved sonic quality. Not everything can be justified with "if it sounds better then..."
Just my 2ct

I'm curious about something; So, you would say that the absolute best sounding gear specifically relates to it having the highest CMRR?

No, I never said that.

It's great that a previous customer enjoyed your modded preamps so much they decided to get rid of their API preamps. That is really what we're all after. Whatever sounds the best to the end user is the best.

As for the argument that the API line input circuit is going to sound worse that modding the stock circuit, I'm not sure how to go about that without getting into subjective territory. Perhaps you could elaborate on your mod and possibly how to improve on the API circuit? Have you tried the API circuit mod?

I didn´t compare the SSM to the API. The owner did that. You´re right, though, percieved soundquality is highly subjective. But like I said. Depending on the scenario technical parameters turn into better or worse circuit behavior, hence soundquality.

 

[jensenmann]

What exactly did you do to mod the circuit around the  SSM2015?

There´s an input filter which I opened up a bit to get rid of HF phaseshift. Then I modified the supply around the chip. There´s some small ceramics across the rails next to the SSM. I replaced these with 100uF electrolytics + bypasscaps. Then I modified something in the network around the SSM, but since it´s many years ago I don´t remember exactly what that was. Of course I recapped everything (usually with larger value caps), added bypasscaps to all electrolytics and replaced all chips downstream. In the end it´s the sum of everything I did which lets the SSM perform better. And it does help to keep the EQ engaged even if not in use since the SSM doesn´t like to see much load at it´s output. The EQ circuit works as a buffer for the SSM this way.

 

[Eliani]

What exactly did you do to mod the circuit around the  SSM2015?

There´s an input filter which I opened up a bit to get rid of HF phaseshift. Then I modified the supply around the chip. There´s some small ceramics across the rails next to the SSM. I replaced these with 100uF electrolytics + bypasscaps. Then I modified something in the network around the SSM, but since it´s many years ago I don´t remember exactly what that was. Of course I recapped everything (usually with larger value caps), added bypasscaps to all electrolytics and replaced all chips downstream. In the end it´s the sum of everything I did which lets the SSM perform better. And it does help to keep the EQ engaged even if not in use since the SSM doesn´t like to see much load at it´s output. The EQ circuit works as a buffer for the SSM this way.

I've read a lot of your old posts regarding the Scorpion mods you performed. I'm not a true electronics engineer, rather an enthousiast. All the projects I did so far are built on trial and error. I took the original API differential circuit and tried it with succes on one of my S1000 strips.

But I am a true audio engineer, and to me this is exactly why I prefer Analog audio including the theoretical imperfections.

 

[jensenmann]

Rereading my first post let´s me think that it sounds a bit harsh. It´s not meant this way. I only wanted to add the CMRR thing for the record. No offence intended. And I´m totally with you about the beauty of DIY to tailor everything to fit one´s needs.

 

[Eliani]

Rereading my first post let´s me think that it sounds a bit harsh. It´s not meant this way. I only wanted to add the CMRR thing for the record. No offence intended. And I´m totally with you about the beauty of DIY to tailor everything to fit one´s needs.

No offence taken!! Don't worry...

I would however like to know how you would handle CMRR adjustment on this concept! I'm here to learn, and if I can improve my project all help is welcome!

 

[jensenmann]

That´s not so easy done in this scenario. Depending on how things are connected to your input it will represent different impedances to the source on + and - inputs. Transformers of course will handle this problem great. But that may not be an option here.
With opamps it could be done by using a circuit like in the latest revision Urei 1176 with two ampstages. Both amps are connected via their - inputs. That means they will show the same impedance to the outside on the + and - inputs of the inputjack, no matter how things are connected. Trying to get a circuit like that based on 2x 2520 footprint DOAs will not fit due to space. You could try to do skip on the 2520 footprint and do a new layout with both DOAs on a single PCB. That´s a lot of work though.
IMHO there are many other corners cut in that desk, which can be adressed for improvments. The input stage being only one of them. Improving other aspects can result in better sound, too, without having to go the long way of designing a new PCB with all the trouble involved which comes along with it.

 

[critterkllr]

That´s not so easy done in this scenario. Depending on how things are connected to your input it will represent different impedances to the source on + and - inputs. Transformers of course will handle this problem great. But that may not be an option here.

What kind of input transformer would do the job? Is it a simple 600:600 that would do it?

 

[Eliani]

What kind of input transformer would do the job? Is it a simple 600:600 that would do it?

Not for a line-source, you would need something like 10k input impedance if i'm not mistaken

 

[jensenmann]

10k:10k