RIAA pre-compensation with Presto 92A, or forget it?

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EpicenterBryan
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RIAA pre-compensation with Presto 92A, or forget it?

Post: # 31025Unread post EpicenterBryan
Sat Aug 16, 2014 8:46 pm

Hi guys,
I’m new to the forum. Thank you to Steve Espinola for allowing me to join.
I hope not to abuse this honor, and hope to be able to contribute in the future as I come up to speed on this rather daunting learning curve. Please excuse me if this topic has come up before. I have not found specifics in prior posts. I really have made an attempt to find what I’m looking for.

My first big question has to do with RIAA pre-compensation. I have a Presto 92A amp, and a Presto 1D cutting head. The Presto has an EQ flat setting in addition to an NAB pre-comp curve for output.

Assuming the built in NAB pre-comp is correct (I have not tested yet), is it better to run flat and do an inverse RIAA in front of the Amp with an equalizer, or use the NAB pre-comp, and do a “make NAB pre-comp an RIAA pre-comp curve” with an external EQ in front of the amp? The amp with NO EQ is crazy flat. +/- 0.5 dB from 20hz to 20Khz.

The reason I’m asking is that I have two 1/3 octave graphic EQ’s I could use, but neither can go +/- 19 db (for the RIAA curve). The largest delta I have is +/- 16db for a digital one (and +1111111111111111111111 is very bad) and +/- 12 db for the other Analog unit. I could use 2 channels in series since my system is Mono (for now), each with ½ the required boost or cut for the desired result. I have read people prefer to cut rather than boost for phase error related issues. I am not clear on that and will need to look into it…

The situation may get worse later when I plan on building a stereo DYI cutter partly made on my 3D printer. It will need additional EQ…

I am wondering if anyone has a table or a program that could take the RIAA 1/3 octave EQ data and convert it to something where I could use a multi band parametric EQ. I have one that is 12 bands and one that is 20 bands. Both are stereo and all bands for each channel can be messed with on their own. The parametric EQ's can go +/- 60 db as I recall.

My gut says a parametric EQ should be able to do the job with only 4-5 bands (looking at the curve)? A few poles, a few zeros? I’m sure I can play with the Q and such with several bands and check with my RTA but I wonder is someone has already done this. Again, I hate to be a taker (most newbie’s are), and I hope to be a contributor some time…

I’m a former Analog and Digital circuit designer but never designed audio equipment and I’m 20 years out of date related to modern day Op Amps and all. I don’t trust myself to design something from scratch these days unless it’s “indication only”… Inputs from the forum would be greatly appreciated! And thanks for not flaming me as a newbie.

Bryan

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Radardoug
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Re: RIAA pre-compensation with Presto 92A, or forget it?

Post: # 31027Unread post Radardoug
Sun Aug 17, 2014 2:31 am

I would not recomend using a third octave eq to try and create the required curve. Better to use a simple network with 2 or 3 poles.
First suggestion, do a test cut and see what you are getting in the two positions you have. Then you can work out what correction curve you need, assuming you have an accurate playback system.

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markrob
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Re: RIAA pre-compensation with Presto 92A, or forget it?

Post: # 31035Unread post markrob
Sun Aug 17, 2014 7:01 pm

Hi,

I would use the flat setting on the Presto and build up an outboard analog RIAA EQ. Use the 1/3 octave EQ along with the constructed outboard preamp to add any extra corrective EQ needed to flatten the overall response, season to taste, or cut excess highs/lows out of the cutting signal chain. Also note the need for a series resistor with 1D head (the value depends on the impedance of your 1D). This provides the 500 hz turnover in the standard RIAA curve. So, you really only need the pole at 2122 Hz (75 us RC time constant) and zero at 50 Hz to do the entire curve (you should also provide for a roll-off at the ultra high end as the RIAA does not spec this). You might even be able to get away with just the 2122 Hz turnover. There are several circuits detailed here from simple passive filters with large losses to op-amp based designs. It should be easy to modify them to include only the turnover frequencies needed. If you have some experience with op-amps, then you should have no trouble getting something up and running. There are some newer op-amps with great audio performance, but the basics of designing with them have not changed in the last 20 years. Good luck with the project and let us know how you fare.

Mark

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Snakeheadfishlab
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Re: RIAA pre-compensation with Presto 92A, or forget it?

Post: # 31036Unread post Snakeheadfishlab
Sun Aug 17, 2014 8:14 pm

I will be dissecting the last statement for the next month,thank you markrob....

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Steve E.
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Re: RIAA pre-compensation with Presto 92A, or forget it?

Post: # 31053Unread post Steve E.
Mon Aug 18, 2014 1:04 pm

I messed with my 92A, with a lot of help, adding resistors, and managed to get a good RIAA curve into the EQ. Unfortunately, what worked for me may not work for you, as my 92A had already been modified a lot, and was NOT flat (it had a consistent, linear tilt of +1 db per octave....something like that). There was a lot of trial and error, but I got it. I don't have the data handy. :(

You also want to map the spikes of the head, using an NAB test record of some sort, and create an EQ prior to the RIAA circuit that compensates for its lack of flatness. If you do that, you'll be amazed at the quality of cuts you can get.

I don't use the resistor setup that the Presto manual recommends, which Rob mentions.

92A amps are amazingly powerful. They sound great, and are potentially flat well above our hearing range.

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EpicenterBryan
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Re: RIAA pre-compensation with Presto 92A, or forget it?

Post: # 31069Unread post EpicenterBryan
Mon Aug 18, 2014 11:53 pm

Radardoug wrote:Better to use a simple network with 2 or 3 poles.
First suggestion, do a test cut and see what you are getting in the two positions you have. Then you can work out what correction curve you need, assuming you have an accurate playback system.
Thanks for your inputs Radardoug. The more I read, the more I'm thinking I should start with passive filters (where possible) to get in the ball park, then tweak with special sauce as needed. As far as playback equipment goes, all the sudden I question what I have. I have ordered a passive reverse RIAA from http://www.hagtech.com/iriaa2.html so I can check my SAE Mark XXX preamp and also check a Behringer DJ mixer I have kicking around for a modern reference (of sorts). My one and only turntable is a direct drive pioneer with an M95ED cart. Although highly thought of when I purchased it new in 1975 or so (I was 15, and I still have it at age 53) I have no clue if anything I have in this playback chain is accurate enough to evaluate a disc.

So I'm going to hold off on cutting a test disc as you suggest until I know what is up with my playback system. And thank you for pointing that out!

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tragwag
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Re: RIAA pre-compensation with Presto 92A, or forget it?

Post: # 31074Unread post tragwag
Tue Aug 19, 2014 1:12 pm

I use the same amp, on the FLAT setting with computer RIAA ENCODE (and a HUGE boost of high end in addition, about +5 dB shelf at 8kHz up)
making lathe cuts on a Presto 6N, HIFI stereo cuts on vinylrecorder
at Audio Geography Studios, Providence, RI USA
http://www.audiogeography.com

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EpicenterBryan
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Re: RIAA pre-compensation with Presto 92A, or forget it?

Post: # 31082Unread post EpicenterBryan
Tue Aug 19, 2014 9:19 pm

markrob wrote: Also note the need for a series resistor with 1D head (the value depends on the impedance of your 1D). This provides the 500 hz turnover in the standard RIAA curve. So, you really only need the pole at 2122 Hz (75 us RC time constant) and zero at 50 Hz to do the entire curve (you should also provide for a roll-off at the ultra high end as the RIAA does not spec this).
Mark
Thanks for your inputs Mark. I totally missed that 180 ohm resistor and 4uf cap. I found the full spec sheet for the 1D (I think Steve posted it) on this forum. What a nice find! I have the parts on order.

You are an interesting fellow. The projects you come up with are truly inspired. For one, that record press with the heated plates and Harbor Freight shop press... Did you end up going with a two station heat / cool approach? What is the latest update? I also got a kick out of you using Steely Dan in that test cut.

Also found your DIY stereo head very interesting. If you ever do a Rev 3, I bought some drivers to play with that look very promising. I paid $15 each for them last week, but they are on special for $9.90 today. I ordered a few more for spares.
http://www.parts-express.com/aura-cougar-nsw1-205-8a-1-extended-range-speaker-driver--299-014
They are 8 ohm, 5W, 20W max, 220 to 20,000 Hz. They have Neodymium magnets, rear vented for cooling, and it looks like you could put your ferro cooling goop right in that port if needed. They are slightly smaller than the ones you used, but the mounting bracket is back set so they can be mounted with the drivers closer together. Also, the inverted cones are titanium so my plan is to glue a dome / push rod kind of assembly right on the titanium (I'm going to try 3D printing that in ABS). Here is the frequency response curve. I think you will be happy to see this:
http://www.madisoundspeakerstore.com/images/product/NSW1-freq-imp.jpg

Anyway Mark, I do appreciate your inputs!

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EpicenterBryan
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Re: RIAA pre-compensation with Presto 92A, or forget it?

Post: # 31083Unread post EpicenterBryan
Tue Aug 19, 2014 9:40 pm

Steve E. wrote:I messed with my 92A, with a lot of help, adding resistors, and managed to get a good RIAA curve into the EQ.
When you did your mods on the 92A to make an RIAA curve, did you leave the NAB alone and commandeer one of the other buttons to switch in your RIAA filters? If so, which one did you choose? I'm not sure I want to modify the amp - still thinking about something on the front end for RIAA pre-comp only (accounting for the recommended RC for the 1D on the output), and something for other adjustment like head response once I have end to end response known.

Oh, and thanks so much for having posted that manual for the 92A. NOW I have a schematic I can actually read. Life is good.

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Re: RIAA pre-compensation with Presto 92A, or forget it?

Post: # 31084Unread post EpicenterBryan
Tue Aug 19, 2014 9:51 pm

tragwag wrote:I use the same amp, on the FLAT setting with computer RIAA ENCODE (and a HUGE boost of high end in addition, about +5 dB shelf at 8kHz up)
That is a valid option as well I guess. Humm, I need to think about that. Assuming I have already bounced files from my Protools Mix system, do you use a stand alone program that does the "apply" to an existing file? If so, does it automatically reduce levels so the boosted highs stay below digital clipping?

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Re: RIAA pre-compensation with Presto 92A, or forget it?

Post: # 31085Unread post EpicenterBryan
Tue Aug 19, 2014 11:13 pm

tragwag wrote:I use the same amp, on the FLAT setting with computer RIAA ENCODE (and a HUGE boost of high end in addition, about +5 dB shelf at 8kHz up)
Hey Tyler, I just ran across a video Steve did with you and your two 6N's. Fascinating! I have the same scope on my 8D. Love that Audio Disc Chip brush. Does that do a good enough job that you don't need a vacuum setup for swarf? At some point I'm going to need a 16" tone arm for test playback. Got a spare kicking around?

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tragwag
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Re: RIAA pre-compensation with Presto 92A, or forget it?

Post: # 31094Unread post tragwag
Wed Aug 20, 2014 1:47 pm

I take my already mastered tracks into a special "lathe cuts" project file.
Its a template that has two tracks, one for the cutting program that has RIAA, compression and EQ in the chain already.
One EQ I use to automate HF up towards the inner diameter of the cut.
I usually end up turning down the encoded track about 14 dB so it doesn't digitally clip the channel/master

another track for playback, so I don't have to listen to the encoded audio.

this goes out through an audio interface, into the mackie big knob as my final level adjustment and routing.

I used the swarf brush for a laserdisc thing, but I do mostly embossing on polycarbonate so I don't have a direct need for it.
No spare tonearms, sorry! But personally I've never found a need for hearing my cuts as they're cutting.
It's always been enough for me to set the level and check it afterwards
making lathe cuts on a Presto 6N, HIFI stereo cuts on vinylrecorder
at Audio Geography Studios, Providence, RI USA
http://www.audiogeography.com

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Steve E.
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Re: RIAA pre-compensation with Presto 92A, or forget it?

Post: # 31099Unread post Steve E.
Wed Aug 20, 2014 9:28 pm

EpicenterBryan wrote:
tragwag wrote:I use the same amp, on the FLAT setting with computer RIAA ENCODE (and a HUGE boost of high end in addition, about +5 dB shelf at 8kHz up)
Hey Tyler, I just ran across a video Steve did with you and your two 6N's. Fascinating! I have the same scope on my 8D. Love that Audio Disc Chip brush. Does that do a good enough job that you don't need a vacuum setup for swarf? At some point I'm going to need a 16" tone arm for test playback. Got a spare kicking around?
A 12" tonearm should do....I think that's what you are talking about.

Check this thread: http://www.lathetrolls.com/viewtopic.php?f=1&t=927&p=29954&hilit=tonearm#p29954

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