music, guitars & other obsessions

Speaker

Modified Crossover for JBL 4730A

I took out the JBL crossover PCB, stripped everything off it and installed the new components on it. Here are the before and after shots:

Bare PCB

Bare PCB

Stuffed Board

Stuffed Board

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VT15 Loaded With JBL 2118H

I replaced the stock speaker with the JBL 2118H that I took out from the 4311A cabinet – what a difference! Compared with the stock speaker, the 2118H has a much improved frequency response, in fact,the amp can’t be cranked up full, because the bass rattles the poor little combo cabinet, this is a 200W speaker designed for heavy duty PA work after all…

20141024_124435

20141024_112924


What To Do With the JBL-4730A?

As mentioned in the previous post, I have a few options to improve the performance of the 4730A, I chose to start with the easy one – getting rid of the bullet tweeter and the mid-woofer and replace them with the Markaudio Alpair 12 Gold, since the 2226H is a lot more efficient than the Alpair, I had to add a pad on the 2226H to bring its level down. While the Alpair is brand new and not broken it at all, a quick listen showed some promise, although when A/B between this setup and the Tannoy DMT12, it revealed that the JBL/Markaudio was far inferior, which was not really surprising since the Tannoy is a great studio monitor, unlike the kludgy solution that I came up with for the JBL/Markaudio.

2226H with Alpair 12 Gold

2226H with Alpair 12 Gold

Moving on… I went looking for other alternatives, and guess what, one of the more popular designs for Econowave uses the JBL 2226H driver – the 4Pi design by Pi Speakers seemed to be a winner, and I even have the D220Ti driver with the horn, how perfect is that?! I just need to order some parts to update the crossover and get some cabinet to house the horn, since I want to keep the 4730A original – but I will remove the existing crossover board from the speaker cabinet to make it easier for tweaking.  Another bonus, most of the components on the exisiting crossover can be re-used in the new 4Pi design, so that will save me some money on parts.

4730A Crossover Board

4730A Crossover Board

[Added on 9/24/2014]

I ordered some crossover components for the 4730A, here is the modified crossover:


New Speakers – JBL 4730A

[Update 7/31/2014]

After listening to the speakers for a few days, I could not stand them! So I had to take them down – the frequency response was just too far off for the EQ to work out the kinks. They were very hard to listen to, sigh… So there are few things that I can try to address the problem:

  1. re-work the passive crossover to smooth out the response – however I don’t think this is a trivial problem and would require a lot of measurements and component fine-adjustments;
  2. re-wire the speakers for bi-amping with electronic crossover (via foobar2000’s DSP) – this probably is a bit easier to do than the passive crossover, since I think a lot of the problem were due to the poor interaction between the drivers – which could be potentially dialed out with DSP;
  3. replace either the woofers or the mid-woofers – but this kind of defeat the purpose of having “JBL” speakers;
  4. get rid of the bullet tweeter – I think they are too directional in my small space;
  5. a combination of 3 & 4 above.

In any case, the poor frequency response was not easily dealt with equalization as I originally thought, oh well, worst case, I can always part out the drivers, which are actually worth quite a few dollars… seems a shame though.

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Scored a pair of vintage JBL PA speakers yesterday, the JBL 4730A is a 3-way design with 2226H 15″ woofer, 2118H 8″ mid-woofer and the 2402 bullet tweeter replacing the original 2403 super tweeter with bi-radial horn in the original design.

4730A With & Without Grill

4730A With & Without Grill

The first impression upon listening to them was that these speakers were too bright, a quick check of the data sheets of the tweeters revealed the reason why – the 2402 was 5dB more sensitive than the 2403, so the HF response was drastically exaggerated from about 2.5kHz and up.

2402 Bullet Super Tweeter

2402 Bullet Super Tweeter

So right off the bat, the crossover needs to be modified a bit to tame the 2402.  In the meantime, I am just using the built-in EQ of the audio player; which reveal another thing, that these speakers were very analytical, i.e., they reveal the recording – warts and all – many recordings were just poorly done and very hard to listen to without some equalization so forget about “natural reproduction”…


Projects On Deck

Since it is the start of a new year, I am taking the opportunity to outline the projects that I am currently working on:

Amplifiers

All Octal Amp – using the parts and chassis from the 6N8 6P3 kit, I intend to turn this into a dual channel guitar amp, so far I have sim’d a few designs, and the most promising one seem to be based on the Top Boost AC4 from Tubeswell, it will include a tremolo circuit, since I have not done one yet.

Tweed Overdrive Deluxe – I got the design from Hoffman’s forum, and it will give me a chance to try out pentode pre-amp distortion (with the 5867), the chassis and parts were salvaged (ok, I stripped it) from a el-cheapo Audio Space Houston Mini-2. I always like the looks of the Mini-2 more than its sound, anyway, it is way too powerful for my hi-fi, so why not turn it into a stereo/dual channel guitar amp? I incorporated a lot of switching function into the design – each channel can select either the clean or OD pre-amp, then the Phase Inverter can be either Paraphase, Long Tail Pair or Cathodyne, and finally the output tubes will be different for each channel – I plan to have 5881’s on one and 6V6’s on the other. Furthermore, each channel will drive its own 10″ speaker, again, two different ones are available – Celestion and Jensen are being considered. So that’s 2x3x2x2=24 combinations per channel! Probably way too many to be useful, but it will provide a good platform to try out various settings all in one go.

6N2 6P1 Mod – I am thinking to change the circuit to Loftin-White just to see how it sounds, the amp is so simple, so it will be easy to put back if the experiment does not work out.

Speakers

I have drawn up a few designs over the past few weeks, they will be built after the Chinese New Year when the workers return.

Asbury KO – I took the dimension from Woden Design’s Victory posted on the web and scaled it up slightly to fit the Alpair 12 (which why it is the Asbury).

Fritzmaurice FX10 KO – I used some photos posted on the web and tried my best to guess the dimension for the FX10, the trick was to get the cross-firing and the tilt angles, which I estimated at 17 and 15 degrees respectively, as they say, it is close enough for rock n’ roll.

AX84 Cabinet – I am having another one made to match the one that I already have, so they could be placed in a stereo fashion or used by another amp.

CNC

As usual, I am finding the biggest challenge to be in the hardware – the chassis, the faceplate, and the cabinet, like they say, 80% of the work for amp building are not electrical/electronic related, no kidding! That’s why having a small CNC machine that can do metal, wood and plastic milling is so attractive to me, it will save so much time (well not counting the learning curve on CNC itself), so that is another project all by itself.


OB Speaker Build 1

It took awhile, but I finally managed to complete the OB speakers for my study. A prototype was made with MDF first and tested to verify its performance (listening tests only), I then had the carpenter make another pair but this time with plywood, alas at the end it was still not done properly, as I will discuss shortly.

The  OB design was done with Martin King’s great Quarter Wave software. The speakers used were Fostex 126En and Eminence Beta 15A. It’s an active 2-way design, Bessel 2nd order crossover with LF at 200Hz and HF at 400Hz. Below are the screen shots from Quarter Wave, one thing to note is that, while the frequency response looks very flat, it is very sensitive to placement in the room, in the simulations, small movements of the speaker caused the peaks and dips in the mid band to change pretty drastically, primarily due to wall and floor reflections. But the transition from LF to HF showed the design had a pretty good chance – it won’t “suck”. When I get a chance, I will try to measure in-room response to see how closely (or not) the actual frequency response matches the simulation.

Thiele-Small Parameters

Frequency Response

Impulse Response

As mentioned earlier, the construction could have been done much better – the mistake I made was being talked into “saving money” by the carpenter, he suggested using layered 12mm plywood for the woofer and a single 12mm for the full-range, instead of my original idea of just using a sheet of 18mm plywood from top to bottom. Oh well, that was another penny-wise, pound-foolish decision I regret making. The next build will be either 18mm or 20mm plywood, it will cost more, but should sound better too from the increase rigidity of the panel, and better time-alignment of the drivers. Here are some pictures of the finished speaker:

The front was covered with “white spruce” veneer then clear coated, I think it looks ok, but perhaps a bit IKEA-ish… You will also note that both drivers were rear-mounted, which was not based on my original design – the Fostex should be flush mounted on the front, to better time-align the two drivers. By the time, the carpenter brought over the speakers for me to check, it was already too late to correct the mistake, unless I wanted to scrap them and start all over again. Since it had taken more than a month just to get these darn things made, I decided just to take them as-is.

Front View (Left)

The transition from the lower 2x12mm woofer panel to the upper 12mm panel is disguised by the use of tapered cuts of plywood, I think it looks pretty cool and adds some rigidity to the overall construction.

Side View (Detail)

Full side view clearly shows the upper section is only 12mm, which is sufficient for the small Fostex driver, although as mentioned, it would be better had the whole panel be constructed with 18mm plywood top to bottom. Next time…

Side View

The signal chain is as follows:

Source -> Foobar 2000 with DSP crossover -> M-Audio Revolution 7.1 Soundcard -> S/PDIF carries the LF to digital amp -> Eminence Beta15A, and analog out carries the HF to DIY 6P1P amp -> Fostex 126En

I found the setup particular well suited to blues and rock music, well balanced… Not so good with electronic though. It shows up poor recordings easily, the difference between MP3 vs. lossless sources can be clearly heard. It is surprising how many albums are very poorly mixed, which usually lacking highs and rather compressed. But on properly mixed material, these speakers are very good sounding indeed – open, transparent, dynamic with decent soundstage. Overall, I think they are very easy to listen to, but perhaps lacking a bit in stage depth, I need to investigate further, the lack of depth could also be caused by the soundcard or the cheapo 6P1P amp that I am using. Current grade: 85/100.

For comparison, the 10-year old Usher speakers that I was using before would probably only score 70/100, it sounded boxy and lifeless compared with the OB. So at a minimum, the key feature of  a “no box” design, and all the online ravings that prompted me to build them in the first place, has not gone un-rewarded. It was well worth the effort. Now I am ready for build 2 😉 Better wait awhile and enjoy these first…