Descrete jFET versus IC op-amp quality - diyAudio
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Old 16th June 2011, 02:07 PM   #1
W1DAN is offline W1DAN  United States
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Default Descrete jFET versus IC op-amp quality

Hi folks!

I am newly registered here and have looked around a bit, but not comprehensively, so I apologize if this has been covered already.

Since about 1980 or so I have occasionally built audio devices with op-amps. These include a radio station style audio console, a multiband audio compressor/limiter, an NAB preamp, and especially phono preamps. These used the NE5534, TL074, '84, and the AD797, usually with feedback style EQ, but sometimes flat where I EQ in a computer.

Last summer I found at a yard sale a Borbely EB585 (Old Colony KT-2) home preamp ($10.00!) that includes a discrete RIAA preamp (see Audio Amateur magazine 04/85 and 01/86). This preamp uses a single set of 2SK147/2SJ72 jFET's in the front end and passive EQ.

I finally fired this unit up a couple of months ago and was very surprised at the clarity of the sound over my past IC op-amp units. The Borbely preamp is not the quietest, bus the instruments now sound like instruments and the sound stage is accurate.

So I am now on a trek to learn why technically the discrete preamp sounds better than a well-made IC unit. This is with the idea of specs like open loop bandwidth, slew rate and distortion that are comparable with the discrete jFET unit.

I have read Erno's newer design articles and am wondering if I should chase after the idea of breadboarding his last RIAA design, or just get some of the newer op-amps. I see a few newer op-amps out there and wonder if these can match or exceed the performance of the Borbely unit. I have also enjoyed a few designs posted here-some excellent work!

Thank you for your thoughts!

Dan
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Old 17th June 2011, 01:33 AM   #2
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Originally Posted by W1DAN View Post
Hi folks!



So I am now on a trek to learn why technically the discrete preamp sounds better than a well-made IC unit. This is with the idea of specs like open loop bandwidth, slew rate and distortion that are comparable with the discrete jFET unit.

I have read Erno's newer design articles and am wondering if I should chase after the idea of breadboarding his last RIAA design, or just get some of the newer op-amps. I see a few newer op-amps out there and wonder if these can match or exceed the performance of the Borbely unit. I have also enjoyed a few designs posted here-some excellent work!

Thank you for your thoughts!

Dan
You can try hanging arounf the Pass part of this forum under amplifiers. Nelson Pass also has a dislike for Opamps, and his preamps and I/V converters are all discrete. If you go to his websites, he will discuss a bit more on his approach .

Oon
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Old 17th June 2011, 02:45 AM   #3
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Originally Posted by oon_the_kid View Post
You can try hanging arounf the Pass part of this forum under amplifiers. Nelson Pass also has a dislike for Opamps, and his preamps and I/V converters are all discrete. If you go to his websites, he will discuss a bit more on his approach .

Oon
which kind of misses the point if the poster is really looking for balanced opinion on the relative merits of op amps isn't it?

mc phono pre requires discrete or xfmr front end to get acceptable noise performance

for mm cartridges some of the lower noise jfet op amps can be used

at line levels op amp noise isn't an issue, many applications can use op amp based circuits with measurable errors held to below the noise floor

many will still be using strawman examples from the 1st few generations of op amps from the 70's to "prove" the superiority descrete circuits in any audio application

technolgy has considerably advanced since then, if you want a contrasting expert opinion you might also seek out posts by Scott Wurcer for one


another intellectual line to pursue is perceptual psychology and psychoacoustics which suggest much "just listen" opinion is naive and likely overwelmed by other considerations than actual circuit properties
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Old 17th June 2011, 03:12 AM   #4
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I'm always curious about the why of things. The first thing I'd do is compare the RIAA response. It would be unusual if the two units matched closely. There's also the noise issue. The presence of broadband noise changes how we perceive response. I assume I don't even have to mention gain, but if one unit tends to get set higher than the other, it will affect the preference. Everybody says louder always sounds better, but I don't know if that's entirely true. Then there are things like output impedance and how it drives cables. And on and on.
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Old 17th June 2011, 03:23 AM   #5
W1DAN is offline W1DAN  United States
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Hi:

Thanks for the replies. I'll look at Nelson's section.

I should define this a little better. I am not worried about noise and assume the RIAA EQ differences are minimal.

I am looking for things such as distortion, Class of operation such as A and B. Is there a valid technical reason I should be working with discrete JFETS in an RIAA preamp if there is an excellent IC op-amp out there that can match the performance of discrete transistors?

Working with an IC is easy, discretes require more effort.

Thanks,
Dan
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Old 17th June 2011, 09:50 AM   #6
jcx is offline jcx  United States
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http://www.fortunecity.com/rivendell...o/aaphono.html

is one example that was designed by someone I would characterize as having "subjectivist" leanings with extreme tweaking tendencies


all op amp stages except the output typically operate Class A - the output may be ccs biased for Class A operation up to some load, Vswing requirement
I have used composite op amp circuit which operate the output push-pull Class A up to 400 mA - multiple op amps used in composite architectures for a "single function" can improve greatly over single op amp circuits
“unmeasurable” low distortion is possible with composite op amp circuits
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Old 17th June 2011, 09:56 AM   #7
AndrewT is online now AndrewT  Scotland
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have a look at Salas RIAA
many are building this.

It effectively uses two discrete amplification stages in place of two opamps.
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Old 17th June 2011, 10:20 AM   #8
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Originally Posted by W1DAN View Post
Working with an IC is easy.
Says who ?
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Old 17th June 2011, 10:31 AM   #9
EUVL is offline EUVL  Europe
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If you want to try your hands on a discrete JFET RIAA, I would suggest this (I did) :

All JFET open-loop RIAA pre-amp

You can replace 2SK389/2SJ109 with 2SK170/2SJ74 matched pairs.

The next best thing to 2SK389 / 2SJ109 ?


Patrick
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Old 17th June 2011, 10:36 AM   #10
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May help.

http://www.firstwatt.com/pdf/art_diy_opamp.pdf
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