MPP

New Year

I was still not totally satisfied with the sound of the Inductive Shelve RIAA.
In my attept to DC couple i had lowered the series resistor from the servo to 900 Ohm. I found out that this influenced the Inductive part too much. At higher frequencies this resistor is in parallel to the coil and that was the last reason for the somewhat sharp sound. I put in a 100kOm, wraped it only around the Inductive stage because there was some instability when i included the output stage, AC coupled the MPP and suddenly everything clicked into place. You have to use a precission opamp in the output to do that. I use a LT1468 and offset was under 0.5mV.
Sound is now extremely neutral and stiil the transparency and quick timing is there. I got quite a shock on Petruschka on Athena with Ernest Ansermet. Mag Selma with the Art Enseble of Chicago was a showcase of resolution, space, dynamics and detail. I can now listen to slighly bright recordings without fatique. I will post the current circuit here. It works very well and i recommend it without ceveat.
What come next ?
I react well on critisism usually so i will try to improve the ECHO to a point where even critical people will be happy with the measured performance i hope. So i post here the ECHO Extrem. It has 4.5dB less voltage noise and the lower bias on each transistor should lower the current noise too. Noise resistance is around 13 Ohm now. Equivalent input voltage noise is 0.6nVQhz under this conditions. I will use Fet current sources and an anhanced current mirror for better stabilty, PSRR and protection of the cartridge coils in case there goes something wrong in the powersupply. Anyway i never had a fried cartridge with DC couple bipolar input stages and i build some really weard ones over the years ! Happy New Year ! I wish you all the best and may this website prosper.
 

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Joachim
that is the spirit - allowing others to use your design posted here as well provide your own layout containing PCB' with components you feel that works the best as an alternative. And as Scott points out - I do not feel anybody would object to your offerings. This forum is great as it has many international contributors and members except some are **** how to say? - I wonder why on earth an experienced Engineers feel offended by questions from non engineering DIY audio enthusiasts or feel bad about subjective observations or preferences? No two human beings hear same sound similarly which is known scientific truth. I can always defer somebody else' view in a gentle manner.
I will try some of your circuits very soon as I do not have any MC cartridges on my turntable yet.
kannan
 
Kannan i will be exited to hear the results !
One last thing before i celebrate New Years.
I found the reson of the anoying cracking noise in one channel that i had contributed to flicker noise. I was a half loose cable in the Inductor docks. I had used solid core there and had put the dock on and off many times. I resoldered that connection and now everything is fine. White noise on both channels.
Another scoop: i am working with a well respected maker of transformers and inductors for the audio industry on a mu metal shielded version of the inductor. Unfortunately this will not be cheep. I still recommend my ferrite inductor. I use it and it sounds just fine, is has to be shielded though and put away from tranformers.
I have an idea to use that inductor in a hum bcking arangement of four. I will tell you more tomorrow.
 
I wonder why on earth an experienced Engineers feel offended by questions from non engineering DIY audio enthusiasts

Most don't, fortunately. We are fortunate to have some of the best electronics designers in the world on this forum, and most of them are incredibly nice people who are glad to help us non-engineering types.

Joachim, a question that I've never been able to get a satisfactory answer to (you're not the only person who does this)- why inductive? Even top quality inductors are much less ideal than even moderate quality caps.
 
I react well on critisism usually so i will try to improve the ECHO to a point where even critical people will be happy with the measured performance i hope. So i post here the ECHO Extrem. It has 4.5dB less voltage noise and the lower bias on each transistor should lower the current noise too. Noise resistance is around 13 Ohm now. Equivalent input voltage noise is 0.6nVQhz under this conditions.

Good :)

Now, simply paralleling more bipolars while lowering the bias won't help much. Voltage noise goes up with decreasing the bias, and goes down with paralleling devices ('cause the equivalent Rbb' decreases), however, the current noise goes up with paralleling more devices, as it is proportional to sqrt(Ib).

In this mess, there is an optimum (or compromise) where the bias is Ic=Vt*sqrt(Beta)/Rsource, where Vt=0.026V, and Rsource is the source impedance (the MM cartridge). E.g. for Beta=100 and Rs=10ohm, Ic=26mA (not coincidentally, the HPS 4.1 bias value). For these values, the total noise (voltage noise plus noise current*Rsource) are at a minimum. You can of course split this bias over several paralleled devices, to benefit from the reduced Rbb' (and therefore a lower voltage noise).

About the input bias current, in an asymetrical configuration, I don't think there's much to do; you have to live with it, at least during transients. Never connect the cartridge before powering up, and provide a DC path for the input cap. Some input bias current cancelling methods (servos) can be used, but none of them comes without a noise penalty.

BTW, a BC550 has at best 2.2pA/rtHz noise. Nine of them paralleled have 20pA/rtHz, which is already huge. A 10ohm MC adds 0.2nV/rtHz at low frequencies, and at 1KHz could be, due to the cartridge inductance, over 1nV/rtHz. There goes your nice 0.6nV/rtHz equivalent input voltage noise :)

Noise budgets are perverse :)
 
Is that correct? I would think their noise currents would be uncorrelated so you'd have power addition.

That is right, but I have found recently the source impedances of exotic phono front ends make things more complicated than one might expect. Mainly the virual ground inputs are not always the best choice for very low Rs (real part of souce impedance) carts.
 
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May anyone point me to the mistake(s) I must have made? The results I get are not very promising:
echoex.png
 
inductor

The inductive solution has only 8 ohm impedance in the bass region so johnson noise is lower there. I get inductors from a supplier that are better than 2% in tolerance. The active inductive version that i have shown here had RIAA precision better then +-0.15dB over the 20 - 20kHz range. The shelve version i have posted here got not measured because i am in vacation and have no access to my measurement setup in the factory. I simulated the result and it sounds fine to my ears. Subjectively (sorry, forget about than when you do not beleave my ears) the incuctive version sounds more transparent and open. More like seeing the sound then listening to the sound.
The constant current sources in the Echo Extrem are not BF types. I work with Tina and that has only a limited amount of options concerning types. OOPS, yes, the current noise is higher. About what cardtridges you are talking about syn08 that are low impedance MC and have high impedance at 10Khz ?
 
Syn,
I didn't bail out of our discussion by choice, I got sick and couldn't easily read & type. Better now.

One thing, you say you can hear MC pre noise in 97dB speakers at 100 watts - I should think so, a deaf man with effective hearing protection could hear noise under those conditions!

I for one wouldn't want to be in the room when the real music started - never liked discos or rock concerts, and used to take ear plugs to the aerobics gym! I use ESL's at maybe 85dB/watt at work and Apogee Scintillas at 80-85dB.w at home. Work gets 18 watts, home gets 100 watts. No noise even vaguely apparent.

But certainly some people are incredibly sensitive to noise - we have a client who complains about the line stage noise in a SVP-2 preamp on 87dB/w speakers and 26 dB gain poweramps. I sure can't hear anything under those condidtions - maybe it was my years of playing with guns and loud cars...

I'm interested in your suggestions on how to improve my RTP3 circuit - but it needs to be a practical sonic improvement, not just measured.

Regards, Allen
 
I thought when Accuphase can parallel 8 long tail pairs with great measured result i could parallel 12 in single ended

Joachim, if that is any help: I was spying at pictures of the stuffed Accuphase C27 boards and see that they bias each differential pair by a 1K8 resistor. If I assume that they have a little less than 10 Volts across the transistors and a only couple of volts on the collector resistors I arrive at ca. 3 mA bias current through each transistor (I could not figure out what they are using, e.g. 2SC2545..7 or similar) which should be about optimal.

There is a design called “Crop” phono stage or similar by PigletsDad on this forum (did not dearch for the link). His input stage is very similar to the Accuphase, though he does not use paralleled transistors. He somewhere mentions this interesting detail - quote:

"Resistor noise is a factor but the big one is avalanche noise in the front end transistors - if you put more than about 10V of Vce on a bipolar, an additional noise process starts to intrude."

Btw., does anyone have any information on the above noise process?
 
"Resistor noise is a factor but the big one is avalanche noise in the front end transistors - if you put more than about 10V of Vce on a bipolar, an additional noise process starts to intrude."

Btw., does anyone have any information on the above noise process?

Depends obviously on the specific devices, but in general I don't think it's a big issue. There are other noise mechanisms (like 1/f and shot noise) that are more important in the usual bias range. Avalanche noise becomes important when breakdown voltages are approached (and mostly when the base is driven by high impedances), not a typical situation in a MC pre.
 
Having used very low output cartridges (eg: Kondo Io, Yamamura MC1) over many years I have come across only one step-up which was quite enough with 94Db speakers. The 'trade-off', for there is always one in the balancing of compromises involved, was that the dynamics of the music were suppressed and the immediacy of the music was severely limited. I would add that that MC stage was still of very high quality and measured superbly: it just did not work in an otherwise high resolution system.

For the same reasons as Allen [+ advancing years], my hearing can stand that small noise content provided the sound of the music is credible. The brain can filter this as those who have been trained to deal with tinnitus can attest. Sit in a concert hall and listen to the background noise created by a few hundred people in the audience! It tends - even during the quitest passages of music - to encroach far more than does a little low volume electronic noise in a well matched and developed MC based analogue system. But we can and do filter that distraction of random noise pretty effectively in a concert hall. As for rock gigs, I have heard Deep Purple at Knebworth along with several other high volume bands. The ambient noise from a few thousand fans was filtered adequately effectively to be part of the experience as opposed to being a distraction from the performance. I can't understand why we need to be too precious about listening to records in privacy when we can deal with potentially far greater distractions listening in situ to real performances!
 
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Having used very low output cartridges (eg: Kondo Io, Yamamura MC1) over many years I have come across only one step-up which was quite enough with 94Db speakers. The 'trade-off', for there is always one in the balancing of compromises involved, was that the dynamics of the music were suppressed and the immediacy of the music was severely limited. I would add that that MC stage was still of very high quality and measured superbly: it just did not work in an otherwise high resolution system.

For the same reasons as Allen [+ advancing years], my hearing can stand that small noise content provided the sound of the music is credible. The brain can filter this as those who have been trained to deal with tinnitus can attest. Sit in a concert hall and listen to the background noise created by a few hundred people in the audience! It tends - even during the quitest passages of music - to encroach far more than does a little low volume electronic noise in a well matched and developed MC based analogue system. But we can and do filter that distraction of random noise pretty effectively. Why do we need to be too precious about listening to records?

Strictly from a sound perspective, and beyond noise (which indeed I can't stand) about the only thing that I could correlate with the measured parameters is the dynamic range. The ability of handling "pops" and gracefully recovering from overloads are critical for the sonic performance. Tubes shine here, and that's also the reason why I designed my stuff for an almost insane (for SS) dynamic range (32dB).

See my comments here and on my website. More to come in the Conclusions.