A new sontec. PRELIMINARY OVERVIEW

So i'm going to take the plunge and build the sontec clone.. With a few changes that is. I decided to log my progress because of the lack of info on these units, both with build info and info on the unit itself.

The biggest change will be the use of the FETbloak DOAs which are of course current feedback. My second choice will be one of the AD81x series of opamp from AD. This brings me to the next point.

Apparently the original opamp used in the Sontecs were very fast, around 600v/us and extremely stable. From reading GM's statements and as much info as I could, this has a great effect on the quality. This happens to be one of the applications that these video opamps shine at. But again the ones I like to use are currentfeedback as well.

Either way, I will be adjusting the feedback circuitry to remove the caps from the feedback path and other necessary value changes.

I would like any opinion/suggestion you might have. I'm building my BOM right now but it will be only the parts I need to buy in order to get the unit running so it won't be complete.

:thumb:

Check BUF634 for a speedy IC! 2000V/uS

Svart's not a pussy. :grin: :thumb:

I have been looking for a fast BJT opamp with low input noises and low offset but can't find anything that touches the currentfeedback opamps so far. If I could find the optimal BJT opamp I might try it too.

But right now, the most stable opamps that are also the lowest offset and lowest noise are the CFB opamps, thus the reason for my choice.

I'd love to see how they compare to the original's sound.

Hey Jaako, did you ever compare your unit to the real thing? I started that thread over at the other forum you suggested but didn't get to read the whole thing yet..

[quote author="Svart"]Hey Jaako, did you ever compare your unit to the real thing?[/quote]

Nope. All I know is my EQ sounds incredibly sweet & detailed, and probably nothing like a real MEP-250A. FET amps all the way.

There really is no weak spot in the sound. But if I had to choose, the midrange would be my favourite, though the highs are silky smooth, too..

that BUF634 is a good looking opamp. I'll be sure to check it out thoroughly.

:thumb:

[quote author="Svart"]If I could find the optimal BJT opamp I might try it too.[/quote]

There is a French guy on ebay selling GML 8802 Opamps. I bought a few out of curiosity, but haven't used them yet (still trying to figure out the pinout). Maybe these would give roughly the original sound?

Here's the link: http://cgi.ebay.fr/Set-of-2-GML-8802-Discrete-OP-AMP-8200-8900-U47-VF14_W0QQitemZ150050778178

Olaf

I'll check those out, however I read that units that get replacement opamps from maselec and GML don't sound the same. I'm sure that anything fast and stable will be excellent even though it won't sound the exact same.

Keep the ideas coming!

That BUF634 is a good looking opamp.

Click to expand...

I guess you realised that this is not an opamp but just a buffer?

Apparently the original opamp used in the Sontecs were very fast, around 600 V/us and extremely stable. From reading GM's statements and as much info as I could, this has a great effect on the quality.

Click to expand...

Where do you have that info from? The datasheet (of the EQ) just says > 200 V/us. Would you mind pointing us to the GM statements?

It looks like the original opamps are current feedback--allready 200 V/us is very hard to get with voltage feedback without serious noise penalty or greatly increased complexity (such as class AB input stages).

Samuel

Yes, i realize that they call it a buffer and that it's only unity but it's simply an opamp with a fixed gain. I also understand it's not a good setup for this application but I may choose to use it for this or another project.

I'll find the info for you Samuel, I read it yesterday afternoon after searching a lot of information. I have it stored at home.

The schemos that I am seeing have feedback caps, a big no no when it comes to CFB. There are cases where you *could* do this but I don't see the need here.

I am also checking out the AD818, seems like a decent opamp, VFB but needs at least a gain of 2 to be stable.

http://www.analog.com/UploadedFiles/Data_Sheets/36128148110432AD818_c.pdf

or

http://cache.national.com/ds/LM/LM6171.pdf

or

lt1360

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,C1022,P1111,D2555


someone give me their opinion.

all of those are VFB btw. in case the CFB amps change the circuit too much.

all less than 5$ each too.

Hello,

I began to reunite the pieces to construct the project of the EQ Sontec, but I have some doubts and wanted to solve them before continuing. I put a consultation message on my doubts in the other thread of Sontec... does some time, but there was no answer some. If somebody knows something, or can give some light me on my doubts...

http://www.groupdiy.com/index.php?topic=36&postdays=0&postorder=asc&start=165

Thanks.

bumpity bump

Svart, it is not clear to me which opamps you want to replace--let's refer to the original schematic: MEP250a_schemo.jpg

IMO it is not wise to replace all opamps with the same type. IC7 is used as an integrator which is probably better left to a VF opamp. I'd say that speed isn't very important here anyway, but if you want something more fancy than the original NE5534 I'd go with an AD797 (beware though that you need to add a little resistance in series with the inputs to make it stable) or OPA627. Same for IC8.

IC4 and IC5 can be CF if you really like them. I'd be very carefull though as IC5 might see a lot of stray capacity.

Samuel

I was intending to change all of them except the servos.

I understand about the integrator, that was what I mentioned about having caps in the feedback path of the CFB opamp. I've witnessed firsthand the oscillation that can happen with low inpedence inputs and caps in the INV feedback.

I think the FETbloak would work ok on IC4/5 with feedback adjustments.

For VFB with low slew, I really like the AD825 but it's soic only.

Research has led me to the best course of action, having opa604s on the filters and having FETbloaks on the I/O and running +-24v.

any opinions on this?

EDIT: maybe not, everyone seems to be out of opa604..

[quote author="Svart"]Research has led me to the best course of action, having opa604s on the filters and having FETbloaks on the I/O and running +-24v.

any opinions on this?

EDIT: maybe not, everyone seems to be out of opa604..[/quote]

http://focus.ti.com/docs/prod/folders/print/opa604.html

Maybe Arrow has them?

I already looked at their site, they don't show any in stock, they show a lead time of almost half a year...

purchased SMD parts and will make them work.

Most everything else ordered for this too.