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Old 10th June 2005, 08:05 PM   #1
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Default riaa circuit

I hope this has worked ok for all.

A very good sounding riaa, I think I have the values right, but if not, please let me know, thanks

Click the image to open in full size.
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Old 10th June 2005, 08:26 PM   #2
Netlist is offline Netlist  Belgium
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I could offer this spreadsheet, perhaps you know of its existence.

/Hugo
Attached Files
File Type: zip riaa curve optimal noninv.zip (22.1 KB, 254 views)
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Old 10th June 2005, 09:03 PM   #3
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another one!!

Click the image to open in full size.

The output is actually taken bottom right via a series 51 ohm resistor, not shown
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Old 11th June 2005, 12:16 AM   #4
jcx is offline jcx  United States
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your output offset servo destroys the gain matching condition for the output active compensation - R14 is too small and should be moved, why not 1 meg into U2 +in, 1 Meg hardly effects component tol error at U2 +in


not that I think active compensation is particularly useful in any event, Jung's multiloop composites are likely to be more useful in this location as the phono pre out may drive cable C and variable impedance - which also detracts from the active compensation

your images are so large so you can't tell but jpeg really sucks for denser drawings with small text - use .gif
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Old 11th June 2005, 01:00 AM   #5
andy_c is offline andy_c  United States
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I've got some info on my web page about one popular active RIAA equalization circuit, including getting the circuit to match the Neumann time constant, computation of RIAA equalization errors using SPICE, as well as separating out the op-amp induced portion of the error. The link can be accessed via the WWW button below this post.
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Old 11th June 2005, 02:46 AM   #6
Leolabs is offline Leolabs  Malaysia
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How long u have been working on this???
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Old 11th June 2005, 08:48 AM   #7
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they aren't my designs, but they both work and sound v good. They are both well known circuits of items.
Its my first attempt at uploading, so be patient...any tips gratefully received

Any comments about the top circuit (should I post in the vinyl section?), it uses single supply and is my fave phonostage to date.
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Old 11th June 2005, 02:11 PM   #8
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Hi Andy,

Excellent investigation.

My best sounding phono pre-amp split the RIAA process into two so that one IC was not working over such a wide gain/frequency range.

Following a separate buffer with pre-setable variable R + C cartridge matching, the first active IC created the low frequency lift to 20Hz ( not 50Hz ) plus the gain necessary to acheive the required midband output level.
There was separately switchable 'rumble' filtering for discs where the extended low frequency response shook the room.
All ICs were run flat gained through mid to hf, so no equalisation could go wrong at any stage, just plenty of headroom via the psu rails.

This was followed by precision resistor-capacitor passive filtering for the upper RIAA characteristic, and of course it simultaneously cut high frequency noise and hf distortion products from the first two stages.

Finally there was an IC output plus transistor buffer, which could now cover the Neumann time constant.

It sounded superior, no matter what the dB figures suggested, and where every dB of signal to noise ratio is so important it was fractionally quieter too.
It's low impedance output was totally isolated from the RIAA equalisation IC/circuitry so the sound could not be 'sucked' or dynamically challenged by power amplifier or cable impedances.

I had bought our home and little feet had arrived so I needed money; I sold it for 100 about 25 years ago. 36 of that was for the ICs alone, and this was a fair expense in those days, but I'm blowed if I can remember their make or number now; they were the latest high spec of their day. The guy who bought it kept coming back to tell me how he was amazed by the open-ness, clean-ness of attack, stability of sound and sheer clarity.
Aspects not heard via conventional 'all-in-one' RIAA equalisation stages, no matter how good the base amplifier.

I would love to see your type of investigation cover something like that, maybe including noise performance as well as amplitude/frequency.


Cheers ......... Graham.
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Old 11th June 2005, 02:28 PM   #9
tlf9999 is offline tlf9999  United States
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As to the newman constant, has it been proven that we can actually hear the difference between two circuits where one is corrected for it and another not?

Some spice programs (later OrCAD versions for example) come with optimizers and one may be able to just use that function to let the computer find optimal values for the equalization network. Not sure if that has been done by anyone.
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Old 11th June 2005, 03:02 PM   #10
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Yes, that's been done by Jim Hagerman. There's a link to his analysis in my article, but for convenience I've duplicated it here. http://www.hagtech.com/pdf/riaa.pdf

Closed-form solutions are nice because you can just take the formulas and put them into Excel, MathCad or whatever. One of the issues with optimization is that there's not always a way to constrain the optimizer so that it is forced to pick R and C values from a list of standard values. Often the deviation of the calculated value from the standard value can be much more than the tolerance of the component itself.
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Old 11th June 2005, 04:07 PM   #11
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Along the lines of what Graham suggests, I have found the best sounding phono stage to use "split passive" EQ. Isolating the two halves of the RIAA. Using non-feedback means of creating the rolloff. Art Loesch got me into this idea.

The current rage is "T" inductor networks, but having not played with them, no opinion is offered.

For solid state I'd give some serious consideration to using a discrete gain stage and higher B+ with class A biased output devices. I am reasonably certain this is what Spectral did in their model 10 & 20 preamps some time back. Doing this I like FET & Mosfet, but you may not think the same way about it.

Even using this method, and given that LP isn't an inherently "accurate" medium in many ways, it's my view that tubes do this job way better from a purely subjective listening perspective. Seems like more music comes out when well implemented.

When using tubes, I still use a FET for low noise in the front end...

as always Ymmv.

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Old 11th June 2005, 04:26 PM   #12
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Graham...what's your background, I have seen your name around here.
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Old 11th June 2005, 08:00 PM   #13
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Hi lt,

That's a strange question for me to receive, for I wonder what relevance background has.
Some excellent audio/electronics has come out of 'arty' folk.

The two people who inspired me most.

Sir Douglas Hall, for his circuit imagineering capabilities.
See
http://freespace.virgin.net/spontaflex.reflex/

My high school A-level Physics teacher, who showed me the importance of understanding basic fundamentals.

My most inspiring read
The Romance and Reality of Radio, by Captain Ellison Hawks, 1923

I value genuine interest and dedication more than background; mine is somewhat varied, but 'hands-on', and my iron is still smokin'.

Actually its a wonder I don't have lead poisoning!


Cheers ......... Graham.
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Old 11th June 2005, 08:05 PM   #14
andy_c is offline andy_c  United States
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Quote:
Originally posted by Graham Maynard
Excellent investigation.

[...description of another phono preamp circuit...]

I would love to see your type of investigation cover something like that [in reference to circuit described above], maybe including noise performance as well as amplitude/frequency.
Hi Graham,

Thanks for you kind words. To tell you the truth, I don't even have a phono setup at the moment. My records are mostly worn out ones that I had in high school. I do have a Linn Sondek LP12 that's dead, so I should really fix it or get it fixed.

That writeup on my web site is something I originally dreamed up when I was in grad school twenty-five years ago (sheesh, am I really that old? ). The teacher did a Foster's expansion of an RC impedance and it looked just like an RIAA network. So I decided to investigate. I had thought of trying to publish the results until I looked in the AES journal and saw the article by Lipshitz. Needless to say, I decided against that idea.

But since I hadn't seen any published reference showing a closed-form solution that included the Neumann time constant, I decided to put it up on the web. It took a lot longer to write up than I thought it would.

Oh well, maybe someone can make use of it.
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Old 11th June 2005, 08:24 PM   #15
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Hi Andy C,
why was this Neumann thing put there?
Does it give any advantage?
And that 20Hz roll off is balderdash (good english word from a Scotsman).
I support the toplogy of splitting the time constants and putting the 75uS last even without a final buffer, provided you keep the next stage impedance high (say>= 50k).
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Old 11th June 2005, 08:46 PM   #16
andy_c is offline andy_c  United States
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Quote:
Originally posted by AndrewT
why was this Neumann thing put there?
Does it give any advantage?
The background is described in the article a bit. There's a link to Allen Wright's web page where he goes into more detail about it. The idea is that the equalization actually used is not exactly what the RIAA specifies because it's impractical for the high-frequency boost of the recording to continue to too high a frequency. So it seems logical to compensate for the actual equalization used in the most common cutting lathe.

Quote:
And that 20Hz roll off is balderdash (good english word from a Scotsman).
Yes, I agree there.

Quote:
I support the toplogy of splitting the time constants and putting the 75uS last even without a final buffer, provided you keep the next stage impedance high (say>= 50k).
Sure. The purpose of the article was to describe how to get a specific configuration to have the most accurate equalization possible, not to advocate any particular configuration. If you're interested in building such a circuit, it may help you. If not, it makes no never-mind to me
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Old 11th June 2005, 11:05 PM   #17
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I don't see any point in adding the cost/complexity of extra active stages and noise/distortion input stages to something that can be achieved simply with one full RIAA feedback stage with a small added hf loop zero and following passive rolloff.

I have done this in a commercial preamp with a single op amp with the RIAA network declining at hf to about 2K2 ohms and a passive rollof immediately after. this way you have only one op amp noise/distortion source, retain full (maybe 40dB) overload margin, ....

Greg
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Old 12th June 2005, 08:19 AM   #18
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Hi Andy and Andrew,

I believe I would still do the 20Hz roll-off today, but have it switchable.

Is it wrong to have such an option ? Especially for rock and pop ?

Have you actually heard it, so that you have experienced the sound with real ''Hi-Fi'' loudspeakers ?


Cheers ........ Graham.
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Old 12th June 2005, 08:38 AM   #19
AndrewT is offline AndrewT  Scotland
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Hi Graham,
yes, I've tried them both.
Conrad Johnson into Michell monos into AE1s. without roll off.
Audiolab into Crimson/Michell into AE1s/TannoyHPD385. with roll off.
20hz roll off kills the music, particularly pop & rock.
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Old 12th June 2005, 03:10 PM   #20
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Quote:
Originally posted by amplifierguru
I don't see any point in adding the cost/complexity of extra active stages and noise/distortion input stages to something that can be achieved simply with one full RIAA feedback stage with a small added hf loop zero and following passive rolloff.

I have done this in a commercial preamp with a single op amp with the RIAA network declining at hf to about 2K2 ohms and a passive rollof immediately after. this way you have only one op amp noise/distortion source, retain full (maybe 40dB) overload margin, ....

Greg
Of course, you have to be happy with your own system.

But there is really no extra noise added that is worth a damn, since the noise floor of your LP is WAY, way higher than the noise floor of any half decently designed and built circuit... especially in solid state.

Since the implementation is so "simple" why don't you *try* a split passive RIAA?? See if you hear any differences? Maybe you won't. Maybe you will. Of course your implementation will make a difference. At least it has appeared so to those who have rather thoroughly investigated many variations on this theme... maybe you will see something in it?

As always ymmv.

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Old 12th June 2005, 10:07 PM   #21
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Hi Greg,

My first IC was to buffer/match the cartridge and provide frequency invariant source impedance for the 1/2 RIAA IC.
So that the 1/2 RIAA IC would work accurately, it was then not load burdened by the 1/2 RIAA passive filter.
This too was then buffered by a final IC which drove output transistors for 50 ohm line level output with good headroom. ( I had 20ft coax/BNC to LS sited monoblocks.)

A single IC would have had source impedance and noise level varying with frequency at differential input, and line driver loading could affect RIAA feedback accuracy/capability.

There was not any extra noise or distortion, and when there are not any commercial limitations you can build better. The 1/2 RIAA IC followed by a 1/2 RIAA passive/ground connected filter was actually quieter, as well as cleaner/clearer sounding, than full direct differential RIAA on a single IC. Additional noise after the passive filter was negligible.

Modern ICs are now so much better, and I note there are several commercially available single IC based 'phono boxes' costing up to circa 150. Many makers also go on about component selection and tolerance, but that does not make them the ones that get top flight reviews.


Hi Andrew,

I think your reply covers my point about it 20/50Hz roll-off being optional.

Some others clearly think it worthwhile offering that facility too.

Yes 20Hz could kill music, and a brick wall mounted deck becomes the minimum essential, but not everyone can afford to buy or house Tannoys etc, and thus the 20Hz option can have its place.


Yes Bear, I was happy with it. Silly me for selling it !


Cheers ......... Graham.
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Old 12th June 2005, 10:46 PM   #22
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Hi Bear and Graham,

When I designed the Eidetic GB1a preamp back in 1989, I used the newly released LT1028, at odds with my past designs which used discrete (2SB737 and/or 2SD786 from Rohm and various discrete and integrated topologies).
I tested it thouroughly for input impedance interaction which was I believed to be virtually solely the domain of single ended input stages. It is virtually eliminated by diff'l inputs isolating the input from the feedback and the large amount of loop gain available for feedback from such a chip. So the input Z was, to all intents and purposes, identical to the defining components R,C on the inputs.

In my case this stage was switchable for 3 R (100K,47K,100)loadings plus a user define addition (solder pegs), 3 C (100p,200p,320p) loadings plus a user define addition(solder pegs), and three loop gain settings (34dB,44dB,54dB).

A diff'l input will always degrade the noise performance by 3 dB over a single ended stage, but it was still some 20dB lower than record surface noise so considered acadaemic, still achieving 92dB weighted by Australian Hi Fi on test (whole pre/power input to output) and specified at 76dBA (re 1mV) MC and 84dBA (re 5mV) MM.

Australian Hi Fi singled out the phono preamp stage performance as -
"The RIAA response is 0.4dB high in the bass region then tightens up to be +/- 0.2 dB across the rest of the audio band.....the overload margin was an excellent 37.84 dB - one of the best results we have seen for quite a while"

In summary input impedance interaction, the discovery of which spawned the buffer and multiple stage approach is easily eliminated through diff'l input and modern low noise monolithic chip technology.

Greg
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Old 13th June 2005, 08:49 AM   #23
Werner is offline Werner  Europe
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Quote:
Originally posted by andy_c


The idea is that the equalization actually used is not exactly what the RIAA specifies because it's impractical for the high-frequency boost of the recording to continue to too high a frequency.
I don't know about the SX74/VG74, but on the schematics I saw
of the older VG66 cutting rack there was a first-order at 50kHz,
in addition to a second order low pass at about 30kHz. And that is then without the treble limiter. All of these there to protect the cutting coils.

I conclude from this that above 10kHz there is no such thing as a RIAA standard.

I acted by modifying a cheap Rotel phonostage I had lying around for a variable 75us de-equalisation, offering 5 steps of each 1dB boost at 20kHz. During replay I can click through them and listening to voices it is easy to find the correct replay curve.
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Old 13th June 2005, 10:28 AM   #24
tlf9999 is offline tlf9999  United States
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amplifierguru made an excellent point.

There are many factors that could potentially have an impact on an electronic device. Some of them are major, some of them are minor. It is the responsibilities of a good engineer to figure out what to prioritize and focus only on those that are important.

Otherwise, we will end of chasing down things that are sound sophisticated in theory but bear nothing in reality.
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Old 13th June 2005, 03:20 PM   #25
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ampguru,

I'm really not questioning your product's specs or results... and quite honestly, I beg forgiveness for not really looking at either LTs or AD's chip offerings much for some time now... so I'd have to go and pull the book or CD and see this chippie! Sounds nice as you describe it.

My point is simpler than this. It is that there may be "hidden" sonic nirvana in the "split passive" approach to RIAA. Indeed in any given systems any "improvement" may be moot (?) but the experiences I've had purely on the listening end indicate that in the same way there are (oddly) audible diffs to be percieved in "ultra low distortion" power amps and with solid state vs. tubes, a similar set of things happens in LP reproduction.

The first point I made is that since LP is hardly a "technically accurate" medium, that may be justification for simply "making it sound good."

For the solid state implementations, the best I've heard so far are split passive FET/Mosfet designs, but they rarely are as nice to listen to as are split passive FET/tube RIAA implementations! In each case the deviation from the RIAA curve is not likely to be a major contributor to the percieved quality. And in any event, there wasn't much in any implementation I refer back to.

Hope this clarifies?

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