API 560 vs Urei 535

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dandeurloo

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So, I am a real novice at this stuff. But I have a Urei 535 and have been looking over the schematic.  I am wondering what the difference is between the Urei and the API.  They share the same freq. bands.  I am curious what mods could be done to the Urei to make it sound more like the API.  I am guessing that the API uses a bunch of IC's as well specially with the size of the unit compared to the Urei. 

Any thoughts.
 
With equalizers not only should the filter frequency centers be the same, but also the bandwidth affected by that EQ aka Q.

There is no accepted standard for how Q is defined for peaking type EQ sections so unless you use the same circuit topology and component ratios they will sound different.

JR

Note: I contacted the AES standards committee about this lack of standard Q definition. They agreed and forwarded it to a sub committee. They later bounced it to another committee and I haven't heard anything in over a year.
 
I would think that if the output was re built like an API and maybe a few IC upgrades it would sound pretty similar.  Does anyone know of good upgrades for the old tl074 quad chips in the urei?  I have a friend I think who can help me with the API outputs.  Which would add the 2520 flavor.  I am assuming the 2520 on the API 560 eq is in the output section of their eq and the rest of the opamps are IC's.  I can't find an API 560 schematic to compare.

As far as Q. I don't see how the Q is set.  Is that from the tolerance of the actual cap filters and resistors?  I was thinking about upgrading a few of the caps and sizes in the power supply as well.  Any other caps that could use an upgrade in these eq's? 





 
The Q or bandwidth is the spacing between the LF and HF skirts of the bandpass filter that generates the boost/cut.  The filter shape (Q ) will surely make a bigger difference than just the opamps used, unless those opamps are total crap. I suspect the API path is pretty good, so after you get the filter shapes the same, evaluate the Urei path for good signal integrity.

JR

 
who knows?  Here is the manual and schematic if anyone is interested.

http://www.dandeurloo.com/DIY/UREI_535.pdf
 
I've got a 535 as well (or is it a 530 ?!?... I'd have to check).

a Q factor of 1 is pretty common.
(The bandwidth is variable in regards to the centre frequency, which means, that the bandwidth will get larger when the centre frequency rises).

You can read more about the Q factor in wikipedia :
http://en.wikipedia.org/wiki/Q_factor

So... I'm curious for the schematic of the 560. The fact that both EQ's use the same bands doesn't mean that they have the same circuitry. You can't turn a behringer composer compressor into an Avalon either just because they have the same contols, right ? (a question :  don't use all the octave EQ's the same frequencies ?!? )
 
Nice !

Well, the filtersection is really identical, with the exception of ALL values. In other words, if you're willing to replace all components (including the sliders), then you're able to do an almost complete copy without cutting or adding tracks. I say "almost complete", because the API uses 4558 dual opamps, and the Urei TLO84 quad opamps. But, both are in the same league, and making these high quality wouldn't make any difference to the sound since they're used as active inductors.

The amplifier section between the 2 eq's are different though.
The input section of the API is way simpler, and uses only 1 opamp of high quality (LF356). The 535 has an additional input gaincircuitry, so, this inputcircuit uses 4 opamps (TLO84).
For the make up gain, the 535 uses 3 (TLO84) opamp stages for the filtersection, the API uses 2, but from a higher quality (LF356).
And the outputstages use a 2520 in the API, and a TLO84 plus a pair of transistors in the 535.

Now back to your question of changing the circuitry of the 535 to a 460.

Personally, I wouldn't do that, but a recap and changing the opamps in the line amplifier will make a difference.
Especially because the TLO's are cheap stuff. There are a variety of opamps available that can be swapped with the TLO's. The TLE2074 / 2084 could be used, OPA 404, OPA 4134 , etc. 
I'd keep the TLO's where they are used solely as active inductor (IC 2 and 4), and replace all the others. You could also think about replacing the output circuit with the API 2520 output circuitry.

And I just checked my Urei EQ : It's a 530, it uses the different opamps RC 4136......

But, looking at the API schematic, it should be easy to make a DIY project out of it !
 
well i just bought a number of the OPA 404.  I will do those mods and the API 2520 output stage.  Should be a pretty big difference. 

Any bigger cap values i can change?
 
Q is the ratio of reactive to real energy in a resonator.

Q is the ratio of center-frequency to bandwidth.

In most audio, it is simplest to use John's definition, "spacing between the LF and HF skirts", whenever it gets out of AES subcommittee.

Q < 1 is very mild. Q => 1 is ringy.

In an octave-centers EQ, you can argue for Q of 0.8 or 1.5, but if it isn't close to 1 it will be goofy.

> I don't see how the Q is set.  Is that from the tolerance of the actual cap filters and resistors?

You don't "set" something with tolerance.

In a simple L-C-R resonator, you find the L-C resonance frequency, where reactance of L and C are equal, and compare to the resistance. (Some adjustment for high Q, which need not matter here.)

The fake-inductor plan is only slightly trickier, especially if there's numbers on that thing. The "C" is the first cap off the +/- slider, which is marked with a frequency, so we know the reactive impedance of C -and- L from inspection. About 9K. The L-C-R tank resistance is both series and parallel: the to-ground 240K resistor, the 3.9K series resistor, and the boost/cut network. This can be solved easily, but not on one cup of coffee.

BTW: unless AES comes back with an odd definition of Q, the Q of this EQ changes with +/- setting. That's almost universally true, and euphonious. This box's "1-octave" bandwidth is only true at the specified +/-8dB setting. Q is somewhat higher at full boost/cut. However "all" general-purpose octave EQs do it this way, and professional EQs tend to similar extremes. (I've seen $99 consumer Graf EQs with Q>3, to be sure the buyer heard the effect.)

> Urei to make it sound more like the API.

Change the faceplate. That has a major effect on the sound of studio gear.

Can you list the ways the two sound different?

> I think the ICs are complete crap.  Hahaha

There are newer, more fashionable, opamps. And '084 can be abused, with high gain, high signal level, and low load impedance. But the Urei did not fall into these traps. And a LOT of GOOD gear, vintage and modern, uses '08X amps similarly without disgrace.

The output amp, where the heavy work happens, is well-buffered and unstrained. Also (unless you run it into less than 500 ohms at over 10V peak) the chip runs class A here.

Measure DCV across R56 and R57, you want at least 25mV across each one. I suspect they won't be equal (OT DCR); that's probably not important.

> the API uses 4558 dual opamps

The 4558 amp is very significantly slower than the '084. 4558 is kin to 741 which gave chip opamps a bad name in audio.

I'm wondering if you LIKE the slow 4558's mush on rapid drum transients. 4156 is in the same class, will replace TL084, but is now rare.
 
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