electronaut

Optical tube compressor discussion
« on: May 01, 2005, 07:47:54 PM »
OK folks...  I hope I'm not pissing off the moderators by playing hack-moderator a little, but in an effort to keep the discussion of analag's tube opto-compressor on topic, I started a new thread.

Here is the schematic of analag's compressor that he, like so many other generous contributors, offered up for discussion:



Some notes by analag:

- D1 is part of a Vactrol Opto-isolator. R3 is actually a 100k pot with the LDR strapped across it, much like the La2a. R1 and R2 is also a pot.

- I use a tube connected exactly as you see in my schematic, you may use a resistor in series with the LED say 120-470 ohms, this will tune the slope of the unit. In this particular compressor the same tube tube has been functioning since 1998 with no problems, in my studio. This is a rather simple but excellent device in terms of sound quality and compression give it a try.

- Cathode follower and grid feedback aka plate follower equals lower output imedance and very low noise.



Quote from: "gyraf"
Analag - what kind of opto is used?

Some quite slow opto type would probably be needed here in order not to distort at low frequencies..?

I guess that the opto's LDR part goes in parallel with the output level pot (marked "gain") - forming a voltage divider with the 100K resistor that goes to the output pot?

Is there any particular reason why you don't use a simple transistor or opamp stage (maybe even running off heater voltage) for the LED-driving - to save a nice ECC82 tube - and to reduce heat and current?

Jakob E.



analag replied:  VTL5C4/2 optos are used. I use the fastest ones I could find, it does not distort at low frequency. If you notice, the signal that drives the gain reduction amp comes from the output of the first voltage stage or U1 and at the same time the LDR is connected to U1's input. This seems to eliminated the frequency distortion you suggest might be taking place.

Quote from: "chrissugar"
The VTL5C4 is not the fastest vactrol, it has a very slow release, 1.5 seconds. That long release probably simulates in a way the memory effect of the T4B optocoupler from the LA2A and that is probably the reason why you don't have distortion at low freq because it responds slowly.
The VTL5C1 is the fastest vactrol available.



Quote from: "PRR"
> There's also no current limiting resistance in series with the LED...

That 12AU7 is a very effective resistor. Without looking at the curve, I'd guess you can't get 30mA out of it, and that only for peaks: well within sight of the ~20mA rating on most LEDs.

> Current can never flow out of it.

There are such things as "AC" optos, with two back-connected LEDs. That will work. No matter that it isn't shown that way on the schematic: these drawings also don't show which resistor is the opto-R, they are hastily drawn.

Also the single diode WILL reverse-breakdown. At around 7V. So all that changes is that the threshold is higher than a hasty glance suggests. Reverse-breakdown is not harmful (not at the current the 12AU7 will pass).

I think it has a lot more resistors than I would use (even allowing for the several resistors that seem to be merely simulation place-holders). I'm baffled by the use of both cathode feedback and plate-grid feedback. Sure it works, but there are other ways to do it. Is it more euphonic? I don't know. U5 seems to be working awful hard considering its load. The B+ bypass for U1 seems pointless. U6 will suck great spikes of current and should probably be further up the supply chain.


bcarso

Optical tube compressor discussion
« Reply #1 on: May 01, 2005, 08:28:33 PM »
Thanks electronaut.  I'd love to see this new thread get uncontaminated by rancor.

Note as well the one I started on LED behavior in the Vactrols, which has direct bearing on the operation of this circuit.

electronaut

Optical tube compressor discussion
« Reply #2 on: May 01, 2005, 11:45:17 PM »
Analag,

Despite being at this for a few years now, I am still in many ways a newbie.  Would you care to comment on the reasons behind some of your design decisions?

- There are a few series resistors that I don't understand, namely the 49.9K resistors at the 12AT7 grids, and the preceding resistors to each of those, the 8.2K and 100K.  What are the functions of those and why are they necessary?

I guess that's it for the moment.

Thanks,

E.

analag

Optical tube compressor discussion
« Reply #3 on: May 03, 2005, 01:54:31 AM »
The 49.9k is used to obtain a feedback point on the grid where I wrap the output back to the input. This type of feedback in my experience is best at reducing tube noise. The 8.2k stops the LDR from loading down the device that is driving the compressor when the vactrol shorts out the 100k volume control etc. when its compressing. The higher the value of this resistor the tighter it compresses, I experimented and found 8.2k to be satisfactory for taming the ridiculous dynamics encountered in my studio. The 100k resistors after U1 is just for reducing the loading effect on U1.

Analag
Audio engineering suffers from misinformation, disinformation, and downright lying more than most fields of endeavour.

Optical tube compressor discussion
« Reply #4 on: May 11, 2005, 03:01:50 AM »
hi analag

new thoughts and ideas are always welcome...so... i guess ...welcome to the lab..I do have to say that ...time and money are valuable.. I like cut and dried ...pure information....and like others... my eyes suk too..so...
to the point .....be to the point.

that being said welcome

can you post a pic of this unit? I like "d" pics...hehe

sounds like you got a good head on your shoulders (not that I am any "real" judge of intelligence since I am a super newbie. lol)

I'm looking forward to something done "recently" ...it's great this unit has been a workhorse for many years (nice) but you must have done something else since then that is also cool....something with a nice wow
factor...with some pics

anyway looking forward to those pics
later
ts
 
BTW i started to read your post when you first posted it and because of the banter to data ratio i abondoned it before it even got interesting.....somehting to consider...\
ALSO (more imprtantly) noticed a drop off in replies to other posts...which was somewhat disruptive to the flow here....just some observations and food for thought...
AND ALSO BTW  ijust want to see pics of this unit and make a statement that needs no reply ok...so...up some pics OK?  happy happy...joy joy
I need help with  RCA BA-43/ BA-45. RCA BA-25a, RCA BC3 mic pre's, Gates Level Devil, Federal AM-864u, DIY la2a stuff, Altec 436b

analag

Optical tube compressor discussion
« Reply #5 on: May 11, 2005, 08:20:58 PM »
As soon as I get a digital camera or something. I have been updating all my tube stuff lately. Pspice is the culprit behind my new building craze,  I'm achieving a bigger, warmer , crispy sound. This compressor is the newer version as I said in my first post.

Analag
Audio engineering suffers from misinformation, disinformation, and downright lying more than most fields of endeavour.

Kev

Optical tube compressor discussion
« Reply #6 on: May 11, 2005, 08:41:36 PM »
Quote from: "analag"
A transformer input tube pre, with 2 gain stages configured as OPAMPS. Oops I think I said too much.

cool
I've got a single tube followed by gain block proto idea on the go at the moment ... and looking to a switchmode/battery supply
probably will never get off the ground cos of lack of time but there you go.
Kev
DIY Factory

analag

Optical tube compressor discussion
« Reply #7 on: May 15, 2005, 11:31:25 PM »
I have been spicing around with another tube pre, I had all the parts including the power supply, I just stripped out the phantom power section, because I'm only using tube mics, with this pre.
 
Analag
Audio engineering suffers from misinformation, disinformation, and downright lying more than most fields of endeavour.

Opto's LDR part
« Reply #8 on: July 10, 2005, 05:32:21 PM »
Dear friends,
Could you explain where opto's LDR part goes?
It is not clear for me.
Thanks,
J.J.

bcarso

Optical tube compressor discussion
« Reply #9 on: July 10, 2005, 09:06:06 PM »
I believe it is in parallel with the 100k potentiometer on the left of the schematic.

Welcome to the Forum.


analag

Optical tube compressor discussion
« Reply #10 on: July 11, 2005, 10:01:32 AM »
That is exactly where it goes.

That's it by the way, I even got a shoe in there

Analag
Audio engineering suffers from misinformation, disinformation, and downright lying more than most fields of endeavour.

Optical tube compressor discussion
« Reply #11 on: July 11, 2005, 11:57:27 AM »
Thanks friends.
It seems a great design.
regards.
J.J.

mik

Optical tube compressor discussion
« Reply #12 on: July 15, 2005, 03:49:16 PM »
cool work analag, do you think is possible to add a switchable meter circuit to see in/out /Gr ala la2a ?
for in /out no problem but for Gr Im get difficult.
Mik.

analag

Optical tube compressor discussion
« Reply #13 on: July 23, 2005, 07:00:59 AM »
Metering setup is similar to the LA2A which can do all the things you requested.

Analag
Audio engineering suffers from misinformation, disinformation, and downright lying more than most fields of endeavour.

analag

Optical tube compressor discussion
« Reply #14 on: August 18, 2005, 03:04:03 AM »
I've just finished putting a Cinemag CMLI-15/15 line level input transfo into the compressor....the sound is caught somewhere between warm and transparent but leaning slightly towards warm....very nice.
Audio engineering suffers from misinformation, disinformation, and downright lying more than most fields of endeavour.


 

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