Upgrading centre speakers with drop in replacement

Hi all,

15 years ago, just like most inexperienced consumers, I shelled out $200cdn and bought a center center with a brand name on it thinking it would sound great, the Harman Kardon HKC. I came home and connected it up, it sounded thin and lack of energy with very poor top end. I have a pair of the old klipschs KG 3.5 for the fronts. Knowing it's not matched centre but it's still a very poor sounding speaker just be itself.

Lately I figure maybe I can try replacing the woofers instead of buying a new center speaker.

The cabinet is a 0.27 cu ft with a 3.25" straight port 2" in diameter. I calculated the cabinet is tuned at 80Hz.

It houses two 4" JBL woofers and one 0.75" tweeter. It's an 8ohm speaker with XO freq at 3k with first order XO.

HARMAN KARDON HKC TECHNICAL MANUAL Pdf Download.

My question is should I:

1. Just replace the woofers with a FR like the TB w4-2142 and keep the tweeter and the XO assuming having a mediocre tweeter will still sound better the highs from the two FRs.

Or

2. Bypass the XO and disconnect the tweeter and connect the two FR woofers in parallel.

Or

3. Keep the XO and tweeter and modify the XO and delete the low pass filter to the FR woofers so the highs wil be coming out from all three ways.

I am not sure if the w4 is a good match but I assume with a low Qts around 0.3 it should work this bass reflex cabinet.

There's also no room inside to extend the port tube so I assume 80Hz is good enough for a center speaker?
nIZVRI1.jpg

inF24Rq.jpg

jc9h38v.jpg


Thanks in advance!
 
Last edited:
Hi Bchiu2000,
If you are experimenting and if the drivers are in a good condition why you don't test the woofers with no crossovers and go from there. It costs you nothing. If the sound gets better SPL probably you can later adjust with two pairs of new parts inductors/caps or as another test just with the inductors in line and shunting the caps.
Note: don't leave ONLY the caps connected or you might damage the amp.
 
A commercially designed MTM center channel from HK that comes with a crossover schematic and spec of 91dB and 80Hz to 20kHz is probably well designed for its price point. HK as a company values speakers that measure well and they are in business to sell speakers that don’t sound like [email protected] - which is sort of how you are describing their sound. I am not sure you are going to be able to beat what they designed (measurement wise) with a simple swap. It depends on the sound you are looking for. I suspect, you feel that they lack a nice mid range and top end is unsatisfying. I know that 3/4in consumer grade dome tweeters are often crossed way too high because the cheap tweeters have a high natural frequency. 3kHz 2-ways never sounded right to me. I was building an MTM a few weeks ago and tried a crossover at 3kHz. It didn’t sound right even though it measured well. I changed it to 2kHz and it made all the difference. You are probably not ready to redesign the crossover with a new custom tweeter upgrade.

The specs say it is a 5in woofer. Try a pair of know to be great sounding 5in full range drivers. Run them full range in parallel for circa 91dB at 4ohms. Or try pair of PA130-8 in parallel for 96dB and 4ohms. Both of these will have 2dB to 5dB drop due to baffle step falloff. You will be at about 86dB or 90dB overall. Use a super tweeter above 10kHz - about 90 dB sensitivity - like an AMT. Use a simple 1uF film cap as crossover. Leave the 5in drivers full range. You need to do the calculations to redo the vent tuning. The PA130’s will tune nicely down to about 80Hz is my guess. You need a subwoofer for these to sound full.

The cabinet is rather small so your deep bass is limited. Most Tangband full range drivers will require a bigger box. Look at the Vas number. Box should be at least 1/2Vas if you want any appreciable bass out of it. The PA130 is a pro driver and has a smaller Vas suitable for smaller enclosures. It also comes in 16ohms so paralleled gives 8ohms and about 91dB.

It’s not expensive so easy to try. I really like how they sound as a fullrange. The 16ohm in parallel will be about 86dB after baffle step losses.

Dayton Audio PA130-16 5" Full Range PA Driver 16 Ohm

Use something like this AMT for the tweeter, it may need a 2.2ohm resistor pad it down to 86dB in series with a 1uF cap.

Dayton Audio AMT Mini-8 Air Motion Transformer Tweeter 8 Ohm
 
Thanks for all the great ideas and information.

I had an electrical engineering degree long ago. But I still remember the basics.

I'm trying to understand the HKC XO schematic included in my first post. For the mid-range woofers, I see they use a simple inductor as a low pass filter. And I assume the capacitor in parallel is to send all the high frequency to the ground if there's any.
And as for the tweeter, they did the opposite and use a cap as a high pass filter, and connect to the inductor in parallel to drive all the low-frequency to the ground if there's any.

The 3.6 Ohm resistor is just for padding tweeter db level to the woofers.

If all my assumption is correct, then I can simply replace the 8uF with a 12uF to change the cut off frequency from 2.5K to 1.5K?

B
 
Last edited:
Ignore my last post. After doing some researches, here's what I found.

For the mid-range woofers, I see they used a 2nd order LC low pass filter. The cut off frequency is 1.5kHz if my calculation is correct.
And as for the tweeter, they used a 2nd order LC high pass filter. I calculated the cutoff frequency is 4 kHz.

The 3.6 Ohm resistor is just for padding tweeter db level to the woofers.

I hope I'm not missing something. What happened to all the signals between 1.5kHz to 4kHz? I assume the roll offs are pretty quick for 2nd order filters.

B
 
Get the free Xsim program written by Bwaslo. Put the Xo in and you will see quickly. But you can’t just change the values to lower the XO. You need a tweeter that can cross lower. They cross at 3k because the tweeter can’t go lower. You could try changing the tweeter to a better dome that can cross low. A very good sounding yet reasonably priced one is the Dayton DC28F. It can cross at 2k easily. Still, you are getting into XO tweaking which should be done with a microphone, REW software, Xsim software, and go to Multi-way Forum to read up on.

Dayton Audio DC28F-8 1-1/8" Silk Dome Tweeter