Impermanence loading a Phono cartridge?

[rwgast_lowlevellogicdesin]

I have been contracted by an employer to build a low and hi end phono pre-amp. I jumped at it thinking this will be easy stuff, being use to RF  and faster digital I thought piece of cake. Which it really is I think I am just reading way to much down the Hi-Fi Audiofile rabbit hole....

So it turns out there is no real standard in the record industry to make cartridges or the the magnetic head behind the stylus a standard impeadence, 47k has been a pretty standard value used in most pre-amps but its not always the correct one. For the higher end amp I plan on using an AD chip to just read its impeadence and then match appropriately. On the lower end side I was thinking about just slapping a 100k pot in there with a line on the case at 47k ohms. Then I started reading that you can hear  resistor noise and blah blah, is this even true I always thought Johnson noise was so low you couldn't even distinguish it on a scope, I mean were not talking about burning power resistors here, at a few mV a 1/4 watt pott leaves plenty of thermal headroom! Secondly the the cart usually calls for some captive as well as resistive loading, apparently the capactive loading can cause all sorts of problems with frequency roll off if it is not right so why the hell use it at all?? I mean Xc will vary with frequency while a purely restive load will have a flat response correct? So i would buffer the load then throw a low gain stage at it after which I would use the 100k pot to make sure the insertion loss is in negligible. Apparently buffers are bad and create noise....  ? WTF if that were true why would every respectable head phone amp use buff634 chips for current driving?

I would rather just wrap a god damn transformer but audio frequency is so low that it make L circuits cumbersome and I dont know how to make a fully variable transformer anyways, especially in the cheap end model.

My main competitor is this heap of crap....

http://www.amazon.com/Madisson-Preamplifier-Turntable-Amplifier-Pre-Amp/dp/B003UPTE4K

On request I can provide pictures of circuit and reverse schematics when Im done but lets jus say this thing sells becuase of price Im trying to hit 30 dollars. This heap of crap uses a switching supply through a 12v zenez as its main power source with old ceramic disc caps in the audio chain, which we all know have bad piezo effects and no AudioFileBS just arent suitable for audio, dave even has a video on this. The worst part is the opamp has an awful slew rate it is like a dual 741..... common guys. There is nothing in the 30 dollar level right now I dont know about you but I would pay 15 bucks for metalized film at least a long with a real regulated power supply (which will probably use two 9vs to save cost) and an opamp of at least marginal quality like a TI or NE55.

Any intelligent info would be appreciated to me the impeadence loading make all the difference in the acually frequency response and things like crappy RCA files to the amp along with Cart can effect it's level. I am just looking for an inexpensive no bs way to get a flat response before the RIAA eq. Unfortunately Im getting sucked in to this whole dont use buffers you need teflon caps but hey just stick a 47k to ground in there meet load requirements hole. God audio SUCKS!

[Lightages]

You are wading into the wonderful world of voodoo when it comes to vinyl and cartridges. It is a complex reaction of mechanical systems, magnetics, and electronics. There is no 100% single solution for all cartridges. This thread can go on for 100s of pages discussing all the nuances.

Every cartridge will have its own characteristic impedance. It needs to be matched at the input of the preamp so as to manage ringing and damping of the electrical-mechanical characteristics of the cartridge. There I have scratched the surface of an iceberg! 

[f5r5e5d]

phono cartridge preamps are a bit worked over technically speaking - you can find a lot of audiophile nonsense surrounding too

but they can be demanding designs for low noise, especially the RIAA bass boost making 1/f corner critical - and as you've already found wide range of impedance and signal level

the distinction between amplifier input noise voltage and current becomes important, changes the preferred front end device types over the range between few tens of Ohms moving coil input vs moving magnet where LRC can be |kOhm| over audio

Douglas Self has a small signal audio book that shows a few tricks http://www.douglas-self.com/ampins/ampins.htm

http://linearaudio.net/ has some phono articles, many authors active at http://www.diyaudio.com/index.php I believe you can now purchase individual articles from Linear Audio


the RIAA EQ network passive cost alone can eat your budget depending on accuracy - even finding 1% speced caps, much less "audiophile approved" dielectric, construction...

quality MC step up transformers are over US$30 apiece in OEM qty

you really need to understand the market, the cost/performance - competing with a US$30 finished cost requires dropping most all concerns you mention

[Pjotr]

Good luck finding your way. As mentioned, there is an endless number of different cartridges. Ranging from 200 uV to 7 mV at 0 dB nominal. You are riding on the edge of what is technical possible with the low outputs and what looks technically an optimum doesn't mean it sounds optimal. This is an area of both the real art of analogue electronics and loads of experience if you goal is high quality. High-end audio is a weird thing, even without all those voodoo.

[TimFox]

A good phonograph cartridge data sheet will specify the recommended load resistance and capacitance.  47 k is one popular value (from which we may infer that 10% tolerance is sufficient, since that is a "10% value"), and the Shure cartridges typically wanted 400-500 pF in parallel with that (including cabling and tone arm).  The old Shure data sheets indicated that resistance up to 70 k would have negligible effect on the frequency response.

[rwgast_lowlevellogicdesin]

LOL great this is suppose to be the technical non BS audiophile place! I realize some of what I am talking about cant be implemented in a 30 amp but metal film caps a decent opamp and a 100k pot could. Im not going to go all crazy Im doing two amps like I said, the hi end one will implement DSP along with an AD584.

What I am really wondering about is what the deal is with not using purely resistavie loading, maybe some of those papers will explain it to me. Im shooting in the 30-50 range the guy backing me is in the industry he use to work for Dolby and and is now is at DTS. He has ins to Amobea and manufacturing people. Basically I am going through with this despite my non audiophile enthusiasm because in some ways it falls in line with certain interests of mine like small signals and and DSP, and he has some heavy connections! Ive pretty much got a decent design in my head I think Im just having trouble with understanding the loading and why any capacitive loading is necessary especially when the RCA connections will carry some small capacitance.

[Lightages]

A cartridge is not just a coil, it is an electro-mechanical device that has multiple interactions in the coil, the coil mounting, the magnet armature, the stylus compliance, the interaction between the vinyl and the diamond tip, etc... All of these relate to the correct value of the input load. It will affect the resonance of these parts as well as the actual electrical characteristics of the the coil. Leaving the load as purely resistive can have un-forseen consequences in the response of the whole cartridge. You cannot have a a purely resistive load anyway. The cable from the cartridge to the preamp will add capacitance no matter what you try. So you need to control the capacitive load on the cartridge beacuse if you don't, other circumstances will and not to your benefit.

[Audioguru]

There are two completely different types of magnetic phono cartridges. Moving magnet is (was?) the most popular but moving coil is (was?) also available.
The impedance/resistance is never matched!. The coils in a Shure moving magnet cartridge are about 1500 ohms and the load must be 47k ohms. The resistor will make some noise only when the cartridge is disconnected.
EDIT: What about the important RIAA equalization? The record is recorded with the bass frequency amplitudes reduced so the stylus does not need to move far and the highs are recorded boosted. Then the equalization boosts the bass and cuts the highs which reduces hiss while it returns the frequency response back to normal.

[Pjotr]

I have been contracted by an employer to build a low and hi end phono pre-amp.

LOL great this is suppose to be the technical non BS audiophile place!

It is. However designing a decent high quality phono pre amp is NOT a piece of cake. This subject has a long history and there a tons of examples on the web and app notes, describing it well. For the low end simply copy something from an app note, a single op-amp will do. But for the high-end it is different matter.

[rwgast_lowlevellogicdesin]

Well for low end im targeting mm carts only but they also vary widely in impeadence that is the issue. For the low end i plan to load then amplify a little next a passive riaa eqq followed by a final gain stage. On the hi end amp im goning to use an ad854 to read the cart and cables z and load as exact as possible with a digital pot. As far as riaa im looking in to advantages of using a stable vref to do dsp with a cpld or dsp processor (this is where the real r and d comes in). My issue is ive read your riaa eq can be pretty off (low tolerance components) and still sound better than a perfect eq stage with a junk loading stage. So my main concern is really figuring out the best cheapest way to do correct mm loading on the cheap version. Would using a pot with a 47k mark and no capacitance be a decent way to give the user a cheap way to load there mm accurately or to there taste? I have tons of hi end and some low end tables/carts to test on with some frequency sweep records and a spectrum analyzer.

I did a ton of reading on riaa eq and active vs passive etc before i even realized 47k is not an acuall represantative number of all mm carts. Im sorry i should have explained i have a few cheap ideas to deal with the whole amp but whats bugging me is figuring out the loading stage for mm only and how to get the user a close to perfect match so there is no hi end roll off or hi end peaking. I doubt an mc guy would touch a 30 to 50 dollar amp anyways. I dont need audiophile grade i just want the most important part of a phono preamp to be a well thought out solution and not just follow the crowd. If the loading is correct and the opamp(s) are decent i think that is going to be the best bet and give it a better sound. I really need to read the articles linked to above before going on but ive heard rf is voodoo so many times and i was always scared vut it just wasnt true in the end. Btw im also planning on using two 9vs as a dual rail supply that should cut any real psu cost while maintaining a clean and steady power source with no opamp based vground, so that gives me a little head room!

[dmills]

You are thinking of the matching problem in RF (power transfer) terms, audio does not (usually) work that way....
Consider that the source is highly inductive (Hundreds of mH in series with maybe 500 ohms), and that if you were trying to do a power transfer style match, you would be looking for a conjugate match over ~8 octaves, good luck with that!

47K is the de facto standard load impedance (NOT at all the same as the winding resistance!), and most things will be designed to work into that load, "Small signal audio design" has some more details on this, and the old classic "Radiotron designers Handbook" has more then you ever wanted to know about how record players really work.

I would note that there are ways to reduce the noise contribution of the 47K load by enough to make a dB or so of difference, by increasing the resistor value and then boostrapping the far end with an amplified and inverted version of the cartridge output, Self gives more details. 

Headroom is surprisingly problematic here, especially if the disk has taken some damage, and overload recovery can be worth some consideration, especially if you are feeding this into a digital recorder where the ability to do automatic scratch concealment needs good headroom so that the software can see the large amplitude scratch pulse .

Have fun, but forget about power transfer matching, that is not how audio gear plays the game.

Regards, Dan.

[Pjotr]

Indeed Dan. Also not all op-amps are the same, even if they are marketed as precise and very low noise. You have to select an appropriate one carefully. They have voltage noise and they have current noise. Current noise interact with the cartridge impedance heavily. In general: For high impedance carts current noise is dominant and for low impedance, voltage noise is dominant.

[dmills]

Also, be very careful about any opamp with a 'bias current compensation scheme', these tend to have an elephant in the room in the form of correlated current noise in both inputs, so they are only low noise if the input impedances can be matched, clearly not the case here with a highly reactive source impedance in one leg.

Gram amps are not anything like as simple as they first appear.

Regards, Dan.

[retrolefty]

 Why don't you stand on the shoulders of a giant?  Rod Elliott does a great job of explaining the topic and has several example projects to follow along with.

http://sound.westhost.com/articles/cartridge-loading.html

http://sound.westhost.com/project25.htm

[rwgast_lowlevellogicdesin]

Hey thanks for all the info guys, Im going to start reading it all today. Im glad I asked on this board I had no idea the "voodoo" involved but on the other hand its is great to get resources rather than audiofile crap on the net. It seems like any project involving audio is really hard to get real solid information on. NWVA guy was a great recource no BS all measurement and double blind tests, unfourtantely most of the hi end audio community would rather use adjectives than gear to gauge there designs.

Along with reverse engineering that pretty bad 15 dollar amp I have an old technics receiver I plan to take apart and look in to it's phono stage I would imagine technics should have a decently designed pre amp system.

[jaunty]

yes - for god's sake - look at some (esp) of the better designs out there... there are tons and tons of them - many very different. Most are ripoffs of eachother but there are some very clever ones too.

[dmills]

Yep, lots of audio design is as much about fashion and pandering to peoples misunderstandings as it is about engineering in the usual sense (I do need to take out a hit on the people who write textbooks on digital audio with stupid stair step diagrams of quantisers (With no mention of dither) and words like 'resolution', it is getting annoying) .

No voodoo, but down within a few dB of the theoretical noise floor, and with huge dynamic ranges in play things can get 'interesting', obviously the dynamic range at the output is not that great but the rather extreme gain range imposed by the RIAA stage and the ~20dB range of cartridge sensitivities does make the problem more interesting then it first appears.

Technics has a discrete semiconductor design that used a ~130V rail! Pure marketing crapola of course, but they did it. 

Oh, watch the 1/f noise corner in whatever you use, the LF gain can be high enough for this to matter.

Regards, Dan.

[rwgast_lowlevellogicdesin]

I got douglas self's small signal amp design, and am digging in to it. Great stuff!

This wouldn't be in the price range on the low end of course but it is something i am rather curious about. Would using DSP either a cpld or psoc5 to do the riaa eq network be a much cleaner way than using passives that will drift? The spice models of the very best components still drift, i dont think it is all that bad, but could dsp get a perfect curve every time?

I kind of plan on prototyping this thing in a bunch of modular pieces with different techniques so i can have the client do a blind test and mix and match the modules for the different price targets.

[Pjotr]

Of coarse you can do the RIAA equalising with a DSP but realise it takes app. 40 dB span. So you will need a large dynamic range for the AD converter, incorporating the minimal 20 dB "scratch" headroom. If you look at the curve it is overall a 1st order low pass with 2 "bends", at 500Hz and app. 2kHz. If you take the DSP road, take best of both worlds to keep dynamic range manageable. Do the overall low pass the analogue way with a simple 1st order low pass. And then the final RIAA curvature with a DSP and IIR bi-quads.

[Wirehead]

Dip your toes in the water with something like Rod's phono preamp or RJM's VSPS. Then try to improve loading (which is cartridge dependent). Find out that less capacitance on your phono leads is desirable. Find out you do need to take into account Johnson noise. Try to lower noise with a different input stage. Find out that for higher impedances you need FET inputs. Find out that for MC's, you need 60dB of gain. Damn, there's that noise floor again. Rework your grounding scheme. Oh hi! There's RF popping up in the inputs.

Phono pre-amp design is fun Keeps you tinkering