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Mackie 808M power amp in jack not isolating?

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Mackie 808M power amp in jack not isolating?

Postby Soundside » Thu Jan 17, 2019 5:42 am

Hi all, newbie here,
I'm trying to put an EQ between the line out and power amp in on the 808M per the manual. But I noticed that the power amp in jack does not appear to isolate the amp from the mixer as the manual states. It says that when you insert a plug into that jack, it breaks the connection between the mixer and amp. But with a signal in an input channel, if I plug a plug into the power amp in jack, I can still hear the program from that channel loud and clear.
Now, if I plug a signal straight into the amp in jack, it does in fact pass the signal to the amp. But I'm not getting the expected behavior out of the EQ so I'm wondering if I have a problem with the jacks or something. Although, these jacks have hardly ever been used, they are tight, no signs of mechanical issues.
Any guidance is appreciated.
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Re: Mackie 808M power amp in jack not isolating?

Postby Hugh Robjohns » Thu Jan 17, 2019 11:28 am

As you say the socket is tight, I'm wondering if you're not pushing the jack plug fully home to disengage the internal switch contact.

It might be worth trying with a different brand of jack plug as the dimension tolerances do vary between brands.

H
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Re: Mackie 808M power amp in jack not isolating?

Postby Wonks » Thu Jan 17, 2019 11:58 am

The block diagram does show the jack tip breaking the through connection when inserted. Although a TRS socket, it's an unbalanced input and the R connection is taken to ground.

As Hugh says, feed a signal in to the mixer then try some different jacks in the socket, TR or TRS types, it doesn't matter. If you've got any contact cleaner, I'd spray some in the socket and push a jack in and out a few times. You don't need any signal on the jack. If you can get the output to cut out, then the jack is (finally) doing its job.

Is this a new or a used mixer? If someone has had it before you, then someone may have linked across the power amp in jack socket terminals to increase the reliability. Those switched jacks can become unreliable over time if not used and the contacts can corrode. So it's possible that someone has done some DIY on those sockets.

If the sockets are remote from a PCB, but wired to one, then there is always the faint possibility that some of the wires were crossed during assembly.
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Re: Mackie 808M power amp in jack not isolating?

Postby shufflebeat » Thu Jan 17, 2019 4:17 pm

I had the stereo version, well thunk through and solidly built. Great little box with many loyal tributes (cheap copies).

My 808s had the same "feature" as yours which required fettling by a chap. Simple procedure but ultimately possibly not as much use as you might think.
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Re: Mackie 808M power amp in jack not isolating?

Postby Soundside » Thu Jan 17, 2019 8:05 pm

Thanks all for the comments. I admit I had to go look up "fettling".

I agree this is a great little box. Smart features and solid build. It has never been repaired or modified. I had the box open not too long ago because I had some issues getting signal out.... it had been a number of years since it saw action, and some of the switches had gotten some corrosion, so I opened it up and cleaned everything really well, exercised the buttons and it worked fine. I hate to have to open it up again (and I have a gig tomorrow), but it looks like it might be necessary. Might just be corrosion or other minor issue with the jack.

I didn't do it yet, but one thing I want to do is just try it with the monitor channel. It has the same features. And for now, it is actually more important, because I want to put an EQ in there for better feedback control in the monitors.

The main reason I want to use this feature on the mains is to insert a compressor/gate. I know the amp has compression, but I like being able to control the gain reduction and such.
I know what you mean about tolerances on the plugs, that is sometimes irritating on some guitar jacks. Chinese manufactured stuff sometimes not up to snuff.

While I have your ear, there is another issue with this head. One channel is showing a clip light, almost all the time, whether there's anything plugged in or not. It doesn't create any problems (not noisy), and I don't need the channel at the moment, but I wonder what it is, and I wonder if it might eventually cause some horrendous screaming sounds or damage some other part of the amp. I've exercised all the pots and nothing seems to affect it.

Thanks again,
Keith
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Re: Mackie 808M power amp in jack not isolating?

Postby shufflebeat » Thu Jan 17, 2019 10:33 pm

Got it.

My reason for getting the work done was to but a graphic EQ between the mixer and amp sections. I was on a bit of a mission at the time to make the most of the system I had. What I subsequently learned was that the 808 was fine for what it was but if I wanted something substantially better I would have to upgrade my system for something substantially better, designed for the job. Tinkering with the Mackie would achieve only minimal improvement which only I would notice.

It was an interesting learning process but probably more drawn out and expensive in the long run than necessary.

There was much head scratching when we were discussing the mod as it seemed obvious that the system had been designed to insert an inline processor. The only justification we could imagine would be to add some extra channel count from an external mixer but it turned out that this was an anomaly unique to early units which was put right before too long.

As far as controlling monitor feedback is concerned, the basic built in EQ is pretty good. I would suggest, beyond that, good mic/monitor choice and placement would be much more effective than finer resolution control.

Hope that helps.
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Re: Mackie 808M power amp in jack not isolating?

Postby Soundside » Fri Jan 18, 2019 12:47 am

Shufflebeat, you are on target I think. I know there are only marginal returns available by increasing the EQ resolution, but I already had the EQ and I thought it was going to be an easy no brainer. In fact, when we used to gig with this amp we ran a compressor in this spot and that's what I was first trying to do.
We may just keep it simple and not worry with it. As you say, the amp is pretty good for what it is, and with some care, we can get good sound from it. An upgrade to more "modern" hardware is probably in the cards for the future, assuming our little band achieves some success and stability.
Thanks again,
Keith
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Re: Mackie 808M power amp in jack not isolating?

Postby Soundside » Fri Jan 18, 2019 12:53 am

Just an afterthought. I think there are a number of options for this arrangement on the amp. One thing you can do is easily add more power amps to expand the sound level. The line out jack is not circuit interrupting, so, you can tap this jack and employ multiple amps to give lots more horsepower to your program. And of course it allows you to just use the amp section of the box with an outboard mixer.
It's a nice little workhorse of a mixer/amp, perfect for a small acoustic act like ours.
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