Wanted: Anything involving the RIAA pre-emphasis

[davideakin]

Hi everyone!
I'm new here, and I know very little about disc cutting. I'm an electronic engineer, not a recording engineer, but this looks liek the place to ask some specific questions:

Recently I have become interested in RIAA equalization and the various arguments that exist, such as the "Neumann 4th pole", innacuracy (or not) in cutting amplifiers, and the exchange that took place between CD4cutter and Cohearent a few years ago:
http://lathetrolls.com/viewtopic.php?f=1&t=1417
(I still can't decide who was right!)

I don't know much about lathes, but I do know about circuits. I would therefore love to see some schematics of the inverse RIAA filters using in cutting amplifiers, so I can make up my own mind about these arguments. Is anyone willing to share with me? If you don't want to send whole schematics that's OK, I would be happy with cuttings of just the important filters.

As an experienced analog engineer, maybe I can reciprocate by helping out with any electronic questions others may have on the forum

[opcode66]

The recording curve is an emphasis. It is the RIAA curve. Playback phono pre-amps have a de-emphasis.

Most RIAA eq curves, be them emphasis or de-emphasis, are inaccurate to some extent.

[davideakin]

The recording curve is an emphasis. It is the RIAA curve. Playback phono pre-amps have a de-emphasis.
Most RIAA eq curves, be them emphasis or de-emphasis, are inaccurate to some extent.

That is what I would like to examine- the accuracy of the emphasis circuits in some typical cutting amplifiers.

Does anyone have a schematic of the Neumann SAL74B cutting amp, showing the RIAA pre-emphasis?

[opcode66]

The schematics are available in a cd sold on ebay

http://m.ebay.com/itm/161352273238?nav=SEARCH

[davideakin]

The schematics are available in a cd sold on ebay

http://m.ebay.com/itm/161352273238?nav=SEARCH

Well that covers one type of lathe. I cant believe they're the only schematics that exist, I mean, there were many lathe brands

[opcode66]

You were asking for the RIAA EQ curve implementation SAL rack. The CD has exactly what you're asking for. I don't know what else to say.

If you are looking for the implementation from Westrex or Ortofon, you might want to ask some of the other members on the board directly who specialize in those particular Amp racks.

Flo has a preamp board for Caruso cutterhead that has riaa on it. He has posted board images, and possibly schematic. You could ask him about his implementation. Also the one from vinylium.

When all is said and done, the majority of modern made professional cuts were done on a Neumann or a Sculy/Westrex. So, my feeling is, you'll want to investigate those.

By the way, it is hard to find some documentation on lathes and cutting amps. Some is lost to the ages. Other documentation is held and not released by existing lathe techs. They wont share because they feel it would take jobs away from them, and that is how they support themselves and their families

[davideakin]

You were asking for the RIAA EQ curve implementation SAL rack.

So is SAL the same as VSM (as listed on the DVD)? Sorry, I'm not familiar with the various discrete parts that make up a lathe, and their names (am I even talking about it the right way, or is 'the lathe' one very specific part of the system?)

[opcode66]

This is why I'm saying plunk down the $20 for the CD-ROM. Everything is explained within the materials on the disc. It's less than going out to eat a meal these days. And, it is orders of magnitude less than nearly anything else you have to purchase for a studio or a lathe. I can't understand why people are so cheap about not wanting to buy this CD. Not saying you in particular. It has been a common theme here. In general, so many people could have their questions answered AND MORE by simply employing one good ol' Andrew Jackson. It contains most all of the original manual with setup instructions, a Q & A, pictures, diagrams and schematics. Even includes the schematics for the Neumann recording consoles.

The only things that are missing from the CD are the things that make most lathe techs unique. If the other missing parts were widely distributed, then these people would lose a good part of their livelihood. Certain Gotham bulletins, and papers from Temmer are not widely distributed. Also, for instance, I've never seen anyone post the entire documentation on the Tracing Simulator. Even though no one uses it. I think that it was close to being functional, and a modern equivalent could be made. Likely as a plugin. But, without all of the original doc and schematics, that would be very hard to do. I have them. I also have all the original doc and tech notes for several pitch systems. I consider myself extremely fortunate to have this information seeing as though few people do. After Al passed there were promises by a few that all of his documentation would be scanned and made available. Yeah, that didn't happen... This stuff to some degree is kept very well guarded.

However, what you are looking for is all on the cd. And some is explained here. Please use the Search feature on this site. It is helpful.

SAL is the Amp Rack for the cutterhead. Every part of the lathe has a 2 or 3 letter designation, generally followed by a number value. Even the connections as well as the cables themselves. So, SAL74 is the amp rack that was sold with the VMS70

There were different versions as well. So, SAL66 or SAL74. To screw things up even more, Neumann made an improvement kit for the SAL74. So, my amp rack is actually a VG74B. Through the years, there were some improvements made. Or, it seems, they couldn't get the same parts always. So, I've seen variations in implementations within the exact same product line. A lot of the cards in the lathe body itself are identical from the 60's VMS model to the 70's. VMS is a name for the complete system: ZT cabinet AM body, VA pitch box, Lyrec motor, PS Pluggable speed box, variously named cabling, SX68 or SX74 cutterhead, and SAL or VG amp rack.

FYI, the one thing that is not directly interchangeable between the actual lathe body or cabinet of a 60's VMS and a 70's VMS is the SA Suspension box. They are physically/mechanically identical. However, the circuitry inside that allows the amp rack to know if the head is up or down changed significantly. You could convert a 60's box to a 70's or vice versa. But, you can't use one on the other directly. If you connect a 60's suspension to a 70's amp rack, the Cutter Off light will never go off when you put the head down and therefore the SEL's won't open up on the drive lines. Which is not a good place to be in for transcribing audio to disc.

Finally, Neumann no longer in any way supports this gear. If you call them asking about recording lathes they will just hang up on you.

Brief VMS Cheat-sheet:

VA - Pitch Box
NG - Power Supply
SV - Pitch Computer
AS - Motor Control and Switching
AM - Lathe Body
SA - Suspension Box
ZT - Chassis
SU - Carriage Lever Switch

162 MT -30 pin tuchel for preview audio and tape remote
302 - Connects AS to SV Pitch Computer
303 - Connects AS to VA Pitch Box
304 - Connects AS to AM Lathe Body
307 - Connects AS to NG Power Supply
308 - Chassis cabling to PS Programmer Box

[davideakin]

Brief VMS Cheat-sheet:
VA - Pitch Box
NG - Power Supply
SV - Pitch Computer
AS - Motor Control and Switching
AM - Lathe Body
SA - Suspension Box
ZT - Chassis
SU - Carriage Lever Switch

So to that list you could also add 'SAL', which is the amp rack? And it is the only bit that I need to consider if I'm interested in the pre-emphasis circuit? (I don't want to spend $29 importing a DVD to discover it doesn't include the post-it size bit of circuit I actually want to see, and have someone say "oh, you meant that bit. That bit is in an entirely different bit of kit that isn't even part of the lathe. You're not even on the right forum, douchebag" )

(I'm not sure if I made it clear at the start of this thread, but I don't own a lathe, and have no intention ever of owning one, and most aspects of cutting are of only trivial interest to me. I have more of an academic interest in circuits, and to appreciate fully the de-emphasis requirement I am investigating the details of pre-emphasis circuits actually used by manufacturers.)

[opcode66]

That cheat-sheet is just the components in or on the ZT cabinet. So, that is why I didn't include the SAL/VG.

Since the amp rack is a completely separate unit (literally a rack that stands next to the lathe), I have a separate cheat-sheet for it. It contains many individual components as well, all with abbreviations just like in the cabinet. Things like SEL, SAB, etc.

I already emailed you a copy of the riaa posted by Flozki on this site. But, here it is again. Many thanks to Flo, who by the way I have been an ass to in the past. I apologized to him directly. May as well do it here too. Hey Flo, sorry for being a jerk. Inexcusable. We all appreciate you, your work and talent. Sometimes, we all get wrapped up in our own poop.

riaa_enc_flo.jpg

I will go out of my way to find the right part of the schematic in question for you. It will take time. It will take effort. So, some patience may be required on your part. Ok?

Note: If you are an audio guy, and you work with or repair audio gear, the schematics on this cd could be leveraged enormously. Many interesting bits of circuitry. With tax, in the city of Chicago, you can barely buy 2 packs of cigarettes for $29. I can't think of anything in my studio that I use that is less than $29. Not even my cabling.

[davideakin]

I will go out of my way to find the right part of the schematic in question for you. It will take time. It will take effort. So, some patience may be required on your part. Ok?

That's very kind of you! I'm in no rush. Hope you don't feel like I've badgered you into it...

[emidisc]

"Many thanks to Flo, who by the way I have been an ass to in the past. I apologized to him directly. May as well do it here too. Hey Flo, sorry for being a jerk. Inexcusable. We all appreciate you, your work and talent. Sometimes, we all get wrapped up in our own poop"

It takes a man of integrity to apologise in in a forum like ours well done!
Emidisc

[opcode66]

I took some time out of my busy weekend to answer your question in full. Please take into consideration that a few trolls here have accused me of knowing nothing about electronics. So, proceed at your own caution. I don't believe them. My guess is, you won't either. On to the answer and more!

First, here are a couple of links to pages containing excellent explanations of both the recording emphasis (hereafter referred to as RIAA) and the playback de-emphasis (hereafter referred to as Revers RIAA). This is sort of confusing terminology in that it strictly refers to the high end. The recording emphasis curve actually de-emphasizes the low end and the playback emphasizes. As you can see in the image below. The zero point is 1KHz.

http://www.beigebag.com/case_riaa_1.htm

http://english.cxem.net/equaliser/equaliser4.php

On all of the following schematics, I am only showing the relevant circuitry. I am not including the full schematic. Just excerpts. If you want the full schematic, then plunk down $20 on eBay. Each one is a lot bigger than the pieces shown. I included the information from the schematic so you know which one I pulled this from exactly.

riaa_1_1.jpg

Below is an excerpt from the Block Diagram for the SALB/VB/VG Amplifier Rack. You can clearly see the modulation input goes through the BSB Acceleration Limiter (if you have one) and then immediately through an RIAA EQ curve. Then on through the rest of the system. There is a switch on the rack to enable/disable the RIAA EQ curve. That is used only for calibration purposes and is left enabled for normal operation. What is interesting to note is that the RIAA switch enables/disables both the RIAA and the Reverse RIAA for Feedback/Pickup. The same EQ circuit is used for Feedback as it is for the tonearm because only one channel is sent through the circuit at a time selected by the 3 position knob on the Monitor Selection card MVS.

Amplifier-System-Block-Diagram-Segment.jpg

Below is the RIAA circuit in the Singal Processing of the SAB unit that is used to affect the Modulation signal that is destined for a drive coil in the cutterhead. So, everything to the left of R22 is part of the switching for enabling/disabling the RIAA curve. To the right of R22 is the RIAA implemntation. Following R38 is an op amp and the rest of the signal path. When RIAA is disabled, signal flows directly through the next op amp in the chain (not shown).

SAB-Modulation-RIAA.jpg

Finally, below you will find an excerpt of the schematic for the SAB Feedback processing. This is the Revers RIAA implementation for the Feedback and Pickup monitoring.

SAB-Feedback-and-Pickup-Inverse-RIAA.jpg

I find it curious that no one else on this forum actually answer this question. Not even the guys who have said I know nothing. Hmmm. Coincidence???

[opcode66]

--== Addendum ==--

The RIAA and Reverse RIAA circuits above can be described thusly:

1. Demphasis of a frequency band which is either below or above 1KHz is achieved via a passive RC filter.
2. Emphasis of a frequency band that is either below or above 1KHz is achieved via an Op-amp and and RC filter.
3. Altogether, the collective circuit implements an EQ curve that approximates the image below.

Note: You can use Resistors and Capacitors paired in various ways to achieve a passive demphasizing EQ circuit. The timing for the filter cutoff is determined by the value of the resistor and capacitor. Please see the following URL for more information on passive RC filters. http://en.wikipedia.org/wiki/RC_circuit

All you need to do to implment a particular curve is build circuits that are passive for demphasis and active via Op-amps for emphasis. Each one represent a band end point in your EQ (or point in the overall EQ curve).

EQ-Time-Constants.jpg

[davideakin]

Hi Todd, that's excellent! Exaclty what I was looking for, many thanks!

Could you perhaps fill in a couple of details? On the Modulation RIAA EQ schematic, does R38 (at the output) feed directly into the inverting input of another opamp? It looks like it ought to do something like that. And are there any notes about C11,C12,R29?
Also, on the inverse RIAA, does the output feed a resistance to ground? Do you know the value of this resistance? It looks like it might be about 47k.

[opcode66]

Could you perhaps fill in a couple of details? On the Modulation RIAA EQ schematic, does R38 (at the output) feed directly into the inverting input of another opamp? It looks like it ought to do something like that.

Followup-RIAA-Forward-of-R38.jpg

And are there any notes about C11,C12,R29?

It is odd. I've gone over the schematic several times. There are no notes pertaining to those capacitors anywhere on the document. This can mean two things. It was an error. It wouldn't be the first time things were mistakenly omitted on a Neumann schematic. Or, different models had different values??? I'm not sure. At this point, the only way for me personally to answer this question would be to pull this module from the amp rack on one channel and inspect the capacitors on the board. Possibly someone else knows before I have to go to that length to answer your question???[/quote]

Also, on the inverse RIAA, does the output feed a resistance to ground? Do you know the value of this resistance? It looks like it might be about 47k.

Well, there is R65 which is a 1M Ohm to Ground that is visible in the schematic excerpt above. The image below is what follows R68 in the schematic.

Followup-Invere-RIAA-Forward-of-R68.jpg

[davideakin]

Wonderful!

It is odd. I've gone over the schematic several times. There are no notes pertaining to those capacitors anywhere on the document. This can mean two things. It was an error. It wouldn't be the first time things were mistakenly omitted on a Neumann schematic. Or, different models had different values???

I have been simulatin the circuit. Interestingly, the replay curve is not quite identical to the preamphasis curve. When combined, you get rougly a 1.5dB shelf (loss) at 20kHz, rather than a perfectly flat response. Those unlabelled components have the potential to improve the match. Presumably that is what they are for (?) but why they are shown dashed and unlabelled is curious. The ~47k to ground that I was expecting to see would also do this, but evidently that's not how it is actually being done.

[opcode66]

If you look at the block diagram for the amp rack, what is labeled Amplifier System VG74, after the block that I marked with red text "RIAA For Recording", there is a box that represents filtering. High pass at 20Hz and Low pass at 17K. FYI, almost all cutters roll off around 16K or 17KHz. This is built into the modulation signal processing in the amp rack that supplies the ac voltage to the drive coils of the cutterhead. This might have something to do with what you are observing in your test circuit.

[davideakin]

If you look at the block diagram for the amp rack, what is labeled Amplifier System VG74, after the block that I marked with red text "RIAA For Recording", there is a box that represents filtering. High pass at 20Hz and Low pass at 17K. FYI, almost all cutters roll off around 16K or 17KHz. This is built into the modulation signal processing in the amp rack that supplies the ac voltage to the drive coils of the cutterhead. This might have something to do with what you are observing in your test circuit.

Hmm, that would tend to add to the droop! Although that block shows it will also do boost as well as cut.

Does it happen to say what sort of opamp IC1 is? That would give some indication of what stabilisation it may have needed.

[opcode66]

IC1 is an LF 356