Official Forssell Limiter Build thread

[soundguy]

I put a 10K (I thnk) pot in parallel around the feedback resistor in the output opamp. If it changes the bandwith I cant really hear it monitoring music through it, good enough for me.

dave

 

[DrFrankencopter]

Hi Guys, just an update here.

I'm just about finished my FF opto comp, and have it up on the bench supply. With the board stuffed as per the schematic/layout I get V+ and V- that are too high (about +/-26V instead of +/- 24). I am using 1085 and 1033 Vregs as per Fred's original schematic. I double checked the resistance calculations for R15 and R16 and found out that they are not correct.

I replaced R16 and R15 (were 6.19k per the schematic) with 5.4k, and now have +/-24V (well 23.98).

Just a 'heads up' on this one...

Cheers,

Kris

 

[Svart]

wow, i just read this whole thread once again.. my head hurts.

I have tried to follow this but i am not understanding what has transpired i think..


did some PCB modification have to happen?
aren't some values of resistors incorrect on the BOM?
when using the 5534s do we have to use different rail voltages?
if i don't plan on using stereollinking, bypassing or the onboard PSU, i trust that I can leave all those parts out as long as i take the proper precautions?

I downloaded both versions of parts lists and they don't jive.. the more complete bom in XLS(sorry i don't remember who did this one) is missing some parts and has some extras..

I have been trying to order parts for this thing all day! uggh damn those pots. damn them.


i need a :guinness:

 

[DrFrankencopter]

Hi Svart...

I've been mulling the FF opto threads over for the past few days...here are some tidbits that may help you out.

1. The current limiting resistor (R28, I think...the one that attaches to the ratio pot) should be between 100-200 ohm (I chose 100).

2. If you are using the Discrete op-amp output you'll need an extra .68uFd cap for the output servo (wasn't on the parts list that I had).

3. If you're using the 5534 based output you'll need to double check the pinout of your FET...and you may need to mod the PCB.

4. The power supply resistors are not correct for generating +/-24V. If you're using LT1085 and LT1033, use 5.4k resistors in place of the 6.19k's.

5. There is no makeup gain for the discrete opamp...best way to do this is to use a 100k log pot or attenuator just after the opto, and mod the feedback resistor of the output stage to give something like a 10-15 dB fixed gain. Fred mentioned this in one of the related threads (the even longer one on the FF opto)

6. I couldn't get the 5k rev log pot so I ordered a 2 pole 12 posn rotarty switch and will use fixed resistors. I have the boards on the bench right now and am using a regular 5k pot to tune the resistor values. Stay tuned and I'll post a resitance vs rato chart. The rest of the pots can be obtained from digikey...

Cheers,

Kris

 

[Svart]

ok here is what i'll be doing.

no bypass, external psu(so no psu on boards), no stereo link, and using the IC I/O.


also for the 22uf caps, can i just use 47uf?

 

[DrFrankencopter]

I've got mine running on the bench....haven't heard any output yet, but have seen it pass signals nicely on the scope.

First thing I noticed, is that the output is out of phase with the input....I think I hooked the input and output up correctly. This is an easy fix, but it's worth a check for those just finishing the project.

There's a gain of 10 (20 dB) right away...so, the raw board is ready for an attenuator.

Next thing to note is that measuring the ratio will be difficult since it appears to never really be a constant slope....even at near zero ratio there is some compression happening at the far end of the scale.

I've decided instead to just choose 10 resistance values log spaced from 0 to 5 k (really more like 2.5k to 5k, as there's not much change in ratio below 2.5 on my test pot).

Oh, and it compresses....I ran a 1k tone through it and it does indeed work as a compressor.

The OPA604's get quite hot though....I don't think they're oscillating, but maybe that's just the way they are when running at max voltage. Additionally, the JFET992 in the output section is getting warm. The input DLR 992 is cool though.

That's all the testing I can do today....

Cheers,

Kris

 

[Svart]

nice!

ok i am building up another BOM for my version of the board without bypass, stereo link or on board PSU and using the IC I/O.

i should have it done tommorrow if anyone wants to post it

:thumb:

 

[Svart]

oh and Soundguy, you built the version with the Forssell opamps? have you compared it to the one with the 5534's? i'm wondering if the Forrsell I/O setup might affect the sound more than the compressor part actually does..

 

[DrFrankencopter]

Subbing 47uF for the 22uFd input coupling caps should be no problem at all....

Also, regarding the sound of the compressor....well, I think most of it's behaviour is defined by the sidechain, whereas the audio path with the 992's should be quite transparent.

If you're omitting the stereo link you may have to find a way around one of the OPA604's....I think that one of these IC's sits in the sidechain to sum in the other channel's signal. You may be able to bypass it with a jumper wire (if it's a unity gain summer....I forget) U9, I think it is.....but that's jsut from memory.

Cheers,

Kris

 

[soundguy]

hi svart-

everything you need is in this thread. I went with a 100 ohm resistor for the ratio slope and besides that one cap for the output amp, everything else is in the BOM. I would just follow the schematic closely though, either way, everything you need to complete the build is in this thread.

The entire sound of the limiter IMO is in the amplifiers you chose, I dont think the side chain is an incredible source of coloration in this design. I tried a few different opamps in mine, I had used a 992 on the input with a 993 on the output and tested that for a while and eventually decided to go with a 993 on the input and output. With 992's limiter had a more forward feel to it. I havent compared it to dip packages but its my guess after my testing that whatever opamp you use is going to play a very big part in the sound of the unit. I settled in on mine with marinair input transformers and reichenbach output transformer, love it to death.

thanks again to fred for dropping this one on us. Everyone here should consider building this limiter, it sounds awesome, its easy, its inexpensive and it sounds awesome. There isnt anything out there that I know about that does what this thing does, its sooooo cool that we have a project thats totally unique, so much cooler than just building a clone of something you can go out and buy one version or another of. This is a REALLY GOOD sounding limiter folks. Check it out if you ever get the chance, its pretty much everything Ive wanted to get out of an optical limiter.

dave

 

[soundguy]

[quote author="DrFrankencopter"]Also, regarding the sound of the compressor....well, I think most of it's behaviour is defined by the sidechain, whereas the audio path with the 992's should be quite transparent.

If you're omitting the stereo link you may have to find a way around one of the OPA604's....I think that one of these IC's sits in the sidechain to sum in the other channel's signal. You may be able to bypass it with a jumper wire (if it's a unity gain summer....I forget) U9, I think it is.....but that's jsut from memory.
[/quote]


hi kris, guess we posted at the same time!

Funny how we had opposite reactions about the sidechain. I suppose everything effects everything! I foudn the sidechain to have a transparent feel compared to say a neve 2254 which has a crazy colored side chain. when I swapped out different opamps (tried 992, 993 & JLM 99v) the sound of the unit changed quite a bit, even putting a 992 input and 993 output versus 993 input with 992 output, the sound would change a bit. Maybe thats just my impression but try it out and see.

I originally tried to bypass the dip in the stereo link and couldnt get the unit to work by jumpering it (on tommy's boards), havent looked closely at the schem in quite a while, but I think you may need that opamp there.

dave

 

[Svart]

yeah, i was going to look at it and see if i could get away without the IC completely.

:guinness:

EDIT: thanks Soundguy, I assumed that the I/O was going to have more color than the sidechain but not having heard one yet i thought i would at least ask.. and you being a very outspoken.. well almost spokesperson like :green: and alas, one of the few who has actually complete this project, i thought i would ask you but it was a general question for all too.

I am excited as this will be my first optical compressor and having new toys always brings great deals of creativity for me at least. If this is even half as good as you say it is then i will be greatly impressed! assuming i don't fugger it up somehow.

I'm not sure i caught the models of the trafos that you used, what were they again? I'm also not sure i am going to go trafo I/O either since these are going to likely be inserts but i think i am going to start building everything with balanced i/o from now on(and retroactively)

thanks for the info!

 

[DrFrankencopter]

I dunno...when it comes to compressors, the character of the unit while compressing is dominated by the design of the sidechain. Fortunately here, it's a really cool sidechain design, being partly feedback, partly feedforward. The main path is pretty clean...listen to it in bypass and see! Me, I put black gate nonpolars for the input couling caps, and with the servo output + no transformers it's pretty hi-fi.

Soundguy took a different approach, with Xfmer I/O and using a pot in the feedback path of the output amp (which changes the bandwidth, resulting in a slightly different sound at each makeup gain level). Nothing wrong with this approach, it's just less like a 'wire with gain'.

FWIW, you can impedance balance the output by removing the trace between gnd and the output pin 3, and replacing this by a 49.9 ohm resistor between pin 3 on the output XLR and ground.

Cheers,

Kris

 

[Svart]

so if i use the 5534 setup I am going to have to run this at +-15v correct?

 

[Svart]

also I have a 15-0-15 50 VA toroid.. i'm pretty sure that I can use this for the +-15v after rectification as it should be around 20VDC and the VA is sufficient to uphold this.. any thoughts?

 

[DrFrankencopter]

I'm measuring 100ma draw per leg of the P/S at +/-26.5Volts input to the regulators. Thats about 5 VA total.

The 5534's can run at up to +/-22V, but I'd stay below 20 to keep it safe.

Cheers,

Kris

 

[Svart]

TI states that their 5534 can only do 30V differential...

EDIT: i take that back, their datasheet says +-20 but their webinfo says +-15??? hmm I'll be running them at +-18 i think.

 

[DrFrankencopter]

30V differential between the input pins, but not 30V between the power rails.

On their data sheet is says max +/- 22V (top of page 3) http://focus.ti.com/lit/ds/symlink/ne5534.pdf

Though they recommend +/-15V.

Cheers

K

 

[Svart]

yeah my mistake i was reading too quickly.

should i push for +-20 or stay at +-18 do you think? I don't think i'll notice much difference.

i'm wondering if i should try that trafo or just get a higher output one..

also i have a BOM somewhat complete, minus the parts i already had. features parts from mouser, digikey and newark for the US people. just need a host!

 

[DrFrankencopter]

Honestly, the difference between even +/15 and +/24 is pretty minimal (not even a 3dB change in S/N or headroom).

Stick with +/-15 or +/-18...whatever the trafo will be comfortable driving (it should do 18 nicely).

Cheers,

Kris