Forthcoming Opamp review

[CaptBubba]

I'm going to spend the next few few weeks getting together a review of a few different opamps to hopefully help people decide what to use.

I have and plan to review the following:
OPA134a
OPA227
OPA228
AD843
AD845

I will use the same setup for all of them. A multiloop amp (same type as META-42) using 1 BUF634 per channel in high bandwidth mode The opamps are single package DIP-8. 17.4V unregulated power supply with a high b/w buf634 as the ground driver. I hopefuly will be able to configure it to test chips in both class-A and normal situations, but I'll have to match up some JFETs and figure out how to switch class-A on and off.

I have just quickly listened to the opa134a and the AD873, and my first impression that the AD873 is more detailed, and the opa134 has a warm, smooth sound. I will say no more until everything is done. I will use both SACD and redbook discs.

Wish me luck.

 

[CaptBubba]

Ok, I finished up the amp, this is what I'll be using to test with.

The two pins by the opamps can be jumpered to put the opamp into class-A operation.

Ack, A bit of reflection there, but you get the idea.

 

[tangent]

FWIW, there's no problem switching the class A bias in and out while the amp is running. Hooking your jumpers up to a DPST switch will give you to clearest picture of how the sound changes when the bias is present vs. absent.

By the way, it's "843" and "845", I suspect, unless you're pioneering some new chips for us.

 

[ppl]

Thanks tangent i was just going to search the AD site for these but i think you also noticed the Typo.

CaptBubba> if your going to compaire the AD-843 & AD-845 with and without a current source the Curent must be at least 5 Ma since these deviced are heavaly Biased in the Output stage and Walt Jung sugested to me that a current source is not needed in the AD-845 since it is allrady Class A. I have found this to be true as current loading changes the sound of the AD-845 but i don't think it made it any better. I also did mini reviews of Both of these Opamps and a search will show what i think of them as well as what i think of the OPA-132. However i look foward to seeing what you think of these.

 

[Budgie]

Is that a Sesame Street amp? I see it includes the alphabet and counting.

 

[tangent]

No, it's an amp-by-numbers. Like with paint-by-numbers, except that instead of "19-blue", it's a bit more complicated: "A6-red-black-black-gold".

 

[geom_tol]

Quote:

Originally posted by CaptBubba 17.4V unregulated power supply with a high b/w buf634 as the ground driver.

Why will you use an unregulated supply and ground driver?

 

[CaptBubba]

Quote:

Originally posted by geom_tol Why will you use an unregulated supply and ground driver?

Because I really hate voltage regulators. They just screw with the sound in many ways to my ears at least. I didn't use batteries because I didn't want to worry about charging the batteries before each listen as it would just be one more variable to take into account. It isn't true to say the supply is unregulated, it is actually choke regulated, but it does not have a traditional voltage regulator. You can see the PS on the left of this picture:

Thanks for pointing that out tangent, I just got a bit ahead of myself typing I guess. I also know that it is possible to switch between class-a and unbiased without turning off the amp, I plan on using the dual outputs of my ns500v to play the same channel through both sides of the amp, compairing between the two chips, in addition to listening to all of them for a very long time, so quick switching is not that important.

ppl, the current source I have right now is biasing the chips at 8.84ma and 8.89ma for the left and right channels, respectivly.

Budgie, you inultin' my amp? You looking for trouble or somethin'? Really, I don't pay attention to the letters, but I did use the grid marking to make sure I could fit both channels into that space.

 

[geom_tol]

Quote:

Originally posted by CaptBubba Because I really hate voltage regulators. They just screw with the sound in many ways to my ears at least. I didn't use batteries because I didn't want to worry about charging the batteries before each listen as it would just be one more variable to take into account. It isn't true to say the supply is unregulated, it is actually choke regulated, but it does not have a traditional voltage regulator. You can see the PS on the left of this picture:

That's fair enough. But what about the ground driver? It messes with the sound to...

 

[CaptBubba]

Not really, it just keeps the power supply from becoming unballenced when a lot of current is drawn from it. All it is is a buffer for the virtual ground.

 

[dta116]

I for one would be interested in seeing the schematics on this.
Think you could post 'em.

'Preciate it.

 

[ppl]

I agree with CaptBubba, I also hate Voltage regulators of the IC type however while some of the new ones show promise thay are only rated for less than 1 amp of current. I would not have used any regulators at all in this test not even just the One. I would have insted used a battery pack or two like a pair of those 9.6 volt RC car packs or a 16 Cell AA Ni-MH. After all to test an opamp you must have a power supply that is pure. BTW CaptBubba, I would not go any higher than what you are using for a current source. Also are those TO-220 BUF-634's on the Output? what no heat sink at high Bias?

 

[CaptBubba]

dta116, are you asking for a schematic as in the general connections in the amp or do you want a layout diagram of how I pladced everything in this amp?

ppl, I have a few 1800mAH RC packs, but I'm not going to use them. The amp sounds very clean, and I would be stupid to think Icould eliminate all the variables. Istead I'm opting for a mid-range approach. My thinking was basically "if someone has $150 to spend and wants to make their own amp, what chip would be best suited to their needs?". In reality I just want an excuse to listen to a lot of music

Its not like I'm driving speakers with the things, they get just warm while in use. The opamps (in class-a) get warmer than the output buffers,which isn't saying much. I need to upgrade the transformer though as it is getting borderline hot, which I don't like.

 

[dta116]

Capt' Bubba,
The schematic will do, I can see the layout in your post.
I am interested in both the PSU and the op-amp prints.

Thanks.