A15Mk1 FXloop

Re: A15Mk1 FXloop

Looking at the small schematic available on the madamp site, I see that the FX loop on the A15 is very simple. It's just a buffer on the fx send. A more advanced loop design would use another tube triode to amplify the fx return signal back up to preamp levels.

A guitar tube preamp can produce signals of over 100V peak-to-peak. A typical rack effect processor will probably output less than 10V. So about "half" the volume (10dB lower) seems reasonable.

Re: A15Mk1 FXloop

Hi,
d95err I think you've missed the point here
Yes, a tubepreamp CAN produce upto 100V peak2peak, but the A15 won't. The signal level at the send is about -6 to +6dB, so this won't be an issue for an fx processor. The Digitech 2120 is specified with +18dBU max at 10k (input) and +18dBU max at 470 Ohm (output), so this should work fine.
david_djd Are you sure, the output level is set appropriate? I've read in the manual, that the output level is switchable. Is the input LED flashing, when you play?
Regards,
Martin

Re: A15Mk1 FXloop

Zitat von MAD

Hi,
d95err I think you've missed the point here
Yes, a tubepreamp CAN produce upto 100V peak2peak, but the A15 won't. The signal level at the send is about -6 to +6dB, so this won't be an issue for an fx processor. The Digitech 2120 is specified with +18dBU max at 10k (input) and +18dBU max at 470 Ohm (output), so this should work fine.

OK, should be allright then. I said I didn't know the levels of the A15, so it was just a guess. With four gain stages, I though the signal would be pretty maxxed at the send jack, but I suppse there is a lot of attenuation between the stages and the tone stack takes the level down a bit too of course.

Re: A15Mk1 FXloop

Zitat von david_djd

last night I have tried all the type of connections to the 2120 (A15 send > input of 2120 with the pré bypassed>output of 2120> A15 Return... A15 send>return of the 2120 (with the tube and EQ bypassed to go direct to the FX)>output of 2120> A15) and I have the same problem.. in all the options the volume decrease a lot.. I can get almost the same volume..connecting in the input and increasing the pot input but the 2120 clip's (a lot!! the red led is always on)...I've got to much input in the 2120 (send of the A15) and not enought output (return of the A15) to drive the poweramp section of the A15...

PS: the output of the 2120 is not switchable.. only the FXloop have a switch (to drive a stomp or a external digital FX unit) I even try to put the pre section of the A15 in the FX loop of th 2120.. the problem is the same..

Without any effects... this little amp sings!!.. ::) and I love Him..

EDIT: I play usualy the 2120 with (my other amps) a Marshall EL84 20/20 or with a Seymour Duncan Convertible 2000 (conecting the 2120 in the FXloop of the Seymour)..

EDIT2: The A15 Fxloop is a série Fxloop? am I right?.. is it suitable modding it to a paralel Fxloop? how?

Have you tried the same setup on the 2120 with another amp effect loop? Have you tried any other rack gear in the FX loop of the A15Mk1? If you can try that, you could verify if the problem is with the A15Mk1 or with the 2120. It is possible to set the 2120 in bypass mode? Does it still lower the volume?

For a paralell FX loop, you really want an active mixer stage on the FX return. This would require one tube triode (an extra tube or sacrifice one of the existing gain stages), or solid state components.

If you want to learn more about FX loops I really recommend "The Ultimate Tone" by Kevin O'Connor. It's *THE* bible of guitar amp construction and has a long and very good chapter about different types of effect loops, both tube and solid state.

An extremely simple passive paralell FX loop (3 100k resistors) is used in some Orange amps, but I have no idea on how it would work in an A15Mk1. I've been thinking about trying it in my M15Mk1 though...
http://www.schematicheaven.com/newamps/orange_mkii1972.pdf