Opa2228

[schnullimaus]

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]

The OPA2228 is FET-input no bipolar. Also, remember that the 2228 is only stable with a gain of more than 5. If the application requires lower gain, use the 2227 instead.


/U.

[schnullimaus]

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

[Nisbeth]

Well I was sure that it was FET-input but now I'm not After re-reading the datasheet it certainly looks like you're right. Sorry!


/U.

[Dominique]

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

[schnullimaus]

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

[Dominique]

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

[schnullimaus]

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...s/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

[BrianL]

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.

[Dominique]

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