Presonus Eureka Mod

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Presonus released a 'Hot Rod' version of the Eureka which has the following mods done

http://www.audiomidi.com/Eureka---Special--Hot-Rod--Edition-P10583.aspx

There is also a thread over here about modding it

http://www.gearslutz.com/board/new-product-alert/181827-hotrodded-presonus-eureka.html

I have been offered an Eureka for a very low price and am tempted to buy it and do the mods.
I already own a Tampa, DMP3 and Yamaha MLA-7 and am unsure if there would be any point in buying the Tampa and modding it?
 
Hi,

I'm considering pick up a really cheap secondhand one of these because I quite fancy a channel strip to go with my Focusrite ISA One digital, GAP PRE73 and M-Audio DMP3.

I'm also interested in hearing various peoples experiences with different mods and whether it makes a big difference?
 
I noticed that in the Eureka HotRod they use the Linear Technologies-1357CN8-ND opamp.

How does this compare to the Burr Brown OPA 627 opamp?

Which would be the best opamp to use in the Eureka?
 
Five year correction: OPA134 is not quieter than NE5534. Doesn't really matter much in this amp though, because of the input transformer.
 
I was speaking to one of the guys from Presonus who has tried various opamps in the Eureka and he said that the LT 1357 are the quietest ones to use with the Eureka and reduces any noise with the Eureka even more!

Will be trying this when the Eureka arrives!

Also may try the full Hotrod mod at a later date ( Jensen Transformer + various caps & resistors changed ). Could be interesting!
 
Be aware the limiting factor should ultimately be the input transformer. The Jensen in question (I believe the stock generic trafo was a copy) has thermal noise of about -121dBV with a 150 ohm source. LT1357 has noise of 8nV/Hz^0.5, which translates to -119dBV. So I would seriously question the conclusion that LT1357 leads to the best possible noise performance; clearly a quieter amp would get closer to realizing the transformer's potential.

That said, -121dBV summed with -119dBV yields -117dBV, or -120dBA, less transformer gain of 10dB is EIN of -130dBA. That is plenty quiet.

Still, there is too much voodoo involved in these swaps. I mean, given that the transformer has limited frequency response, what is the point of a 600V/usec amp? Quite honestly, I think I just insulted voodoo . . .

Y'know, I have very successful used LT1167 (different topology, an instrumentation amp, won't work here) in a product that proved quite popular. For those that choose amps based on speed, that fact should be shocking.
 
Hi mshilarious,

Thanks for the reply and info.

Just been reading this post and this guy says that the OPA 627's make a noticeable improvement!?

seveone said:
Now, The Eureka with the mods chips (OPA627AP), didn't seem at first like a night and day difference. Until, I A/B'ed the two Eurekas. HUGE difference. The low end is really well rounded and not muddy at all. The mids seemed clearer, but not OMG! clearer. The highs poped out as much as the low end, really clear. overall much more clear and punchy. The EQ and Comp really work that clear signal. Almost as big of a difference as A/B'ing the board to the original Eureka.

Overall this experience was painless - AND FREE. I'm not an electronics know-it-all. Really pretty ignorant. But I know how to change out chips. Just like upgrading a proccesor, or swithing RAM, just smaller. If anybody is interested in doing this, Texas Instruments sent 2 samples of each OPAMP that I wanted, overnight, early morning delivery - FOR FREE. Can't beat that. I almost paid $18.46 a piece plus delivery just on the OPA627AP's. And based on my results, I would have been very happy doing that.

It'll be a little while before I post samples. I'm not a bass player, and nobody needs to hear my voice. In a couple weeks I'll have something.

Thanks for the help mshilarious!

7
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From what I've been told the OPA627 is a noisier chip than the LT1357.

What I don't want to do is make the Eureka any noisier with the newer chip. How does the OPA 627 compare to the stock chip?

How does the Eureka compare to the DMP3 noise wise with and without chip upgrade?
 
I just heard from one of the guys at Presonus and he mentioned that the OPA 627BP is a low noise version of the OPA 627 AP!

Would the OPA 627BP be a better option? Is there anything better out there? Does THAT make a drop in replacement for the NE 5534AP?

I will hopefully get some TI free samples for the OPA 627BP chips to see how these sound!
 
You ought to read the datasheets, it's all in there. AP: 5.6nV/Hz^0.5, BP: 5.2nV. 1357: 8nV. 5534: 3.5nV. So all of the chips being discussed are noisier than 5534, and the 627BP is especially expensive for its 0.6dB improvement--it is intended more for applications that need extremely tight input offset voltage, which you don't really.

Again, the transformer should make even 8nV noise not an issue, but that will clearly be noisier than the stock 5534. If you are experiencing a problem with noise, it isn't because of the mic amp section. Could be the mic, could be the EQ/comp. Could be something else entirely . . .

Edit: OK, you want a recommendation? Try OPA1611. But I'd also check Analog's site to see what they have these days . . .
 
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Thanks for the info. To be honest I'm not really sure what these specs mean?

When I was recommended the LT1357 by the guy from Presonus I was told that this would improvement be noise performance?

I've actually only just got the Eureka so I don't have any real idea as to what it's capable of yet so the 5532's may fine but if it's possible to make any performance improvements by upgrading the opamps I would like to do so?
 
Well, I hate to say it but if that was the comment by the guy at Presonus, he's just flat out wrong. LT1357 will degrade noise performance by about 3dB (this is assuming that the Presonus has the transformer feeding the opamp, it's been so long since I owned one I don't remember the circuit). I could explain noise analysis, but it's a long explanation, and the bit above about the transformer noise is the first step.

5534s aren't bad, really. You have to realize there is like a cult of opamp swappers that is very lean on actual measurement methodology. I didn't know that five years ago, as you can see from this thread. That said, I think it was Samuel Groner that did a very detailed analysis of opamp distortion, and there is certainly better than 5534; but again, does it matter given the rest of the circuit, especially the transformer? These things have to be evaluated in the circuit under test.
 
Spoken to the guy a Presonus again and he said that whilst the 5534 is quieter on paper he said there was noticeably less noise when they put the LT1357's in?
Am going to find out if the transformer feeds the opamps.
 
I've found out that the transformer does indeed feed the opamps but apparently putting in the LT1357's does improve noise performance!?
 
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