Fender ABY signal drop on Y into Vox

Hattigol

Ok, so I have a Vox AC15 HW. I run a Fender ABY pedal into it so I can switch between the normal and top boost channels. That works fine.

The problem is when I want to run both channels together. When I do that, the signal strength/volume drops significantly. 

Is that an issue with the pedal? Would a different pedal do this satisfactorily? Or is it the amp (seems unlikely)?

prlgmnr

You need a buffered switcher/splitter otherwise you're effectively splitting your signal in half in strength terms. Something to do with impedance. Someone who actually knows what they're talking about will fill you in soon.

A buffer in the chain before the splitter might sort it.

joetele

I had the same issue when trying to A/B my guitar and tiny Korg synths via my pedals and amp, especially at the same time, but thought that was more to do with the synths and their volume through guitar equipment

ICBM

I'm pretty sure the two channels feed into opposite sides of the phase inverter on this amp, so a volume drop when running both together is normal.

Sporky

ICBM said:

I'm pretty sure the two channels feed into opposite sides of the phase inverter on this amp, so a volume drop when running both together is normal.

Not being terribly familiar with amp innards, does this mean that an ABY with a phase switch for one channel (and appropriate circuitry inside, not just swapping hot and ground on the jack) would solve the problem?

thermionic

ICBM said:

I'm pretty sure the two channels feed into opposite sides of the phase inverter on this amp, so a volume drop when running both together is normal.

My guess was that the Top Boost channel has an extra triode stage which inverts the signal compared to the normal signal path. If both went to the same side of the PI, would this have the same effect?

Hattigol

My mind is frying....
So it's not the pedal - it's the amp?

ICBM

Sporky said:

Not being terribly familiar with amp innards, does this mean that an ABY with a phase switch for one channel (and appropriate circuitry inside, not just swapping hot and ground on the jack) would solve the problem?

I’m not sure, because it’s not a phase issue as such - although the channels are out of phase as well due to the extra gain stage on the Top Boost channel - it’s because the ‘virtual ground’ on the opposite side of the phase inverter is no longer fixed, which reduces its gain.

Someone who understands the theory better than I do, like @jpfamps or @RiftAmps, may be able to confirm or correct me!

Hattigol said:

My mind is frying....
So it's not the pedal - it's the amp?

Yes. The pedal does nothing more than connect the guitar to both channels.

Hattigol

Just been reading - even more confused.

Saw this on the Radial Bigshot page:

Q: When using two amps, will the original guitar signal be half-strength to each amp or will each amp receive full signal from the guitar?
A: with passive devices like the BigShot ABY, the signal strength from the guitar will basically be reduced by 3dB which technically means half the power. For those that do not want the level to go down, but want more of a wall of sound... products like the Radial Bones Twin City and the Radial Tonebone Switchbone employ a buffer to retain the level so that it is maintained.

That to me suggests it is in fact the pedal?

ICBM

Hattigol said:

Just been reading - even more confused.

Saw this on the Radial Bigshot page:

Q: When using two amps, will the original guitar signal be half-strength to each amp or will each amp receive full signal from the guitar?
A: with passive devices like the BigShot ABY, the signal strength from the guitar will basically be reduced by 3dB which technically means half the power. For those that do not want the level to go down, but want more of a wall of sound... products like the Radial Bones Twin City and the Radial Tonebone Switchbone employ a buffer to retain the level so that it is maintained.

That to me suggests it is in fact the pedal?

No, that is not true. The signal level is reduced *very* slightly by being loaded by the two input resistors, but nowhere near 3dB, or even 1dB. Each channel will still see almost the same signal level as with only one.

ToneControl

The twinstomp ABY does not reduce the volume when using both 

Hattigol

ToneControl said:

The twinstomp ABY does not reduce the volume when using both 

With a Vox?

Hattigol

ICBM said:

Hattigol said:

Just been reading - even more confused.

Saw this on the Radial Bigshot page:

Q: When using two amps, will the original guitar signal be half-strength to each amp or will each amp receive full signal from the guitar?
A: with passive devices like the BigShot ABY, the signal strength from the guitar will basically be reduced by 3dB which technically means half the power. For those that do not want the level to go down, but want more of a wall of sound... products like the Radial Bones Twin City and the Radial Tonebone Switchbone employ a buffer to retain the level so that it is maintained.

That to me suggests it is in fact the pedal?

No, that is not true. The signal level is reduced *very* slightly by being loaded by the two input resistors, but nowhere near 3dB, or even 1dB. Each channel will still see almost the same signal level as with only one.

@ICBM I'm not one to question you - but Radial wouldn't say that unless it were true....would they? Read a similar thing somewhere else referring to a 6 dB drop.

I'll change the pedal if it's that.

ICBM

Hattigol said:

@ICBM I'm not one to question you - but Radial wouldn't say that unless it were true....would they?

Radial are the company which prints ‘True Bypass’ next to the switch on a pedal which isn’t, so interpret that how you wish...

Hattigol said:

Read a similar thing somewhere else referring to a 6 dB drop.

That would require the signal voltage to be halved, which is even more wrong.

Hattigol

Thanks @ICBM ;;

Ok, so it is now fixed. I turned it on last night, tried A then B then both - lo and behold, the two channels were singing together wonderfully with no drop in volume - just how I wanted it.

Was then adjusting my settings and realised that my 'always on' pedal, the Rockett Josh Smith Dual Trem, which runs in the A (Normal) channel, was off (my six year old is a git for playing with my pedals). Turned it back on and noticed that the volume drop when using both channels was back. I'm not being a halfwit here - I didn't have the volume down on the pedal, it was at 1 o'clock which gives it a great slight boost. Turned it off again - both channels working fine.

So, I moved it in the chain to immediately before the ABY switcher and BOOM, it has fixed it.

Presumable something complicated and electrical in that pedal is causing this to happen when it is after rather than before the ABY. It is  beyond my comprehension entirely but there you go. Sorted.

Thanks for the advice, folks. It does work brilliantly kicking in the B (Top Boost) channel...

ICBM

Hattigol said:

Ok, so it is now fixed. I turned it on last night, tried A then B then both - lo and behold, the two channels were singing together wonderfully with no drop in volume - just how I wanted it.

Was then adjusting my settings and realised that my 'always on' pedal, the Rockett Josh Smith Dual Trem, which runs in the A (Normal) channel, was off (my six year old is a git for playing with my pedals). Turned it back on and noticed that the volume drop when using both channels was back. I'm not being a halfwit here - I didn't have the volume down on the pedal, it was at 1 o'clock which gives it a great slight boost. Turned it off again - both channels working fine.

So, I moved it in the chain to immediately before the ABY switcher and BOOM, it has fixed it.

Presumable something complicated and electrical in that pedal is causing this to happen when it is after rather than before the ABY. It is  beyond my comprehension entirely but there you go. Sorted.

It's not complicated - the pedal also has a simple phase reversal when it's on. That will reverse the phase *again*, on top of the channels being out of phase and feeding into opposite sides of the phase inverter - which they are and do, I found the schematic.

I was slightly wrong about part of it - if the inputs to the phase inverter are on opposite sides *and* out of phase, it will work correctly because the two effects cancel out, which is what you've now found by taking the extra phase reversal out.

Which also proves that Radial are wrong about the 3dB drop. (And anyone else about 6dB.)

Hattigol

Wis awarded, thanks again.

ChrisRG

Great info on this thread and glad you got the issue sorted. 

I was just about to recommend a Radial Bigshot to a mate who wanted to run 2 amp separately and together.  It seemed to have all the right options and was within his budget but I started to get worried that it would drop the signal with both amps on as mentioned above.

Yet again, @ICBM is the voice of wisdom and knowledge.  This place is great.

*goes off to send some cash to tFB which he meant to do ages ago.....*

thermionic

Thinking about it, it makes a lot of sense to have channels that are out of phase sent to opposite sides of the PI. That way the phase at the power amp, and most importantly, speakers, will always be the same. If you were using such an amp in a two-amp setup, miked up...

ICBM

thermionic said:

Thinking about it, it makes a lot of sense to have channels that are out of phase sent to opposite sides of the PI. That way the phase at the power amp, and most importantly, speakers, will always be the same. If you were using such an amp in a two-amp setup, miked up...

If only all amp manufacturers thought about this! But absolute phase is not considered as important in guitar equipment, if it's even considered at all.

There are plenty of amps which reverse phase - at a guess it could be a completely even split, there's no real reason to think otherwise - and many which have more than one channel which are out of phase with each other, both traditional parallel-channels like Fender BFs with reverb, and channel-switchers. This makes combining them in multi-amp set-ups complete pot luck as to whether they will work together or not.

Pedals too, as here.

Ossyrocks

Just for info. I get great results with two amps and a Radial Twin City. Prior to this, I did notice a gain reduction when using both.