forr:
thanks for your effort. that hasn't happened before, let me see what's going on ...
mlloyd1
thanks for your effort. that hasn't happened before, let me see what's going on ...
mlloyd1
forr said:
Charles,
I may be open to correction, but I seem to recall JA talking about the 20Khz headroom capability on this phono amp - not general overload capability. I doubt JA would make a mistake about signal matching. I'll try to find the actual review in my pile of Stereophiles in the next week or so.
Lineup,
the first cct you show is very similar to the phono amp topology in the Marantz PM7000. I guess the basic premise for MM inputs is use a JFET because the current input noise is lower. Different story with MC.
I may be open to correction, but I seem to recall JA talking about the 20Khz headroom capability on this phono amp - not general overload capability. I doubt JA would make a mistake about signal matching. I'll try to find the actual review in my pile of Stereophiles in the next week or so.
Lineup,
the first cct you show is very similar to the phono amp topology in the Marantz PM7000. I guess the basic premise for MM inputs is use a JFET because the current input noise is lower. Different story with MC.
Bonsai said:
I have spent a little bit of time searching the Stereophile website for an explicit description of how JA makes this measurement, but without success.
However, I have found examples of products that show that (IMO), JA is misinterpreting the results of these tests. And I have also found examples of products that seem to have poorly designed phono stages.
First of all it should be noted that the measurements are made from the phono input to the tape output. So I will start with a product I well know, the original Ayre K-1 preamplifier I designed 13 years ago:
http://stereophile.com/solidpreamps/609/index6.html
Fig.4 Ayre K-1, distortion (%) vs output voltage into 100k ohms (from bottom to top at 1V): line balanced, line unbalanced, phono.
From this graph we can see that the THD+N of the phono stage (measured at the single-ended tape outputs) is quite similar to the THD+N of the line stage (measured at the single-ended outputs) except that there is more relative noise due to the much lower input signal used for the phono stage.
Later the review states:
"Finally, the phono overload margin of the K-1 was also very good; 1% THD+noise was reached at an input of 17.2mV at 1kHz, 161mV at 20kHz, and 1.63mV at 20Hz. The input signal used for these measurements was unequalized, and the variation with overload margin with frequency is due to the characteristics of the RIAA curve."
In other words, the limiting factor of the "phono overload margin" is simply that the circuit used in the K-1 will only output about 7 Vrms single-ended. Furthermore, this limit is reached regardless of frequency.
Compare this to the comments made in the measurements section of the Musical Fidelity Tri-Vista kWP preamplifier:
http://www.stereophile.com/tubepreamps/104mf/index7.html
JA writes:
"Whether measured via its MM or its MC input jacks, the kWP had the highest phono-stage overload margin in the midrange and bass I have ever encountered. Assessed at the tape out jacks at 1kHz, for example, the MM input overloaded (1% THD) at 439mV, which is 38.9dB above the reference 5mV input, and equivalent to an extraordinary 43V output!"
So you can see he is clearly confused about the meaning of "phono stage overload margin". The fact that a preamp can output 43 Vrms at the tape outputs (instead of, say, the 7 Vrms provided by the Ayre K-1) is absolutely meaningless in the real world. JA should know better than to make congratulatory statements over a meaningless measurement.
On the other hand, there are poorly designed phono stages that apparently can have internal overloads at specific frequency ranges. This is something that can potentially be a bad thing, possibly causing distortion in the real world.
For example, please refer to the measurements of the Audio Research Reference phono preamplifier:
http://www.stereophile.com/phonopreamps/200ar/index4.html
Looking at Figure 5, we can see that there is some part of the circuit that internally overloads at high frequencies compared to mid- and low frequencies:
Fig.5 Audio Research Reference, Low Gain, distortion (%) vs output voltage (V) into 100k ohms with (L–R) 20kHz, 1kHz, and 20Hz signals.
Now, I'm not sure that this is really such a problem in the real world, as the unit can still output 7 Vrms at 20 kHz (the same that the Ayre K-1 did at *all* frequencies). But it does suggest that there is some capacitance that is being charged by a source with limited current drive capabilities, leading to something similar to slew-rate distortion at high frequencies.
But the bottom line is that in his measurements for Stereophile, JA does not generally distinguish between a high "overload margin" due simply to an excessively high output level capability versus a unit that truly has a problematic phono stage that can run into *internal* overload problems under certain circumstances.
And if I recall correctly, the review you mentioned with the "6 dB" overload margin was one of those that simply had a relatively low output level capability (but still sufficient for any real world application), and JA incorrectly blamed this on a poorly designed phono stage with a low "overload margin".
Once we find the review in question, we should be able to confirm the actual situation.
This post is an addendum to the above post concerning Stereophile's measurements of phono stages.
In the test of the $28,000 Boulder phono stage:
http://www.stereophile.com/phonopreamps/621/index6.html
JA makes the following statement:
"As a result of the Boulder circuit's extraordinary linearity and dynamic range, the MM overload margins were the highest I have ever measured: 42.1dB at 20Hz, 41.5dB at 1kHz, and 41.1dB at 20kHz, each equivalent to an output of 30.5V! Partly, this superb performance will be due to the lower-than-usual MM gain. But even when the module was set to MC, the margins were still superbly high, at 32.9dB, 32.0dB, and 31.8dB, respectively, and again equivalent to an output voltage of 30.5V."
So once again, we can see that a high output level capability (*far* beyond anything that would be beneficial in the real world) leads to unjustified praise for a high "overload margin".
Furthermore, he implicitly admits that he does not take into account the gain of the phono stage when making these measurements. So what we are left with is a hodge-podge of numbers that are exceedingly difficult (if not impossible) to interpret accurately, yet they are assigned qualitative values (eg, "good", "excellent", "high", "poor", et cetera), presumably without taking into consideration the actual factors of importance.
In the test of the $28,000 Boulder phono stage:
http://www.stereophile.com/phonopreamps/621/index6.html
JA makes the following statement:
"As a result of the Boulder circuit's extraordinary linearity and dynamic range, the MM overload margins were the highest I have ever measured: 42.1dB at 20Hz, 41.5dB at 1kHz, and 41.1dB at 20kHz, each equivalent to an output of 30.5V! Partly, this superb performance will be due to the lower-than-usual MM gain. But even when the module was set to MC, the margins were still superbly high, at 32.9dB, 32.0dB, and 31.8dB, respectively, and again equivalent to an output voltage of 30.5V."
So once again, we can see that a high output level capability (*far* beyond anything that would be beneficial in the real world) leads to unjustified praise for a high "overload margin".
Furthermore, he implicitly admits that he does not take into account the gain of the phono stage when making these measurements. So what we are left with is a hodge-podge of numbers that are exceedingly difficult (if not impossible) to interpret accurately, yet they are assigned qualitative values (eg, "good", "excellent", "high", "poor", et cetera), presumably without taking into consideration the actual factors of importance.
Charles, do you care to elaborate what you call 'input overload'?
Are you saying, that many input stages can't cope with those tiny --admittedly not yet equalized-- signal voltages? I would think that the phase and amplitude shifted signal (and it's potential 'spiky' nature) from the cartridge would benefit from a fast, wide bandwith input stage with some slew rate, but would guess (hope?) that most modern designs reflect this.
thanks,
Rüdiger
Are you saying, that many input stages can't cope with those tiny --admittedly not yet equalized-- signal voltages? I would think that the phase and amplitude shifted signal (and it's potential 'spiky' nature) from the cartridge would benefit from a fast, wide bandwith input stage with some slew rate, but would guess (hope?) that most modern designs reflect this.
thanks,
Rüdiger
JA wrote: "The MM gain was slightly lower than usual, at 34.2dB (1kHz). The MC gain was higher, of course, at 63.7dB," (I assume 1kHz again, i.e. 83dB LF approx.)
We may assess his "headroom" meaurements regarding these numbers.
We may assess his "headroom" meaurements regarding these numbers.
Onvinyl said:
Well, the basic idea has some merit. It's just that so many people don't understand it. The only magazine that measures it is Stereophile, and they do so in a way that is misleading.
The basic idea is really similar to TIM -- ie, the fact that a circuit can have internal overload problems.
Take for example a Dyna PAS-3 phono stage and compare it to the phono stage of an Audio Research SP-3. (I'm sure the schematics are somewhere out there on the web.) Both have feedback from the output to the cathode of the input tube that uses RC networks to create the RIAA equalization.
But on the Dyna, the output is the plate of the second tube, while on the Audio Research a cathode-follower is added. It doesn't take much imagination to realize that at high frequencies the impedance of the feedback network (connected to a low-impedance node) is low enough that it will cause distortion when driven from the plate of a high-impedance 12AX7. On the other hand, the Audio Research's cathode follower could drive that network much more easily and with less distortion.
I've never measured either one, but I would bet that the Audio Research would have significantly lower distortion at high frequencies and high levels than the PAS-3.
But to me, this is so elementary that it's not worth wasting much time on. If your circuit has trouble driving the EQ network, then you shouldn't be designing circuits. There are much more subtle points to work on and achieve real improvements than to waste time on these types of basic problems that have been understood for decades.
Bonsai said:
Quite right Bonsai & PMA.
It is that obvious it does not need told.
Especially the first input stage of MC vs. MM has got very different requirements.
Due to the low source resistance level of Ceramic cartridges.
Like for example 50-100 Ohm
Let me quote myself:
So, my post recommends AN-222 reading and have a look at very good lownoise circuits there.
As a sidenote, I post a low noise input stage suitable for Magnetic Pickups ( MM ).
I guess not many posting here bothered to download and read the 2 papers I point to ??
Maybe because you have already all the knowledge .. and maybe even more ...
But for all others 100-s of members/not members from all over this world,
including CHINA TAIWAN, reading our www.diyaudio.com forum :
If you really is interested in doind some low noise designs of you own,
go get some good old litterature. Will save you from trail & error exprerimenting.
================================
I attach the most interesting circuit, in my opinion, form AN-222.
When it comes to Head Amplifier for MC cartridge.
It was written in the days ago, when LM194 / LM394 was the top of the line for lownoise MATCHED monolithic dual devices.
Today we have devices with considerable better performance.
Like Analog Devices http://www.analog.com/
-------------------------------------------
MAT02, MAT03, MAT04, SSM2210 and SSM2220 ( 0.7nV/Hz )
MAT04 is a QUAD, 4 transistors.
Suppose we parallell the 4T in a SO8 capsule. Will reduce input stage noise with a factor.
And at such a small area as an SO8 there would be very short leads to do the parallelling. Shorter leads = less target for inherent noise production.
The National AN-222 really deserves a study!
Even the very experienced designers of this board ( read Curl, Pass )
may pick up some ideas / details .. or maybe refresh old discrete design techniques.
--------------------------------------
As I said,
I attach I attach the most interesting circuit, in my opinion, for Head Amplifier for MC cartridge.
This is The FIRST, very important stage in MC RIAA amps.
Lineup regards
Attachments
Onvinyl said:
The real problem with overload isn't so much with the normal levels of the musical content as it is with the ticks and pops. They can easily exceed the voltage produced by the music by a couple of orders of magnitude. It is often noted that ticks and pops are far less intrusive through tube phono stages. This is (at least to a first approximation) due to the fact that tubes can easily swing hundreds of volts without even batting an eye. Solid state excels at current, but doesn't do so well at swinging voltage. Of course with tubes, you're battling noise from the word go, so...it's a question of tradeoffs, as usual.
Life would be easier if cartridge manufacturers would adhere to, say, two or three output level conventions. But every cartridge is different and designers (Charles Hansen, in this case) are left guessing as to what sort of gain the end user might need. 40dB? 50dB? 60dB? 80dB? There's not necessarily a one size fits all phono stage, but somehow Charles, et. al. are expected to provide one.
I've got Grados, Koetsus, and everything in between. There isn't a single phono stage that would work perfectly with all the cartridges I have on hand. I'm forced to compromise. Charles is faced with an even greater quandary and I, for one, don't envy him.
Grey
I am just designing a commercial product, and the customer wants for me to cover and switch for different MC sensitivities and load impedances, just as you are saying, Grey.
On the other hand, do not forget that pops and clicks with fast rise times are high frequency. They inherently lay in lower gain part of transfer char. Of course it depends on circuit design, but I do not think it is that difficult to solve clipping behaviour.
You can do the same thing with IC opamps
You can do the same thing with IC opamps (or any opamp for that matter). All you have to do is run 47 ohm resistors in their outputs to equalize the currents. Not only does it lower noise 3 db for every doubling of amplifiers, but it also boosts the current drive capability of the composite amplifier that results.
lineup said:
You can do the same thing with IC opamps (or any opamp for that matter). All you have to do is run 47 ohm resistors in their outputs to equalize the currents. Not only does it lower noise 3 db for every doubling of amplifiers, but it also boosts the current drive capability of the composite amplifier that results.
Janasek's paper, while OK in principle, is based on 40 year old knowledge, and we can do better, today.
222 paper is crude, yet is a good noise example. I work a 0.4nV/rt hz, direct input and output coupling, and have done so for the last 35 years.
222 paper is crude, yet is a good noise example. I work a 0.4nV/rt hz, direct input and output coupling, and have done so for the last 35 years.
john curl said:
Thanks for your comment, Curl.
Yes, using capacitor coupled output is not Optimal.
As most any capacitors adds a level of distortion that should be to consider in very Hi-fi apps.
The example from AN-222 was not ment to be a SOTA ( State Of The Art ) contribution.
More so, I wanted to show, that when we hunt for the ultimate low noise
there is nothing to beat a discrete design.
A transistor circuit that is customly built for a special case. A specific source in question.
This will apply to such critical amplifiers as MC cartridge amps.
For an unexperienced DIY-er using a toplclass IC ( e.g. op-amp ) is much to recommend.
There are very good RIAA Op-Amp amplifiers applications to find around the net.
A hybride, using discrete transistor based first stage + Op-Amp(s) for filtering/buffering, is one way to go.
And can give a very good result.
--------------------
Further, when designing MC amplifiers, as well as RF-circuits,
The Practical layout and build is AT LEAST 50% of the trick.
This involves good shielding of input cable and input stage
+ a very good Power supply, that matches the quality of the amplifier.
GROUNDING issues must be well taken care of, too!
Lineup, regards.
Yes...
When I build something, I try to use an RF layout design.
Most times this involves breadboarding with either a piece of copper strap or unetched circuit board to serve as a ground plane. The ground plane sits atop the circuit board and all the grounds connect up to it.
When I build something, I try to use an RF layout design.
Most times this involves breadboarding with either a piece of copper strap or unetched circuit board to serve as a ground plane. The ground plane sits atop the circuit board and all the grounds connect up to it.
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