OT: Gpa

**LICORNE** · March 4th, 2015, 04:14 PM

How is GPA doing these days !

Should I consider making a purchase now just in case of the future!

As you know Altec 416 B and Altec 6048g have te lowest FS and QTS meaning tey reproduce very low frequencies and have a big magnet . A driver with a low Qts of around 0.20 would have a large magnet and be able to move the cone with a lot of force. This makes for a tight driver. A driver with a Qts of 0.45 would have a smaller magnet and less control over its cone

Plus they are underhung voice coils.

**joyspring** · March 5th, 2015, 03:36 AM

Originally Posted by LICORNE

As you know Altec 416 B and Altec 6048g have te lowest FS and QTS meaning tey reproduce very low frequencies and have a big magnet .

Depends on the alignment. Generally very low Qts implies either a weak, drooping passband response or a high f3 for maximally flat alignment.

A driver with a low Qts of around 0.20 would have a large magnet and be able to move the cone with a lot of force. This makes for a tight driver.

Which implies a higher frequency response and suitability for horn loading. It's usually detrimental to achieving low f3 in flat response sealed or vented enclosures.

A driver with a Qts of 0.45 would have a smaller magnet and less control over its cone

Qts is affected by far more than magnet weight or gap flux density. And alignment+tuning in concert with Qts dictates the amount of control *necessary* in the frequency range of interest.

Plus they are underhung voice coils.

Which necessarily limits undistorted output level at low frequencies as excursion quadruples with each lower octave.

FWIW, I use extremely low-Qts 604-8K-equipped UREI 811As with properly-aligned high-Qts SUBWOOFERS.

BobR

**604man** · March 6th, 2015, 04:29 PM

You can adjust the driver "Q" with a series resistor , An Ohmite wire wound
rheostat comes to mind...

On another note, while digging through AK threads on on 604's, An author stated
that the horn acts as A rudimentary phase plug for the bass cone.

Check out Lowther Loudspeakers The type PM4
has a plug fitted for what I would assume pattern control .

.....GC

**LICORNE** · March 8th, 2015, 01:47 PM

Originally Posted by 604man

You can adjust the driver "Q" with a series resistor , An Ohmite wire wound
rheostat comes to mind...

On another note, while digging through AK threads on on 604's, An author stated
that the horn acts as A rudimentary phase plug for the bass cone.

Check out Lowther Loudspeakers The type PM4
has a plug fitted for what I would assume pattern control .

.....GC

Did he mension if plastic or metal made a difference !

..........

**604man** · March 8th, 2015, 07:10 PM

No mention was made if the horn was plastic
or metal or what vintage ..

It would be interesting to compare the 515 pattern Vs
the 604 LF driver pattern if they correlate or not .
............GC

**joyspring** · March 8th, 2015, 09:14 PM

Originally Posted by 604man

You can adjust the driver "Q" with a series resistor , An Ohmite wire wound
rheostat comes to mind...

For a LF driver? That's a lot of power to dissipate even through a large VR... not to mention added inductance from the winding.

Originally Posted by 604man

On another note, while digging through AK threads on on 604's, An author stated
that the horn acts as A rudimentary phase plug for the bass cone.

Check out Lowther Loudspeakers The type PM4
has a plug fitted for what I would assume pattern control .

Have heard of Lowther loudspeakers before but never have had a close look. Whizzer cones... at a price. Interesting.

The plug has no effect on pattern control -- rather, it addresses the fact that the driver is tasked to reproduce frequencies that have wavelengths half (or smaller) of the diaphragm diameter. The plug reflects out-of-phase waves converging in front of the driver outwards preventing destructive interference and attendant frequency response anomalies, same as a compression driver.

It would be interesting to compare the 515 pattern Vs the 604 LF driver pattern if they correlate or not .

The 515 is intended to be crossed-over at 500 hz so its increasing directivity will match the horizontal coverage pattern of a 90 deg. horn.

The poor match between the LF and HF sections has always been a problem for our 604s.

FWIW, David Gunness of Fulcrum Acoustics (formerly Eastern Acoustic Works) has been addressing this amongst other coaxial issues via DSP (TQ):

Temporal EQ | Fulcrum Acoustic

Building a Better Coax | Fulcrum Acoustic

Presonus Sceptre range of control room monitors leverages TQ DSP:

PreSonus | Sceptre

Nifty technology and these monitors do sound great! That said, I still prefer the far less expensive JBL LSR 305s.

BobR

**604man** · March 18th, 2015, 01:06 AM

Originally Posted by joyspring

For a LF driver? That's a lot of power to dissipate even through a large VR... not to mention added inductance from the winding.

If you are using a tube amp , these already have a high output Z . a series resistor
would be unnecessary.

A small value series resistor may be needed to trim the Q with a SS amp.

Check out Rod Elliot's article on output Z. Impedance, and how it affects audio equipment

Ideally , driver Q would be measured through your speaker leads + through your
crossover + interconnecting leads to the driver. I have a 604 E that I would like
to run tests on... When I have the time.

Wire wound sandbar resistors, wire wound L pads, wire wound cathode bias resistors
and wire wound emitter resistors have been used for years .
Is it a problem ? yes it could be.

Have heard of Lowther loudspeakers before but never have had a close look. Whizzer cones... at a price. Interesting.

The plug has no effect on pattern control -- rather, it addresses the fact that the driver is tasked to reproduce frequencies that have wavelengths half (or smaller) of the diaphragm diameter. The plug reflects out-of-phase waves converging in front of the driver outwards preventing destructive interference and attendant frequency response anomalies, same as a compression driver.

It would be interesting to see patterns with and without the plug and patterns
on a 604 with and without the horn . This data may exist in some engineering log book.
it may be insignificant. This speculation .

The 515 is intended to be crossed-over at 500 hz so its increasing directivity will match the horizontal coverage pattern of a 90 deg. horn.

It is generally accepted that the 515 and the 604 LF are one in the same. I don't
know if patterns exist to confirm this.

The poor match between the LF and HF sections has always been a problem for our 604s.

True..

FWIW, David Gunness of Fulcrum Acoustics (formerly Eastern Acoustic Works) has been addressing this amongst other coaxial issues via DSP (TQ):

Temporal EQ | Fulcrum Acoustic

Building a Better Coax | Fulcrum Acoustic

Presonus Sceptre range of control room monitors leverages TQ DSP:

PreSonus | Sceptre

Nifty technology and these monitors do sound great! That said, I still prefer the far less expensive JBL LSR 305s.

BobR

BobR Thank you for your input.......GC

**Alien_Shore** · March 19th, 2015, 06:32 PM

Originally Posted by joyspring

The plug has no effect on pattern control -- rather, it addresses the fact that the driver is tasked to reproduce frequencies that have wavelengths half (or smaller) of the diaphragm diameter. The plug reflects out-of-phase waves converging in front of the driver outwards preventing destructive interference and attendant frequency response anomalies, same as a compression driver.

This is interesting!
Bob, do you know if there are any research or design materials available that go into this in more detail?