JBL C-38s - 3 way?

[cjwebber]

Hey guys -

I have been listening to my JBL C-38s for the past few weeks, and have been LOVING them.

They have the 001 load (130a, 175, n1200). They have a midrange that is simply gorgeous and easy to listen to.The midrange and aesthetics actually ousted the 4430s from my living room.

I am looking at doing some additions to dial things in for a wider range of music types.

My thoughts are:

• Buy a pair of n8000s (or 3106)
• Buy a pair of 2405s
• Add a subwoofer crossed around 50

A couple of things:

• Any general thoughts?
• If I install the 2405s in the open port, the cabinet will be sealed, as there are no other vents. Will I need to cut another hole for venting?
• If I do buy a n8000/3106 I may just create a whole new back panel so I can cutout two crossover holes and a vent (if necessary). I'd like to keep the back panel original.

I don't have internal photos right now - just one of the the outside. I've attached two other photos (not mine) that show the layout of the baffle and the layout of the back panel

 

[wweigle]

I would think a subwoofer or two would be required to reach down with those speakers and round them out with modern music. My guess like most speakers of that era they had limited bandwidth, with the focus on the midrange, very nice midrange. I think if you fill the port you will need to provide a similar volume port to maintain the same enclosure tuning.

 

[Zonker92]

Can you simply replace the 075s with 2105s? I was listening the other day to that same 001 load in custom cabinets, and I couldn't believe how beamy those 075s are. All I had to do was move my head a foot or so from their axes and the highs just disappeared.

 

[cjwebber]

The last two pictures aren't mine. I don't have 075s.

Mine have 130a's and 175s. I'm hoping to add a 2405 to increase the High frequency extension.

 

[toddalin]

Better sound is easy to come by.

The N1200 crossover in that combination yields a frequency response that looks like a suspension bridge. You can get the horn dialed in much better by leaving the horn set to max setting and using an L-pad to get the levels to match.

The 2405/077 will add lots of intelligability and voices will become easier to understand, much more so than with the 2402/075. The easy way to add one is just to use a 1 or 1.5 mfd cap in series and an L-pad on the horn circuit and let the horn roll off naturally. Also put a 20 ohm, 10 watt resistor in parallel across both the horn and tweeter leads to stabilize these loads.

The 077 will be much quieter than the remainder of the combination. While many will poo poo this next idea, give it a try.

Put an 8 - 16 ohm resistor in series with the woofer. Use a high wattage, non-inductive resistor. This will both reduce the woofer down to a level suitable for use with the 2405/077 and dramatically increase the bass below ~50 Hz relative to the rest of the frequency response. The higher the impedience, the more bass you will see, but obviously you trade overall volume.

I've actually done this with an L-pad used as a variable resistor while watching the results on a 61-band RTA and you can watch the frequency response change as the resistance changes.

I've done with my 2235 woofers in my L300s, and when you get to the right resistance, all of a sudden everything comes into focus, intelligability goes up, and you hear things you've never heard before. You really need to try it.

Unfortunately, at that point you've given up a lot of volume, and I settled on 8 ohm, 100 watt non-inductive resistors that I can switch in/out. These flatten the response to where the low frequency roll off is shifted from 60+ Hz to ~40 Hz.

 

[Zonker92]

The last two pictures aren't mine. I don't have 075s.

Mine have 130a's and 175s. I'm hoping to add a 2405 to increase the High frequency extension.

Ah so; sorry, I read 175 as 075.

 

[cjwebber]

The N1200 crossover in that combination yields a frequency response that looks like a suspension bridge. You can get the horn dialed in much better by leaving the horn set to max setting and using an L-pad to get the levels to match.

The 2405/077 will add lots of intelligability and voices will become easier to understand, much more so than with the 2402/075. The easy way to add one is just to use a 1 or 1.5 mfd cap in series and an L-pad on the horn circuit and let the horn roll off naturally. Also put a 20 ohm, 10 watt resistor in parallel across both the horn and tweeter leads to stabilize these loads.

The 077 will be much quieter than the remainder of the combination. While many will poo poo this next idea, give it a try.

Put an 8 - 16 ohm resistor in series with the woofer. Use a high wattage, non-inductive resistor. This will both reduce the woofer down to a level suitable for use with the 2405/077 and dramatically increase the bass below ~50 Hz relative to the rest of the frequency response. The higher the impedance, the more bass you will see, but obviously you trade overall volume.

Intriguing!

a couple questions:

• What difference will I see between a 1 and 1.5 mfd cap?
• Clarification: You suggest adding the L-Pad to the 175, right?
• Can you explain what "stabilizing the load" means? (in reference to the 20ohm resistors)

 

[toddalin]

1 mfd will roll off higher, than the 1.5, but you loose more volume.

Yes, use the crossover terminals to feed the L-pad, and tap the horn off that.

The 20 ohm resistor smooths impedience peaks and was implemented in JBLs later offerings and shows on their schematics.

 

[cjwebber]

1 mfd will roll off higher, than the 1.5, but you loose more volume.

Yes, use the crossover terminals to feed the L-pad, and tap the horn off that.

The 20 ohm resistor smooths impedience peaks and was implemented in JBLs later offerings and shows on their schematics.

Awesome - this is all very helpful information - Also much cheaper than a pair of n8000/3106s.

Any insight on what port(s) should be included on a new rear baffle?

 

[toddalin]

Not off-hand.

There are free programs that do box tuning and include the T/S parameters for most major speakers. (The name escapes me, but someone may chime in.) Of course if you add the resistor, you change the T/S parameters.

 

[Andyman]

I think if you fill the port you will need to provide a similar volume port to maintain the same enclosure tuning.

Indeed!! I see you would have to as there is nowhere to mount a mid and keep the front mounted port, but moving it to the back has to have some sort of effect. In a hobby where we detect changes in tubes, wires, caps and other parameters, I would expect moving the port would have some effect too.

Plus you'll muck up the cabs unless you cut and fit a new motorboard. I'd look at addressing crossover and tweeter changes first and see how those suit you.

 

[cjwebber]

Indeed!! I see you would have to as there is nowhere to mount a mid and keep the front mounted port, but moving it to the back has to have some sort of effect. In a hobby where we detect changes in tubes, wires, caps and other parameters, I would expect moving the port would have some effect too.

Plus you'll muck up the cabs unless you cut and fit a new motorboard. I'd look at addressing crossover and tweeter changes first and see how those suit you.

I suspect moving the port the the rear changes things a bit, as well. How much? I'm not sure.

I will not make any changes that are not reversible - I just do not roll that way!

My plan is to save the original back panel and create a new back panel to allow for crossovers, ports, and L-Pads.

I had thought about creating another cabinet for the tweeters and setting that on top - but I love how clean and minimalist these cabinets are, so I'm trying to keep things that way.

 

[wiredbecker]

My plan is to save the original back panel and create a new back panel to allow for crossovers, ports, and L-Pads.

I like the way you're thinking. You could turn the speaker around, replace the back panel entirely, block all openings on the front from the inside (no one would be the wiser) and simply reconfigure a new baffle as needed! Don't like the sound? You can easily swap back to the stock configuration.

 

[cjwebber]

I like the way you're thinking. You could turn the speaker around, replace the back panel entirely, block all openings on the front from the inside (no one would be the wiser) and simply reconfigure a new baffle as needed! Don't like the sound? You can easily swap back to the stock configuration.

Unfortunately the back is not as pretty as the front...but maybe that's subjective...

 

[wiredbecker]

Dunno, I think it could look, and more importantly, sound grrreat. The lip on the front of the cabinet creates audible distortion. Flush mounting of drivers on a new baffle would clean that right up. How much do you really fiddle with the L-Pad? You could run your speaker cables through the existing tuning port on the front! Fun.

 

[cjwebber]

On the hunt for some 2405s. There's some on eBay - I gonna wait a bit for some alnico...

Anyone have any insight on what I should do about a port or vent?

 

[Xtinc]

Have you considered building a separate smaller baffle just for the 077/2405 and placing on top? I think this would allow for better driver alignment and allow you to keep the vent original. I see lots of people stacking on their JBL cabs with good results. You may be against for aesthetic reason though.

 

[cjwebber]

Have you considered building a separate smaller baffle just for the 075/2405 and placing on top? I think this would allow for better driver alignment and allow you to keep the vent original. I see lots of people stacking on their JBL cabs with good results. You may be against for aesthetic reason though.

Definitely a consideration! My first choice would be inside the cabinet, though.

 

[BMWCCA]

My suggestion would be to add an UHF driver (either JBL version) and get a larger cabinet (C35 or C37) which have the proper port already and offer more room for you 130A to perform. Then I'd just enjoy them for the fine vintage speaker they are without a sub. The empty cabs are cheap if you don't have to ship them and they come up quite often as many part-out these old systems rather than ship them.

 

[cjwebber]

My suggestion would be to add an UHF driver (either JBL version) and get a larger cabinet (C35 or C37) which have the proper port already and offer more room for you 130A to perform. Then I'd just enjoy them for the fine vintage speaker they are without a sub. The empty cabs are cheap if you don't have to ship them and they come up quite often as many part-out these old systems rather than ship them.

I've got a pair of C-37s loaded with the 001 system as well. Unfortunately, I do not have the do not have the horizontal space that the C-37s require. When I use them, I use them standing up in C-35 position - which leaves a whole ugly side without veneer exposed. I've been looking for C-35s or C34s but have not been successful. The C-38s have been the most widely accepted (aesthetically - by my wife and I) of any of my speakers, so that is why I am doing some work to get these sounding more versatile.