Tuning 604 Project - High Q and High Output Impedance?

[Seadweller]

Greetings,

Having been an audio nut for 30+ years, I finally decided to take the plunge, and build my own pair of speakers...They are based on the Great Plains Audio (Altec) 604 coaxial driver, along with a tested cabinet design. I'm using the stock Great Plains crossover, which was evidently modeled for a cabinet of a similar baffle width, but I'm still struggling with the "tuning" of the system. Let me say right off the bat that I'm a complete novice, but I've done quite a bit of research to try and educate myself. I now have more respect than ever for speaker designers! It’s certainly a far cry from putting a driver in a box!!

With that said, I still need to be fed information at the kindergartner level. What I am faced with is a system that has what seems to be the typical baffle step issues. Output from the vocals up is extremely forward and overwhelms the rest of the frequency spectrum, to the point of being “shouty.” I have experimented with a baffle step correction circuit, as detailed by the very kind and generous Martin King, but I don’t believe it applies to a two-way driver, as I’m experiencing odd results (vocals become recessed, but the remainder of the spectrum remains elevated).

A couple of folks have mentioned that the 604 driver is “high Q,” and designed to be used with an amplifier with high output impedance. The cabinet design evidently was designed around this parameter as well. Sadly, I am in the dark with this.

My current solid state amplifier is an obvious mismatch for this system, so I am wondering if moving to a tube amplifier with high output impedance would change the character of the system? If so, how (in laymen’s terms if possible)?

Thanks so much!!! Despite my lack of knowledge, I'm having fun, and am determined to make the system right!!! There's magic hidden within this system that I want to break out...

[Seadweller]

My apologies....

Evidently, the 604 driver has a LOW Q, not a high Q....

[Scottmoose]

Right. Most SS amps have a high output impedance (or low damping factor if you prefer). With a low Q driver, which is essentially over-damped in the bass, you will have a relatively weak LF response, because there's only so much the cabinet can do to boost things, and it's not getting any help from the amplifier. Shifting to a lower output impedance / higher DF amp will bring things back into line. Meantime -you could try sticking a resistor in the hot lead to the driver, which will artificially lower the amp's DF a bit.

In the long term, you might want to change your amplifier -doesn't have to be to tubes -Nelson's First Watt F3, or a couple of his F4s with a high-gain pre-amp spring to mind.

604s. Oh well, one day, when I'm rich.

[GM]

I believe you mean tube amps and shifting to a higher output impedance/lower DF for low Qts drivers since SS amps typically have an output impedance < 0.1 ohms, ergo are best suited for > 0.5 Q systems........ http://www.audioasylum.com/forums/hu...13/131306.html

GM

[Scottmoose]

Er, yes. Ye Gods, I can't believe I wrote that! This is what happens when I'm not concentrating -I shall now go & iron my hands in punishment. Not one of my better days, this. Thanks for picking up on it Greg!

[Seadweller]

Thanks everyone.....

Is there a straightfoward way to determine the critical damping factor of the driver (or my system in total) in order to identify the proper amplifier match?

Once identified, I would assume that I could calculate the optimum output impedance, correct?

Or, am I looking too far into this? Do most tube amplifiers have sufficiently high output impedance to accommodate?

Thanks again!

[MJK]

By using a BSC filter, you have already simulated the impact of the low damping factor that can be achieved with a tube amp. Above a few hundred Hz, you essentially have a DC resistance downstream from your SS amp in series with the driver. This is the effect you would be looking for in a tube amp. However, your DC resistance is adjustable since it is not amp design dependent.

The other property of a tube amp that might be of assistance is a rolled off high frequency response. This might help eleviate a painful top end. At one time I had an old Adcom GFA 555 amp which was a very dark sounding amp. At the time, my Focal inverted metal dome tweeters sounded very nice. When I switched SS amps to a newer vintage the tweeters sounded too bright.

In effect, you could tune your speaker further by selecting a tube amp. But which one? I would recommend trying before buying to make sure that the tube amp's high frequency properties compliment the speakers high frequency response.

Or, if you want to experiment some more add a low pass filter at some higher frequency to roll off the highs gradually. This would definitely be a trial and error effort and might take some time. I have no idea what starting values to recommend since I have never worked with this driver.

Hope that helps.

[nullspace]

Of course, it could just be that you are one of the legion that find the Altec 'house' sound aggressive or 'shouty'. It's not as though they don't have that reputation.

Regards,
John

[Seadweller]

Thanks again everyone.....

Regarding the BSC filter duplicating the effects of a lower damping factor, what value of resistance would provide an accurate representation of having an amplifier with a higher output impedance? Would 2ohm's of resistance be a fair representation, or would it be much higher, on the order of 10ohm's?

I still have the circuit in place, so it would be a simple matter of changing out the resistors.

I am also consider a different crossover, likely the Jeff Markwart design, where L-pads can be used to attenuate the higher frequencies. I'd prefer to get the new amplifier in place first to determine its impact on the system.

Thanks!

[GM]

Originally posted by nullspace
It's not as though they don't have that reputation.

The 604 series don't and one reason why they were the recording studio standard for many decades, it's the old 300 - 800 Hz sectoral and multi-cell CD horns that do when driven with a low output impedance and no CD horn EQ, so it's how you implement them that determines whether they 'shout' or reproduce a signal more accurately than all but a very few of their contemporaries.

GM