Official Forssell Limiter Build thread

[Jonathan Hayward]

Nele, if you mount the big caps on the board, you won't need external ones. It's only the transformer and rectifier that really need to be external with Tommy's boards.

 

[soundguy]

Ok, just went through the thread and have to ask-

anyone have any tips calibrating the LED meter array?

Im having a tough time getting the meter calibrated to anything that makes sense. I have the optos calibrated right but the meter tracking is killing me right now. I think I may have used two different types of LED's that have diffferent forward voltage for them (just used what I had laying around), this is likely to cause me problems, right? Id like to get 1 dB steps out of the meter...

dave

 

[Nele]

Ok, thanks. Figured as much but wanted to make sure.

corneel

 

[Nele]

Hi,

just powered up my first unit. for the power supply I used 4,99K resistors instead of 6,19K and this is what I measured:

+ 21,9V
- 22,6V

is this ok? negative voltage seems a bit off...

thanks,
corneel

 

[Jonathan Hayward]

Nele, it's fine if the supplies are a bit different, maybe not ideal though.
Is 22V not a little hot for 5534s? I know some people run them higher, but maybe they don't mind replacing them more often.

 

[Nele]

If I read the datasheet correctly the ne5534 can run on +/- 22V so should be just ok... maybe I'll put in some even smaller resistors to lower the voltage a bit.

 

[Jonathan Hayward]

22V is the absolute maximum, 18V is typical.
Even so, I would err on the side of caution.

 

[soundguy]

hi folks-

Ive got my box up and running and it really is nothing short of fabulous, Im planning on building a second stereo unit for sure. I still need to work out unity gain with input and output transformers but do have one thing I cant figure out. I just read through fred's explanation on page 7 of how the sidechain works and the lightbulb isnt going off yet-

how can I adjust the threshold to open a bit more gradually? Currently, the limiter pretty much goes from 1dB to 10dB of limiting instantly. The whole throw of the threshold pot isnt being used at all, Im getting maybe 1/5 of the action out of the pot as I would like. Ive started to change values of R7 (have 100ohm right now) but so far that hasnt addressed the problem. Would changing the value of the threshold pot to a lower value help this, or is there somewhere else in the circuit I should be adjusting?

dave

 

[hitchhiker]

hi Dave,

I also find the threshold not good. I have nothing to offer as a fix though?

Maybe Fred will come by?

Lance

 

[DrFrankencopter]

Dave, the threshold is set by how hard you drive the sidechain, so all you need to do is adjust the gain of the first op-amp in the sidechain path (just going off memory here).

Try changing R3/R4 to give you about 1/3rd of the current amount of gain. Use the formulas for a non-inverting amp and plug in the resistances that you're currently using.

Cheers,

Kris

 

[soundguy]

thanks for the suggestion, I'll see what I can do there.

A friend suggested to me that using a linear pot (which is what Ive got in there) is the cause and right now thats making the most sense, gotta find a log pot and see where that puts me.

dave

 

[DrFrankencopter]

Yeah, a linear pot in front of that opamp would do it....it's acting like a volume attenuator, and really needs to be log.

Cheers,

Kris

 

[soundguy]

If anyone happens to have any clarostat 308 series in audio taper, let me know, need 100K. These were available a few places a few months ago and foolishly didnt order any, Ive called a bunch of places today and feel like Im reinventing the wheel, clarostat has a minimum 100pc order with a 2 month wait.

boo.

dave

 

[hitchhiker]

I found that I too had a linear pot in there . I put in the audio taper

and it's so much better. :thumb:

Lance

 

[edanderson]

not quite the same thing, but much more easily available than the claro 308 series:

91A1A-B24-D20

bourns 91 series, conductive plastic, 5/8" box pot, 1/4" shaft, the D20 means log taper 100k. available from mouser and allied for ~$5.50ea.

ed

 

[soundguy]

thanks ed. I actually have some of those here, my front panel is drilled for the 308 and because of the layout I did, I need to find something that size. frustrating. The 91 is too big even if I opened the mounting hole a bit, the centers are different.

dave

 

[hodad]

Here's a question for anyone who's built this with DOA's:
Did you use sockets for the DOA pins? If so, which? Did you have to drill out the board to get them to fit?
I ask because the sockets I have are too large for the holes, & if there are sockets that'll fit without drilling, I'd rather go that way.

Thanks
Tom

 

[soundguy]

I couldnt find any sockets that would fit without drilling, so I drilled the boards. Not the biggest deal, but not the best scenario either. Any trouble I had with the boards came from bad solders on the sockets, I would reccomend soldering both the top and bottom of each socket if you drill (since its a two sided board with plated through holes which you'll be destroying) just to make sure you have good electrical connections.

I cant remember the part I wound up using but they were milmax sockets from mouser. I tried a few, and either they didnt fit the board or didnt fir the pin of the DOA.

I may also just be an idiot but I coudnt find a socket that would work. Drilling worked out fine for me, I love the stereo box I built, Im half way through a second pair.

dave

 

[babyhead]

Drilling is fine, but I have found that a small circular file works best if you don't have a press...

Digikey ED5019-ND

Mfg# 0331-0-15-01-1827100

I had the same problem with Fabios boards... Maybe if I get a press, I can move out of the DIY ghetto!

 

[DrFrankencopter]

Getting ready to finish my FF opto comp, and was wondering if anyone has tackled the make-up gain issue yet? I'm using JFET 992's as an output amp.

My plan is to use a rotary switch to switch a shunt resistor to ground just after R8 (right before the opto element in the audio path). I figure if I set up the output amp for about 10dB gain I can set up the switch as a pad. This would give me a range of say -2dB to +10 dB for make-up (1dB steps on a 12 posn switch, probably good enough for my needs).

I figure this approach is better than messing with the feedback of the output amp, as that will change the bandwidth, and sound as gain is changed.

I gave up on the 5K rev log pos, and bought 12posn switches for that one too. I figure I'll just try out some values, and see what works out best. I'll probably only use 6 ratios at most

Cheers,

Kris