[Sampsa]

I'm thinking of upgrading the phonostage in my integrated amp. I'm already replacing the op-amp (most likely to OPA637) and associated caps. But would it make sense / is it feasible to upgrade the RIAA correction resistors and caps?

[Dan Banquer]

Use 1% tolerance or better on the resistors and caps. Metal Film RN55C or Vishay Bulk Metal Foil resistors (these are expensive!). Caps in the signal path should be either NPO Ceramic, COG, or a metalized film for low Dielectric Absorption. Warning! if you start straying from the 1% tolerance you may find your frequency response straying also.
             d.b.

[Sampsa]

Thanks! Do you have any suggestions as to the capacitor brands? Also, what kind of sonic changes should I expect from this?

[Dan Banquer]

Roederstein  has a good reputation for film caps. There are others but their names escape me at the moment. For resistors, any RN55C classified resistor should be the same for this application except for some foaming at the mouth audiophiliacs. If your present phono stage uses carbon resistors I would expect a significant change in the S/N ratio when you change to metal films, or vishay bulk metal foils. If your present phono stage has capacitors that are not tight tolerance or have poor Dielectric Absoption spec's than I would expect a better frequency and transient response.
                 d.b.

[Sampsa]

Thanks! Those changes sound good. If I change the RIAA correction resistors and caps together with the op-amp (from JRC5532 to 2 OPA637s) and the electrolytics (from very cheap Teapos to Elna Cerafines), will the phonostage improve dramatically?

The amp in question is Musical Fidelity A300 integrated, by the way, and I'm modding a whole lot of other things in it as well (pre-amp op-amp from MC33079P to OP467, power supply caps, rectifiers, possibly pre-amp regulators, etc.).

Here's what Soundstage reviewer said about the A3.2 integrated, which most likely has the exact same phonostage:

"Based on the quality of the rest of the integrated amp, I expected big things from the A3.2’s phono stage, but unfortunately the results were only very good, and not excellent. Admittedly, my low-output Roksan Shiraz isn’t exactly the type of budget cartridge that’s likely to find a home with the A3.2, but it’s what I’m used to. The Shiraz’s 0.3mv output is on the low side, but the A3.2 had plenty of gain and was very quiet, with no hiss and negligible hum. That’s a big plus in my book.

Bass via the A3.2’s phono stage was good, with the utmost extension missing in action, but the region just above, say 50Hz, was as firm and taught as the rest of the integrated's sound. The basic character of the A3.2, namely its slightly lean, dynamic upper bass and midrange, carried through via the phono section. While the A3.2 imaged superbly though the line stage, the phono section presented instruments in much more of a two-dimensional manner. Some of the holographic depth that I value so highly was missing in action in comparison to my Sonic Frontiers tubed phono stage. Also in absolute terms, the highs took on a grainy character that added an aggressive edge to percussion and female vocals."

Here's what Stereophile's measurements said about the A3.2 integrated, which most likely has the exact same phonostage:

"Phono frequency response (fig.1), again measured at the preamp outputs, was characterized by a slightly shelved-down treble, with the ultrasonic output rolling off above 20kHz. Given the wide frequency range covered, the treble shelf will be audible as a slight warming of the sound. The MC phono overload margins were good, at 24dB at 20Hz, 18.1dB at 1kHz, and 17.1dB at 20kHz, though slightly less good via the MM setting at 15.5dB, 14.8dB, and 15.1dB, respectively, due to the higher-than-usual gain. The phono stage's A-weighted signal/noise ratios were excellent, at 84.4dB (MM) and 68.7dB (MC), though the unweighted, wideband figures were less good: 68.7dB and 56.4dB, respectively."

[Dan Banquer]

I would put in 8 pin dip sockets where the op amps go (if this is through hole) and experiment with the OPA637's and the AD797's. The electrolytic supply decoupling caps shouldn't matter much; just make sure they are bypassed with a 0.1 uf film. Your frequency response should be close to the orignal.
            Good luck;
                    d.b.

[Sampsa]

Thanks! I already put in a socket and am currently using an OPA2134. The OPA637s should be arriving soon and I'll have to get a BrownDog adapter (single-to-dual). The AD797s seem like worth a try also.

[andy_c]

Careful with the OPA-637, it's only stable with gains of 5 or more.  Without having seen the schematic of your phono preamp, I can't say whether or not this will be a problem.  If the configuration is the same as this circuit: http://home.mindspring.com/~andy_c/PhonoPreampDesign.htm then it's necessary to have R4 >= 4 * R3.  Many phono preamps have R4=0 which requires unity gain stability.

[Sampsa]

Very interesting page! Thanks, andy_c. I have to dissect the design to see what values they have. I'm getting OPA627s just in case.

After a brief skim of your page, it looks like the non-ideal characteristics of the op-amp can have an effect on the equalization. Could the above-mentioned divergence of my phonostage be due to the crappy 5532 originally in there?

[andy_c]

Oh, the 5532 is a dual op-amp, while the opa627/637 are singles.  You'll need to find a dual op-amp with a compatible pinout.  The 5532 datasheet is here http://focus.ti.com/lit/ds/symlink/ne5532.pdf .  The OPA2604 might be a possibility http://focus.ti.com/lit/ds/symlink/opa2604.pdf

Whether the op-amp contributes to the equalization error depends on the configuration of the circuit.  If it uses passive equalization, then the op-amp has a negligible effect on equalization accuracy.  It's the active equalization as described on my page that's susceptible to these types of errors.  Not knowing what type yours is, it's impossible to say.  If you could reverse engineer it to get a schematic, that would show what's going on.

[Sampsa]

I plan on using BrownDog single-to-dual converters to get two single-channel op-amps into the space of one dual-channel. Standard practice. I'll have to reverse engineer it.

[andy_c]

Oops, I didn't know about those adapters.  That's a cool product idea - gives a lot more flexibility in op-amp choice, that's for sure!  If it does turn out to be active equalization, the AD797 that Dan recommended has about the lowest op-amp-induced equalization errors around.  The huge DC open-loop gain (146 dB typ) will give very good low-frequency equalization accuracy, while the very large gain-bandwidth product (over 100 MHz) will give very accurate high-frequency equalization.  The OPA627 has also has a large DC open-loop gain of 116 dB, but its gain-bandwidth product is lower at 16 MHz.  The disadvantage of the AD797 is that it's difficult to get stable becaus of its wide gain-bandwidth product.  The 5532 has a gain-bandwidth product of 10 MHz, but I can't find a spec for its DC open-loop gain.  In terms of objective performance, the 5532 is actually no slouch, but it is an old design from the '70s.  This site http://www.dself.dsl.pipex.com/ampins/webbop/opamp.htm has some op-amp distortion measurements.  The 5532 actually does quite well in the distortion tests.

[Sampsa]

Yep, the BrownDogs are nice. Some others, like Audiocom sell complete packages with decoupling capacitors included.

Regarding op-amps, someone else suggested INA103. What do you think of it? Also, do you know anything about quad op-amps? I'm replacing a quad MC33079P in my pre-amp and thinking of using OP467. I'd love to use AD8004, but it requires much lower supply voltages than the current +/- 15V.