604 Conundrum

I just got referred here from the Audiokarma forum. . . lovely place!

A couple years ago, I inherited a pair of Altec 604 speakers in 9 cubic foot baltic birch cabinets from a retired electrical engineer in Silicon Valley. I'm a musician, songwriter and producer of independent label artists, so I was fascinated to dip my toe into the aural world of the famous 604 sound.

Except for a strange frequency response difference between the left and right speaker, I thought they sounded amazing. The midrange detail was unlike anything I've ever heard. I was told the speakers themselves were 604 B and from the late 1940s.

Then things got more interesting. When I opened the cabinets, I discovered one speaker was, in fact, a pristine Hollywood 604B with beautiful N1000-B crossover network.

The second speaker turns out to be an Altec Duplex 604 20 Ohm, with no letter demarkation and a serial number in the 3400s. It is beautiful, has an accompanying N1000-B crossover network.

So, now that I know what I have. . . what should I do?

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Originally Posted by Lofiluther

Except for a strange frequency response difference between the left and right speaker, I thought they sounded amazing.

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Can you be more descriptive as to frequency response difference? Is it with music or test tones or both? Since you have identical crossovers you could swap them to see if there is a problem with one of them. And welcome to the forum!

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Originally Posted by Lofiluther

So, now that I know what I have. . . what should I do?

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check the phase?

Sounds like the 2nd one is an original 604 and you would expect it to sound different from the 604B, no?

I would guess the 604B sounds better but the original might be worth more?

Dunno what I'd do.

I haven't tried switching the XOs yet, but what I've noticed is that the 604B seems to have extended range at the top and bottom of the frequency spectrum, while the 604 20 ohm displays more of a prounounced midrange presence. Again, this is what they sound like through the N-1000B crossovers which are likely 60 years old, so . . .

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Originally Posted by Lofiluther

So, now that I know what I have. . . what should I do?

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Hey Lofiluther, I see you made it ! I would keep them ! If you do sell them do not sell them cheaply they are worth probably several thousand dollars each or more!As stated in the AK thread the Crossovers should be rebuilt.And obviously the one speaker doesn't have the original Crossover as it is 16 ohm and the speaker is 20 ohm.I'm linking your post from AK for the pictures.I will PM Steve Schell, he should know more about these particular drivers.

AudioKarma.org Home Audio Stereo Discussion Forums - View Single Post - Altec Hollywood 604 Conundrum

All The Best ~ AB

Lofiluther I once had an original 604 in a 605A (fish hoops grille) cabinet that was a bit older than yours, serial #3385 I think. It had the N-1000B network, so evidently they changed the network before going to the B series driver. I've never studied the 604 series changes in depth, but they sure jumped around with impedances and especially crossover frequencies. The first Duplex, the 601 field coil unit, had a 12 ohm, 1200 cycle crossover. The very first 604 units also had a 1200 cycle crossover, but were changed very soon to the 2000 cycle. Anyone wanting to research in depth can find relevant catalog pages here:

ALTEC CATALOGS

Thanks Steve for your insight's on these ! ;)

My pleasure, AB. Lofiluther's comments on perceived frequency response of the two models are indeed interesting. The 604s in general were known for an exaggerated upper midrange response. In this catalog page (probably more like 1947 than 1949) the response graph looks pretty hot from 3kHz. to 6kHz. Nevertheless 604s were the standard in most recording studios for decades and everyone accomodated the eccentricities. When JBL tried to crack the monitor market in the 1960s they at first contoured their response to imitate the 604.

http://www.lansingheritage.org/image...1949/page3.jpg

From the text it is hard to tell exactly what changes were made from the 604 to the 604B. The crossover frequency was halved, the voice coil impedance was lowered, yet they seem to imply even more. The lower impedance might provide better top octave performance due to the lower voice coil inductance. A friend of mine has studied/collected/repaired 604s extensively; I'll have to pick his brain about all this stuff.

First you must decide : do I want to listen to them or do I want to sell them. If it is to sell them then fix the XOs and leave the speakers alone. XO recap ; replace the large paper caps with poly's. and leave the bath tubs alone. When you have the XOs open check to see if they are the same.

Pick one speaker and listen to the XOs they should sound alike. If you do not have any test equipment ,listen to some pink noise.
Now should you want to keep your 604s and listen to them , fix the XOs send the speakers to GPA for reconeing, and re-magnetizing .
When the speakers are rebuilt they can probably be built as 604Bs. I recommend against the option of making them for bi-amping,
because of resale. At this point, you will have a pr of 604s .

Don