EL84 with opamp driver?

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EL84 with opamp driver?

Postby ro7939 » Sat Sep 06, 2014 6:52 pm

I humbly and respectfully request one of the designers here post whatever comments they please Re. a new power amp with this opamp driving dual EL84s: http://sparkoslabs.com/discrete-op-amps/ I need only 15-20W.

Roger Modjeski makes dual EL84 with SS input. I suspect SS driver has strong potential to outperform tube driver.
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Re: EL84 with opamp driver?

Postby Geek » Sat Sep 06, 2014 9:03 pm

ro7939 wrote:I suspect SS driver has strong potential to outperform tube driver.


Depends on your tastes and how hard the load is to drive.

As for discrete opamps, opamps are designed and tweaked to the nines... hard to beat their performance. Maybe add a discrete jFET front-end to an already spectacular opamp (like a 5534) will rock the socks off a tube for low-level, low noise stages like a moving coil phono input, but to beat an existing OPxxx, LTxxxx or ADxxxx with discrete is I think a bit of a pipe dream.

:2c:
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Re: EL84 with opamp driver?

Postby Sparko311 » Sun Sep 07, 2014 5:43 pm

Hi. I am the designer of the SS3602 discrete op amp. What questions did you have?

Discrete op amps can outperform AD, LT, etc in numerous ways, including output current (drive) capability, higher internal bias currents and higher class A bias in the output stage. These things are due to the fact that discretes can dissipate more power than their ICs counterparts. They also allow for far better compensation capacitors and compensation schemes. Capacitors are difficult to fabricate in ICs, and the dielectric they are made from is inferior to a discrete NPO or C0G capacitor. With discretes - one can use 2 pole compensation which allows for vastly higher loop gain (140dB out to 600Hz) that can be sustained deep into the audio band. IC op amps can not do this because they are mostly all single pole compensated due to the fact that they can not fit an additional large value capacitor inside the IC that is required for a 2 pole scheme. This comes with several drawbacks that are explained in the following white paper (if anyone is interested) -

http://sparkoslabs.com/audio-op-amps-ga ... bandwidth/

Jfets typically have higher noise than BJTs. I would think putting a JFET input stage on the front of a BJT input op amp would worsen the noise performance. The SS3602 uses a BJT input and has a noise of 2.9nV / Root HZ in 20KHz bandwidth. This is superior to most "audio grade" IC op amps.
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Re: EL84 with opamp driver?

Postby Geek » Sun Sep 07, 2014 8:46 pm

Sparko311 wrote:Jfets typically have higher noise than BJTs.


N-jFET, yes. Consider P-channel....
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Re: EL84 with opamp driver?

Postby Shannon Parks » Wed Sep 10, 2014 7:17 am

Geek wrote:
Sparko311 wrote:Jfets typically have higher noise than BJTs.


N-jFET, yes. Consider P-channel....


Sadly P-channel JFETs are going the way of the dodo. Stockpile now - or rather years ago before the lowest Rds ones disappeared. :'(

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Re: EL84 with opamp driver?

Postby Geek » Thu Sep 11, 2014 12:32 am

Actually I Googled some university physics papers mentioning several modern N-jFET's and even MOSFETs as being a fraction of the noise of BJT's.
(I didn't share the link because it was HUGE and wouldn't post right. Use "jfet noiser than bjt?" as the search term)

But nevermind that, just Google for low noise MC phono preamps to see complete circuits with them :$
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Re: EL84 with opamp driver?

Postby EWBrown » Sun Sep 14, 2014 2:35 pm

Tube op-amps:

http://www.computerhistory.org/revoluti ... position=1

GAP/R Vacuum Tube K2-W Operational Amplifier

Image

Image

Image


It used two 12AX7s.

These units were readily available for dirt cheap as surplus in the late 1960s / early 1970s.

/ed B
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Re: EL84 with opamp driver?

Postby Shannon Parks » Mon Sep 15, 2014 10:49 am

Definitely pick up one of those Philbrick op amps if you see one! Super cool technology and collectible.

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Re: EL84 with opamp driver?

Postby Geek » Mon Sep 15, 2014 5:54 pm

Those were the first things I thought of, Ed.... but I couldn't find my bookmark at the time (sick)
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