Line out from OT voltage ratio question and such

[nsubulysses]

A friend gave me a blue Matamp to look at. someone added a line out along the way and he doesn't know if it actually works. It works, but from looking at other schematics it seems the voltage divider off the OT secondary is a pretty high ratio, in this case 6.8K and 470K, 69-1.

Viewing schematics of other amps I'm seeing these types of dividers in ratios of about 10-1 to about 50-1. I'm not sure if I'm looking at it the wrong way but does this line out output seem low compared to the output into speaker load? Only about 230mV from the lineout when the amp is putting out full volume at clipping? Also why do the two waves look so different after clipping if they come from the same source?

This shows output into a 4 ohm load with the yellow trace, and line output on the blue trace. Below the waves you can see RMS output in the corresponding colors.

[Justin Thomas]

There was a pretty good explanation here about how to figure it out, and the chart is a lifesaver... this is for a line out off of the OT secondary; is that what you've got? And generally you'd want around 1 volt from the line out for general purpose use.

http://music-electronics-forum.com/t10582/

And another...

http://music-electronics-forum.com/t10985/

I've never had any complaints about noise with these from soundguys; never used them for recording, though...

Justin

[Chuck H]

I've never had any complaints about noise with these from soundguys; never used them for recording, though...

Matchless includes a .022uf or .047uf cap from the line out jack + lead to the - lead. I simulated this and found it doesn't really do much. Roll off is well above guitar frequencies. That said, I started including it on any customs I build since I include a line out on all of them. The line out would be going into a very flat response amplifier. I haven't tested if it makes a difference in that application, but it can't hurt.

My absolute favorite use for an OT/speaker driven line out is to run it into effects and then run that to a board or separate amp. This allows you to add effects like reverb to amps that clip the power tubes and would just garble many effects to death if they were built in or run through a loop.

[nsubulysses]

Great links and totally helpful thanks Justin. I see Chuck has answered this questions many times over the years

I tried to search but the word "out" is not searchable, making line out hard to search for.

I found this too which I thought was good -- Doug Circuits

[g1]

I tried to search but the word "out" is not searchable, making line out hard to search for.

I rarely get any satisfaction from the forum search function. Use google advanced search and specify music-electronics-forum.com on the line listed as "site or domain".
To get into google advanced search, click on the gear icon on the regular google page.

[J M Fahey]

I rarely get any satisfaction

Hey !!!!!
That line sounds good !!!!!
Do I have your permission to use it (or some slight variation) for a song title?
I guess such a song might become very popular, even famous.
What do you think?

[Tom Phillips]

I am wondering why the "line out" signal shown in post #1 is noisy, distorted and why the phase is shifted so much if it has simply been attenuated by a resistive divider. Something seems amiss.

[Chuck H]

I am wondering why the "line out" signal shown in post #1 is noisy, distorted and why the phase is shifted so much if it has simply been attenuated by a resistive divider. Something seems amiss.

nsubulysses was wondering the same thing but it hasn't been addressed yet. I can't think of a good reason unless there were some other components in the circuit not mentioned. Sometimes I use the line out on one of my amps as a wave form reference point. It is identical in shape and phase to the speaker output.

[nsubulysses]

Yes was wondering about the phase shift too. There are just two resistors coming off the speaker jacks, 6.8K from ground and 470K from tip. The ground of the speaker jack goes to ground of line out and tip for line out comes from the junction of 6.8K and 470K resistors.

I won't be able to check this again til Sunday or Monday. I have only lightly persused those links too so I gotta get to that, will report back

[Enzo]

Are both scope traces synced to the same source and to the same level?

The circuit might be a simple pair of resistors across the load, but is the bottom end of that pair grounded to the speaker jack or is it grounded to chassis elsewhere that the speaker negative lead?

[Dave H]

It can't just be a simple resistive attenuator because of the phase shift and the way it is differentiating the edges of the square wave. There must be L and or C in there. As the phase is lagging perhaps the load is inductive? The phase shift is 90 deg so the test frequency is well away from the -3dB point which could explain the high attenuation. Try higher and lower test frequencies to see if the attenuation changes.

[Dave H]

I rarely get any satisfaction

That's far too posh and gramatically correct for an R&R song Juan

[Justin Thomas]

It can't just be a simple resistive attenuator because of the phase shift and the way it is differentiating the edges of the square wave. There must be L and or C in there...

Wirewound resistors? They're basically a coil, right? I used one once to approximate an open coil in the active tone controls of a Super Twin. It worked kinda, and got the amp sold...

Justin

[nsubulysses]

Enzo, the video above uses scope set to sync on channel 1 which is yellow. Yellow trace is scoping a 4 ohm dummy load and the blue trace is channel 2, scoping the line out. I am just scoping it at the jack though, it is not plugged into anything. The resistive divider is grounded at the speaker jack, not at a separate place

Dave, pretty sure there is no L or C in there unless it is the capacitance of the wire running along the length of the chassis. I'll check again tomorrow just in case I missed.

I know it's hard to see but in this pic you can see the voltage divider in top left, right across the speaker jack, the two wires from it run down to bottom left of pic to the line out jack.

[Tom Phillips]

My input is that parasitic capacitance from the wiring and the small inductance of any wire wound resistors will not be an issue in this audio frequency case. They certainly would not cause the drastic phase shift shown.