Official Forssell Limiter Build thread

[khstudio]

Kevin, what did you do to add gain control of the discrete output stage? I have read some of the previous posts on this and I'm not sure I understand correctly - replace R6 with a 10k log pot like this?

Yes & ground one side. Seems to work OK.

Fred said you could go directly into the 4.99k for un-balanced input but the more I think about it, I think i asked him about an input cap & he said something small... like 10uf or so. Not sure about a resistor to ground.

A small bypass cap across it... not sure, have to listen.

The 2604 DOES GET HOT... I think it's normal & should be fine.

BYPASS - I don't think you can get a full signal in bypass mode with this circuit unless you make a TRUE BYPASS WITH A SWITCH OR RELAYS.

The only way I found to work is run a test tone & note the level, THEN run it thru the FFO & adjust the output gain control (R6 - 10k pot) so the level coming out matches. (WITH GAIN REDUCTION OFF).

MARK THIS SPOT ON YOU GAIN CONTROL!

This was also helpful when comparing the RAW signal to what's going thru the FFO circuit. With NO GR, your just listening to the AMP SECTION of the FFO & can hear what IT'S doing to your signal. :idea:

 

[martthie_08]

Kevin, thanks for your quick reply, very helpful :thumb: I'll try it out soon!

Yes, the bypass simply cuts the signal to the sidechain section, just like on the GSSL, except for make-up gain defeat.

The thing is I prefer to have unity gain on the compressor with make-up gain fully CCW, I'll look for a solution...

 

[khstudio]

I just checked & my 10kA gain pot is set about 60-70% for unity.

Be careful changing the gain of the actual output AMP... someone mentioned that changing it too much could affect the authority of the DC Servo.

 

[janczmok]

Hi,

sorry for my stupid questions (maybe) - i need some help from you guys...

i nearly finished assembling my two Forssell optocomps (see photo) , but i stumbled across some things, which i would like to have clearified before i set the box on fire.

Pre-Info:

- Board Version of Rev 1.C, green
- i did fix the 2N5457 on both boards,
- i built it with the Bendo Output Driver + Input Buffer Option 2
- U5 / U6 are NE5534's instead of OPA02604

Now my questions:

- C18 (in the LED Array Metering Part) as 220uf/35V, but i am not able to find this on the PCB, can this be omitted or is there another error ?
- how should the stereo link be wired between both opto pcb's ?
- R28 is not found in the docs, what is the correct value for it ?
- is RV1 == 10K Ohms ? But what about R_Adj ?
- Is the GAIN POT stuffed ? What is the pot value ?

Whan can/should i use for the Descrete Amp ? any recommendations ?

I aquired 4 parts of Bourns LM-NP-1001-B1 transformers, how should those be wired to the PCB ?

Also what is the correct power i should feed both PCB's with ? You mentioned sometimes 15V other times 48V --> i have build a greenpsu, is those feasable ? or what alternate ?

Any advice is highly appreciated.

 

[ChrioN]

Anyone making PCBs for these suckers?

 

[janczmok]

See my mid-post on this page . i updated my questions regarding the pcb.

 

[janczmok]

Did somebody could give me assistance or help / hints to my questions regarding the optocomp ?

 

[martthie_08]

Hello, cannot give many answers, as I've built the opto with discrete I/O sections in mono only...

- C18 (in the LED Array Metering Part) as 220uf/35V, but i am not able to find this on the PCB, can this be omitted or is there another error ?

it was ommitted in your version of the pcb, look at the schematic of the led metering circuit, it was for decoupling the V+

- R28 is not found in the docs, what is the correct value for it ?

this resistor sets the maximum slope, set to 100R

Whan can/should i use for the Descrete Amp ? any recommendations ?

a 990 will work (although it's not really discrete), but better get in touch with Fred and get one of his jfet DOA's

I aquired 4 parts of Bourns LM-NP-1001-B1 transformers, how should those be wired to the PCB ?

maybe try with output transformers only at first. Hookup is easy, check your transformer datasheet and the excellent
overlay

Also what is the correct power i should feed both PCB's with ?

in your case you'd have to go with something like +/-15V to +/-18V, check if the +/- 24V designed metering circuit will need adjustments though. (I have used 990 and 2604's so I'm on +/-24V)

You mentioned sometimes 15V other times 48V

48V is def. wrong

--> i have build a greenpsu, is those feasable ? or what alternate ?

sure, that one should be fine, hook it up for +/-15V or get a JLM AC/DC or a Keith PSU.

hope this helps a little

 

[janczmok]

Martthie: Thanks for the info, i'll try this tomorrow after my live gig...

 

[pepe]

I’ve built two channels, and each is significant different than the other in gain reduction characteristic (both have been rigorously adjusted). Is it a known problem to mach opto channels based on the Vactrol VTL5C1?

 

[3nity]

Do this boards really work?
i'm gonna try to build 1....

 

[3nity]

One question more!
can i use NE5532 instead of OPA2604?

thank you.

 

[Nele]

yes, they'll work, but you have to watch the voltages. Go for + and - 18V, not higher (like + / - 24V)...

change R15 and R16 to 3K9 and you should be fine.

 

[3nity]

thanks Nele seems that is near completion project!

 

[Johan]

[quote author="pepe"]I’ve built two channels, and each is significant different than the other in gain reduction characteristic (both have been rigorously adjusted). Is it a known problem to mach opto channels based on the Vactrol VTL5C1?[/quote]

I dont know why Fred choose such low resistance for the series resistors(input-to-output), but it will make matching/selecting nececery if you build more than one chanel since it will require the LDR to work in a region where it is very close to its limmit ( lowest possible resistance). try increase the 4k99 and 1k resistors to 49k and 10k and the difference between different indevidual opto's will be much smaller...I'm sure it might affect other things like the responce curve, noise and how you set up the gain recovery amp due to different impedance, but that's what I'd try...
j

 

[3nity]

i powered my forselll today.

it passes audio but i cant hear it compress.

it's very noisy too.

I dont think the controls work!

any ideas please?

 

[3nity]

Alright its working right now good levels.
I was asking my self do all LDR's should be powered? cuz at my PR1 theres no voltage.

 

[3nity]

just did some audio tests on my unit and its sounds amazing totally addicted.
Even only passing audio trough it with light compression works wonders...I'm using a Hammond 804 600:600 interstage transformer to choose with or without. Both are wonderful.
Thanks to mr. Forssell. :thumb:

 

[vertiges]

Any pic 3nity ? :green:

eD))

 

[3nity]

still on the bech....not racked yet! let me see when i'll do it!