MC head amp

[Nordic]

So far in the race:

AD811
60dB
2500V/µs
INPUT BIAS CURRENT: 2µA
INPUT OFFSET VOLTAGE 0.5mV
NOISE: 1.9 nV/√Hz
DISTORTION: 0.0003% at 1kHz

OPA604
100dB
25V/µs
INPUT BIAS CURRENT: 50pA
INPUT OFFSET VOLTAGE ±1mV
NOISE: 10nV/√Hz
DISTORTION: 0.0003% at 1kHz

AD797
114dB
20V/µs
INPUT BIAS CURRENT: 0.25µA
INPUT OFFSET VOLTAGE 25µV
NOISE: 0.9 nV/√Hz
Distortion -120 dB

OPA228/2228
120-130dB
10V/µs
INPUT BIAS CURRENT: 10nA
INPUT OFFSET VOLTAGE 0.75µV
NOISE: 3nV/√Hz
DISTORTION: 0.00005 at 1kHz

 

[Onvinyl]

Ok,
and me findings *sonically* were: of these, the AD811 was best in 1st stage, AD797 did not work without radio broadcast in my circuit, OPA604 was a lot worse than the original op27. if you don't care about a real good psu, the extra money for the AD811 is not well spend (compared to the original op27).
Rüdiger

 

[Calvin]

Hi,

I wondered about are we still talking about MC-Headamps here?
As phn mentioned before...is there any positive in using such a lightning quick OP in the audio frequency range and especially for this drastically bandwidth limited vinyl?
I´d keep an eye on parameters as noise, offset and input bias current. ´Speed´ is a rather useless parameter here since the needed slewrate is very very low (less than 1V/µsec)
For MC uses FET-OPS are principally out. Amongst the bipolar ones one should look at low 1/f corner frequencies and lowest noise voltages (current noise falls out of interest in first place). Some video OPs state excellent noise values in their interesting freqency range, but are quite noisy in the audio range (high 1/f corner freq).
The input bias should be low since this current flows though the tiny coils of the pickup. Not only don´t I like the idea of stressing the precious pickup with unneccessary current (heat) but this current generates an offset voltage that is greatly magnified by the circuits gain.
With regard to this even an old LT1028/LT1115 outperforms most of the more modern designs when it comes to noise. The AD797 is imo one of the best choices among the newer designs and outperforms the AD811 with regard to noise, bias and offset clearly.

jauu
Calvin

 

[rdr]

Hi,

I wondered about are we still talking about MC-Headamps here?
As phn mentioned before...is there any positive in using such a lightning quick OP in the audio frequency range and especially for this drastically bandwidth limited vinyl?
I´d keep an eye on parameters as noise, offset and input bias current. ´Speed´ is a rather useless parameter here since the needed slewrate is very very low (less than 1V/µsec)
For MC uses FET-OPS are principally out. Amongst the bipolar ones one should look at low 1/f corner frequencies and lowest noise voltages (current noise falls out of interest in first place). Some video OPs state excellent noise values in their interesting freqency range, but are quite noisy in the audio range (high 1/f corner freq).
Don not The input bias should be low since this current flows though the tiny coils of the pickup. Not only don´t I like the idea of stressing the precious pickup with unneccessary current (heat) but this current generates an offset voltage that is greatly magnified by the circuits gain.
With regard to this even an old LT1028/LT1115 outperforms most of the more modern designs when it comes to noise. The AD797 is imo one of the best choices among the newer designs and outperforms the AD811 with regard to noise, bias and offset clearly.

jauu
Calvin

I could not agree more.

I have some questions on the idea of using a frozen dead cartridge in the feedback loop to cancel out non linearities. I know there is a patent on this.

http://www.freepatentsonline.com/4622660.html

Do not the moving coil system non linearities come from - a moving coil? The magnetic field strength could be non linear over the coil displacement. As far as I know coils immersed in an unvarying magnetic field do not generate distortion unless there is hysteresis and I wonder if that is a significant effect here due to the very small coil currents and the relatively large static magnet.

 

[valveitude]

Do not the moving coil system non linearities come from - a moving coil?

I *think* that the idea is to have a current flow through the feedback cart so that it acts as a motor, rather than a generator, inducing comparable coil movement. This current would be miniscule..I'm only guessing here.

-Casey

 

[Onvinyl]

Hi,
regarding the phonoclone, I did some 'research' in this thread including frozen card and op-amp choice.

See the linked threads in the linked thread as well, quite some good information.
regards,
Rüdiger

 

[rdr]

Hi,
regarding the phonoclone, I did some 'research' in this thread including frozen card and op-amp choice.

See the linked threads in the linked thread as well, quite some good information.
regards,
Rüdiger

You found no improvement with the frozen cartridge feedback? I could not find the post. The Needle Doctor is offering me a 20% discount for my DL-103D body, and I may take them up on it. Question is 103 or 103S. I'm not sure if I want a spherical tip, but the 103S is the successor to the 103D.

I am certainly going to refer to that thread when I build mine.

 

[tvi]

I thought the 103S had a Shibata stylus shape

Regards
James

 

[Onvinyl]

You found no improvement with the frozen cartridge feedback? [/B]

As I wrote. But I will someday repeat some research with alternated layout; I wasn't able to run the OPA627 with the cart in the feedback, for instance, it gave me excessive offset in both in- and output of the op-amp, it was probably oscillating. It was *only maybe* a bit better for the op27 with 'frozen cart', but not better than the AD811 alone, which did not benefit from that arrangement.
regards,
Rüdiger

 

[rdr]

I thought the 103S had a Shibata stylus shape

Regards
James

Sorry. I meant 103R. My other option is the 301II. Any opinions on plain 103 vs 301II?