John Curl's Blowtorch preamplifier

[syn08]

I haven't done envelope capture nor do I have a digital scope to do it with (maybe next year). But published data I have seen showed a 22dB peak over the 5cm/s rating in an MC caused by dust, the maximum energy being at the tip-vinyl resonance. This particular cartridge was a fairly well damped one- I would want at least 6-10dB more at ultrasonic frequencies to ensure that with less-well-damped cartridges, the preamp wouldn't overload.

That's about my understanding as well; that's why I thought 28dB would be good enough. Ill look into pushing this a little up, but I stll believe 36dB as PMA is assuming is a huge overkill.

 

[AndrewT]

I like the logic of Sy's last two posts.
Vinyl needs >=30dB of overhead.
35dB is better, some strive to approach 40dB but never get there.
Sy's explanation of why 40dB is not needed over the whole bandwidth seems a sensible way to avoid impossible to reach numbers.

 

[scott wurcer]

I haven't done envelope capture nor do I have a digital scope to do it with (maybe next year). But published data I have seen showed a 22dB peak over the 5cm/s rating in an MC caused by dust, the maximum energy being at the tip-vinyl resonance. This particular cartridge was a fairly well damped one- I would want at least 6-10dB more at ultrasonic frequencies to ensure that with less-well-damped cartridges, the preamp wouldn't overload.

I have some pictures somewhere, but they are 22.05kHz rolled off. There seems to be a "signature" for a lot of the bad pops. My completely unfounded sense of what's going on is that the tip actually looses contact and slams down again. In this case I see what looks like an elastic relaxation with ringing. I saw no evidence of preamp misbehavior even on ones that were 95% of full scale.

 

[SY]

Physically, I think that's very much what's going on.

What sort of cartridge was used for your pictures?

 

[scott wurcer]

One of the wooden Grados. I even have some almost perfectly mono pops showing some quadrature crosstalk (again total conjecture). Still it's interesting to see a fullscale signal in one side and almost nothing in the other.

 

[john curl]

I am both impressed and amused. The foundation for this argument was published over 30 years ago, by me, in an IEEE publication. I didn't just think about it, I MEASURED it. Then I published it.
Syn08 is correct about the 330 ohm. More resistance would give LESS noise in this location because it reduces the noise gain of the current mirror.

What you need in order to be up to MC cartridge mistracking is to duplicate a 10us risetime square wave at the OUTPUT of the phono stage. This I call TIM(30,30) and published it in the IEEE paper. If you cannot pass this signal effortlessly, then you will add even more distortion to the phono playback process to a significant degree.

I measured a number of MC cartridges and found that ALL can mistrack with normal quality records within their recommended stylus force range. How well they handle this mistracking is an important aspect to how the record will be reproduced.

 

[Edmond Stuart]

Alberto Bilotti (sp?) wrote a fairly tedious hand analysis of current mirror noise in 1968 that should be in some IEEE archive.

Thanks for the ref, Scott.

Can anybody send me a pdf of that article?
Thx in advance.

Regards,
Edmond.

 

[syn08]

I like the logic of Sy's last two posts.
Vinyl needs >=30dB of overhead.
35dB is better, some strive to approach 40dB but never get there.
Sy's explanation of why 40dB is not needed over the whole bandwidth seems a sensible way to avoid impossible to reach numbers.

I'm still not convinced that 35dB is required for any practical reasons (still can't find any commercial example of such) but I would concede that anything over 30dB can't be realistically built with regular opamps. High voltage and reasonable noise parts exist (e.g. OPA551) but they obviously lack output current.

I'm gonna try a LM3886 power amp 😀. It might be actually pretty good in replacing the AD797 in the input stage! 😀

 

[scott wurcer]

Thanks for the ref, Scott.

Can anybody send me a pdf of that article?
Thx in advance.

Regards,
Edmond.

Sorry that was 1975 his 1968 work was on analog multipliers.

 

[scott wurcer]

I am both impressed and amused. The foundation for this argument was published over 30 years ago, by me, in an IEEE publication. I didn't just think about it, I MEASURED it. Then I published it.

Do you have a reference? Your name does not show up in the IEEE data base. I did find an article that deals with rbb' in making low noise amplifiers from 1963., but stuff is pretty sparse.

 

[asnakeon]

Irrespective of the headroom issue, could some kind soul provide a concise analysis (or pointer to) of how to think about the practical gain limits of current mirors for these first-stage, small signal, low noise applications?

 

[gerhard]

I really don't understand what this so-called headroom business is about. When I give +- 1V to my power amp, it will pass +-40V or so to the speakers. That's 400W and should be enough.

Does anybody here really think that a preamp that has 30 dB of "headroom" does any good? Something like 70V out? Or 400 KW to the speakers?

Looks like there is a limit somewhere. I would not want to remove the B&W tweeter membranes from a wall of my room, and if limiting must be done,I'd prefer this not to be done by driving the emitters of 2 dozens of TO-3s against the rails.

If limiting must be done, it's done better in the preamp; preferably by a stage that's back in business 1 usec after the overload is over and where at least the ultrasonic power can be kept at bay.

The main reason for preamp 'headroom" is to give a number to brag about.

regards, Gerhard

 

[SY]

You miss the point. How badly does the preamp clip? How long does it remain clipped? Does it block or stick? Oscillate on recovery? And as I pointed out, even a huge signal at the input gets knocked down dramatically if there's a passive RIAA- not so if the gain reduction is via feedback...

 

[scott wurcer]

Irrespective of the headroom issue, could some kind soul provide a concise analysis (or pointer to) of how to think about the practical gain limits of current mirors for these first-stage, small signal, low noise applications?

Since you can't area ratio discrete devices your options are more limited. I'm afraid most references will assume that you are on an IC process.

 

[john curl]

For Scott, try this: CH1285-6/78/0000-0263$00.75@1978IEEE
I don't know what you can do with it, but that is what is on the first page.
Title 'Omitted Factors in Audio Circuit Design' John J. Curl, Audio Design Consultant pp. 263-266.

 

[john curl]

Scott, you have to look also at the IEE, not just the IEEE for low noise design info.

 

[john curl]

Asnakeon, you should be able to ratio current sources. Isn't that a 2N2219 as your moniker? Do you know what it is? Do you have a chip book?

 

[asnakeon]

Asnakeon, you should be able to ratio current sources. Isn't that a 2N2219 as your moniker? Do you know what it is? Do you have a chip book?

Yes, this was taken from one of the 60's promotions for the (then) new STAR geometry.

 

[john curl]

I used to have those pictures, also. Mine got burned up in 1991, in the firestorm. However, the idea is still important with current mirrors. More chip area gives more current, when the 'diode' is a small area device.

 

[1audio]

Output of moving coil cartridge

I decided to measure the output under mild duress in a way that may help this discussion. I grabbed what I had available int he shop-
Technics sp15, generic tonearm of unknown origin, cartridge: DL 103 GL (Gold Limited) – special black housing, output 0.25mV, 40ohms, spherical stylus, 5cu, VTF 2.5 +/- 0.3grams, coils 4N High Purity Gold Wire 99.99%, ltd. production of 2000 or so the spec from the web says., connected directly to a Tek 7A22 set to 10Hz-500 KHz. Played a test record -0JVC CD4 Quadradisk- to get some baseline- and a reference Recordings RR-11 45 Berliotz since it has some pretty high modulation.

What I noticed first was the pops and ticks could easily hit 2-3 mV peak, with a very short duration (need to squint and stare to see them). Peak output on the loudest transient passage was around 4 mV peak or 8 mV P-P.

Output on the CD 4 carrier (30 KHz) was a steady .3 mV rms or 1 mV P-P. Some spoken dialog on the test record was up to 1.5 mV peak.

I know its not easy to relate the unequalized output of a phono system to equalized levels so the numbers should not be taken straight, BUT its an indication of the actual levels of a cartridge with a rated output at "reference level" of .25 mV rms or signals with easily more than 20 dB higher level than "reference".

Playing through an equalized preamp there is significant output (20 dB above the noise floor) up to 50 KHz that is musically related with peaks of energy at around 15 KHz and 30 KHz. Given the rolloff of 6 dB per octave starting at 2.2 KHz that is a lot of HF energy. It may all be distortion products but its from back at the cartridge.