Far too lean 604-8Gs in 620 kabs

GM · September 10th, 2011, 09:27 PM

Originally Posted by amandarae

To me, the 620 cabs bass is bloated and undefined(could also mean less depending on room interactions) and something is amiss on the midrange compared to the 612C or Open Baffle using the 604-8G's.

Yeah, long, boring story about design cut short, moving the vent to the extreme bottom will cure these problems for most folks, though some [like me] need to additionally critically damp the vent to get a fast transients 'sealed with extra gain' response.

GM

**Altec Best** · September 9th, 2011, 03:33 PM

You should be getting better bass then your getting especially in a 620 cabinet.I don't like MDF it just sounds muddled to my ears.Any void free plywood sounds the best to me IMHO.With Baltic Birch being tops in my book.If your HF is too bright it will have the effect of your bass being a bit shy.

I also have 604-8G's in 612C cabs that are a bit smaller than the 620.The 620 A/B is supposed to be the right cab for the 604 G,H so it will be your preference to which would sound best.To me I thought my 604's sounded bass shy in the 612 but when I popped a 416B in there, there was plenty of bass.So I'm thinking it is just the 604 duplex that is bass shy IMO.

GM · September 10th, 2011, 03:04 AM

Do you have the HF shelved down [AKA baffle step compensation or BSC] to tonally balance it? This will make it sound too lean if not. Ditto if there's any cab leaks which will turn a reflex alignment into a lossy sealed one with a very high F3/Fb. That said, for deep bass, a very large cab tuned to Fs is required, especially when driven by a high output impedance tube amp.

GM

**KBergsson** · September 10th, 2011, 04:17 AM

Originally Posted by GM

Do you have the HF shelved down [AKA baffle step compensation or BSC] to tonally balance it? This will make it sound too lean if not. Ditto if there's any cab leaks which will turn a reflex alignment into a lossy sealed one with a very high F3/Fb. That said, for deep bass, a very large cab tuned to Fs is required, especially when driven by a high output impedance tube amp.

GM

Hi GM, im using the J. Markwart filter unchanged (using the 8g suggested values) as published here " Simplified 604-8H EQ Crossover ". I welcome any suggestions to modify this filter.
There could be a small leak due to a small damage on the gasket (between cab and unit) around one mounting hole on one of the speakers.
Im not expecting deep bass and they seem to go reasonably deep if there is real deep bass on the record, but it is more that the bass is very dry and attenuated compared to the mids and top.
Regards
Kjartan

**Todd W. White** · September 10th, 2011, 09:32 AM

This was a problem with some of the post-Electro-Voice takeover 604's, but should NOT be a problem with yours. I have personally made side-by-side comparisons of a 604E that had been freshly recharged and reconed by GPA and a brand new GPA 604-8H-II, using the same crossovers (designed by Jerry K. Hubbard), and, though the HF sounded a bit different between the two, as one would expect, the LF was identical. Something else is going on here...it's not the speaker's design fault.

**VolvoHeretic** · September 10th, 2011, 01:53 PM

Are you guys sound junky purists who use no tone control? I’m a low brow stereo aficionado and use a heavy dose of bottom and top EQ and my A7’s play plenty low, although I have to adjust for every different song and volume change and they sound great whether playing classical, jazz, fussion, or rock. Or play them flat and just stand 100 feet away and then they have all the bass you could ever want.

**bowtie427ss** · September 14th, 2011, 12:21 PM

That said, for deep bass, a very large cab tuned to Fs is required, especially when driven by a high output impedance tube amp.

Quoted for emphasis.

A graphic analysis of the frequency response characteristics of the amplifier/s would probably help shed some light on "where's the bass?".

GM · September 10th, 2011, 10:23 PM

Originally Posted by KBergsson

Hi GM, im using the J. Markwart filter unchanged.........

There could be a small leak...........

Im not expecting deep bass............ the bass is very dry and attenuated compared to the mids and top.

I don't have my notes available, so don't recall 620's exact tuning, etc., but it should do 40 Hz solidly. To go lower requires either a larger cab tuned lower or a lower vent tuning and corner loading for the 620.

Even leaks around screws can ruin a reflex's tuning, causing a much higher F3, Fb.

Regardless, once you're sure there's no leaks, Jeff's XO has an adjustable HF shelving pot same as the OEM unit, so as it is dialed up, the tonal balance shifts to an increasingly mid-bass heavy one and if you don't like what it does to the extreme HF you can experiment with higher/lower by-pass cap values [c4].

WRT weak magnet testing, I know of no way for the casual DIYer to test them, but the dead giveaway is that it increases the driver's Qts a lot, meaning you'd be here complaining about having a 'fat'/'slow'/'boom' bass due to the 620 being too small and too high tuned for them without plugging up the vent and even then it maybe too small.

GM

**KBergsson** · September 13th, 2011, 01:59 PM

Thanks for all the good suggestions. Work has been disturbing my hobbies the last couple of days

I haven't had the time to dismount the speakers, take pictures of the membranes and remove some of the fiberglass so I only have top, side and back covered.
I have however been playing with the shelving pot in the JM filters and it does not have a huge effect, everything is a little more up front on the one extreme setting and a little slower/darker on the other extreme. I use the middle position for most of my listening.
If removing damping from the cabinets is not enough i might try closing the front port and cutting a port on the bottom (I have the 620's mounted on wheels so my wife can move them around) and see what that does.
Regardless of the missing bass output, the Altec's still sound quite good and are smooth, detailed and fast.
Regards
Kjartan

**KBergsson** · September 10th, 2011, 04:05 AM

Originally Posted by Altec Best

You should be getting better bass then your getting especially in a 620 cabinet.I don't like MDF it just sounds muddled to my ears.Any void free plywood sounds the best to me IMHO.With Baltic Birch being tops in my book.If your HF is too bright it will have the effect of your bass being a bit shy.

I also have 604-8G's in 612C cabs that are a bit smaller than the 620.The 620 A/B is supposed to be the right cab for the 604 G,H so it will be your preference to which would sound best.To me I thought my 604's sounded bass shy in the 612 but when I popped a 416B in there, there was plenty of bass.So I'm thinking it is just the 604 duplex that is bass shy IMO.

Thanks Altec best, maybe the 604s are more bass shy than I expect or would like, I have regrettably no comparison near by (i.e. another set of 604?s)
Regards
Kjartan