lf response of 291 Driver

[kevin@kab]

Hi,
I have had some time to listen to the 803B/291's
I find one horn goes decidedly lower in frequency than the other. Have not measured it but it is clearly audible. I am crossing these at 350hz 24db/oct. This means the output is -6db at 350hz.
I noticed when inspecting the diaphragms, that they are original Symbiotik. one had some very slight marks on it not really deep impressions. I cannot say if that is the one that is misbehaving or not. I did not mark it.
I do plan to replace these with GPA 390's since they are guaranteed to 300hz.

Any speculation why one driver would not go as low as the other?

Kevin
Nov 23 2010

[bowtie427ss]

Active or passive filters? If passive, measure the DCR of both phragms to ensure they're fairly close.

My other thought is a weak magnet.

Any evidence of either driver being dropped or otherwise suffering an impact?

[Steve Schell]

Usually a weak magnet will take away response toward the top end, as the mass break point is shifted downward. If the response is inhibited on the low end I would suspect a rubbing condition, either junk in the gap or a misaligned or out of round voice coil. If this is the case you would hear it on a very low power sweep of the lower frequencies.

[kevin@kab]

Hi,
Ok, here is the follow up. I measured them tonite with pink noise gen.
Response is OK.
One driver is 4-5 db lower in overall output then the other.
I re-balanced the system and they sound much better.

How much variation would you expect to see between drivers?


the Eq is Marchand active crossover 24db/oct 350/3500
the amplifiers are Glow 11 watt P-P pentode amps.

Kevin
Nov 23 2010

[bowtie427ss]

I guess what i was really wondering is if one of the diaphragms had been replaced at some point in time with an 8 ohm version(assuming they were both 16 ohm originally). That was the basis for my curiosity about the DCR of the coils.

[kevin@kab]

they were changed at some time yes. both are 8 ohm symbiotik's. One reads 7.5 and the other 7.6 ohms. I mentioned one dome had a very slight narrow line depression in it.
Considering all this, I would lean torwards low magnetic strength, but I defer to those on the board who have much more experience with these than I.

Kevin

Nov 23 2010