Official Forssell Limiter Build thread

[cstella]

Hey Arno if you are going to run your discrete amps at +/- 24V then you have to use OPA2604's in the SC. 5532's or TL072's won't handle that voltage. As for sonic differences they are just in the SC so the audio is not running directly through there except as a control signal. In terms of the OPA2604's being better I think it's more a matter that they can run at the higher voltage.

 

[Nora]

aren't there any other opamps that will run on higher voltages ?

The opa2604 just seem so expensive to be used in a sidechain circuit.

the ne/tl 's are at least 3 times cheaper.

Maybe that's the reason why they're so expensive ?


thanks,
-Arno

 

[cstella]

I am using the OPA2604's there may be other IC OA's that can run at +/- 24V but I am not aware of them. The Forssel DOA's will run at anything from 12-24V so you could run your system at a lower voltage and use the other IC's within their tolerences. The DOA's performance should not suffer you just won't have as much headroom before clipping. But my advice is if you are going to spend the money on the DOA's you might as well spend a little more and get the OPA2604's and run everything at 24V. My opinion of course. But at $4.28 a piece from Digikey they are pricey. Also check TI.com they may have samples for free. Best, Charles.

 

[Nora]

Oh well I guess you're right.
I'd better not get into the trouble of changing everything to lower voltages and get some 2604s

Thanks for the advice,
-arno

 

[Infernal_Death]

Well with the recent board selling thread i thought i bump this thread. I just made some listening tests with my forsell (actually i have 4 boards, one stuffed with the ic option and one with discrete melcors which isn't fully built).

Here are some pics:

My high-tech listening lab:

Actually it's just the floor of my room with a el-cheapo active pc speaker, a oscilloscope which is so old it could have been used by jesus and the new amorphis running through the forsell.

Two additional pics:

Yes there are some wires loose. I didn't connect the stereo link switch yet. Maybe this is a little inspiration for all the other people to finish their forsells. There have to be way more boards out there than what we saw until now as finished versions.

Oh almost forgot i don't have any output transformers yet. I am still looking for a source in germany for those 600:600 OEP's (i think something like A262A7E)

Flo

 

[ciminosound]

Flo, BEAUTIFUL looking box!

:sam: :thumb:

 

[matthias]

hi flo,

this looks very nice

you can get the 262a7e from rs-components

have you done any listening tests yet??
I'm really interested in the sound of this unit...
maybe you can make soundsample (pref. some drums)


mat

 

[Infernal_Death]

Hi Matthias

Thanks for the source, i need to see if this time rs components will sell me something. Last time they denied because i am no reseller :roll:

I just found some minor mistakes. One resistor in the output stage had the wrong value and the bypass switches were wired incorrectly. I also implemented the make-up defeat like Frank describes in his pdf file.
Everything seems fine now. I need to install the unit in the rack in the cellar studio. Maybe i find some time to do a sound sample (i only got some programmed drum tracks for some songs i write at the moment, nothing mixed so keep this in mind :green: )

Flo

 

[matthias]

I'll start stuffing the boards now...

thank you frank for this nice pdf:
http://www.nrgrecording.de/tempfolder/forsell_opto_rev1.pdf

are there any news about linking and calibrating the unit ??

thanks,
mat..

 

[Svart]

I was wondering about using a current feedback discrete amp like the FETbloak on these boards.. anyone try it yet?

 

[matthias]

I just went through the schematic , part list and franks summed up pdf...

and there are a few things that are still unclear for me..

do I need to stuff the parts for the discrete input/output buffers, when I want to use the 5534 option? (I would say "no")


r29 / r30 / r31 / r32 are 22k with the 5534 option (10k in franks pdf)
c28 = 22µF (part list rev3) and 10µ in franks pdf
c4 = left on the schematic, 33p in franks pdf, but 680n in the schematic..

and don't forget to swap the pins of the 2n5457 (q3) because
it doesn't fit to the symbol printed on the pcb


and another question: I don't know who designed the pcb (tommy?)
but it would be great, if there's a comlpete schematic that's equal to the pcb...


mat

 

[Svart]

i can tell you that when you use the 553X parts you don't have to stuff the supporting components for the discrete parts.

Tommytones designed the PCB, from modular schematics, not one whole schematic.

 

[Greg]

matthias, you are correct... for the discrete output option, you don't need to put the 5534, output transistor, and supporting circuitry.

 

[tommytones]

Looking at my parts list, each I/O section is divided...parts needed for discrete and parts needed for IC. Just follow. The SC & PSU section are universal to every board.

 

[Lowfreq]

Ok peeps,

I've bought the tommytones board off Matthias, and I thought while it's in transit I'll get myself clued up on a few things. Matthias has done most of the hard work so this should be easy peasy. :wink:

I'm lookin at which transformer to get for the output, and so far I'm thinking of gettin the Cinemag CMOB-2H, which is 600:600ohm, 1:1.
Now from the schem and what I've searched, that'll be sweet, but...................as always I thought i'd just check with you guys first.
I'm goin with the cinemag as I wanna order a couple of other trannies for my api pre at the same time. But any other suggestions would be cool.

regards,

Steve

 

[matthias]

hi steve, I used an oep 262a2e which worked very fine...

mat

 

[Lowfreq]

oep 262a2e

Cool, I've got one of those around here somewhere. Cheers

Any other suggestions?

 

[enthalpystudios]

I have a board for this guy, but I feel like I have about 4 parts lists, which are real helpful, but just to illuminate:

output transformer only need on the IC version, right? Is it need on the discrete, or can one be used? I have some stancor wf-30's, how would these do?

Is U9 needed on a mono board, or just a jumper?

Reading through the thread, it seems there are a couple mistaken parts..... are those mistakes on the parts list that references pcb designations (C24, R14, etc), or on a different parts list?

Just trying to figure it all out,

thanks,

billy

 

[DrFrankencopter]

Back on page 15 of this thread I wrote the following observations about the ratio control:

10K = 1.97:1
6k19 = 2.2:1
4k02 = 2.5:1
3k01 = 2.75:1
2k59 = 3:1
1k78 = 3.8:1
1k33 = 4.3:1
1k = 5:1
825 = 6.4:1
750 = 10:1
619 = 9.2:1

Just a note on the ratio...it's hard to define at low ratios becasue the knee is pretty smooth. I think I wrote the procedure I used to get these numbers in the thread somewhere....I think these would correspond to at an input that's at least 10 dB over the threshold setting.

The attack and release can't really have ranges because the compressor is feedback based (well there's some feedforward in there too)....these will vary as a function of the ratio.

Cheers,

Kris

 

[hejsan]

Hi, I'm getting ready to stuff some boards, but vtl5c1 seems to be out of stock.
I do however have some vtl5c9 around so I'm wondering if I can use them?

Here are links to their datasheets, I just don't know what to look for. The off resistance is the same for both, but I don't understand the rest..
http://optoelectronics.perkinelmer.com/content/Datasheets/vtl5c1_c2.pdf
http://optoelectronics.perkinelmer.com/content/Datasheets/vtl5c9_c10.pdf

Please advise, could I use them with the circuit unaltered?

hejsan