John Curl's Blowtorch preamplifier part II

[kgrlee]

JC-3 phono preamp measurements from STEREOPHILE.
Please note: No complaints about S/N
By the way, I did NOT use the AD797 in the final design.

Wanna explain what possessed you to use AD797 in the evil JC2 original?

YOUR poor design and YOUR inappropriate choices are NOT reasons to dismiss excellent devices (wrongly used) and claim they are mid-range, evil, must have unacceptable 7th harmonic distortion .. bla bla

Wanna show us the poor reviews of the original JC2?

 

[john curl]

I was a fool, who still believed in Scott Wurcer. '-)

 

[john curl]

Please do NOT confuse the JC-2 with the JC-3.

 

[Max Headroom]

I do, I have my own measurements of dozens of opamps under normal conditions, with standard load like 2k - 10k, 2Vrms, gain 1 - 10. None of the good opamps (OPA627, AD797, OPA134 ...) has any residual 7th above -120dB. They do not sound same, but the 7th is not the reason. John's fairy tales about the 7th are marketing claims. We can easily find the 7th above -120dB in many so called High End products, as I have linked earlier.

Hello to Pavel and all others here, and not the least hello to you JC .
As a long time looker and listener to this thread, I and I am sure most others here have gained very much valuable and insightful knowledge....the value of a true collective.
Recently there has been much debate about the deleteriousness of presence of 7th harmonic production, at what would seem to be close to infinitesimal levels.

Pavel, could you please elaborate on your above statement (my bolding) in objective and/or subjective terms ?.

Thanks, Dan.

 

[PMA]

JC-3 phono preamp measurements from STEREOPHILE.
Please note: No complaints about S/N
By the way, I did NOT use the AD797 in the final design.

But Atkinson wrote

As with John Curl's legendary Vendetta phono preamplifier, the Halo JC 3 was superbly quiet. The wideband, unweighted signal/noise ratio, measured with the input short-circuited, was 78dB (MM) ref. 1kHz at 5mV, and 67dB (MC) ref. 1kHz at 500µV.

78dB ref. 1kHz at 5mV with input shorted is a high noise. With proper design, you can get near 90dB under these conditions. As Richard has said, you have lost some 10dB due to your circuit design.

 

[john curl]

Well, so be it.

 

[john curl]

test2:

 

[jan.didden]

Is that a slide rule you're holding John? Great pic.

Now that I'm here, can I interrupt the great entertainment with a tech question to the assembled intellect here:

I have a project using some Bourns series 3296 25-turn trimmers. I've been trying to find a spec for the max wiper current but it's not on the Bourns data sheet; it only lists max dissipation. An email to tech support was met with deafening silence. Google couldn't help either.
Anybody has any info on that?

jan

 

[john curl]

No Jan, I had an Apple II at the time. 3 years later I got Saul Marantz to buy me a MAC.

 

[31697B]

The wipers can take quit a bit of current as they are metal. Its the resistive material that cant take as much current.

However, for distortion, the contact point in the presence of dc current thru it can be a source of distortion. The design of the contact wiper will determine to a large extent the distortion from that point contact. -RNMarsh

 

[kgrlee]

I was a fool, who still believed in Scott Wurcer. '-)

JC, if you believed in him, you should have read his datasheet.

The best OPA or even Golden Pinnae stuff, hand carved from solid Unobtainium by virgins, won't stop poor design from losing 10dB of signal noise .. or introducing distortion and instability with poor decoupling and layout.

Mostly explained in the AD797 datasheet.

 

[mikelm]

So do most people here believe everything they read in data sheets ?

 

[SY]

They're not always 100% accurate, but one would be foolish to not use them as a starting point. The 797 datasheet is a particularly good one.

 

[jan.didden]

@ RM: Wel if I stay with the total current through the device below the stated dissipation, than that's taken care of I guess.
Like a 5k pot in mid position, with 7 mA entering at each end terminal, and the wiper sinking the total. Dissipation is just 250mW, but the wiper current is 14mA. That OK you figure?

jan

 

[stormsonic]

I have a project using some Bourns series 3296 25-turn trimmers. I've been trying to find a spec for the max wiper current but it's not on the Bourns data sheet; it only lists max dissipation. An email to tech support was met with deafening silence. Google couldn't help either.
Anybody has any info on that?

jan

http://www.digikey.com/Web%20Export/Supplier%20Content/Bourns_118/PDF/Bourns_TrimmerPrimer.pdf?redirected=1

http://www.bourns.com/pdfs/OnlinePotentiometerHandbook.pdf

too big files to be attached

 

[jan.didden]

Many thanks! The first doc gives me what I was looking for. Seems I am in the clear, which is good since I already ordered the boards

jan

 

[Esperado]

I have a project using some Bourns series 3296 25-turn trimmers. I've been trying to find a spec for the max wiper current but it's not on the Bourns data sheet;

It depend of the value. I would use max 10mA for a 1/25 W of 1K, but 3ma for 10k, 1mA for 100k etc and this, for the long run
If you are in concern with noise for DC and distortion for signal , better divide those value by 3. Just my two cents.

[edit] Oh, I don't saw the previous, hope it correlate.

 

[john curl]

It's only a schematic, guys! At first I could not understand what you were referring too. Then I realized that you meant the 500 ohms that gets paralleled with 50 ohms to get the higher gain for MC input. I consider it a good compromise, so that I don't load the OPA.

 

[jan.didden]

Not bad:

Rtrim 0.25W 0.5W 1W
10 OHMS 100 MA 100 MA 100 MA
100 OHMS 50 MA 71 MA 100 MA
1 K OHMS 16 MA 22 MA 32 MA
10K OHMS 5.0 MA 7.1 MA 10 MA
100K OHMS 1.5 MA 2.2 MA 3.2 MA
1 M OHMS 0.5 MA 0.7 MA 1.0 MA

jan

 

[john curl]

The photo that I put up was: Me arguing with Saul Marantz in 1982, with Noel Lee looking on. I guess that I will argue with just anybody. '-)
3 Years later, I was hired by Saul as VP of Engineering for Lineage.