John Curl's Blowtorch preamplifier part II

[AndrewT]

a bit better than the resolution of 24bit @ 192ksamples/s can manage.

[john curl]

We should look into Barkhausen noise.

[Rod Coleman]

Doyen of MC magnetic design, van den Hul, has no doubt about the difficulty of conveying low-level cartridge signal through a magnetic circuit.

THis from his 'Phono FAQ' -

[SY]

Originally Posted by Rod Coleman

Doyen of MC magnetic design, van den Hul, has no doubt about the difficulty of conveying low-level cartridge signal through a magnetic circuit.

THis from his 'Phono FAQ' -

van den Hul doesn't sell transformers.

His points on shielding are well taken. Very tight balance is also important for noise rejection. It's hard to get a quiet phono stage when the raw supply is in the same cabinet.

[john curl]

I spoke at length with Mitch Cotter today about audio transformers. He agrees that a low effective resistance is possible, and the transformer that I have selected so far leaves a little room for improvement.
He also independently (of VDH) warned me about ground loop problems and related hum pickup with transformers. We shall see.

[1audio]

Input transformers are quite the complex brew with more options than a Chinese Restaurant menu. Also interesting is the effect of different source impedances on transformer response. The chart for the SRS box shows quite a difference between 0 Ohms, 1 Ohm and 10 Ohms source impedance (the range that MC cartridges represent). With a smaller ratio there will be less variation.

While the lowest noise will be had with a transformer the real limitation is the surface noise of a disk. Even a very quiet disk (fresh cut and unplayed lacquer) will be noisier than any of these numbers. A good design will start with the complete envelope of requirements of the application and then balance that against what the technology can support. Aside from getting the basics right (no hum, correct RIAA, enough gain for the system) its all about the designers particular mix of tradeoffs. The same as a good Chef.

[john curl]

I agree Demian. It is not just the S/N of the transformer, but the need for variable loading, and what the effect of the .5 Hz warp signal passing through the transformer along with the music.

[john curl]

I should be pointed out that it was not just the transformer's S/N ratio that got us on the track of attempting to REMOVE the transformer from the input audio path of many components.
It was really low frequency distortion, and the addition of high frequency noise due to eddy current losses in many commercial transformers, that led the way. While the RIAA tended to roll of the high frequency noise from both the record or the transformer, the NAB or IEC tape equalization did just the opposite.
This is where I found myself, back in 1968, looking at the transformer noise problem.
One way to evaluate this is with a Q meter that measures the ratio of inductive impedance to the EFFECTIVE resistance of the transformer with frequency. I used a GR unit for my measurements. The main transformer that we were looking at was a 10:1, or so, Beyer transformer from Germany, that looked pretty good from the outside, and was well shielded. However, on the Q meter, I saw bad things. The effective Q at 20KHz or so was only about 1 or 2, as I recall. This meant that the effective resistive noise at high frequencies went up several times over its DC value. Not good for tape recording. This is due to the magnetic materials and the THICKNESS of the laminations that made up the core assembly. They used 6-10 mil laminations as I recall, AND I could easily see the problem with my Q meter.
This was a VERY GOOD reason to both improve our tape heads AND remove the transformer, if possible, from magnetic recorders.
Getting back to phono, this problem would not be so pronounced, but it is still real.
For phono, another problem becomes VERY SERIOUS. This is the ultra low frequency garbage put out by the record warps and passed through the transformer. This could be deadly, and what I will look at, primarily, with my transformer sample.
Initially, the challenge of making a transformer eliminator to make direct connection with a MC cartridge possible, was considered almost impossible. In fact, I was virtually 'shown the door' at Ortofon in 1967 for even considering it. It had been 'proven' by their best engineers that it was impossible, I was told.
Well, it wasn't impossible, just impractical. With the JC-1 released by Mark Levinson in 1973, I got just revenge.
It must be said that at that time, the transformers used, were inferior to what we can get today. Now, we have true competition, given that low noise transistors and jfets are getting rare and expensive, and transformers are improving.

[Terry Demol]

Originally Posted by john curl

I should be pointed out that it was not just the transformer's S/N ratio that got us on the track of attempting to REMOVE the transformer from the input audio path of many components.
It was really low frequency distortion, and the addition of high frequency noise due to eddy current losses in many commercial transformers, that led the way. While the RIAA tended to roll of the high frequency noise from both the record or the transformer, the NAB or IEC tape equalization did just the opposite.
This is where I found myself, back in 1968, looking at the transformer noise problem.
One way to evaluate this is with a Q meter that measures the ratio of inductive impedance to the EFFECTIVE resistance of the transformer with frequency. I used a GR unit for my measurements. The main transformer that we were looking at was a 10:1, or so, Beyer transformer from Germany, that looked pretty good from the outside, and was well shielded. However, on the Q meter, I saw bad things. The effective Q at 20KHz or so was only about 1 or 2, as I recall. This meant that the effective resistive noise at high frequencies went up several times over its DC value. Not good for tape recording. This is due to the magnetic materials and the THICKNESS of the laminations that made up the core assembly. They used 6-10 mil laminations as I recall, AND I could easily see the problem with my Q meter.
This was a VERY GOOD reason to both improve our tape heads AND remove the transformer, if possible, from magnetic recorders.
Getting back to phono, this problem would not be so pronounced, but it is still real.
For phono, another problem becomes VERY SERIOUS. This is the ultra low frequency garbage put out by the record warps and passed through the transformer. This could be deadly, and what I will look at, primarily, with my transformer sample.
Initially, the challenge of making a transformer eliminator to make direct connection with a MC cartridge possible, was considered almost impossible. In fact, I was virtually 'shown the door' at Ortofon in 1967 for even considering it. It had been 'proven' by their best engineers that it was impossible, I was told.
Well, it wasn't impossible, just impractical. With the JC-1 released by Mark Levinson in 1973, I got just revenge.
It must be said that at that time, the transformers used, were inferior to what we can get today. Now, we have true competition, given that low noise transistors and jfets are getting rare and expensive, and transformers are improving.

WRT HF noise and Q, did you adjust the transformers response to
perfectly flat in the passband and Bessel rolloff with proper transient
response?

Have you tried the Jensen JT-346-AX. It has about the lowest LF distortion
I have seen from a standard MC TX .

T

[Allen Wright]

Originally Posted by SY

Let's get a feel for the signal numbers:

Take an MC with a typical output, say 0.2mV at 5 cm/s and 1kHz. The cartridge is likely to have a self noise of -70dB below that, which is about 0.03uV. Now, for argument's sake, let's pick an effective load of 100R. That means the signal current is about 3 x 10^-10A.

By definition, an ampere is about 6 x 10¹⁸ electrons per second. That means our signal current is about 1.9 billion electrons per second or 1.9 million electrons per cycle. That's not enormous, but it's certainly bigger than "about one."

Assuming you have understood his paper better than me, I ask my question again with different numbers:

Can a transformer ACCURATELY pass on the signal change from (say) 1.9 x 10pr9 to double that, 3.8 x 10pr9?

There must be a lower limit where a traffo simply stops working accurately, or doesn't work at all.

Any thoughts? Any theory? Any tests?

Regards, Allen