Altec 604-8g recone question

[jaaronb]

Happy New Year!

Quick question for the Altec crowd here:

I have (and love) a pair of 604/8gs, both of which have been reconed by Bill and co. at Great Plains Audio within the past few years. They're currently in original 620 cabinets that I've sealed and rehabbed as best I can.

I've been running Markwart 604-8g simplified phase-corrected crossovers to mostly great effect, although at times I feel like I'm still getting a bit more upper MF than I need, even with the mid-range attenuating L-Pads turned to about 9 oclock, sometimes lower. I may raise the series resistor from 3uf to 4uf and see how that works. Anyway, that's not why I'm posting today.

Wondering about the LF/HF balance on these speakers, and how to get a bit more on the low-mids got me thinking:

If I'm running recently GPA reconed 604/8gs, am I really even running 604-8gs on the 15 anymore?

Markwart's simplified crossover calls for a 1mh series inductor and 25uf paralleled cap on the LF circuit for the 604-8G, for the 604-8H, it calls for a 1.7mh inductor and 16uf cap - for the exact same circuit.

Is the 15" on my reconed 604-8G still a 'G', or is it now closer to an H, or something completely different?

Have I been missing something this whole time using the network for an original, unaltered 604-8g?

I can't go into the new year not knowing.

 

[GordonW]

The difference between those two L/C pairings, is primarily damping and/or characteristic load impedance.

They are virtually exactly the same frequency- but the 1mH/25uf pairing looks to be made for a LOWER impedance woofer, than the 1.7mH/16uf combo, all else equal (same woofer construction other than the impedance). A lower impedance woofer needs a smaller inductance coil, and a higher value cap, to have the same damping at the crossover (resonance) point, as would be used on a higher impedance woofer.

It's just a matter of what the effective impedance is, on the newly reconed woofer, compared to the original 8G.

That would be a good thing to ask the people at GPA...

BTW: The 1.7mH/16uf combo WOULD act to reduce the highest frequency output of the woofer, compared to the 1mH/25uf combo. Given the same woofer impedance, it has higher damping, which reduces the energy at resonance (which makes the woofer "roll off" a bit faster as it gets close to the crossover frequency). This could be what you're hearing- the current L/C value pairing doesn't have enough damping with the the current woofer effective impedance... but the increased inductance and reduced capacitance of the other pairing, might have the correct (higher) damping for the current woofer impedance...

Regards,
Gordon.

 

[jaaronb]

The difference between those two L/C pairings, is primarily damping and/or characteristic load impedance.

They are virtually exactly the same frequency- but the 1mH/25uf pairing looks to be made for a LOWER impedance woofer, than the 1.7mH/16uf combo, all else equal (same woofer construction other than the impedance).

It's just a matter of what the effective impedance is, on the newly reconed woofer, compared to the original 8G.

That would be a good thing to ask the people at GPA...

BTW: The 1.7mH/16uf combo WOULD act to reduce the highest frequency output of the woofer, compared to the 1mH/25uf combo. It has higher damping, which reduces the energy at resonance (which makes the woofer "roll off" a bit faster as it gets close to the crossover frequency). This could be what you're hearing- the current L/C value pairing doesn't have enough damping with the the current woofer effective impedance... but the increased inductance and reduced capacitance of the other pairing, might have the correct (higher) damping...

Regards,
Gordon.

Thanks so much for this. I'll definitely give Bill a call. I don't think I have the correct value inductors on hand to tack the 604-8H LF section onto my crossovers.

 

[jaaronb]

The 1.7mH/16uf combo WOULD act to reduce the highest frequency output of the woofer, compared to the 1mH/25uf combo. Given the same woofer impedance, it has higher damping, which reduces the energy at resonance (which makes the woofer "roll off" a bit faster as it gets close to the crossover frequency). This could be what you're hearing- the current L/C value pairing doesn't have enough damping with the the current woofer effective impedance... but the increased inductance and reduced capacitance of the other pairing, might have the correct (higher) damping for the current woofer impedance...

This is especially interesting and I do wonder if this is the...objectionable....thing I've been hearing (particularly since I rehabbed and sealed the cabinets and moved them into a new room.

In particular, check out the 1-1.5k crossover region here, you can tell when where I have the mids attenuated after the crossover, but the bump *at* the crossover region is pronounced. These were taken at the driver...I think my bass issues are room/placement related.

 

[GordonW]

That is very close to the exact area, that would be affected by the Q (damping) of the woofer lowpass- just above the electrical crossover point for the woofer...

Regards,
Gordon.

 

[jaaronb]

That is very close to the exact area, that would be affected by the Q (damping) of the woofer lowpass- just above the electrical crossover point for the woofer...

Regards,
Gordon.

Well I ordered a couple of the 1.7mh air core coils from Parts Express. Should have them by Friday. I'm still planning to call Bill tomorrow to see what he has to say about the specs of the reconed G

Regardless, I'll be trying the LF portion of the Markwart crossover for the 604-8H.

Now I wonder... will changing to the LF section of the H network mess up the phase alignment of the crossovers?

 

[GordonW]

Now I wonder... will changing to the LF section of the H network mess up the phase alignment of the crossovers?

Well... if, by doing this, you're putting the damping in the correct range- if it's acting as a minimum-phase filter (which it it SHOULD be), then by correcting that part of the transfer function, theoretically, you SHOULD be correcting the phase as well. In short, it may be CLOSER to optimal phase...

Regards,
Gordon.

 

[jaaronb]

Well... if, by doing this, you're putting the damping in the correct range- if it's acting as a minimum-phase filter (which it it SHOULD be), then by correcting that part of the transfer function, theoretically, you SHOULD be correcting the phase as well. In short, it may be CLOSER to optimal phase...

Regards,
Gordon.

Outstanding. I'll have the correct coils tomorrow and will report back as soon as I get them installed. I haven't had a chance to call Bill at GPA yet.

 

[elitopus]

I’d be interested to hear what Bill at GPA has to say to your question. And I’m also curious if you have emailed Jeff Markwart to ask him.

I’m in the same boat as you (same speakers and crossovers) and am thinking maybe I should make the change as well. Pls update the thread with your listening impressions when you get the new coils installed

 

[jaaronb]

I’d be interested to hear what Bill at GPA has to say to your question. And I’m also curious if you have emailed Jeff Markwart to ask him.

I’m in the same boat as you (same speakers and crossovers) and am thinking maybe I should make the change as well. Pls update the thread with your listening impressions when you get the new coils installed

Will do. I have not emailed Jeff - I'll try to do that today. Hopefully I can also get ahold of Bill. I'm about to send him a couple 416-8Bs to refurb anyway.

 

[elitopus]

Jeff has always been super helpful when I’ve emailed him with questions about his crossovers. And he helped me work out a few changes to another crossover I was working on.

It seems like the Markwart xovers are the standard nowadays versus the factory xovers. Most people who are posting about 604 and wanting to get the best sound are building his xovers. With the different options for the 604, it would definitely be good to know which is best for the application. And with these drivers getting older, there are sure to be more and more GPA recones.

 

[Nyjblbob]

Jeff has always been super helpful when I’ve emailed him with questions about his crossovers. And he helped me work out a few changes to another crossover I was working on.

It seems like the Markwart xovers are the standard nowadays versus the factory xovers. Most people who are posting about 604 and wanting to get the best sound are building his xovers. With the different options for the 604, it would definitely be good to know which is best for the application. And with these drivers getting older, there are sure to be more and more GPA recones.

I’ll be interested as well
Waiting to have mine Reconed with Gp cones

 

[jaaronb]

Ok... Interesting results, and not positive on the whole. The 604-8H LF network actually seemed to lower the overall bass output somewhat. So while it did seem to improve the 1200-1600 bump some, it also diminished bass output and articulation to a degree I found unacceptable.

So my main question now is, what's the best way to go about trying to reduce this MF bump?

Could I sightly push the HF crossover point higher? Or spread the MF attenuation freq band 'wider?'
Which HF component values would I change, and in which direction, to achieve this?

I feel like these would be just about perfect if I could tame this narrow errant bump.

 

[GordonW]

What's the DCR of the 1.7mH choke, compared to the 1.0mH choke?

Wonder if a difference there, is tripping it up...

It may be that the 1.7mH/16uf combo is TOO much of a change. Maybe something like a 1.25mH and a 20uf, or something of the like, may be the right balance...

Regards,
Gordon.

 

[toddalin]

So my main question now is, what's the best way to go about trying to reduce this MF bump?

eq.

 

[Nyjblbob]

eq.

Either that or hours and hours trying to get it just right
Oh yeah Who has dozens of Chokes coils and Caps to try out other than speaker companies

 

[jaaronb]

So I went back to the original 604-8g crossover specs. I swapped in some 1mh air core inductors in place of the copper foil, which sound a bit better on the LF to me. More heft.

I also installed 8 ohm L-pads into the front of the HF circuit, so I'm able to attenuate the HF, and hardwired the mid-range attenuation circuit.

They sound incredible - orders of magnitude better than any of the previous high dollar/hi-fi speakers I've owned in the past.

Now I'm moving on to a 416-8b/Onken/802/811b project, using Model 19 crossovers (eventually I'd like to do the Hiraga/multicell 500hz crossover).

I'll probably keep the pair I like the best and sell the other. I'll also throw the 604 into the Onkens to hear how it sounds.

As much as I love what the 604s do, I miss the impact of the 811b horn (properly attenuated), at ear level, as it will be positioned with the Onkens.

I'll start a new thread for all that.