Calling all Electrical Engineers who aced AC circuit analysis.

[exhilaration157]

Or even if you failed AC circuit analysis, you still know more than me.

I recently started measuring the frequency responses of speakers around the house. I had replaced some capacitors in the Tangent TM3s (a British bookshelf speaker with KEF drivers). They used to sound really dull in their old age (they are 30+ years old). I replaced the bipolar electrolytic caps with metalized PP. I replaced C1 with a 51 uF 200V PP cap. I replaced C2 with a 6.2 uF 200V cap. I was advised not to get rid of c4-c7 alone since they are Mullard polystyrene caps.

The thread about this is here: http://audiokarma.org/forums/showthread.php?t=200726

After this change they sounded bright again, but a bit too bright, harsh even. Now I know why. Check out their frequency response. When I say Hi-Pass in the graph title I mean Low-Pass.

To see my measurement methods refer to this thread: http://www.audiokarma.org/forums/showthread.php?t=266217

Now I know why they sound harsh. Look at that peak at 3 kHz, and the dip at the next octave (6 kHz). It makes the attack on instruments sound really cool sometimes, but its fatiguing.

The XO frequency is supposed to be 3000 Hz, and the nominal impedence on both drivers is 8 Ohms. Any speculation on how I could smooth that out. Does anybody think its a XO issue, maybe a phase shift in the XO? or a shift in XO frequency?

 

[bowtie427ss]

Few things would be as useful now as a FR plot from BEFORE you recapped.

How long have the Solens been installed and used?

IME, newly installed Solens have seemed a bit edgy and harsh only to mellow into something quite wonderful after some hours of use.

Just my .02..............

 

[Nat]

This is the blind leading the blind, but one of the specs that matters with caps is ESR, which is Eqivalent Series Resistance -- caps have reactance, which is why they block low frequencies, but they also have some resistance. When speakers are designed carefully, as Tangents were, that resistance is taken into account. When caps are replaced one reason for them to sound different is not only that the new caps are closer to the original capacitance, but also different ESR.
I don't know if the dramatic difference in voltage rating is significant, but 50 to 200 seems a significant jump, and I wonder if it affected other parameters.
But as I say, I am not knowledgeable on this topic.

 

[GordonW]

A WAG here... have you tried reversing the polarity of the tweeter? Are you sure it's the same polarity as original?

Regards,
Gordon.

 

[Zilch]

You're measuring one speaker at a time, I presume?

What is the spec'd crossover frequency? It's unlikely anything you did in the lowpass would mess up the VHF like that....

 

[exhilaration157]

I'm measuring one speaker at a time with one microphone at a distance of 30 cm.

They were recapped last year. They seemed a bit harsh compared to other speakers I had recapped. I didn't have anything set up to measure them at that time.

The Solens most definitely have a lower ESR since their dissipation factor (which is directly proportional to ESR) is ~0.0002%, where the bipolar electrolytics are around 5%. In some cases that doesn't effect the XO frequency at all, I recently did recap ENERGY point 1e bookshelf speakers, and I measured their response before and after, aswell as control speaker and experimental speaker. The data is in my freq. measurement post linked above.

The ENERGY's response wasn't changed at all, but they sounded better, as proven by single blind listening tests with my extremely large sample size of 2.

The specified crossover point of the speaker is 3 kHz. There is some weird stuff going on at 3K, and 6K. So it might be a phasing thing with the tweeter, but I'm pretty sure no one but me has touched these since they left the factory. They were my grandparents, and they never took them apart believe me. I could try inverting the tweeter leads and see what happens.

I know I'm light on info in this. I just thought if there was anything obvious that I did wrong someone might catch it. I'm curious about the integrity of the polystyrene tropical fish caps. But without measurements from every component in the crossover as they are now, you can't tell much. I don't have any measurement instruments past a multimeter though.

 

[Zilch]

Pull back to ~1m and remeasure.

Buy a $45 Dayton measurement mic, too; needs phantom power. Cardioid's probably helping with room reflection issues, but it's not "standard" procedure....

 

[Pete B]

It is possible for C2 to resonate with the VC inductance of the B200 woofer. The ESR helps to reduce this effect, try 1-2 ohms in series with C2. It's possible that the reduced ESR introduced a peak at 3 kHz with phase shift that causes the dip at 6 K. You could also take a measurement with C2 disconnected before you add the resistor.

Can you measure the woofer + XO independent of the tweeter?

Might also be diffraction, is this grille on or grille off?

 

[Jcricket]

Zilch and GW are on the case - you will be hard pressed to find better help. I did read something today - skimmed over it actually.
If you have a 2nd order xover, one of the mids(IIRC) needs to have a reverse polarity from the other. Otherwise you do get a phase shift that will casue a huge dip in that range. Hopefully, zilch or grodon can calrify this and make me sound almost intellegent(good luck with that). It was on the DIY audio help page for xovers.

Mark

 

[dlp4341]

Measure the woofer with the tweeter disconnected. And tweeter with the woofer disconnected. If you still have the old caps , swap them back in and remeasure. Also you could measure with the polarity reversed on one driver. Let us kwow what uou find.
Don

 

[AngelRa]

The measuring method discussed in the link is severely flawed. You cannot measure with a microphone in a listening room and expect the measurement to reflect the speaker response alone. The room acoustics are in play and changing the response violently. What you got is the room response to your speakers. Just move the speakers or the mic around and re-measure, you see what I am talking about. The graph will not resemble the one you posted.

Speakers are measured in an anechoic chamber. Now, you probably do not have one, in that case you can only measure room+speakers response. In this case, measure at the seating position. You need to take at least 10 measurements in 10 different spots around your sweet spot, then average.

Also, you cannot rely on a studio microphone, the response is not flat. A measuring mic is needed. Check Behringer measuring mic for a cheap option.

 

[Zilch]

Speakers are measured in an anechoic chamber.

Not necessarily; we also do it quasi-anechoically, in-room.

Exhilaration is aware of the limitiation due to reflections, and is measuring in the nearfield, at 30 cm. I'm suggesting that may be too close for a system measurement; we'll see in the 1m result.

If he takes some precautions with respect to reflections, the direct reading may be valid. Yes, spatial averaging would help in this situation, but that would represent power response, including the room.... :yes:

 

[Gang-Twanger]

Please excuse my newbie ignorance. What is a WAG? I'm assuming this is some nickname for a member of a particular audio society or perhaps for a speaker modifier/designer (??).

 

[whoaru99]

Wild-ass guess. This is a somewhat lower confidence interval than SWAG...scientific wild-ass guess.

 

[GordonW]

Please excuse my newbie ignorance. What is a WAG? I'm assuming this is some nickname for a member of a particular audio society or perhaps for a speaker modifier/designer (??).

Wild A** Guess.

Meaning, that without being there, I don't have all the info... but that's the first thing I would check.

Pete B. may have a quite valid point. People do mess up sometimes substituting caps... ESR can be, and has been, designed into the proper transfer function of crossovers, in several cases I've seen. May not require as much resistance as he mentions (sometimes, even 1/2 ohm or less can make a difference), but that's an interesting tack...

Regards,
Gordon.

 

[Fisherdude]

I have been looking at measurement mics, and several mention "phantom power". What does that mean? I assume it means that the mic needs an external power supply, but none of the ads I've seen mention what kind, do they sell it, etc.. Wall-wart? B+? Type of connector? For example, the Behringer ad on PE simply gives the voltage, and no other information whatsoever, even in the product specs pdf.

I took a look at the s/w used, and the full package is quite expensive. Does anyone have a suggestion for a reasonably priced s/w app to do the minimal FR charting, etc.?

Tanks!

 

[exhilaration157]

Yes, phantom power is a 48V power supply that runs from the mic preamp through the mic cable to your microphone. Most mic preamps contain a phantom power supply.

I know the studio mic is cardiod and not perfect and probably introducing phasing issues since that's what unidirectional mics do, but I have measured other speakers in the same way and found the results to be decently reliable. By reliable I mean that I have managed to get quite flat response curves from speakers with flat responses. If you are seeing a ~20 dB swing there is something up.

I may try some of this stuff closer to Christmas.

As for the diffraction theory... The grille was on. They grill fits in against the baffle, and the cabinet juts out (~1.5" or 3.8 cm) to be flush with the grille. It looks really sharp, but with the cabinet jutting out you can't really get rid of diffraction. So diffraction is undoubtedly an issue.

 

[whoaru99]

I have been looking at measurement mics, and several mention "phantom power". What does that mean? I assume it means that the mic needs an external power supply, but none of the ads I've seen mention what kind, do they sell it, etc.. Wall-wart? B+? Type of connector? For example, the Behringer ad on PE simply gives the voltage, and no other information whatsoever, even in the product specs pdf.

I took a look at the s/w used, and the full package is quite expensive. Does anyone have a suggestion for a reasonably priced s/w app to do the minimal FR charting, etc.?

Tanks!

Room EQ Wizard is free and certainly a good/great start for getting your feet wet with similar types of measurement technique.

 

[Zilch]

I took a look at the s/w used, and the full package is quite expensive. Does anyone have a suggestion for a reasonably priced s/w app to do the minimal FR charting, etc.?

Plenty of freeware available, and some of it is quite good. This one is winning widespread acclaim in the DIY community, but it's not "entry level," meaning it's sophisticated, with a learning curve:

http://www.holmacoustics.com/holmimpulse.php

See also ARTA, TrueRTA, REW, and a bunch of others.

The Behiringer mic is pretty much standard with DIYers, but Dayton just came out with an apparent rebrand for slightly less $$$ and perhaps a bit better accuracy:

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=390-801

These measurement mics are electret condenser type that require an external excitation voltage; most pro gear provides that, as condenser mics are common in the industry. It's called "Phantom" because it is DC riding on the input cable; no additional wiring is required between the mic and preamp.

Some compatible mic preamps with phantom power:

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=248-576

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=245-8688

http://www.bhphotovideo.com/c/produ...6_00_Audio_Buddy_Microphone_Preamplifier.html

Preamp with phantom power mic inputs and USB "sound card" in combination:

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=246-328

http://www.bhphotovideo.com/c/product/393375-REG/M_Audio_9900_51076_00_Fast_Track_Pro_USB.html

Parts Express is rumored to be putting together a "bundle" including mic, preamp, and Praxis s/w.

For now, I'm setting up another measurement system using the Dayton mic, Fast Track Pro, and HOLMImpulse....

 

[Fisherdude]

Zilch, thanks a ton!! Exactly what I was looking for. I appreciate the time you put into answering my question.

I'm gonna have to put a bookmark on this post.:yes: