Bridged Output Jack for ADA Microtube 100

[allsignalmustpass]

Hello everyone,

Would someone mind checking my selection for this part? I'm replacing the bridged output jack on an ADA Microtube 100. The output jack is an unusual Switchcraft enclosed jack. It doesn't seem like anyone stocks the PCB mount version but I thought I had found a version with solder lugs that might work with is the N113EX. I've got it installed and I'm still having very low output from the bridged section. I've got output from either side in mono. I'm attaching the best schematic I can find (sorry it's a bit burry). I'll also post the Switchcraft master data sheet. If anyone can help me out it would be much appreciated!!

[nosaj]

Hello everyone,

Would someone mind checking my selection for this part? I'm replacing the bridged output jack on an ADA Microtube 100. The output jack is an unusual Switchcraft enclosed jack. It doesn't seem like anyone stocks the PCB mount version but I thought I had found a version with solder lugs that might work with is the N113EX. I've got it installed and I'm still having very low output from the bridged section. I've got output from either side in mono. I'm attaching the best schematic I can find (sorry it's a bit burry). I'll also post the Switchcraft master data sheet. If anyone can help me out it would be much appreciated!!

Low output from the bridged section? Is it low when it gets to the jack ?
Was jack broken?
nosaj

[Enzo]

N means plastic - as in insulated - bushing. In bridge mode we can't have either side of the output grounded.

IF I READ IT CORRECTLY, the extra contacts open to put the unit in bridge mode. That means to me the channels are inverted one from the other. A out becomes the hot, and B out becomes the sleeve in the bridge jack. Using A or B outs leaves those contacts closed thus grounding the bridge sense line. SO it seems to me if you simply temporarily wire up a simple jack with B to sleeve and A to tip, you OUGHT to get good bridge out. Hopefully that works. If it does, then what we need is for the jack switching to work. The extra contacts must open when a plug is inserted.

I think you need N113EPCX to fit, while N113EX ought to work if wired right.

[The Dude]

^^^^^^ Agree with all of that. I'll add, it looks like the original part has 2 switches, but one is unused, so you don't need to worry about it.

[Enzo]

Yeah, the tip cutout contact doesn't matter. But it is crucial that the extra set be independent, ie not electrically connected to the signal path.

And make darn sure you are using the correct terminals for the wires. It is not hard to get confused looking in there. Don't ask...

[allsignalmustpass]

Thanks for the quick and helpful replies! I'm pretty sure I've got the N113EX wired correctly (no one seems to carry the PC-mount version). I'm starting to suspect that my problem is in the switching. When I first turn the amp on, the mono input and bridged output produce signal. After 15-30 seconds the output drops to almost nothing suddenly with maybe a small pop. Interestingly, when I probe with a scope the visible signal on the jack doesn't change when the audible signal drops out. I'm wondering about the 4053 IC but I don't see how it could be causing trouble when I'm seeing big signal swings on the output. What's the best way to measure the power output of the "floating" bridged output? I've swapped speaker cables and speakers.

[Enzo]

Frankly I like the test I proposed, remove wires from 113 jack for now, tack a couple wires to the two outputs - A and B - and connect them to an isolated jack in the air. Does it produce bridged sound or not. That will be proof of function. Then also, the bridge sense line. Grounding it or not should turn bridge off and on. Does toggling that to ground cause the test output to come and go?

The two outputs will work in both modes, they will sound and appear on a scope. What won't be apparent is that in bridge mode, one will have reversed its polarity. If the two signals are at the same polarity, then there will be no difference between them, so in the bridge jack, no sound. Invert ONE of them and now the bridge works. So the fact your new jack is in there, and I will presume correctly, tells us either one of the outputs is not reaching the jack, OR the sense line is not causing the one channel to invert. We need to find out which.

Got a two channel scope? Scope each output A and B on one channel of the scope. Put it in dual display. Pump a sine wave through. DO the two look opposite in phase?

[allsignalmustpass]

Thanks again. With the A and B outputs soldered to a regular switchcraft jack floating in the air and the C3 and C4 isolated switch connections desoldered and also floating I get out of phase signals on the tip and sleeve when the amp is first turned on. Within 15 to 30 seconds the signals pop into phase. C3 appears to be the bridge sense switch? Grounding either C3 or C4 doesn't make any difference.

[Enzo]

C3 appears to be the bridge sense. Note it follows up and over to R212, which is pulled to -12A supply. That controls the gate at U201A. SO with C3 floating, does a negative voltage appear there? If the control voltages seem to work, sure, I might replace U201. I stock 4053.

[allsignalmustpass]

With C3 floating, I have -12v on pin 10, which is the sense for u201a. Connecting C3 and C4 doesn’t change the voltage, which seemed weird to me. Digging deeper, my meter showed continuity from C4 to the star ground on the chassis, and continuity from C3 not to pin 10 but to pin 11 on the 4053 which seems incorrect looking at the schematic. Pin 11 is the sense for U201B. That one seems to be stuck near 0v. I’m going to double check the voltages on 11 tomorrow morning and see if they change with the signal popping into phase shortly after startup. Maybe R212 is failing?

[g1]

I'm not sure how getting in phase would make you lose your output. Worst case is the bridged out becomes the same as a single channel, not some lower weak output.
Does the amp function perfectly on both channels at high power when in normal, non-bridged mode?

What was the original fault reported?

[Enzo]

Think of a typical stereo power amp. For bridge you use the two POS terminals. One channel is inverted, so the NEG side of the speaker is connected to POS of one channel and the POS terminal of speaker goes to the POS of the other. If the two are in phase, then both sides of the speaker receive the same signal - and so ther is no difference between them to put a voltage ACROSS the speaker. It is the opposite phase of the two side that create a signal ACROSS the load.

Likewise here, in the bridge jack, the sleeve of the bridge out is wired to the POS of one channel, while the tip is wired to the POS of the other. If both have the same signal phase, then there is zero across the bridge out.

[g1]

Ok, I see that. But then it shouldn't it be zero, not 'low'?
My big concern here is that the load is not truly floating, and some other piece of equipment is grounding one side. Then, depending on the sophistication of this circuit, maybe the amp 'senses' it and flips the signals back in phase?

The test procedure in the factory literature is to set it up stereo (both channels loaded), go in the mono input, and stick a dummy plug in the 'bridged' output jack. If amp delivers full power stereo out of phase, you are good to go.

[Enzo]

Should be, but that assumes the two channels have zero differences. Yes, the test procedure is consistent with my theory.

I don't see anay circuit that would detect anything and veto bridge. ANy protection would most likely just cause a shut down.

In any case. my test was two fold: find if the thing worked in bridge at all, and find of the control worked to switch it.

[allsignalmustpass]

Update: Upon further investigation all of the mechanical switching worked properly when testing for continuity. However, when probing voltages the chip I found that the phase would switch semi-randomly sometimes when I touched the DMM probe to pin 10. I checked for bad solder joints and everything looks ok. New chip ordered. I'll get it installed and report back.