Reply
 
Thread Tools Display Modes
  #1   Report Post  
Posted to rec.audio.opinion
Bret L Bret L is offline
external usenet poster
 
Posts: 1,145
Default Interesting article on Altec 604 (The Marilyn Monroe of Speakers)

Jeff Markwart's Corner
Altec Lansing 604-8H Crossover Revelations

Altec Lansing 604-8H Crossover Revelations

"Enter the H model


In early 2004 I acquired a pair of 604-8Hs, the last ALNICO magnet
duplex model Altec produced before they switched to ferrite magnets.
It was also the first Altec duplex model that combined the Tangerine
phase plug with a constant directivity high frequency horn and a
crossover that utilized dual equalization of the compression driver
output. I swapped out my 604Bs for these Hs using the same cabinets,
and was immediately impressed by the differences in output - well
controlled upper mids, significantly more high frequency extension,
improved imaging detail and more apparent bass & mid bass. What was in
that Dual EQ crossover? Could it be adapted to older 604s as well?

Crossover Topology

I spent a few hours tracing out the crossover circuit, and enlisted
Jim Easley's help to organize it into a usable and readable format.
Jim did an excellent job.

Follow the link on the Vintage Altec 604-8H Schematic page to
download our reproduction of this vintage circuit in a PDF format. I
have also included a link for a Jim Easley supplied schematic of the
Vintage Altec Model 19 crossover circuit for comparison.

At the basic level the 604-8H filter is a parallel 3rd order HP and
2nd order LP filter, with L pad shelving of all horn frequencies when
the EQ button on the front panel is in the released position. Pretty
much like a stock 604-8G filter. However, when the EQ button is
clicked inward to the depressed position, the similarity ends. The
older style shelving circuit is disabled, an adjustable, frequency
dependent, contour circuit is pulled in for the upper mids, a high
frequency trim circuit becomes active, and a driver-side series
resistor further shapes the rising HF slope. At the recommended dial
settings the upper mids are appropriately depressed relative to the
high frequencies, and results similar to the famous Mastering Labs and
UREI 604 filter implementations are achieved. Unfortunately, this
Altec filter is not time domain corrected like the others - so I
naturally wondered just how would it sound if applied to my phase
corrected 604B filters?

MF EQ on a 16 ohm 604B?

Adding the mid frequency EQ function to my 604B circuit was pretty
straight forward. The old style shelving circuit was deleted and a
tweaked contour circuit was substituted in its place. I used a 7uf
capacitor instead of Altec's 6uf, and modified the resistor values
slightly to achieve a -3dB to -9dB MF EQ normal operating range with a
16 ohm speaker load. I chose not to add the high frequency trim
circuit or driver side resistor as I had found myself running the
604-8H HF control wide open. Yeah, yeah, I know what you're
asking.....but how did it sound?!? To make a long story short, I put
the 604Bs back into the cabinets and immediately heard all the
improvements listed above for the 604H, plus razor-sharp transient
attacks from rim shots and piano. The improved transients were
probably due to the phase correction through the crossover region.
Don't get me wrong - I'm not saying the B was now as good as the H! -
but it was much improved over its standard shelving circuit/L-pad
equipped brethren. It was still a bit too hot above 4kHz for my taste
however, so I added a 2 ohm series resistor after the contour circuit
to shape the rising HF slope a bit, and also lowered the resistance
value between P1 and ground to achieve a wider -5 to -15dB MF EQ
normal operating range. These two changes put the MF EQ's pot dial
near its midpoint for normal listening, and really dialed in the
compression driver balance above 4kHz to my ears. Feel free to vary
the series resisitor following the contour circuit between 0 and 3
ohms to find the the high frequency slope that suits your ears and
room.

Follow the Altec Lansing 604B Phase Correct Crossover with MF EQ link
to view the circuit schematic

MF EQ on a 16 ohm 605?

The previous paragraph applies as well to the Altec 605A or B,
although I didn't have either of these drivers on hand to audition. I
believe you can build it with confidence however, as feedback from the
first MF EQ equipped 605 phase-correct crossover constructor has been
positive. Or, if you have already built the original 605 crossover
contained in these pages, I urge you to remove the L-pad and
substitute the 605 EQ circuit components in its place.

Follow the Altec Lansing 605A/B Phase Correct Crossover with MF EQ
link to view the circuit schematic.

I have also added a note to this schematic concerning differing
component values for users of the early High Frequency voice coil/
diaphragm. See the Recommended Parts - Substitutions - Construction
Hints page for a bit more detail on this, and other topics, that you
may find helpful.


How about MF EQ on a 604-8G, or an 8-H, but simplified and with
better parts and no switch?

I've had several requests from 604-8G users for an 8 ohm version of
the MF EQ crossover. My solution was to simplify the 604-8H circuit by
eliminating the switch and Hi Freq EQ, substituting premium parts, and
drawing it up. Follow the Simplified 604-8H EQ Crossover link to view
an easy-to-build crossover for your 604-8G that includes the Altec MF
EQ circuit.

And finally, a phase-correct crossover for the 604-8H!

This one has it all for the 604-8H - follow the Altec Lansing Phase-
correct 604-8H Crossover with MF EQ link to view this circuit."

http://home.earthlink.net/~jmarkwart/id4.html
Reply
Thread Tools
Display Modes

Posting Rules

Smilies are On
[IMG] code is On
HTML code is Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Marilyn Monroe Jumps Right Out At You [email protected] Audio Opinions 0 May 7th 09 06:00 AM
Marilyn Monroe on the new dime Mark Steven Brooks Car Audio 6 April 23rd 04 11:58 AM
Marilyn Monroe on the new dime Bruce J. Richman Audio Opinions 2 April 19th 04 08:42 PM
Marilyn Monroe on the new dime Sander deWaal Audio Opinions 0 April 19th 04 02:17 PM


All times are GMT +1. The time now is 10:51 AM.

Powered by: vBulletin
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 AudioBanter.com.
The comments are property of their posters.
 

About Us

"It's about Audio and hi-fi"