Here is a good read for info on the T4B module:
T4B
The idea to use a 12AY7 and change R9 and R13 to 100k came up.
To tame some of the gain...
There must be some difference in the vintage units input resistor network...
Saturday, March 28, 2009
Audio Erratum Vol.1 Pt.2 - The Dual La2a
Its Alive!
Here is a good read for info on the T4B module:
T4B
The idea to use a 12AY7 and change R9 and R13 to 100k came up.
To tame some of the gain...
There must be some difference in the vintage units input resistor network...
Here is a good read for info on the T4B module:
T4B
The idea to use a 12AY7 and change R9 and R13 to 100k came up.
To tame some of the gain...
There must be some difference in the vintage units input resistor network...
Tuesday, March 10, 2009
Compressors
Love the squeezebox.
One of the best descriptions I've heard about audio compressors in general is that a good, transparent compressor is like a little guy riding the fader up and down faster than a human ever could. It's such a handy studio tool, but not every type of compressor works the 'mojo' on every type of source.
A good starting place on the subject is to understand the different types of compressor designs.
-Opto gain reduction is accomplished by varying the intensity of a light into a light sensitive resistor (later LA4s) or electro luminiscent panel (older La-2a's T4B module) causing a relatively slow attack and release which is also typically fixed. "Syrupy" and "smooth" are both good descriptions of opto compression. Vocals and bass benefit the most. Teletronix La-2a, Urei La-3a are examples of opto compressors.
-Variable-mu - "Mu" is gain and the compression works by using remote cut-off or re-biasing of a vacuum tube via a sidechain control voltage. As some of the very first compressors ever designed and produced, they continuously increase their ratio the harder they’re pushed. Though faster with attack and release times than opto designs, vari-mu are still slower than VCA or FET compression.
RCA BA-6A, Altec 436b, Fairchild 660, CBS RIIZ, Gates Sta-Level, GE Uni-Level and the Manley "Vari-Mu" (go figure) are examples of vari-mu compressors.
-FET, by definition, is a Field Effect Transistor. Compression is achieved by a FET, which is used as a variable resistor. the FET acts like a resistor whose resistance is controlled by the voltage applied to its gate. The higher the voltage applied to the gate, the smaller the drain-source resistance will be. The greater the voltage applied to the gate of the FET, the less resistance, hence large signals cause the FET to reduce the gain. Urei 1176 is the most obvious of FET compressors, but other FET compressors include the API 525.
-VCA is a Voltage-Controlled Amplifier. The gain is determined by the voltage supplied to a control input. Virtually every compressor technically uses a VCA, whether it is based on FETs, variable-mu tubes, or optical elements. However, in the parlance of audio, the term “VCA compressor” usually refers to a compressor using an integrated-circuit VCA for gain control. Grab and pump are two things done well by VCA compression. DBX 165, 160a
-Diode bridge The compression of a diode bridge circuit is accomplished thus: The diodes in the bridge are forward-biased, and the incremental resistance of the diodes determines the amount of gain reduction. The resistance of the diodes is in turn dependent on the amount of bias current going through the diodes. Bias current is determined by the output of the sidechain control voltage.
At large amplitudes, the audio signal itself can cause appreciable changes in the amount of current flowing through the diodes in the bridge. The audio signal moves the diodes away from their bias points, the incremental gain of the circuit changes. Because the circuit’s gain depends both on the bias currents (determined by the sidechain control voltage) and the audio signal itself.
There is an inherent amount of ever increasing THD in the diode bridge compression circuit which can "soften" audio very nicely.
Neve 2254, Neve 33609 and Vintagedesigns CL1 are all diode bridge compressors.
One defining design element of all compressors is a feed-forward vs. feedback design.
The actual compression circuit, whether a VCA, Vari-mu tube, etc., needs a control voltage in order to respond appropriately to the signal being passed. As is logical to the names, a feed-forward circuit will draw the control voltage from the incoming signal resulting in a greater accuracy whereas the feedback design draws its control circuit from the output signal, thereby letting the circuit breath before applying the control voltage to the compression circuit. Obviously, each of these types of compressions will have their own impact upon attack and release curves depending upon how the control voltage is derived.
Tuesday, March 3, 2009
More than meets the eye
- random notes on audio transformers -
Ground isolation, DC rejection, RF shielding, circuit balancing, impedance matching and line splitting or combining are all very good reasons for knowing about audio transformers. Transformers have grown to become a very integral part of the sound of high-end studio equipment. A magnetized core made up of varying parts of different metals, such as nickel, steel, etc. This "bobbin" is created by laminating thin sheets of metal and stacking them, insulated from one another. This core is wound with a type of insulated copper wire. All in all a very simple design which, fundementally, has remained the same for way over half a century. For more on the fundemental principles of all transformers, see Faraday's law of induction.
Bill Whitlock of Jensen transformers has a detailed explanation of the concept, parts and construction that is specific to audio transformers HERE
RE: 48v and vintage transformers via Oliver of Tab-Funkenwerk:
"Phantom Powering
One of the biggest concerns with these historic microphone pre-amp input and output transformers, is the phantom powering. Developed in the beginning of the 60s, and initially a sub-standard of powering microphones, this has now moved on to become the industry standard, applying 48V via two resistors to each leg of the modulation (pin 2/3) so there is no potential difference and using the shield as return path/0V....
It is necessary to keep in mind that in the 1940/50 manufacturers only guaranteed the PVC coated magnet wire for approx. 24V AC, which is why every single layer was insulated with additional material like oilpaper, etc.
Therefore when you dump 48V into a input or output transformer you engage in a slow process of killing your historic transformer.
DC blocking capacitors do not help at all because the time required to charge the capacitive property is still too much, and every time the microphone gets disconnected while the phantom power is on, the entire energy of those caps is dumped charging the windings of the transformer and pre-magnetizes the core.
If you want to use phantom power then it is necessary to use a ramped 48V supply with a time constance of more than 5sec.
This will take care of the problem posed by this energy burst, simply by having a slower time constance than the capacitive property of the transformer can charge.
But still there is a slight chance that something will get burned in the process, as you are dealing with historic/vintage parts, that had a maximum shelf life of aprox. 25 years."....great stuff, Oliver...
Below are a few of the biggest transformer companies.
Jensen
Sowter
Lundahl
Cinemag
Edcor
Carnhill
Among the vintage brands there are: UTC, Triad, Haufe, Stancor, Marinair, St.Ives, Peerless, Thordarson, Altran, Chicago, Western Electric, Reichenbach, API, Hammond, and more.
Below are some handy wiring diagrams for UTC's if anyone happens to need them:
UTC documents
If you are building a classic studio piece, this link is a great reference for what transformers will work:
Transformer by project list
Ground isolation, DC rejection, RF shielding, circuit balancing, impedance matching and line splitting or combining are all very good reasons for knowing about audio transformers. Transformers have grown to become a very integral part of the sound of high-end studio equipment. A magnetized core made up of varying parts of different metals, such as nickel, steel, etc. This "bobbin" is created by laminating thin sheets of metal and stacking them, insulated from one another. This core is wound with a type of insulated copper wire. All in all a very simple design which, fundementally, has remained the same for way over half a century. For more on the fundemental principles of all transformers, see Faraday's law of induction.
Bill Whitlock of Jensen transformers has a detailed explanation of the concept, parts and construction that is specific to audio transformers HERE
RE: 48v and vintage transformers via Oliver of Tab-Funkenwerk:
"Phantom Powering
One of the biggest concerns with these historic microphone pre-amp input and output transformers, is the phantom powering. Developed in the beginning of the 60s, and initially a sub-standard of powering microphones, this has now moved on to become the industry standard, applying 48V via two resistors to each leg of the modulation (pin 2/3) so there is no potential difference and using the shield as return path/0V....
It is necessary to keep in mind that in the 1940/50 manufacturers only guaranteed the PVC coated magnet wire for approx. 24V AC, which is why every single layer was insulated with additional material like oilpaper, etc.
Therefore when you dump 48V into a input or output transformer you engage in a slow process of killing your historic transformer.
DC blocking capacitors do not help at all because the time required to charge the capacitive property is still too much, and every time the microphone gets disconnected while the phantom power is on, the entire energy of those caps is dumped charging the windings of the transformer and pre-magnetizes the core.
If you want to use phantom power then it is necessary to use a ramped 48V supply with a time constance of more than 5sec.
This will take care of the problem posed by this energy burst, simply by having a slower time constance than the capacitive property of the transformer can charge.
But still there is a slight chance that something will get burned in the process, as you are dealing with historic/vintage parts, that had a maximum shelf life of aprox. 25 years."....great stuff, Oliver...
Below are a few of the biggest transformer companies.
Jensen
Sowter
Lundahl
Cinemag
Edcor
Carnhill
Among the vintage brands there are: UTC, Triad, Haufe, Stancor, Marinair, St.Ives, Peerless, Thordarson, Altran, Chicago, Western Electric, Reichenbach, API, Hammond, and more.
Below are some handy wiring diagrams for UTC's if anyone happens to need them:
UTC documents
If you are building a classic studio piece, this link is a great reference for what transformers will work:
Transformer by project list
Tuesday, February 17, 2009
In a galaxy far, far away...before Pro Tools....
“Every console, I don't give a damn if it's analog or digital-hell, every mixing situation today-is the brainchild of Bill Putnam.” -Bruce Swedien
There are many arguments both for and against the home recording, digital recording revolution. I've heard most of them and they both have their points, though every opinion is just that, because ultimately music is music and a recording is just the style of documenting music. A crappy recording will still convey information almost as well as a great recording.
And with the onslaught of everyone, and their mother, having Pro Tools and booking time in the spare bedroom, I continue to try and learn about what makes a recording great.
Before the days of digital compression, internet forum interns, MP3s going to the mastering house and assistants who refuse to patch in a real 1176 because "the plugin sounds just as good". In the days when you actually had to KNOW about electrical engineering to engineer. Even before the rise of the cassette tape...
All the way back to where the modern recording studio started.
People like Bill Putnam Sr. were running the show.
As the founder of Universal Audio, UREI and other successful companies, his studio circuit designs AND recordings as engineer are still among the most highly regarded.
This story is one inspiration I found to build my own studio gear.
Here is some excerpts from the History of Universal Audio:
..."We had gone in to record with Bill, and Bill had everything set up" recalls Murray Allen, then a member of Kenton's band and the man who would one day own
Universal Recording. "Kenton called in to Bill, 'Bill, how's everything in there?' and Bill replied, 'Everything's perfect until the music starts.'"
Thus began an association that would last until Kenton's death. These two mavericks were perfectly suited for each other. For these sessions, Putnam wanted the sound to be as fresh and bold as Kenton's arrangements. So he began to address several aspects that would have profound effects on how records sound.
Putnam devised a band shell for strings that was a mainstay for almost two decades. In addition, he built a drum shed for the isolation of drums to be used for the Kenton recordings. He conducted the first 8-track experiments, which featured a staggered head with a signal-to-noise ratio of 30 dB. But most significantly, perhaps, was the creation of another "home brew": a custom console, complete with rotary faders, 12 inputs, preamps and dedicated echo sends....
....Putnam, who had been recently divorced from his first wife, was suddenly in sync on every level of his new West Coast life. Demonstrating how a recording facility could be built and run, he'd gone head-to-head with Radio Recorders, the biggest independent studio in Los Angeles, and won. He finally found a woman who "understood what my business was about and what I was about." (They married and produced two children, Bill Jr. and Jim, both of whom would continue the Universal brand to the present day.) And, he was making cool records. Examples of Putnam and Sinatra collaborations at United/Western include Sinatra: Basie and Sinatra & Strings. They're textbooks on how strings, horns, brass, rhythm and vocal should be laid down....
...Talk about shrewd. In the early '60s, stereo was roughly at the same evolutionary spot where 5.1 is today. Label owners finally got hip to marketing and releasing stereo product, except that they had none. Or did they? Like Swedien, Ramone and Tom Dowd, Putnam had surreptitiously been running simultaneous stereo mixes along with the expected mono masters for a couple of years. When the labels got wind of this, they offered to pay for the tape. No dice, said Putnam. "However, I will let you pay me for the studio time."...
For more on Bill Putnam Sr. and the history of Universal Audio, check out these links:
Bill Putnam Story 1
Bill Putnam Story 2
There are many arguments both for and against the home recording, digital recording revolution. I've heard most of them and they both have their points, though every opinion is just that, because ultimately music is music and a recording is just the style of documenting music. A crappy recording will still convey information almost as well as a great recording.
And with the onslaught of everyone, and their mother, having Pro Tools and booking time in the spare bedroom, I continue to try and learn about what makes a recording great.
Before the days of digital compression, internet forum interns, MP3s going to the mastering house and assistants who refuse to patch in a real 1176 because "the plugin sounds just as good". In the days when you actually had to KNOW about electrical engineering to engineer. Even before the rise of the cassette tape...
All the way back to where the modern recording studio started.
People like Bill Putnam Sr. were running the show.
As the founder of Universal Audio, UREI and other successful companies, his studio circuit designs AND recordings as engineer are still among the most highly regarded.
This story is one inspiration I found to build my own studio gear.
Here is some excerpts from the History of Universal Audio:
..."We had gone in to record with Bill, and Bill had everything set up" recalls Murray Allen, then a member of Kenton's band and the man who would one day own
Universal Recording. "Kenton called in to Bill, 'Bill, how's everything in there?' and Bill replied, 'Everything's perfect until the music starts.'"
Thus began an association that would last until Kenton's death. These two mavericks were perfectly suited for each other. For these sessions, Putnam wanted the sound to be as fresh and bold as Kenton's arrangements. So he began to address several aspects that would have profound effects on how records sound.
Putnam devised a band shell for strings that was a mainstay for almost two decades. In addition, he built a drum shed for the isolation of drums to be used for the Kenton recordings. He conducted the first 8-track experiments, which featured a staggered head with a signal-to-noise ratio of 30 dB. But most significantly, perhaps, was the creation of another "home brew": a custom console, complete with rotary faders, 12 inputs, preamps and dedicated echo sends....
....Putnam, who had been recently divorced from his first wife, was suddenly in sync on every level of his new West Coast life. Demonstrating how a recording facility could be built and run, he'd gone head-to-head with Radio Recorders, the biggest independent studio in Los Angeles, and won. He finally found a woman who "understood what my business was about and what I was about." (They married and produced two children, Bill Jr. and Jim, both of whom would continue the Universal brand to the present day.) And, he was making cool records. Examples of Putnam and Sinatra collaborations at United/Western include Sinatra: Basie and Sinatra & Strings. They're textbooks on how strings, horns, brass, rhythm and vocal should be laid down....
...Talk about shrewd. In the early '60s, stereo was roughly at the same evolutionary spot where 5.1 is today. Label owners finally got hip to marketing and releasing stereo product, except that they had none. Or did they? Like Swedien, Ramone and Tom Dowd, Putnam had surreptitiously been running simultaneous stereo mixes along with the expected mono masters for a couple of years. When the labels got wind of this, they offered to pay for the tape. No dice, said Putnam. "However, I will let you pay me for the studio time."...
For more on Bill Putnam Sr. and the history of Universal Audio, check out these links:
Bill Putnam Story 1
Bill Putnam Story 2
Monday, February 16, 2009
12A*7 Vacuum Tubes
I've been compiling a list of the different dual triode 12A*7 preamp tubes and the different versions for use for pro-audio equipment: specifically for mics, micpres and compressors. Of course I screw around with occasional guitar amp as well...
Since 1947, with the introduction of the 12AX7 vacuum tube, there have been lots of interesting variations that can be tried in tube preamp circuits. This list is by no means complete but will provide some quality alternatives to the "Sovtek, Ruby or EH" tube choice that guitar center has.
Most of the below tube types spec close enough to be drop-in replacements (with circuit specification, a certain Mu value tube will be more in focus than other Mu value tubes) but a quick looks at the data sheets is always a good idea for specific biasing, etc..:
Tube Types
Gain (mu) 19
12au7
ECC82
E82CC
ECC802S
5814A
6189
13D5 (brimar)
CV4003 (Mullard)
M8136 (Mullard)
Gain (mu) 41
12av7
Gain (mu) 45
12ay7
6072
7062 (48)
5965 (48)
Gain (mu) 60
12at7
CV455
CV4024
ECC81
6201
E180CC
E81CC
ECC801S
Gain (mu) 70
5751
Gain (mu) 100
12ax7
12ax7a
12ax7WA
12ad7
12dt7
12df7
7025
7729
ECC83
ECC803S
E83CC
CV4004
Some of my favorite are the cleartop Black plate RCA's, Telefunken, Amperex (especially the PQ series), Mullard, GE 5-star, and NOS JAN (joint army/navy- military spec) tubes will be of significant quality. Brimar is still a sleeper...
Sunday, February 15, 2009
Audio Erratum Vol.1 Pt.1 - The Dual La2a
This is an interesting project: a remake of the Teletronix La2a tube opto compressor...in stereo.
Board design is courtesy of [Silent:Arts] via the ProdigyPro forum. Many thanks to the Prodigy community for this one!
I decided to give my version a little taste of the original Teletronix La2a by using UTC A-10 and A-24 transformers. I'm still looking into original T4Bs but, for now, have a pair of clones done by DripElectronics. In my opinion, transformer Lams will have more of an effect on the sonics than the opto circuit anyways.
So far (with no test for power up as of yet) I have the PCB populated, power toroidal, power wiring, all front panel switches and pots and right channel audio wired.
Left to do is left channel audio, meters, power indicator lamp and get some nice tubes!
Here's a sourcing tip: find parts from the Ham radio crowd as opposed to broadcast or pro-audio people: way better deals.
Something to consider: these things have TONS of gain, use a 12AT7 or 12AY7 in the first stage as opposed to the higher gain 12AX7/ECC83.
Variants of the 12AT7 include the Amperex PQ 7062 series (a pin to pin replacement for a 12AT7) which I am looking for now. 5-star 6201's are also worth checking out. Any NOS GE 5-star is good by me as are Telefunken tubes.
I've been putting this together piece by piece over a matter of many months due to the expensive nature of quality parts. Still need to get: one set of UTCs, meters and tubes. Pretty much all the parts that give the comp soul...besides the circuit design of course.
Hopefully it will be finished soon, I have vocals to squash!
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