damping in 620 cabinet?

[lhgin]

Hi guys,

My 620 cabinet finally arrived. My carpenter had taken the initiative to put 2" thick sponge all over the interior of the speaker for damping.

I mounted the drivers and tested it with my 80wpc SS amp and 1.5wpc SET amp. Both came out to have no low bass at all, everything below 100Hz is as good as gone!

I suspect that it's due to the sponges. I'm thinking of removing all the sponges and replace them with a thin layer of wool, about 1cm thick. What do you guys think? Is the the possible cause?

Rgds,
Heng Gin

[billfort]

I lined by cabinets with 1 1/2" unbacked fiberglass batt insulation on opposing walls only (back, top and 1 side). Don't know how this affects bass response but I would certainly not say 604G's in the cabinets I built (300B SET amp)are lacking much in the low end.

Billfort

[jmarkwart]

I suspect the driver's VAS is too low for the 620 cab. Are you using 604Es?
Jeff

[lhgin]

Thanks guys!

I'm using a 604D 16 ohm version. Heard that it's equivalent to the 604E.

I went back and look all over the place for the fault. I've got a reply from BrianC saying that it's probably due to the unclosed opening at the side, about 2"x1.5" hole that's for the handle that I haven't mounted.

I guess, with this 2 holes at the side + a big port in front, the HelmHoltz resonance requirements couldn't be satisfied, therefore the boost at the bass couldn't be achieved.

Am I right? I'm still waiting for the handles to arrive.

Rgds,
Heng Gin

[GM]

All those holes are tuning it higher, plus IIRC it takes 3-4ohms of series resistance to flatten out a 604 in this size cab.

GM

[lhgin]

Hi GM,

What do you mean plus IIRC to take 3-4ohms resistance? Please elaborate further.

Many thanks!

[GM]

These were designed back when high power transformer coupled tube amps ruled so the driver was in series with considerable output impedance, raising the effective Qts, flattening the rising response, and the better impedance matching to the speaker boosted the low/high BW somewhat.

SS amps have so little output impedance that the effect on Qts is inaudible, nor can it 'track' the speaker's impedance so whatever its alignment is it, requiring some form of EQ, such as series resistance. I chose to shape my dual 515s/511/808s entire BW with digital EQ. ~Flat from 16-20kHz in-room.

If series resistance is tried, I suggest a cap in parallel to 'boost' the HF.

GM
----

[lhgin]

Hi GM,

Ooo... I see. What value of cap should I start off with? Is there any calculations to go with the cap?

I've just closed the site ports with handles and try again with the 604. Now the bass is MUCH better. Still, things are slow and muffled.

I'm suspecting that the thick sponges inside is giving me problems for the muddy sound. I'll try to remove them soon and see how it goes. I'll try to replace them with bubble wrap and see how it goes.

Rgds,
Lhgin

[GM]

Try between 1-2uF. Hmm, I missed the 'sponges' part. Tear it all out and replace it with 1" acoustic fiberglass on one wall, top, back, and any cross bracing only.

GM

[lhgin]

Hi GM,

So, it's connecting a 3-4 ohm resistor with 1-2uF bypass capacitor, right?

Where should this be connected? Should it be as follow:

amp +ve --+-- 4R -+-- crossover input
| |
+- 1uf -+

Many thanks!

Rgds
LHGIN