Audio Distortion, The internal "Limiter," and
Input/Output gain modifications.

Sounds with high frequencies sound funny. Seems like there is some sort of clipping?

Clipping in the Echoplex Loop audio is a bit frequency dependent, so sounds with high frequencies will tend to distort before other sounds. It sounds quite bad. If you experience this, you have the input level set too high. Turn it down until the loop audio cleans up. You won't experience any distortion when the levels are set right.

The input LED should help you here. You should never see it turn red, that is much too loud. The best point is for the loudest signal to cause the input to just turn orange.


I heard there's a limiter?!

Well, there sort of is, but not really. We have a hardware limiter in there, but it never actually worked right, so we didn't bother to tell anyone about it. I think it might come on at some point, probably keeping an already distorting signal from distorting even more.

The threshold is frequency dependant. So for a full range signal it works, but for a high pitched or attacky signal, it does not prevent distortion. If you are experiencing ugly distortions with sounds with lots of high harmonics, like cymbals or distorted guitars, turn the input down a bit and it will clean up.

The A/D converter can only deal with signals up to a particular amplitude. Anything beyond that causes ugly distortion. Signals with a lot of high frequency components tend to distort more. If you are getting distortion from the echoplex, your input is too loud!!! You should set the input level so that the loudest signal you ever put in does not cause the converter to distort.

There is a circuit in the Echoplex that would be a limiter if it actually worked. Since it doesn't work, calling it a limiter is not really correct. In fact, discussing it at all is sort of pointless. The purpose of having the limiter is to save people who set the levels wrong from experiencing obnoxious digital distortion in their loops. (make the machine smart so the user can be stupid) Limiting just makes the overload condition more palatable. But really, limiting is still a form of distortion, and if you were engaging the limiter all the time you would still be complaining about the sound and I would still be telling you to turn the input level down.

Another solution, which I prefer, is to modify the input so that it has considerably less gain available. That's the next question.....


I don't like the gain structure in the Echoplex. The input gain is too sensitive and the output gain is not loud enough. How can I change it?

This is easily done by changing a couple of resistors. With the change you are much less likely to turn it up enough to distort the input. This also makes the input level knob more useful since more of its range is available. With this mod you also increase the available output gain to compensate for the lower input. In fact, you increase the output enough to correct another common echoplex complaint, which is that the output level is too weak. With the mod the echoplex can work in -10dBu or +4dBu systems without any problem.

To make the gain changes:

Change R30 from 82.5 K to a 22.1 K 1% metal film resistor
Change R10 from 2.21 K to a 10.0 K 1% metal film resistor

This increases output gain and reduces input gain, respectively.

The input section does have a lot of gain available. Probably too much. The reason for this was to allow a wide variety of inputs. We thought it would be great if low level signals, like mics, could be used with the echoplex without requiring a preamp. In retrospect, this doesn't seem like such a great idea, since most people using mics or other low level signals have preamps for them and would probably prefer that over the echoplex preamp anyway. The number of people having trouble with distortion from turning the input up too much is probably much greater than the number that found it useful.

Oberheim is now including this mod in stock Echoplexes, as of late '97 / early '98.

Static from the Foot Pedal

A static electricity shock happened between my foot and the Echoplex pedal. The echoplex freaked out and killed my loop. Is there any way to prevent this?

This came from Bret Moreland, who lives in Denver, Colorado. The altitude of that city is very high and the air is very dry. Static electricity is a very common phenomenon there, much more so than in other parts of the world. (They also have some truly spectacular lightning) Electronics equipment can be quite sensitive to static electric discharges, and Denver is the sort of place where you discover that. Bret experienced a problem where a static shock from his foot to the echoplex footpedal caused the loop to freak out.

I realized this had to do with the manner in which the footpedal chassis is grounded. Static electic discharges seek the easiest path to earth ground. When a charge hits the footpedal chassis, it's only path is up the cable and through the echoplex. This apparently causes a voltage spike which freaks out the Echoplex's processor.

There are two solutions to this. One is to give a charge hitting the footpedal chassis an easier path to earth ground than through the rack unit. You would do this by connecting one end of a grounding strap to the footpedal chassis and the other end to an earth ground somewhere. (water pipe, earth ground in the electrical wiring, even the echoplex rack unit's chassis)

The other solution is to isolate the jack on the footpedal from the footpedal chassis, so that an electric charge hitting the footpedal chassis does not have a path to the rack unit. You can do this with isolating washers or plastic chassis mounting jacks. This is the method Bret tried, here's his report:

It has been a couple of months since I installed a grommet around the jack on my plex footpedal. This was per your suggestion to resolve the problem I noted with static discharge at the footpedal causing the plex to lose the loops recorded. I am happy to report that I have not had this problem repeat with the grommet in place.

Radio Shack has an assortment package of plastic grommets of various sizes. The largest one in the package fits inside the hole and around the jack of the footpedal perfectly.

                                -Bret

Reliability, Thermal Problems, IC Pin Modifications, Undo malfunctions

I heard that there is a reliability problem with installing the maximum memory on an Oberheim Echoplex, causing thermal troubles. Is this true?

No, this is not true at all. There has never been any problem with expanding memory on the Echoplex. Somebody posted this in a review at Harmony Central, causing endless confusion. That fellow was having the thermal problem above, and mistakenly thought it was caused by a memory upgrade.



I pressed Undo and the Echoplex started Record, which killed my loop! How do I prevent this?

I've heard that the Echoplex has thermal problems and is not reliable, is this true?

No, this is not true. Oberheim fixed this a long time ago. For some reason it keeps popping up on the internet, despite my efforts to correct the endless rumor. I'm convinced this will will follow me for the rest of my life.

There was a small design error that caused strange behaviors when the Echoplex was hot. The usual symptom was that pressing the Undo button would execute Record instead. Basically, a pin on one of the IC's was connected to +5 volts when it shouldn't have been connected to anything. This didn't have any effect normally, but when the unit got hot you would see the problem. Cutting the pin fixes the problem, and the Echoplex works fine at much higher temperatures.

Newer units have this fixed. Oberheim corrected this in production in mid-1996. I don't know which serial numbers would have the mod, but if you bought yours after that it is probably ok. Oberheim now does extensive reliability tests on all units before they ship. As of early-1998, Oberheim has made another production change which dramatically reduces the amount of heat that the Echoplex generates, so the newer ones pretty much run at room temperature.

If you have an older unit with this thermal problem, it is very easy to fix. You can do the mod yourself pretty easily if you feel comfortable working on electronics. You just need to cut pin 5 of U12, the IC with part number ADC0804. If you are facing the front of the unit, pin 5 would be the 5th pin from the left end of the chip, on the front panel side.

If you don't think you can do it yourself, any competent electronics tech should be able to handle it.

Diagram of the IC pin modification: