Urei 811A Monitors and 6500 Amp

[joyspring]

Altec Fan:

Congratulations on winning those UREI 811As; they will easily be the best Altecs you will ever own.

Firstly, the OEM-300 Altec 604-8K drivers are hand-selected by UREI and will match better than any pair of branded Altec 604s. The UREI 800-series was one of the leaders of monitor technology at that time.

The 604-8Ks have ferrite magnets that do not demagnetise like the Alnico 604s do (consistency in a recording reference is important!).

And finally, the crossover is excellent and yields a flat on-axis response from a driver which is not particularly known for a flat response.

Unless you soffit-mount these 811As in quarter-space, expect to use a subwoofer as there's very little under 100hz.

"My question is this: Is the amp worth repairing? How do these things sound? I found documentation and schematics at jblproservice.com but I'd hate to spend a bunch of time fixing it only to be dissappointed that I did. At 275 watts per channel and 84 lbs the specs seem fairly impressive. Anyone ever listen to one of these?"

The UREI 6500 is an EXCELLENT amplifier and definitely worthy of repair! At 275 watts/channel, it'll make a very nice complement to the UREI 811As.

The `Conductor Compensation' circuitry is gimmicky (remember that these amps were introduced at the beginning of the `exotic cable' era) and you really do not need to use it though it will not hurt, nor will it make any sonic difference.

Simply use an appropriate wire to preserve the excellent damping of the UREI 6500 amp. I use 14 AWG zip in my studio for the 811As as the max. length does not exceed 10 meters.

Congrats once again and please let us know how the UREIs work out!

BobR

[Altec19]

Hi BobR,

Just listening casually I've noticed some interesting things with imaging and clarity. I'll need to spend some quality time after the snow flies (which may be real soon now).

I stripped the systems down and found a few things that need correcting. Two different diaphragms, woofers wired out of phase with the compression drivers, etc. These monitors spent their lives in a recording studio and have been repaired a time or two.

In looking at the crossover schematics the Urei woofers are shown as green positive and white negative which is opposite from how this pair was wired and opposite (I think) from the way Altec cabinets are usually color coded. But the schematic appears to be correctly drawn.

Also, the protection light bulbs are not the same from one cab to the other. The schematic says LF is a #1133 lamp and HF is a #81 lamp. You don't happen to have known correct lamps to verify those part numbers???

Here is an excerpt from a reply you made in an earlier post:

"Soffit-mounting in a typical studio is usually somewhere between pi - pi/2 (quarter- to eighth-space) and the UREI 813 is designed to use room loading to achieve flat response."

I understand the bass reinforcement that occurs with wall-floor or wall-ceiling placement but what does "somewhere between pi - pi/2 (quarter- to eighth-space)" mean specifically? Is this a calculation or measurement that is to be made? Can you elaborate?

As for the amp, I'll do some triage and see how bad it is.

Thanks much!

[Altec19]

Well, I'm back from a 2,200-miles-in-three-days Altec road trip. Visited with Bill at GPA, had him remagnetize a pair of 416B's I just happened to have with, and then on to Texas to retrieve a pair of Urei 811A Monitors (using 604-8K's) and a blown Urei 6500 amp. The amp and speakers have an interesting coax feedback connection to "null out" any adverse effects that may be occurring in the regular "zipcord" connection.

My question is this: Is the amp worth repairing? How do these things sound? I found documentation and schematics at jblproservice.com but I'd hate to spend a bunch of time fixing it only to be dissappointed that I did. At 275 watts per channel and 84 lbs the specs seem fairly impressive. Anyone ever listen to one of these?

Thanks!

[joyspring]

Altec Fan:

Here's the LF color code:

green - 604 LF red
white - 604 LF black

red - 604 HF red
black - 604 HF black

Reversing polarity will result in large variations in response up to two octaves below crossover but relatively little above crossover.

"Also, the protection light bulbs are not the same from one cab to the other. The schematic says LF is a #1133 lamp and HF is a #81 lamp. You don't happen to have known correct lamps to verify those part numbers???"

Those part numbers are correct. You can safely bypass the lamps - they were kind of a gimmick anyways and did relatively little to limit current, particularly to the HF driver when being overdriven by a heavily-clipping amplifier.

"I understand the bass reinforcement that occurs with wall-floor or wall-ceiling placement but what does "somewhere between pi - pi/2 (quarter- to eighth-space)" mean specifically? Is this a calculation or measurement that is to be made? Can you elaborate?"

`Freespace' (i.e. - a tower lift) represents 4pi free field; there is no loading from any adjacent plane.

Add a plane (a wall, for instance) and that's 2pi or half-space.

Add an adjacent plane well within a quarter-wavelength distance to the source and you yield pi or quarter-space.

Add another adjacent plane and that's pi/2 or eighth-space. This is synonymous with a corner in a room.

In a recording control room, the monitors are usually mounted near the junction of the front wall and ceiling, flush with the front wall; that's quarter-space mounting.

If the monitors are close to the left and right walls, that approximates eighth-space mounting.

UREI used a slightly overdamped sealed (or resistively-leaky `pressure controlled') enclosure for the 604. That's why the free-field response drops off so rapidly under 125hz.

The 811A depends on the loading afforded by quarter-space mounting to achieve (relatively) flat response below the free-space response roll-off, all the while retaining the excellent transient response of the highly-damped alignment.

This contrasts sharply with the 604-8G in the 620A enclosure, where the LF driver is tuned far below optimum and the LF transient response suffers.

Apart from the flat response, sharp imaging and excellent HF extension, the UREIs were especially noted for their tight bass compared to the 604-8G/620A combination.

Even if you do soffit-mount in quarter-space, I'd still recommend a subwoofer with the 811A. Recording engineer and consultant Bob Hodas has an excellent article detailing the benefits from adding a subwoofer system to a UREI 813-equipped control room:

To Woof Or Not To Woof? A SUBstantial Question

Still looking forward to your opinions on the UREIs.

BobR

[Mirrophonic Sound System]

Bob:

A. What is considered the best large-format studio monitor speaker these days ?

B. Do you agree with the concensus or do you prefer a different state-of-the-art monitor ?

[joyspring]

Mirrophonic:

"What is considered the best large-format studio monitor speaker these days ?"

Not too certain as I make no attempt to keep up with the latest gear; nor do I do much recording in up-to-date studios in LA. I understand that Questeds and large Genelecs are very highly rated `off-the-shelf' systems though I've never done extensive work on them. Many studios now opt for monitors custom-designed and tuned for specific rooms by the acoustic consultants/designers.

I do have my preferences, and they do change depending on the room and type of music.

For classical and small-combo jazz, I'm partial to the B&W Matrix 801 and the unfortunately no longer produced Waveform MC Solo. It's also unfortunate that both are way beyond my budget ;-)

For pop/rock, I like the JBL 4430s + sub, as the room I most often use is equipped with these. Excellent power response and excellent, uncoloured imaging across a large horizontal listening area.

For sound design, mixing, mastering and DVD-authoring at home (where I do the majority of work), I use both the UREI 811As and (borrowed) Meyer HD-1s. The latter are especially accurate self-powered nearfields and are extremely expensive (hence, I'm borrowing a set).

One thing to remember about large-format monitors is that their performance is ultimately dependent on placement and the room.

"Do you agree with the concensus or do you prefer a different state-of-the-art monitor ?"

Is there currently a consensus for large-format systems???

Thing are far different now than in the pre-1970s era when Altec 604s (and a few JBL 43XXs) were the only game in town.

I will say that I have been impressed by the few modern systems I've heard. Obviously, all can boast flat on-axis response (something that older monitors weren't necessarily capable of) and most have excellent off-axis performance as well, owing to flat power response.

These performance characteristics are reinforced by proper room/system interface as well -- while there is an `art' to studio design, many of the attributes that make a room `good' have been quantified over the past fifty years and fewer `bad' rooms are a welcome consequence.

The biggest noticeable improvement is transient capability. Obviously the direct-radiating components in many modern large-format monitors are less efficient than those in Altec/JBL/TAD/Westlake systems, but now 2-5 kW total amplifier power + subwoofer is fairly typical even in mid-size control rooms. The dyanmic range of 24-bit digital audio now requires this much power.

Even so, much improved material science (better adhesives, diaphragm materials/geometry, resonance/node abatement, etc.) offer high power handling and great sound, mitigating most of the gremlins of earlier horn-equipped systems, particularly horn distortion.

That said, I still like my UREIs which are still very high performance monitors in specific applications.

BobR

[scott fitlin]

The Urei 6500.

Considered to make phenomenal sounding bass, I have heard this amp many times! It does make great bass! These were used as an Upgrade to systems having Crown PSA-2,s! The amp has a nice big power supply, and sounds like more than its numerical rating of 275wpc@8ohms implies!

I know for sure its a clean sounding, yet musical sounding amp!

I thinks its worth fixing from a sonic standpoint, and it should serve you well.

[Altec19]

With regard to the UREI 6500: I'm making progress with the amp. This thing really is two completely separate mono amplifiers in one heavy duty chassis. In fact, the only thing shared between the two amplifier modules is the chassis and the AC line cord. I've got both power supplies online and am working to repair a scorched bias adjustment circuit in the one malfunctioning amp module. Then we'll see what else is broken . . .

[Altec19]

BobR:

I've been listening to the 811A's and grinning. I don't know how much of the midrange smoothness and coherency can be attributed to the 604-8K design and how much comes from the UREI crossover network but there is definitely something good going on there. I've been comparing with my Model 19's and my A7-500's. UREI bass is tight but punchy. Slightly more detailed, perhaps. The 19's will reach a little lower but I would hate to try and declare one "better" than the other. The A7's are more of a "broad axe" in comparison to the other two. Crude but effective and fun none-the-less. One has to make allowances for the intent of the design.

It's a dirty job having to make these comparisons but I'm going to do the best I can!

When initially going through the UREI's I found one bad pot and subsequently replaced all six. I'm finding that I prefer all of them set fully CW for my listening environment. Do you do much with your controls or just set them fully to one extreme?

[joyspring]

Altec Fan:

If you've had the opportunity to peruse the UREI crossover network schematic, you'll see that there's quite a bit of mid-HF EQ to tame and extend the Altec 604's non-flat frequency response:

http://www.jblproservice.com/pdf/Vintage JBL-UREI Electronics/UREI-811A.pdf

As I mentioned above, the UREI 800-series is NOT intended for free-standing operation but rather is intended for in-wall soffit-mounting. You'll have to add a subwoofer for flat response below 100 hz.

"When initially going through the UREI's I found one bad pot and subsequently replaced all six. I'm finding that I prefer all of them set fully CW for my listening environment. Do you do much with your controls or just set them fully to one extreme?"

You'll note in the frequency response plot included in the manual linked above that flat response results with the controls at full CW position, which is zero ohms for all three potentiometers (and also note the importance of soffit-mounting as well).

The original 5W 25 ohm pots and even the 12W Ohmite units of the later A- and B-series networks all become intermittent with airborned environmental contaminents.

I bypass the pots altogether. Let us know what you decide to do.

BobR