What's wrong with this Selenium D220Ti?

[keyser]

There appears to be something wrong with one of my Selenium D220Ti's. Both are brand new. One of them seems to be defective, but what could be the cause? Attached is a picture of the impedance plots, but the frequency response curves of the two drivers are quite different too.

[badman]

The problem is in QC, they're inexpensive and thus very inconsistent. I had to go through 4-5 2500tis to get a good impedance match.

Did your FR variations track the impedance changes?

[keyser]

There is a high Q resonance of a couple dB's at about 1650 hz in the defective one. I wonder what could be physically wrong with the driver that makes it perform as it does. Are there any compression driver guru's in the house?

The Seleniums look pretty well built and are finished nicely, but you might be right. PE gave me a refund without asking questions, but what I need is a new tweeter! Shipping costs are considerable, so it'll go with my next order. Let's hope it'll be a reasonable match this time.

[Helmuth]

Ik heb met mijn beyma driver ook problemen gehad met grote verschillen.

Ik heb zitten spelen met papieren pakkingen die er in zaten, Dan kon je de spreek spoel 0,1mm of 0,2mm naar boven of beneden verplaatsen dat maakte veel verschil. Mijn conclusie was dat de airgap en voicecoil en koperring heel precies passen in een HF-driver


And what horn are you using maybe you do not ad a good air-load and is there a xmax problem.

Groeten van de guru

[Ron E]

Originally Posted by keyser

There is a high Q resonance of a couple dB's at about 1650 hz in the defective one. I wonder what could be physically wrong with the driver that makes it perform as it does. Are there any compression driver guru's in the house?

Was the measurement consistent? A friend had an altec 802 with strange inconsistent measurements and he found that a lead was touching the diaphragm. Also, the peaks are caused by the horn loading, so look to your mounting situation.

[gainphile]

One of my Seleniums had strange "distortion" at times. After opening it (quite easy) I found that there was a small sand particle in the magnet gap. The coil was not that round as well. So it was an easy fix.

[pinkmouse]

Yup, first thing to do is clean the gap. You'd be surprised at what gets left in there.

[keyser]

Helmuth, thanks for the tip. I'll check it out. As for your second remark: I was measuring at a very low level. Btw, for those couple people on diyaudio.com who do not speak Dutch, you might want to change your post to English .

Ron, the measurement was indeed consistent. Initially I measured the drivers mounted to the Dayton 12" round WG's. Then I wanted to make sure it wasn't due to some misalignment / non-proper loading, and so the measurement results shown in my opening post were of the nude compression drivers.

[Radugazon]

A diagnostic suggestion :
If you open the drivers, you can test the dome+coil of the defective model on the motor of the safe one, and inverse situation. Could this give a clue ?

[LafeEric]

I received a pair recently and one of them had some bits of glue here and there - a tiny ball stuck to the inside of the diaphragm, one inside of the gap, and some residue here and there.

I carefully inspected them both and cleaned up what I found.