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Old 19th February 2003, 11:02 AM   #1
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Default a heretical preamp

ok.. its sure im certainly in for some flaming with this.. but i cant do elsewise.. i have built an opamp preamplifier (that is, with ic's...) here it is: what can i do to get it better? (only gainstage shown.. supply ordinary LM stuff) resistors cheap metal films hand matched to 0.1% (holco's on the way), decoupling through Elna RJH. pot alps 27mm.. so?
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i am currently using 60,000 uF of capacitance in my preamplifier..
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Old 19th February 2003, 11:17 AM   #2
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I would recommend 10-100 nF polyester 63 V or ceramic, SMD as a complement to 120 µF. In fact it's more important with small high speed caps than big electrolytic. OPA627 is rather fast and therefore you need decoupling which is fast.
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Old 19th February 2003, 06:10 PM   #3
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Default Re: a heretical preamp

Hi,

Quote:
ok.. its sure im certainly in for some flaming with this.. but i cant do elsewise.. i have built an opamp preamplifier (that is, with ic's...) here it is: what can i do to get it better?
I could think of a few things....

Howzdat for freaky?

Click the image to open in full size.

Basic design is an amalgam of stuff I played around with before but with a battery supply, PSU where also tried at some point in time, in a Phonostage. Circuit drawn by Carlos M.

Recommend placement of the feedback resistors (all Op-Amp's) as SMD Resistors soldered directly at the pins. Powersupply is per supplied chip, wiring between regulator out and supplied chip must be minimal and no supply decoupling should be applied. If critical apply a 100pF SMD COG Cap between the positive and negative supply of the amplifier chip.

Pot should be ideally 1K log which is difficult at such low values. Alternatively a 220 Ohm Linear should work okay with a 470 Ohm "build out" resistor, please use cermet track pots if you can, if not carbon. Also, then you need to compensate the gain loss. Maximum output Z with 470R & 220R Pot is around 200R, with 300R & 1K Log it is 230R, both at maximum volume.

Sadly, transformer based attenuators as passive linestage are notably better still....

Sayonara
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Old 19th February 2003, 06:33 PM   #4
Helix is offline Helix  United Kingdom
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Default Re: Re: a heretical preamp

Quote:
Originally posted by Kuei Yang Wang
Hi,
........... no supply decoupling should be applied.

why?
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Old 19th February 2003, 07:04 PM   #5
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Default Re: Re: Re: a heretical preamp

Hi,

Quote:
why?
I thought that would be obvious. The individual Op-Amp used as "regulator" does not tolerate much capacitive load. Up to around 100pF will be okay, much more and the thing will oscillate at VHF frequencies....

Sayonara
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Old 19th February 2003, 07:43 PM   #6
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Default Re: Re: a heretical preamp

Quote:
Originally posted by Kuei Yang Wang
Hi,



I could think of a few things....
The schematics is slightly wrong. The LM431 is a little bit wrong connected. Cathode upwards and anode downwards.
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Old 19th February 2003, 08:09 PM   #7
Helix is offline Helix  United Kingdom
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Default Re: Re: Re: Re: a heretical preamp

Quote:
Originally posted by Kuei Yang Wang
Hi,



I thought that would be obvious. The individual Op-Amp used as "regulator" does not tolerate much capacitive load. Up to around 100pF will be okay, much more and the thing will oscillate at VHF frequencies....

Sayonara

sure, there would be a problem with capacitive load.. but i would put on as much as i could get away with say 68pf

also if u have time.. please discribe your thoughts on those input FETs
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Old 19th February 2003, 08:35 PM   #8
halojoy is offline halojoy  Sweden
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Default Re: a heretical preamp

Quote:
Originally posted by costiss
ok.. its sure im certainly in for some flaming with this.. but i cant do elsewise.. i have built an opamp preamplifier (that is, with ic's...) here it is: what can i do to get it better? (only gainstage shown.. supply ordinary LM stuff) resistors cheap metal films hand matched to 0.1% (holco's on the way), decoupling through Elna RJH. pot alps 27mm.. so?
You are doing fine, costiss!

Very good OPamp OPA627 (peranders has just bought some)
Metalfilm matched resistors
ELNA caps
ALPS potentiometer

And I am sure you will get a good Power Supply.
You already got the best boys as assistance!

Congratulations!

/halo - wish he had some Good Components at home
- maybe time to place an order????
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Old 19th February 2003, 10:58 PM   #9
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Kuei,

Thanks for an interesting design. I have two questions about
the regulator part.

1) It seems reasonable that all the regulators share the
circuitry connected to the positive inputs (ie. the reference
voltage circuitry). Do you agree on this?
2) Would you consider it sufficient that they also share the
capacitors on the preregulated rails, or should these be
per regulator and as close as possible to them?

Just like a previous poster I am also a little bit curious about
the JFET buffer. I assume the purpose is to make the input
impedance invariant of the attenautor. However, I see that
they use a fixed Vgs of 0V and have no feedback. I assume
this particular topology is intended to be very linear, but I
do not quite see why or how (although it is a bit late and I
am tired, I admit). I also guess they have to be well matched.
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Old 19th February 2003, 11:16 PM   #10
Previously: Kuei Yang Wang
 
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Hi,

Quote:


1) It seems reasonable that all the regulators share the
circuitry connected to the positive inputs (ie. the reference
voltage circuitry). Do you agree on this?
Yes. If you need for example for a Phonostage two gainstages per channel you can use dual Op-Amp's, assuming the layout remains tight.

Doing this on PCB's almost literally means using two PCB's arranged with the two soldersides touching, locating each regulator chip output pin directly next to the supply pin driven by it. In short, think 3D not 2D.

Quote:

2) Would you consider it sufficient that they also share the
capacitors on the preregulated rails, or should these be
per regulator and as close as possible to them?
The latter. The smalles (physical & value) capacitor closest to the chip then the larger ones. The PSU Reg Chips do need to be stabilised against oscillation by having their supply pins well decoupled.

Quote:

Just like a previous poster I am also a little bit curious about
the JFET buffer. I assume the purpose is to make the input
impedance invariant of the attenautor. However, I see that
they use a fixed Vgs of 0V and have no feedback.

Well, the input impedance IS invariant of the Attenuator, but as the circuit is following the SPEEDO Axiom and the INVERTING Axiom the input impedance is rather low. The two Idss @ 15V matched (and thermally connected) J-Fets simply buffer the low input impedance of the Op-Amp and provide a high impedance input, in fact pretty much as high as you like (1M no sweat)

Quote:

I assume this particular topology is intended to be very linear, but I do not quite see why or how (although it is a bit late and I
am tired, I admit). I also guess they have to be well matched.
Actually, the topology is not intended to be very linear, it actually is. Driving 3V RMS into 1k will give around 0.008% THD, or better noted -82db THD, and this THD is rather low order and thus of low subjective sonic impact.

Yes, one could have used an OPA627 connected as follower, but somehow this simple J-Fet buffer struck me as more "Zen", just as the J-Fet MC stage on the Phonostage that this linestage was matched with (namely the "analogue addicts phono") seemd a better idea than a gain of 20 AD797. I guess it's a question of "designers fingerprints".

Ciao T
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