avatar

kenfavata

Gold Finger

Posts: 424 Member Since:22/01/2011

#21 [url]

Feb 5 11 8:09 AM

Re-reading this, if I understand correctly, I'm realizing how very different this approach is to the modern VCA designs.  At the core of the GR mechanism is an FET with variable resistance shunting varying amounts of signal to ground.   This is far from  a voltage controlled variable gain op amp.

Is there some key to thinking about how/why this design difference is manifested sonically?

Quote    Reply   
avatar

ssltech

Aqua Marine

Posts: 4,034 Member Since:22/01/2011

#22 [url]

Feb 5 11 9:05 AM

That's right, though other design principles are attenuate-only... Variable-Mu being one, PWM and opto-resistor being others.

I developed and built a working PWM gain cell a few years ago, and sketcheded a compressor design around it, though never put it to the test. PWM has an advantage -just as a VCA does- of being absolutely predictable in the AMOUNT of attenuation per control voltage, so stereo link behavior is completely stable, for example.

The LA-2a and LA-3 are examples of attenuate-only optical compressors, with similar Gain-reduction-metering bridges, driven from the same control voltage.

As to the question of whether the attenuation mechanism has a 'signature' effect on the sound; -nowhere NEAR as much as several factors of implementation, such as feed-back versus feed-forward.

dbX units are RMS sensing and feed-forward, thus use VCAs. ANY sort of feed-forward design requires a high degree of accurate prediction in terms of attenuation-versus-control-input. VCAs have this feature in abundance, and a secondary benefit in that make-up gain can be applied at the same time to the same cell as the attenuation voltage... by adding the two volts together and feeding them at the control port, the VCA does the 'computing' and outputs the correct answer.  (A VCA is in fact a simple analog computer.) The advantage of doing this is that there is no second gain stage in the signal path, with its attendant noise & distortion (both of which increase as a function of gain.)

So many designs are feed-back out of necessity, since FETs, and Opto-resistors are nonlinear in their control reaction and vary dramatically from component to component... even among the same batch. This has a far greater effect than the FET or Opto doing the shunting, as an example.

Keith

-Keith Andrews -If I can't fix it, I can fix it so [i]NOBODY[/i] can fix it!

Quote    Reply   
avatar

ssltech

Aqua Marine

Posts: 4,034 Member Since:22/01/2011

#24 [url]

Feb 5 11 9:33 AM

Exactamente!

The same. -no need to 'wrestle' with the mechanics anywhere near so much.

-But pressing them ALL in makes people FEEL like they're being MORE naughty, so even people to whom I've pointed this out still persist in pushing all the buttons.

-Keith Andrews -If I can't fix it, I can fix it so [i]NOBODY[/i] can fix it!

Quote    Reply   
avatar

kenfavata

Gold Finger

Posts: 424 Member Since:22/01/2011

#25 [url]

Feb 5 11 11:12 AM

.
As to the question of whether the attenuation mechanism has a 'signature' effect on the sound; -nowhere NEAR as much as several factors of implementation, such as feed-back versus feed-forward.

So many designs are feed-back out of necessity, since FETs, and Opto-resistors are nonlinear in their control reaction and vary dramatically from component to component... even among the same batch. This has a far greater effect than the FET or Opto doing the shunting, as an example.
Keith

-ssltech

Ah ha.  This I understand. 
Thanks.

This brings up some interesting questions about the ability of innovative SW implementations to address some of these HW-specific design tradoffs.    e.g. If you are not emulating a phsycal circuit then feedback should not be involved.  It seems there should/could be some killer algos out there...one day.


Quote    Reply   
avatar

danko

Gold Finger

Posts: 524 Member Since:23/01/2011

#26 [url]

Feb 5 11 1:24 PM

Exactamente!The same. -no need to 'wrestle' with the mechanics anywhere near so much.-But pressing them ALL in makes people FEEL like they're being MORE naughty, so even people to whom I've pointed this out still persist in pushing all the buttons.

-ssltech


Well. It's two more. That's like, twice as much. It has to be at least double as much.

Tomas Danko - www.danko.se

Quote    Reply   
avatar

ssltech

Aqua Marine

Posts: 4,034 Member Since:22/01/2011

#27 [url]

Feb 5 11 3:24 PM


Ah ha.  This I understand. 
Thanks.
This brings up some interesting questions about the ability of innovative SW implementations to address some of these HW-specific design tradoffs.    e.g. If you are not emulating a phsycal circuit then feedback should not be involved.  It seems there should/could be some killer algos out there...one day.

-kenfavata


Dead right. 


In fact, the irritating component variation aspect means that things like stereo tracking can be improved, but for other devices like the LA-2a,component aging means that the unit ends up behaving very differently with age.

The T4b module for example, has an enormous influence on how the compressor actually COMPRESSES, and the chemistry of the cadmium sulfide optical cells means that with age, the time constants (whcih are determined SOLELY by the cells... there are no external controls to affect attack and release etc) therefore modify significantly with age. -In fact some T4 modules actually 'overshoot' slightly, and 'nip' into the envelope slightly, punching a small dip in...

My good friend Steve Sykes told me a good few years ago that when you go into a studio with several LA-2a's in the rack, you should listen to find out WHICH of them you like the behavior of. -My contention is that -aside from poor maintenance and things like aging capacitors messing up the signal- the largest effect, and certainly the biggest factor acting upon the actual compression characteristic (as distinct from the output tone, distortion and other characteristics unconnected with actual compression) is the T4 module.

In this sense, the 'modeled' virtual units may not be modeling a characteristic which one actually LIKES... for certain applications. -As part of a recent analysis and investigation into what goes into these things, I've built a box which switches between multiple T4 modules instantly, and allows side-by-side comparison and analysis of the T4 module alone, without necessarily having to have multiple LA-2a's... And even though I DO happen to have a few of them, I'm interested in identifying and observing what characteristics are SOLELY due to T4 modules, and cannot be ascribed to unintentional or unnoticed variations in the 'host' unit.


It seems that people often have a "this one works great on Bass, but this one rules for female vocal" type of experience with owning several of the original units, (as opposed to the reissues, which -being newer- don't tend to have acquired the aging variations) which I believe can -most likely- entirely be ascribed to variations in the T4 module. -unfortunately, the modeling just gives us a single 'unified model' option, and -while these can often be pretty good- they remove the 'individual character variations. This lack of control option seems to be 'necessary' though, because introducing time constant control options on an LA-2a for example, would make the control interface (and therefore the user experience) too dissimilar. -And that's not what will sell to the vast majority of home studio buyers who can't readily own or use the real thing.

Now, let me -in the interest of all fairness- declare right here and now that a driving force behind my present interest in T4 analysis is because I am interested in manufacturing and selling a T4 type module... possibly with some form of switchable characteristic, if that proves possible.

With this in mind and if anyone wants to learn further about these aspects, I'd be happy to start a separate thread about LA-2a's... I've built a few of those in my time also, and they too have grown to become a 'classic' over the years; just as the 1176 has.

-Keith Andrews -If I can't fix it, I can fix it so [i]NOBODY[/i] can fix it!

Quote    Reply   
avatar

jaykadis

Platinum Blonde

Posts: 2,391 Member Since:24/01/2011

#29 [url]

Feb 15 11 3:56 PM


I would love a discussion of T4s.  I have an LA-2a that was borrowed by Universal Audio when they did their LA-2a measurements.  It lived in Stanford's Memorial Church since it was new.  Unfortunately its T4 has ceased to function and I'd like to rebuild it if possible, having had it replaced by a new T4 for the moment.

Quote    Reply   
avatar

ssltech

Aqua Marine

Posts: 4,034 Member Since:22/01/2011

#32 [url]

Feb 17 11 5:03 PM

It looks -at a glance-  like R64 and R65 set and divide the quiescent DC bias, then R63 couples that back to the emitter, helping to set the bias on the first transistor... R68 looks like an overall feedback path.

Haven't had to spend time thinking about those things; it's not really involved with the 'all buttons in' subject, beyond the fact that it's a 4-transistor amplifier with complementary outputs.

-Keith Andrews -If I can't fix it, I can fix it so [i]NOBODY[/i] can fix it!

Quote    Reply   
avatar

david kulka

Tin Man

Posts: 28 Member Since:24/01/2011

#33 [url]

Mar 7 11 3:54 PM

I was curious about the compression ratio with all buttons in so I measured a black face Rev F. With all buttons in, the ratio was about 21:1.

Quote    Reply   
avatar

bob olhsson

Aqua Marine

Posts: 3,300 Member Since:25/01/2011

#34 [url]

Apr 30 11 10:28 AM

I have two LA-3as. I bought one in 1970 and the second in 1972. Each compression knee is quite different however I've never noticed any change in the relationship between the two over the 40 years I've had them. I've also never run into any two LA-2as or LA-3as that were the same.

www.audiomastery.com Bob's room 615 562-4346 georgetownmasters.com Georgetown Masters 615 254-3233 www.thewombforums.com

Quote    Reply   
avatar

s2udio

New Forum Friend

Posts: 6 Member Since:03/06/2012

#36 [url]

Jun 28 12 2:21 PM

Keith,
That was an absolute Tour de Force on 1176ism.

Dave

-mcsnare

The Absolute..

Thanks Keith This is a stickie on my desktop ,,forever !

Quote    Reply   
Add Reply

Quick Reply

bbcode help