Official Forssell Limiter Build thread

[G-Sun]

Does anyone know of/have a picture of the clean pcb?

I've taken over this stuffed board from hitchhiker
and it's hard to see/understand what component is where.

Second: I try to understand the output-sections and my choices.
Any recommendations?
I was thinking IC only,
but transformer out.. whatever gives me balanced out, doesn't break the bank and sounds good 

 

[G-Sun]

Is the bypass just an of/off switch breaking the sidechain?
So, SPST on/off ?

(I bought a DPDT one)

Edit: I see there's a wiring to the maku-op-gain.
But, is it just setting bypass-gain on full trottle,
or is it opposite? Guess so

 

[G-Sun]

I put together my Forssell comp today,
and fuse blew when power up.

I'm using 250mA slow blow on a 230V/25V 30VA transformer,
is that a good fuse size?

psu onboard

wired with dc +-24V to JLM Regurgitator board and power led.

Ah, I made a mistake for the led.
Should have been +24V/0V, not +-24V.
I used 2x 2k2 ohm resistors (parallel) 0,5W for led.
A led I assume is 3V/20mA, but I'm not sure.

Should I expect the led to be cause of the fuse blow?

Edit: Changed led to +24/0V, and fuse blew again..

 

[G-Sun]

Anyone have the schematic for the psu?

Fuse is still blowing for me.
Transformer is confirmed working/wired right.
Regurgitator is not cause for the trouble.
Bouard itself is supposedly tested and working.

So, that points me to some fault in the psu.
Can't spot any shorts,
but will maybe have to measure.
Any parts that I should suspect?

I have a JLM ACDC unbuilt I can replace the psu with for testing.
Maybe that's next step.

Pics:
https://www.dropbox.com/sh/uzvkqu436dvmfkc/AAAL8A6U65kOUFhnt_-y9stna?dl=0

 

[hitchhiker]

Just looked at my one of my Forssell boards. Oddly, these pcb's don't have any rectifier section.  Did you build an off board  bridge rectifier to send DC to the pcb? If not you would be sending AC voltage to the regulator circuit.  I forgot about this because mine are powered from a separate central power supply, and I've never used the on board PS section.

 

[G-Sun]

Just looked at my one of my Forssell boards. Oddly, these pcb's don't have any rectifier section.  Did you build an off board  bridge rectifier to send DC to the pcb? If not you would be sending AC voltage to the regulator circuit.  I forgot about this because mine are powered from a separate central power supply, and I've never used the on board PS section.

Thanks!
The psu-section does have an array of 4 diodes, making a rectifier I presume?

 

[hitchhiker]

I believe if you reverse engineer that section of the pcb you will find that those are protection diodes around the regulator IC's.

Yup. Just confirmed on my unit, all protection diodes, no bridge any where.

 

[G-Sun]

JLM acdc installed
Voltages fine
Passing audio

But threshold not working, no comp-action

Bypass working
Regurgirator working
Gain working
Power-led fine
GR-leds: Light but unsure if correct action

Had C14 backwards, desoldered regulators and C14/C15 (redundant)

 

[hitchhiker]

Maybe bypass switch wiring mistake?

 

[G-Sun]

Maybe bypass switch wiring mistake?

Bypass seems to work fine.

 

[hitchhiker]

does the make up gain control work?

 

[G-Sun]

does the make up gain control work?

Yes

 

[hitchhiker]

make sure all the wires for the threshold are not broken at the pcb and pot ends.

Maybe then drop in  tlo72 ic's into each spot "one at a time" starting with the first ic in the schematic. You will have to lower the voltage to  18 volt rails.  One or more ic's may have not liked the high ac voltage it got a while back.

 

[G-Sun]

make sure all the wires for the threshold are not broken at the pcb and pot ends.

Maybe then drop in  tlo72 ic's into each spot "one at a time" starting with the first ic in the schematic. You will have to lower the voltage to  18 volt rails.  One or more ic's may have not liked the high ac voltage it got a while back.

Yes, this could make sense.
I have some opa2604 free, so I'll try out those.

 

[G-Sun]

Changing the first 3 ICs (from front left)

There are some changes,
but I'm a little unsure about my findings.

I don't have a OPA604 replacement at the moment,
and I don't have more than 3 opa2604.

Seems like my brain is not working for troubel-shooting these kind of things at the moment..

Let's see, ebay, ordering some ICs

 

[G-Sun]

Getting me IC's in the mail,
replacing all of them except the LED ic

No change.

Summary
- Gain is working fine, bypassed on bypass
- Ratio seems kind of working, cw = more gain-reduction, cww = less/none. Yet, it doesn't seem dynamic
But, bypass is not working right here
- Release: No effect
- Attack: No effect
- Threshold: No effect
- Voltages fine
- Power led fine
- Bar-led: Light but static, change when turning ratio

Resoldered release pot as it was broken.

More images: https://www.dropbox.com/sh/a4iyu4g19svs6mt/AAC4kK1rRE0p2JGtbk2DJJ2ha?dl=0

 

[insideout]

some years ago I built a Forsell Opto with the TommyTones board, but never got it going properly. I put it aside to revisit someday, which I now have. I found the TT board pretty hard to work with so here I set out to make things as simple as possible:

JLM ac/dc power supply - but any reliable +/-15 to +/-18vdc will do
the sidechain pcb direct from Fred Forsell's site - I uploaded the gerber to easyeda.com and 5 boards arrived a few weeks later
mnats meter pcb - although I have redone it it use a header for connecting to a meter bar
no input or output amps

Pleased to report this is now working just fine.

I thought I would have problems finding the OPA2604 specified, but an email from Fred clarified that this ic is only necessary if using +/-24vdc. OPA2134 or NE5532's @ +/-18vdc work fine. Also that is fine for the meter's LM3916, which the schematic shows as having +24vdc.

The only problem I had was a solder bridge on the mnats meter pcb - the labeling is on the copper side and "com" is awfully close to the trace and the meter input pad. When shorted, no metering.

Next steps are
- put a JLM 10k rev log wired in parallel to make a proper 5k rev log for the ratio
- add a JLM regurgitator for make up gain

Assuming that works, I'll make another unit and rehouse it all as a stereo unit with a link, and use the nice front panel design (I don't recall who did it). If I was to balance the input, I'd use the little that board from https://www.diyrecordingequipment.com/. Not sure on that yet.

 

[insideout]

well, finally finished this off, is all working nicely though I haven't put it through its paces actually recording anything yet.  It will be interesting to see how the metering stacks up - I have tried to tune it so that 3 bars is about 6dB, most bars is about 10 dB. It's pretty touchy to set, balancing between the output of the  meter opto and the gain of the LED driver. Unfortunately I had 2 pairs of optos from different manufacturers and they don't track the same, so I sort of went for something that seemed to be in the middle.  We shall see how useful it is.

I used the boards made from the gerbers direct off Fred's site, a JLM psu,  JLM regurgitator for makeup gain and balanced output, JLM dual 10k rev log pots paralleled to make 5k, Lundahl 1540 for input transformers, and my own board to drive the LED's (like mnats, just rearranged). NE5532's throughout. I didn't do the stereo link. But bottom line, just building Fred's board with unbalanced I/O will get you a working opto comp.

In retrospect, if I did it again I'd mount the pots onboard and save myself a lot of wiring (doh). So its a bit messier than I would like.....