Sound Quality Vs. Measurements

[bcarso]

Originally Posted by scott wurcer

.5 Ohm @ 77K (SQUID), but the information is there and you should be able to tweak it for a range of MC carts. They offer no data on distortion so this might be a dead end and require Jensen quality transformers anyway.

Thanks. Still interesting, although I'll probably stick with massively paralleled JFETs for low source Z.

It occurred to me the other night that if the source impedance is so low as to shrug off almost arbitrarily high capacitive loading, the falloff in gm with drain current being what it is and the noise voltage going inversely as the square root, one could parallel devices to get down to quite low levels. Of course you'd get there more quickly spending more current, but the idea that phantom power could be used to power hundreds of paralleled BF862s for example... It would be an odd-looking assembly in any case.

[scott wurcer]

Originally Posted by bcarso

Thanks. Still interesting, although I'll probably stick with massively paralleled JFETs for low source Z.

It occurred to me the other night that if the source impedance is so low as to shrug off almost arbitrarily high capacitive loading, the falloff in gm with drain current being what it is and the noise voltage going inversely as the square root, one could parallel devices to get down to quite low levels. Of course you'd get there more quickly spending more current, but the idea that phantom power could be used to power hundreds of paralleled BF862s for example... It would be an odd-looking assembly in any case.

syn08 got .32nV with 32 BF862's (each group of 4 needs a gate damping L).

[SY]

Originally Posted by bcarso

SY, do you have a ref for the preamp you mentioned as being used with V-15's?

I misunderstood Dave's RF jargon (my days as an RF guy ended in the mid-70s, never looked back) and thought of the "other" Norton. As Emily Litella used to say, "That's very different. Never mind."

[bcarso]

Originally Posted by SY

I misunderstood Dave's RF jargon (my days as an RF guy ended in the mid-70s, never looked back) and thought of the "other" Norton. As Emily Litella used to say, "That's very different. Never mind."

Not to worry, that was the only Norton (other than Ralph Kramden's sidekick on The Honeymooners) I knew of. And of course there's the overpriced and substandard Symantec software. But there I go off topic again.

[bcarso]

Originally Posted by scott wurcer

syn08 got .32nV with 32 BF862's (each group of 4 needs a gate damping L).

Groner has been getting some interesting results with paralleled 862s recently. There's a boy to watch . We were both a little puzzled about some of the claims that there might be some sort of "thermodynamic" limit to such reduction with paralleling. He moved more quickly than I (which is not saying much ) to disprove this empirically. IIRC he's using 220nH in each gate lead.

Of course it goes without saying (so why did I say it?), it's easy to mess up such front end performance in the second stage, not to mention with the power supply and about any dang thing else

[ThorstenL]

Hi,

Originally Posted by dvv

As for your "yet another phono stage", may I suggest this:



This appeared in 1990 in a German magazine. The author is one Holger Hermann, who works for Burr-Brown Germany, hence the BB op amps.

There is SO MUCH suboptimal with this from a high performance view, I would not know where to start.

Op-Amp's and full passive EQ generally do not mix well (despite the repeated publications of such circuitry, it seems the "designers" all failed some basic electronics classes or simply copied of each other without ever properly analysing these "designs"). There is a 34dB+ overload margin penalty with this design over active EQ.

If we are going to use feedback and op-amp's, why not at least make sure the feedback factor stays largely invariant across the audio bandwidth and HF distortion is lowered by NFB? Why insist on perpetuating utter sub mediocre performance designs?

With an LT1115 you can make a fine single stage "simple" MC Phono (60dB Gain) that outperforms a lot of very expensive Phono's out there both objectively and subjectively. For MM an OPA637/OPA827 is an excellent choice with similar results. In either case < 0.01% THD at any sensible output level below +20dBu, very high overload margins at any frequency if using at least +/-15V rails.

If we must absolutely have passive EQ because of some fear of negative feedback (which is silly, as the above suggesed passive EQ Phono has ton's of it anyway), then why not use some of BB's "transconductance cell" diamond transistors in parallel into a pure shunt passive EQ, at least we will not loose our overload margin as badly even if they can only take +/-5V (this only gives around 10dB overload margin penalty over active EQ on 15V rails).

There are so many ways to innovate a least a little bit and avoid poor performance, there really is no excuse for such designs in this day and age.

Ciao T

[dvv]

Originally Posted by john curl

This schematic is OK, but not new in any way, even for 1990.

I'm not saying it's a revelation, John, I am simply saying that one has had a long and fruitful life and has pleased many an audiofile. Above all, it's simple to build, and it does have a passive eq.

As I stated before, it will sound better if discrete transistors are added as current boosters - but then, virtually all op amps will sound better that way, no matter what their specs on available current say.

It can additionally be modified as a two stage affair by splitting the rails, allowing the OPA 37 to stay at +/- 15V regulated by a pair of zeners, while for the second stage, an OPA 2604 can be used, again with discrete transistor current boosters, but at +/- 24V. This will significantly increase its overload margin AND give it a better yet sound - I find that OP 2604 has a very warm and natural sound, as opposed to later BB's offerings (after it was acquired by TI and became "The Tucson Operation"), with FET inputs, which I find to be very shrill and irritating.

Ultimately, I am not advocating this particular approach, it has simply proved its worth over the years, I think.

[dvv]

Thorsten, I never claimed this to be anywhere near optimal - but I do say that I have seen and heard at least 15 versions of that schematic, all of which did a good job and in fact did sound better than most other run-off-the-mill fare.

We seem to be drifting towards the exalted here. Which is fine, but there are a lot of folks out there who barely know how to use a soldering iron. They need something relatively easy and simple to build to improve on their usually factory made units, many of which are more complex yet providing worse results in audition.

It's nice when you have a device with a dual FET input duff pair, followed by a dual transistor cascode, followed by a currrent mirror, etc, etc, but one should always remember that such parts are not easy to come by and are not cheap to acquire. Individuals are not companies, which have access to stocks individuals can only dream of.

On a purely personal note, I would rather use discrete components than op amps. Op amps are a blessing to me working as protection circuits, LED array displays and DC servos, but that's about it.

[ThorstenL]

Hi,

Originally Posted by dvv

Thorsten, I never claimed this to be anywhere near optimal - but I do say that I have seen and heard at least 15 versions of that schematic, all of which did a good job and in fact did sound better than most other run-off-the-mill fare.

I have probably heard more than 15 Variations.

They get better if the second stage gets the LF EQ wrapped into the feedback loop, instead of passive EQ.

However I find mixed EQ somehow seems to lack a certain hard to describe quality (coherence?), or it is simply that we still vary feedback factors in the audio range with passive/active mixed EQ and FUBAR the HF overload margin (unless we use John Curl's Vendetta Frontend or some similar open loop transconductance circuitry).

Once we go to a straight active EQ (4 * 10nF Polystyrene plus 100K & 8K2 plus 100R/10R to ground) AND an OPA with high AC performance (LT1115 for MC with 10R and OPA637 with 100 for MM) all things click into place and we have an even simpler design, with superior objective and subjective performance.

Extra buffers may or may not be needed on the output, it depends on a wide range of factors...

For a buffer after an Op-Amp the simplest solution is an IRF710 and an LM317 as CCS on 3W worth of heatsinking each, with 15V rails that allows us around 200mA Iq and full output into 75 Ohm.

Bigger heatsinking may be needed if we want to drive headphones, or we may back off the Iq to 40mA, leave heatsinks off entierly and have only the ability to drive around 400 Ohm to full output.

As my gear is always build "tube compatible" (>10K input impedance) I usually do not need extra line drivers, so I tend to leave them off.

Originally Posted by dvv

We seem to be drifting towards the exalted here.

Hardly, just very basic, common sense and sound (pun intended) design using Op-Amp's. Exalted would look different, very different.

Originally Posted by dvv

It's nice when you have a device with a dual FET input duff pair, followed by a dual transistor cascode, followed by a current mirror, etc, etc, but one should always remember that such parts are not easy to come by and are not cheap to acquire.

Such a device is for example called OPA637 and is easy to acquire, not cheap I grant, but worth the money, used for a 40dB Gain MM Phono.

For MC the AD797 (if you have more luck keeping it stable than I had) or LT1028/1115 make excellent choices for a "single stage, single op-amp" MC Phono.

Originally Posted by dvv

Individuals are not companies, which have access to stocks individuals can only dream of.

The parts needed for my "AAP21K" (read - Analog Addicits Phono for the 21st Century) can be readily obtained from either Farnell, Mouser and similar outlets, at moderate, in not dirt cheap prices. Non of it is seriously "exotic". Non of it is seriously expensive.

The key difference between the "AAP21K" and the original "AAP" is that it drops the MC Pre-Pre (with it's oh so controversial PSU filter choke) and insteads comes in 40dB Gain MM Version (OPA637 or similar Fet Op-Amp) and 60dB Gain MC version (LT1115/LT1028).

It also re-adjust the RIAA EQ Network to use four equal size (10nF), as decent quality EQ cap's are now very hard to come by and often carry larger MOQ's for individuals at places like farnell etc., so one stereo Phono can use most of the usual 10 packs of LCR made polystyrene Cap's (last pair is used to bypass the output coupling cap).

Originally Posted by dvv

On a purely personal note, I would rather use discrete components than op amps. Op amps are a blessing to me working as protection circuits, LED array displays and DC servos, but that's about it.

On a personal note, I would rather use Tubes than op-amp's, discrete or monolithic, but that is another kettle of fish. As would be a discrete Op-Amp version of the "AAP".

Ciao T

[DF96]

Originally Posted by SY

I misunderstood Dave's RF jargon

Sorry, I forgot that there is another quite different circuit also called a Norton amp. Clever people should only be allowed to invent one circuit each!

Just so more people don't get thrown off the scent: by "Norton" I do not mean a current based op amp but a common gate/base low noise RF preamp with inductive feedback. It happened to be in my mind because of a recent discussion I had on a radio forum. Transformer quality could be an issue. In RF a wideband transformer may only have to cover one or two decades in frequency. For audio we need three decades as a bare minimum.

The Norton has some similarities to the output stage with transformer feedback to the cathode, but the Norton takes this to the ultimate in that the feedback is so heavy that it defines the stage behaviour rather than merely modifying it.