Official Forssell Limiter Build thread

[Jonathan Hayward]

LM3X7 can handle 37V so no worry there.

 

[ChuckD]

Has anyone fine tunned this unit yet?
What were the vales for the 3 VRs ??

Thanks

-ChuckD

 

[tommytones]

Which VR's would that be??

 

[ChuckD]

Was looking for your results to set:

VR1
VR2
and

RV1

??? How did you tweak these?

Thanks

-ChuckD

 

[tommytones]

RV1 is only relevant if using the 5534 output. Even at that, you'll need to mod this area and move the pot elsewhere for make-up (look at my mods for reference a couple pages back). If using the discrete output, you're going to have to place a pot probably in the feedback in order to get gain.

VR1 & VR2 you'll have to play with until you reach something you like with the meter. i.e. I tweaked them until I got more of a fine reading from the meter. I only have a 10db range with my meters beginning at 2db @ the 1st led. You're gonna need to mesaure the actual signal to do this. When I got 2db reduction on the signal, I tweaked until I got 2db on the meter. Had to go back and forth a few times but got it stable.

 

[ChuckD]

My DC suppy voltage is +- 27 V DC do you think that is a little too high?
Not sure that the LED IC will handle that much across it Also my Jfet992s are getting really hot.

Should I be concerned enough to divide the voltage down?

Thanks

-ChuckD

 

[tommytones]

2nd that. Too high. I'm running my 5534 comps @21.5v.

 

[ChuckD]

I think the problem is in my Diode rectifier bridge,
When I remove the output DC from the main board and look at the DC voltage I get +- 24 VDC but then when I hook it up to the Board it starts producing +- 33VDC.

What in the PSU circuit is making my rectifier produce more voltage?

I tried a Volatge Divider inbetween the two but that gave me a very small volage output... which means it was seeing a resistance in parallel to it I think.

What is the correct way to produce +- 24 VDC to this PSU?

Thanks !!

-ChuckD

 

[ChuckD]

Yup I'm sure I've done all of that.
Center tap 24VAC - into a diode bridge - outputs +-24VDC.

It's just when I load down the rectifier by connecting it to the main circuit that I get the higher voltage of 33 VDC.
It must have something to do with the inputs of the PSU ??

-chuck

 

[ChuckD]

when I stick a 10 ohm resistor between the rectifier bridge output and the inputs to the board I get a better voltage of +/- 27VDC which seems to be ok.
After the regulators it becomes +/- 25VDC.

However the 10ohm resistors do run very hot! Is there a better way that I am missing?

Effectively I calculated the input to the board must have a low resistance causing the problem I am seeing. By putting an even lower one in series before the inputs I was making a crude voltage divider,

-ChuckD

 

[tommytones]

Hold On Chuck...I think I might know what's going on but i need to check it out 1st. Try flipping the 6k2 and 300R positions. It's important that the 300R be in the proper order otherwise you will see a higher voltage.

I'm gonna look at something to confirm this for you (everyone actually).

 

[tommytones]

Okay, it all checks out good on the pcb. r14/r15. Check to see that you placed the correct values in the right place as I mentioned above.

Other than that, if you're using 4 diodes to make your bridge, make sure they're orientation is correct. Depending on the transformer you use, you'll have to play with the values of the resistors to set the correct voltage but if you're using a 48vct (2x24v) as you say, even with the default values you should be closer than you are.

 

[Jonathan Hayward]

maybe just replace the resistor to ground from each adjust pin with a trimmer (say 1k) and dial up the voltage you want, this is what I did. Can 5534s really stand 24V, I thought it was 21V max?

 

[tommytones]

I did the same but used a small value trimmer (200r) with a small series resistor in order to minimize the range. 5534 can take 24v but I don't think it will last too long though. That's why I adjusted my voltages to ~21.5v. I was worried it would affect other parts of the circuit but it's been working fine.

 

[ChuckD]

BUG Found!! PLEASE TAKE NOTE:

On the 5534 output stage the N-ch Audio amp Farchild 2n5457 (Q3) on the PCB. HAS THE Gate and Source swapped. The Part TO-92 package always goes G S D. The PCB has it wired and screened as S G D.

I swapped mine and now that output stage works! Just have some tweaking on the gain levels for both output stages to match and then it'll be good to go.

Also NOTE:
the 6.18 K resistors disscused above, I had to replace with ~ 4.8K for use with DIGIkey transformer toriod (#TE62072-ND). That should bring down the voltages to ~21 VDC.

Hope to have pictures soon!!

-ChuckD

 

[tommytones]

Gee Chuck, sorry you missed that. It was mentioned somewhere toward the beggining of the thread. Yes, I happened to use the Fairchild data sheet when doing the layout and it seems to be the only one using that pinout. I'm glad you found it though.

Does it compress?? Have you got them working now??

 

[Kev]

ToM,
... should we make reference to this on the web page ?

 

[tommytones]

Probably should make a note to avoid this in the future.

 

[ChuckD]

Yes it works! Dual Channel / Dual output stages each (swicthable) 1 rack space FF-opto.

I put the 100K trimpot resisitor in the feedback path (5534 output stage) and brought the gain to equal that of the 992 output stage. So the output stages are good. The gain output is about x8 to the input... Is this correct?

My Issuses:

I thought I calibrated the opto circuit with the same settings as channel 2 but I am seeing differences when music is put through it.

Tom can you go over a good calibrating technique? I did something wrong here.

I have also noticed that the threshold really kicks in hard at "one o clock" and does a little bit more after that point. By that I mean it is not smooth like I thought it would be.

Finnally the last problem is the Stereo Link... How is this supposed to work when I flip it on. By that I mean do I only use channel 1 to control both? Can you explain for me?

I really like this compressor on Bass Guitar thus far. It really thickens the sound and punches it up. Drums, well I still prefer the Gyraf SSL I built but we'll see when I am done calibrating.

-ChuckD

 

[strangeandbouncy]

Dear All,


Just getting round to doing my Forssell opto comps. I have a question thAt I'm sure must have been answered, but i can't for the life of me find . . . in the parts list R28 is listed as "selected for max slope". How? what order of magnitude should we start with? why not use a preset? Is there some kind of calibration procedure, esp if used in stereo? Again, i have been thru' all the postings, yet i can't find any info.


please forgive me if I have missed the Bleedin' Obvious . . .

Andy P