Opa2228

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Hi Folks,

i am planning to replace NJM4580 opamps
against OPA2228 in a pro-ject phonobox.

Does someone know if the input stage of OPA2228 is npn- or
pnp-based? NJM4580 is pnp-based.

Do you think OPA2228 is a good drop-in replacement
for NJM4580 and the like? Any experiences?

Thank you in advance,

Stephan
 
Nisbeth,

are you sure about opa2228 beeing FET-Type ?

The datasheet doesn't mention it.
For all other Burr-Brown-Parts, which are FET-Type,
the datasheet explicitely mentions it.

Also, the input bias current is in the (very low) nano-Ampere
range, while the FET-Types lie in the pico-Ampere range.

a TL061 (FET) has 5 pA inbut bias current,
a LM833 (BJT) has 500 nA input bias current,
a OPA2228 has 2.5 nA

Also, this can be read in the OPA2228 datasheet, page 12,
Noise Performance:

The OPA227 has very low voltage noise,
making it ideal for low source impedances (less than 20kW).
A similar precision op amp, the OPA277, has somewhat higher
voltage noise but lower current noise. It provides excellent
noise performance at moderate source impedance (10kW to
100kW). Above 100kW, a FET-input op amp such as the
OPA132 (very low current noise) may provide improved
performance


All of the let me think the OPA2228 must be bipolar.

regards,
Stephan
 
Hi,

I like the OPA2228 a lot in my portable mic preamp.
I'm not sure if this will be a remarkable imrpovement, but the 2228 isn't expensive so it's fun to try.

Err, is that phono box a preamp? Do you know how big the impedance of the stage before the op-amp (probably the transducer) is?
Just because I read recently that MM transducers often have pretty high impedances, so a fet-input op-amp may be better in that case.

Grüße
Dominique
 
Dominique,

you are raising a good point.

If you have a Moving-Coil System, the source impedance
is usually yery low, in the range 2 Ohm-100 Ohm.
Probably for this case there are better opamps noise-wise.

If you have a Moving-Magnet System, the impedance is typically
1kOhm .. 47 (?) Kohm. OPA2228 is a very good opamp
for mid-range impedances. For high-range impedances,
probably a FET-Type opamp would be better, noise-wise.

So I think an OPA2228 looks like a good compromise
for a phono preamp which does both MC and MM,
and it seems do be a good drop-in replacement
for NE5552, NJM4580, LM833 and the like.

regards,
Stephan
 
Thanks for the numbers, it's good to know them!
Sounds like the OPA2228 would be a good compromise. Also one of the FET input op-amps with few voltage noise could be fine, like the OPA2132 which is also a dual OP-amp.

In any case, please post your results after you finished!
In the nearer future, I also wanted to build a phono preamp, maybe also with the OPA2228, but for now, I am fiddling around with my mic preamp and the mics...

Best wishes! :)
Dominique
 
Dominique,

in the meantime i've swapped the op-amps in my
pro-ject phonobox:

TL061 => OPA134 (this is a FET Opamp used for Actidamp in
the phonobox)

NJM4580 => OPA2228 (this is the amplification part)

Also, i adjusted a number of resistors and capacitors,
as the phonobox (in some older revisions, including mine)
is somewhat inaccurate.

Pleas refer to the schematics here:

http://www.angelfire.com/az3/dimitri/images/phonobox.pdf

DO NOT connect R9 to ground as it is suggested in the PDF,
but leave it as it is. R9|| R8 = 5003,8 Ohm, together with
C6 gives 75,06 uS time constant !!

From talks to the developer of the original design i learned
that R9, C7 are not meant as a low-pass Filter, but this
is an optimasation noise-wise! My thanks go to Pavel in
czech republic!!

What I did exactly is :

R5 : 1,32 MOhm, was 1,2 MOhm
R6, R7 : 220kOhm , was 330 kOhm
C4 : 0,47 uF , was 0,33 uF
R8 : 5.6 kOhm (selected), was 4,7 kOhm
C7 : 10uF, was 2,2uF
C6 : 15nF (selected), was 18 nF
R10 : 300Ohm, was 270 Ohm
R11 : 2,7kOhm (selected) , was 2,7kOhm
R12 : 2,2kOhm (selected), was 2,2 kOhm
R13 : 47KOhm (selected), was 39 kOhm
C9 : 68nF (selected)

Also I bypassed all electrolytics with WIMA MKS2 capacitors,
approx. 0,33 uF, and replaced the ceramic capacitors
(100pF, 27 pF) against better ones (Glimmer type,
sorry, I don't know the english word)

Altogether if gave a real boost to the phonobox, and in
comparisation to a Lehmann Black Cube the modified
phonobox does't made a bad figure in my setup!

In fact the results triggerd me to replace resistors /capacitors
in the Black Cube for better RIAA accuracy. It is much better
now, also all replaced parts were in their specified tolerance.

I am getting the impression that maximum RIAA accuracy
it most imprtant to good sound. Use selected parts, try
to get better than 1 % tolerance to the calculated
values. On resistors, you could use those (somewhat expensive) 0,1 % precision resistors. At the end, it does't cost
so much more compared to all the other efforts in an DIY project.

I do not think that the usage of "voodoo-parts" will
greaty improve the results.

Dominique, this is ofcourse only for tuning an
existing phono-box. For a new design, there might be
better choices of schematics and parts (including opamps).

Regards,
Stephan
 
even a cursory read of the data sheet will show you that this
is a bipoler-input op-amp, not FET-input. I don't know if one
should care whether it's NPN or PNP input as the input bias
cancellation circuit reduces the input bias to some small
delta around zero.
 
Thanks for the info, Stephan!

I looked at the schematic and indeed, I'd like to build something more simple! :)

Probably I'll simply build the rjm phono preamp.

At my usual electronic store (kessler-elektronik=simons-elektronik=cheap) they only have 2.5% tolerance caps, I guess that it's still ok...?

Regarding bypassing electrolytic caps with smaller film caps, I just tried this 2 times, once it resulted in a audibly better sound, but when I used a better electrolytic cap I couldn't hear any improvement using an additinal bypass... so I guess it depends on several parameters... I'll try and listen some more times.

I'm glad your work was successfull and you were rewarded with a better sound! :)
Happy listening!
Dominique
 
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