opamp rolling?

There was a recent thread started in DIY regarding opamp substitutions that caught my interest.... http://audiokarma.org/forums/showthread.php?t=72508

Many of you will recall that I have a Kenwood KA-7100 amp that has been modded by EchoWars, and now contains an output buffer circuit between the pre-amp and amp stages. I believe it currently uses an OPA2604 chip.

Here is the original thread describing the KA-7100 http://audiokarma.org/forums/showthread.php?t=37748&highlight=opa2604

Anyway, it got me thinking about whether an opamp substitution would have any effect on the sound of the amp.

Thanks to Dantana I happen to have a couple of potential substitutes onhand (OPA2132PA and OPA2132P) and was thinking of giving it a try?

First... I need to verify that these are fully interchangeable, and that swapping them would not have any adverse effects on my amp.

Secondly, can anyone offer any insight into whether I can expect to hear any differences? In this case I believe the opamp is set up for "unity gain" so that should not be a factor.

Echowars, I'd particularly like to have your input before I dare open the hood and consider swapping chips.

Thanks!

Henry

Here are the data sheets for the OPA2604, OPA2132PA and OPA2132P

OPA2604
http://focus.ti.com/lit/ds/symlink/opa2604.pdf

OPA 2132 PA and OPA 2132P
http://focus.ti.com/lit/ds/symlink/opa2132.pdf

First answer is that the pinouts of each chip are the same. The power supply ranges on each chip are similar (+-24 V for OPA2604 and +-18V for OPA2132). They are different, but it shouldnt' matter because whoever put them together surely wouldn't have the 2604 at it's max rating. If you look at the specs, the OPA2132's have a lower distortion, yet a smaller gain bandwidth (20 Mhz on the OPA2604 vs. 8 Mhz for the OPA2132). They both have almost the same CMRR (around 100 dB -- Common Mode Rejection Ratio -- the willingness of the amp to reject signals that are common to both input -- IMPORTANT for audio, you want a high rating). The rest of the specs are similar all around. I don't see any reason to switch them out, frankly, I think the OPA2604 is a much better choice. As far as i can tell, you could safely switch them out, but I would not even bother because to me it looks like you'd be taking a step down.

Of course, Echowars surely knows more about this than me, so listen to his opinion over mine. I just wanted to give my 2 cents

There is a huge thread in diyaudio.com that talks about opamp rolling that was the impetus for the opamp thread in referred to in DIY. Here is the link to the diyaudio thread:

http://www.diyaudio.com/forums/showthread.php?threadid=66526&highlight=

If we assume that manufacturers make different opamps that are truly dofferent and to cater to specific needs, then one subbing one opamp for another should yield different results. Whether these result are sonically apparent and significant is another question.

I bought a bunch of opamps to try in an Elliott preamp board. I figure that whether there is a difference or not, I will still have learned something. And we all know that education costs. But ignorance costs more...

The only other appropriate opamp to use in that 7100 would be the OPA2132 (or the OPA2134, which is essentially the same model in a budget package).

The variable here is the supply voltage. In all honesty, I can't remember where I set the supply voltage for that OPA2604. That opamp has the ability to run off of very high supply voltages (+/- 24V), which very few audio-appropriate opamps can do. The OPA2132 (and '34 variant) tops out at +/-18V. If that is where I set the supply voltages (18V or less), then by all means, experiment. If the voltages are above 18V, then I fear I have left you stuck with what you have, unless the zener reguator is changed to lower the voltage.

Understand that the OPA2604 is a whole 'nother animal at these high voltages. The complaints you see about this opamp are generally from those who are running it at 15V and lower, where it is just a 'good' opamp. Above that voltage (and ideally at 20V or even a tad more) the OPA2604 really sings.

The 2604 also has some of the best DC precision of an audio opamp, equalled (for this application and in the DIP package) only by the OPA2132, which is why I chose it, so I might use DC coupling at its output.

Those are the reasons I chose that chip, and also why I chose the high voltages. Before a swap is made, you need to measure the current voltages, and see if you might get away with using something else.

Echowars...

According to your comments in the original thread on the KA-7100 mods, the voltage was set at 18V.

Obviously I should measure actual voltage to verify before doing anything else.... looks like I have another item to add to my "projects for a rainy day" list.

Well, I could have dropped the voltage to +/-15V or so and allowed easy swapping of opamps, but I wanted to show how good the 2604 is at higher supply voltages...IMHO better than the 2132 at the lower voltages. Several pieces of my own gear use the 2604 at +/-22V.

For an existing circuit where voltage is already set at something lower than +/-15V, I'd recommend the 2132. For a buffer, as the 2604 is in the 7100, the 2604 at a higher voltage is very hard to beat.

Also consider that since the opamp is being used in a 100% feedback situation (unity gain buffer), I'd be totally suprised if you were able to discern any difference at all regardless of what opamp you put in that position.

Do know that opamps using bipolar transistor input may not be totally suitable in the design of the buffer I used in your 7100, as it is DC-coupled at the output, and the 7100 amplifer is a DC-coupled FET input. The higher input bias current of a bipolar input stage opamp can cause the output DC to rise to (what I consider) to be unacceptable levels. Not an issue with an amp using an AC-coupled bipolar input stage, but certainly an issue with a DC-coupled FET input stage such as is used in the 7100.

I have been reminded that the Analog Devices OP275 would also be suitable. It can run from +/-22V supplies, and although its input bias current is 10,000X what the 2604 is (0.1µA vs 10pA), offset voltage is listed as about the same as the 2604 (about 1mV). It is supposed to be unity gain stable (certainly important, since we are using it in a unity-gain application), and can be bought in an 8 pin DIP package.

Next project I will likely try one out.