MM Vs MC Cartridges

[Craig Maier]

Lets look at some numbers, ok? I just looked up the oldest (and lowest performance) production Op Amp - - - the LM 741A in my "National Operational Amplifiers Databook - - - 1995 Edition".

It shows that the LM-741A (which goes back to when I was a kid in college) has an input resistance of 6 Meg Ohms, typical. Pray tell me how that input resistance is going to interfere with the output resistance of a phono cartridge having a few hundred ohms and being terminated with 47 KOhms?

Moving on - - - this op amp has a gain x bandwidth product of 1 MHz. That means that if one used it in a situation covering up to 20 KHz, it would be useful with a Gm (Voltage Gain) up to 50. If 20 KHz it not enough for you, use 4 of them in cascade, yielding a bandwidth of 80 KHz, with each stage having a gain of only 12.5. That solution would cost you about 75 cents.

As for slew rate, this op amp only has 0.5 Volts per microsecond. Please ask one of the 12AX7 tube pre-amplifier companies what their slew rates are. You will find that most tube equipment ignores specs like distortion, slew rate, noise figure, s/n ratio, etc.

So, now take a modern op amp and see what you get!

Craig

[Craig Maier]

Some more numbers

Taken directly from the RCA Receiving Tube Manual, RC-29:

12AX7 (the dual triode tube optimized for audio work in the 1950's)

Equivlant-Noise and Hum Voltage: 1.8 uV RMS (Referenced to Grid, each unit).

Assuming that you are using a MM cartridge, you are probably dealing with a signal level of about 1.5 mV RMS input. The best S/N ratio that you are going to get (according to RCA) will be 20 log 1.5^-3 / 1.8^-6.

That equals 58.4 dB.

K?

Still wanna mess around with tubes? Your soundcard is 24 bit. Although it can not realize that value of dynamic range, in reality, it still represnts a 144 dB dynamic range compared to 58 dB from the best audio tube design ever made. So, which device is determining the system performance, your soundcard or your tube preamplifier?

Thus, why would anyone plug a tube pre-amplifer into a high-tech soundcard?

Please advise.

Craig

ps If you want to add tube distortion (and they have plenty of that), why not add it later on in the process?

[Doug]

I'm thinking it is because the article is referring to linear op-amps with bi-polar transistor inputs instead of a high impedence J-FET input of a TL072 or similar???

Regarding tubes - I like your summary - quite clear, the worst way to go for a good S/N ratio. I think it is amazing that you can spend $40 on a DIY audiophile quality phono-preamp (if not the best design possible) or $90-140 on a DIY valve preamp with specs that would give most people the shits. Buy it pre-made and the cost/quality ratio worsens still! Some of the things I have seen look nothing more than hotted up V8 engines with all the shiny chrome plating (and probably sound little different!).

[Craig Maier]

The tube amplifier and preamplifier market is completely irrational. I think that people buy these things not for the audio performance but for the bragging rights having spent up to $50,000.00 for a pre-amplifier. Also, I have seen mono-block tube power amplifiers producing about 100 Watts of output power that cost $350,000.00 each. Add 'em all up and you have an amplification system that costs $750,000.00 for 200 Watts of audio. I am not kidding. That is probably more than the lifetime earnings of the average individual.

Strange.

[Doug]

It shows that the LM-741A (which goes back to when I was a kid in college) has an input resistance of 6 Meg Ohms, typical. Pray tell me how that input resistance is going to interfere with the output resistance of a phone cartridge having a few hundred ohms and being terminated with 47 KOhms?

Ok Grandpa (just kidding, don't burst a blood vessel), doesn't altering the -ve feedback and thus gain of this type of op-amp alter it's input resistance? (I'm not really sure, but this is what I am lead to believe happens at high frequencies). I guess that is my final question, because if it does, then a single stage preamp could suffer impedence reaction?

I'm really just trying to establish whether this is fact or fiction (as was the case for valve stuff and MC cartridges). It was stated in what I considered an "honourable" Australian journal...but science has taught me that sometimes even that can mean nothing. PS, interesting that a crappy 741 can out perform an audiophile valve phono-preamp ($$ to be made???). Imagine it:
"The LM-741 Audiophile phono-preamp system" - How many people who purchased it for 50K would really know what the name meant anyway? Could make it big, bulky and shiny like a boeing 741.

[Craig Maier]

Quoting Doug;

"The LM-741 Audiophile phono-preamp system" - How many people who purchased it for 50K would really know what the name meant anyway? Could make it big, bulky and shiny like a boeing 741."
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I think that you would have to put some warm, glowy things in the box too. I like that!

Anyway, if an op amp is operated in the non-inverting configuration wherein the signal is connected to the + input and the gain is set via the feedback resistor (Rf) and a shunting element (Rs) to circuit common, there is no direct effect of gain on the input impedance of the circuit so long as the slew rate of the op am is not exceeded. (Voltage Gain = 1 + Rf/Rs). If one was really worried about it, one simply adds a Voltage Follower (unity gain stage up front wherein the - input is connected to the op amps output and ths signal is connected to the + input). This initial stage produces no Voltage gain so you would need a second Op Amp, but may make you feel better. Besides, it cost almost nothing but for (potentially) a very slight increase in system noise.

Keep it simple. There is no "Rocket Science" going on with these things. Notice that "Rocket Scientists" seem to work on rockets and not audio circuits? LOL

[DougMac]

Ah, the warmth

I made the trip to the big city of Atlanta to pick up my fixed NAD AV receiver from the boutique audio store. When I walked in, they were connecting the last speaker lead to a brand new shiny McIntosh power amp. They proudly explained it was all tube, not a solid state device to be found. It pumped out a whopping 75 watts/side. The price? A cool $6k. The irony is that they were playing a Frank Sinatra CD! I'm sure the source chain to make the CD was all tube too! ;-)

They were checking out the unit for a customer who bought a McIntosh setup 40 years ago. He wandered in the store and asked what McIntosh had to offer nowadays. They showed him the catalogue and he did a complete upgrade. If his old gear was in reasonable shape, I'm sure they could blind A/B him and he wouldn't be able to tell the difference.

Unfortunately my interests run where things get the silliest - sailboats, audio equipment and still camera gear.

Doug

[Craig Maier]

Quoting Doug,

"The irony is that they were playing a Frank Sinatra CD! I'm sure the source chain to make the CD was all tube too!"

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I am sorry to tell you that most of the super high fidelity Sinatra cuts that he is known for were made in the 1970's on forward on his Reprise label. These were mixed down on solid state boards. Now, there is the real irony! ROTFL

[Doug]

Keep it simple. There is no "Rocket Science" going on with these things. Notice that "Rocket Scientists" seem to work on rockets and not audio circuits? LOL

Love the quote Craig...still a little confused but I'll get over it. I think the answer I am looking for is in your comment

If one was really worried about it, one simply adds a Voltage Follower (unity gain stage up front wherein the - input is connected to the op amps output and the signal is connected to the + input).

The circuit I have has 2 stages, the 1st having a gain of around 8.3, followed by the 2nd stage with active RIAA feedback. I guess the aim was not to have a straight "voltage-follower" at the start, but to set the gain of the 1st stage to obtain the best S/N ratio for the remainder.

On that topic, I suppose one more question does arise for folks that use RIAA EQ preamps (although no longer important since DC-Art supplies the EQ curves for us). Is there any benefit between passive Vs Active RIAA implementation. I am of the thought that active is better noise wise, but at one stage I recall there was a push toward passive!

[Craig Maier]

On active vs passive, I do not think that it makes a difference so long as the circuits are designed correctly. The results should be the same. The electrons can not tell the difference when flowing into reactive components whether they are in a feedback pathway or in a driven passive network.

[Doug]

Thanks Craig,
I think this has been fantastic for separating fact from fiction, hopefully others will find it as useful as I have.

A brand new shiny McIntosh power amp. They proudly explained it was all tube, not a solid state device to be found. It pumped out a whopping 75 watts/side. The price? A cool $6k.

I think the irony is that it's worth more than my car! (but will it give the same mileage???)

[Doug]

Hey Craig,
Check this MC cartridge out - RRP of over $3k US - I wonder if the wooden jacket of the headshell helps the tracking?

http://cgi.ebay.com.au/ws/eBayISAPI....782718851&rd=1

[Craig Maier]

Quoting this guy:

"...this cartridge have very low hours estimate around 300~ 350 hours"

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As you know, these guys really crack me up. Here is someone selling a dead piece of crap for a lotta money. Here is why I say that:

1. It is hard to get a more significant problem with a MC cartridge than a damaged cantiliver!

2. A few hundred hours on the stylus means to me that the stylus is spent. That is my own personal experience with standard cartridges. I doubt that it is any different with MC types.

3. Since when do you make the housing of a highly susceptable magnetic device out of wood and not mu metal, amorphous metal (ie Metglass), or high silicon content steel? For some bizarre reason, the B-H curve of wood never showed up in my Magnetics Theory class in College.


Anyway, I hope that someone buys it. It give me great pleasure to see uneducated snobs being ripped off by charletons.

[DJBohn]

Hey all, on the subject of high end cartridges, Doug & Craig I'm going to send you via E-mail an un-biased article done in 1988, 5+ years into the Compact Disc revolution, by the legendary Julian Hirsch titled "Feelin' Groovy - Head-to-Head Lab & Listening Test of Five Leading Phono Cartridges" The list price ranges from $115 to $1250 spanning all of the hi-fi budget classes. They include 2 moving-magnet, 2 high-output moving-coil & 1 low-output moving-coil cartridge to represent a fair cross section of cartridge-design philosophies. (The article includes a wooden shell cartridge - the Kiseki PurpleHeart - $1250) Craig, this would be an excellent article to show at the site for anyone thinking of investing into an expensive cartridge. Could prove very eye opening. Due to my E-mail attachment limitations, I will have to send in 2 parts. I will do by the weekend.

[Craig Maier]

Hi DJ

That would be great to read; it would be even nicer to post. I can not do it, but I suppose that maybe you could. (see no evil, hear no evil, speak no evil).

Anyway, I like to think of our bbs user group base as a pretty intelligent group of folks, - - - and by all indications, that is so. We do not aim our products or advice at an uneducated "touchy", "feely" kind of audience. As a matter of fact, you will see that folks are quite blunt here on technical issues. No shrinking wallflowers in the group as far as I can tell.

We believe that one can be very analytical about audio in order to understand what does what and what improves things for the right price. It is more about science than art. It sounds like this article emphasizes an aspect of that. Love it.