Author Topic: Tribute effect loop (Read 351 times)

1dallek1 · « **on:** June 08, 2012, 11:12:16 PM »

hi, this is a tricky question, if you have a trs cable connected to the effect loop jack on the tribute but the other end of the trs is not connected to anything, when you switch on the effects loop using the switch should it be dead quiet or will you still hear some signal from the pickups? I would expect to hear nothing based on my other (o.b.e.l.) guitars, but the fact is there is a week pickup signal. ??

pace · « **Reply #1 on:** June 11, 2012, 04:23:52 PM »

Hmmm.... Is the jumper for the Tribute's loop board set for pre or post master volume???

Also, is the other end of the TRS cable not connected to anything, or is it going to an unused splitter box of some sort???

good luck and let us know what you find!

1dallek1 · « **Reply #2 on:** June 15, 2012, 09:27:32 AM »

the jumper is pre, this is when just a cabel with with or without a splitter but there is no loop connected at the splitter. when the loop is connected you cant hear the issue because you hear the loop. when a trs is connected and the loop switched in there should be no sound as far as i know, does alembic do something different

pace · « **Reply #3 on:** June 15, 2012, 08:02:57 PM »

Have you tried different brands of TRS cable? There might be a physical descrepency between your different TRS jacks, the Alembic being the 'odd man out' .....

1dallek1 · « **Reply #4 on:** June 17, 2012, 07:13:33 PM »

hi i dont want to say this the wrong way, as far as the wires go and break out boxes and frankly the wholr obel thing , thats what i do, the trs wire is tip=send ring=return and ground=ground. that could not be responsible here any way. i was reallky asking first if the alembic techs read this and advise me and if any alembic users please if they have the obel see if theris does this. i will trace the system if i have to but i was looking for others input' please thank you
ns

pace · « **Reply #5 on:** June 17, 2012, 08:30:56 PM »

Cornelius, as far as OBEL's go, that's what I do too.... I understand TRS= tip/ring/sleeve (send/return/ground)..... The reason why I asked about swapping out different brands of cable can be found in a thread as recent as this:

http://club.alembic.com/Images/449/133592.html?1339800547

Apparently, Monster Cables don't take well to Switchcraft jacks.

The fact that your jumper is wired pre-volume, and you're getting a weak signal can only mean that there is either a descrepency between the jack and cable, excess solder on the board, or melted insulation between leads. Knowing the quality of Alembic's work, I find the the last two scenarios highly unlikely.

As far as the techs reading this, shoot Mica an email, and she'll relay your problem to Ron or Tony......

all the best, I know how frustrating these things can be....

1dallek1 · « **Reply #6 on:** June 18, 2012, 02:24:12 PM »

hi ok thanks for the reply, i did not say there was a weak signal though, i said there is a weak signal present when there should be no signal at all. with an unterminated trs in the loop jack and the loop switched in i ger a very weak signal when there should be nothing at all

pace · « **Reply #7 on:** June 18, 2012, 05:14:09 PM »

Yeah, I understand that..... With the loop switch on you are getting a weak signal when there should be nothing..... Sorry I did not take the time to reply verbatim......

Do you own a multimeter with a continuity function? Putting its leads on tip and ring of the open end of the TRS cable should be able to tell you if the signal is 'jumping' on that end of the OBEL's circuit..... If the meter shows no continuity there, then check continuity between leads at the OBEL's bypass switch.

rjw · « **Reply #8 on:** June 18, 2012, 05:42:49 PM »

the small signal present with an unterminated trs cable is normal for the Alembic effects loop circuit. buffer amps drive both the send cable, and provide a hi-impedance termination to the receive cable.

many trs cables have the two signal carrying conductors inside a single overall shield, and thus there is a stray or parasitic capacitance between the signal conductors. with the cable unterminated, the high impedance, (1 MegOhm), receive buffer can hear the signal in the send wire coupled through the parasitic cable capacitor. with an effects device connected, the send impedance of the device attenuates the coupled signal (most effects devices have low output impedances to drive the cable).

while the circuit could be modified to terminate the effects receive line 25 or 50 kiloOhms, to attenuate the crosstalk on unterminated cables by 26-30 dB, the effects device would be loaded by this lower impedance. note that if the receive buffer were omitted (as it is in some other designs), then the the output cable stray capacitance attenuates the crosstalk, but then the effects device has to drive the higher capacitance of both the trs cable and the output cable in parallel.

since guitar amplifier inputs have high impedance, the effects designer may have assumed that his device would be loaded by approx 1 MegOhm shunted by the cable capacitance of a single cable. including the return buffer makes the output signal loading similar to plugging the effects device in-line as it is if there is no effects loop circuit.

if the trs cable is constructed with two separately-shielded cables (which may be side-by-side, in a single overall jacket) then the separate shields give complete isolation between the send and receive signals, and that cable construction would not have the weak signal crosstalk on an unterminated test.

but the crosstalk also disappears when the effect device is plugged in as in normal use, so the unterminated crosstalk is only a special test condition. the engineering tradeoffs of unloading the effects device from the longer cables in series, or loading the input of the buffer intentionally with a lower impedance while eliminating the unterminated cable crosstalk, the option to provide a high-impedance receive buffer ensures that the effects device performs the same as if it were in use with an instrument without the effects loop feature.

1dallek1 · « **Reply #9 on:** June 19, 2012, 11:17:32 AM »

please thanks for taking the time to deal with this, the reply from rjw is what I was trying to confirm. this was going to either be a crosstalk situation ( which would be hard for me to confirm ) or it was hardware related (some kind of shorting). I was trying to avoid probing around the cavity looking for a possible short and possibly making things worse in the process. i was unaware of the buffering on both ends, but this has explained the situation. one more question please, does the tribute use 2 separate devices for this buffering or is it achieved from one unit? again thanks

mica · « **Reply #10 on:** June 19, 2012, 01:37:16 PM »

It's a dual opamp in the buffer circuit card, so both are in one 8-pin package.

1dallek1 · « **Reply #11 on:** June 23, 2012, 10:43:07 PM »

` please one more question, is possible to have an instrument cable (TRS) that could show crosstalk on the 2 conductors tip & Ring? measuring it with an automatic ohm meter. reading continuity across the tip & ring it should always show infinity or no connection at all correct? Or is it possible to have some kind of cross talk there just taking this simple measurement?
ns