Drip Opto4 (LA2A) builders - limiter depth?

[T-rex]

This weekend I will wire up my drip opto4. I am going to use a toggle to switch the frequency response; to go from the traditional LA2A sound to something with a more even or extended high end freq response. So I thought what the hell, I could also do a toggle switch to change the Limiting depth. But I don;t know if that's worth it.

I saw an old post where someone had used 6.8K resistor as opposed to the 2.7 on the schematic to make it squash harder in Limit mode - which always sounds appealing to me. On a normal LA2A there isn't much difference between Limit and compress anyway right? Would it just be better to go a head and put a 6.8K in so it's either Compress or Squash, or is there enough of a difference to have compression and TWO levels of limiting to choose from?

Anyone done this or have an opinion on it?

[mrdibs]

It is my opinion that R7's effect on the circuit has more to do with the envelope and knee of the compression than the "depth" as you put it. It is not a ratio control.

Build it. If you want more compression, turn up the peak reduction knob. It's seriously that simple. There is a reason this design hasn't changed much in 45 years. It just works. If there is something that it isn't doing for you when you are done, you can mess around with it then. I doubt you'll tinker with it too much once you've heard it, tho...

[T-rex]

No question if I kept it was totally stock I would be stoked. But that's the beauty of diy, like the frequency response; I have a choice of the original value or one that can add some more top end than a real la2a.

I fire it up in a few hours!

[mr.adambeck]

Just a question, how much would you say the total cost of this was, including all the components/parts?

Thanks!

[T-rex]

It was pretty much right around 900

Here is my breakdown including shipping and everything:
Case 160
PCB 150
Opto 100
Tubes and tube sockets 70
Transformers 190
Power Transformer 30
Hairball VU meter 40
Parts (resistors, caps etc) 190

If you go to the Drip forum, in the Opto4 section there is an updated BOM you can download. It lists everything you need, where to get it and the pricing; including a whole section on what to buy cheap in order to save money. You can easily shave several hundred bucks off of my costs and more if you buy a generic case. I bought a premade case cause I hate metal work, I suck at metal work and I don't have any of the right tools for metal work. The one hole I had to drill became a major time investment because I didn't have a big enough bit and had to "improvise".

You could also easily spend twice that much if you go with all audiophile type parts or find some original transformers. I went middle of the road; mid-line tubes, Alps blue velvet pots etc.

So far I am loving it, it sounds pretty much exactly the way I wanted it to sound. I put in a switch to go from the traditional value for frequency response to a pretty much flat frequency response, in case you want to dig in with compression and not lose some of the high frequency info. I am not going to put in a switch for the limiter depth, but I am going to increase the resistor just a bit to make the limiting more pronounced.

I highly recommend it as a project. Easy to build, easy to source the parts, easy to mod it a little (as above) or add some more options and it sounds amazing with a very low noise floor. Of course, it is several hundred volts at points in the circuit so you could severely injure yourself if you are not careful.

[millzners]

One mod I believe I'll be making (haven't got all the parts in) is adding a third position to the Limit/Compress switch for 'Preamp' which will bypass some of the resistor network to give the La2a improved performance as a preamp. You can already pretty much use it as a medium gain tube preamp, this just improves that functionality.

[niccolo gallio]

Hi,
I think I've used a 6.8k in mine, but it's still not so obvious the difference between comp and limit.
Audible but not day/night.

[Jerry]

Hey T-rex, where did you get your Alps Blue Velvet pots?

[T-rex]

Actually I just picked them up at Mouser. You may be able to find them cheaper, but when you include shipping it was easier to just throw them in with the rest of my order. I mean I can't do a direct comparison, but the unit sounds amazing and the pots feel amazingly smooth and nice. I keep meaning to replace the output on my 1176's with better pots like these.

[Jerry]

Thanks for the tip. I wasn't aware that Mouser carried Blue Velvet pots. I never thought to look there - too obvious I guess.

[evets]

I am thinking of building one of these, where did you get your pre made case from?

[niccolo gallio]

Don't know where T-rex got his one but I got mine from Tat Purusha Das:
http://www.diy-racked.com/diy-talk/view ... p?f=7&t=16

[T-rex]

Don't know where T-rex got his one but I got mine from Tat Purusha Das:
http://www.diy-racked.com/diy-talk/view ... p?f=7&t=16

Yep, Same Place. He Has a Custom Black Case Just For The opto4 with all the options available that are on the pcb or some that look more traditional.