Multiple effects loops in an HRM

[Bob-I]

I'm transplanting a Dumble HRM ckt into an old Peavey Deuce for a friend. He requested an effects loop on Clean, OD and global. All would be passive as he's an effects builder and will build buffering loops for his effects.

My thought is to break the ckt just AFTER the master volumes, add another relay so that the clean loop can be bypassed in OD mode.

The question I have is with regards to the loop specs. I have no idea what the level and impedance would be. I've successfully built both a Dumbleator and a parallel loop based on the D-Lator CF section.

Any thoughts would be appreciated.

Thx,

[heisthl]

According to HAL it's 2.5v RMS -.5Mohms

[Bob-I]

Thx, that's excellent info. I'm assuming you've seen a data sheet that I haven't so I appreciate your response.

[ayan]

Thx, that's excellent info. I'm assuming you've seen a data sheet that I haven't so I appreciate your response.

Bob, I think those specs are just what Dumble wrote on the back panel by the FX loop jacks. The impedance is on the order of 500K if you have the volume control at 50% to ground (it is a 1 Meg pot), but I believe most of us run the master halfway up which would be more like 100+K, which is better for an FX loop send. As for the signal level, you can definitely measure that with a signal generator and a multi meter. The signal level will vary quite a bit, the highest being clean + PAB -- OD compresses tremendously, so you are not as likely to get clipping of the FX that way.

IMHO, padding down and buffering the FX itself is half of the battle, since the signal level has to be restored to the nominal value (call it 2.5 VRMS) before being re-inserted into the power amp to get a full nominal 100W out of the amp. That will pose some limitations when using battery operated FX... If you build something running on a 9V battery, you probably will not have enough heardoom in the power supply to accommodate the large signal swing required to get 2.5VRMS out of the thing -- you will need like 20+ dB of gain. Something for your friend to consider when building his FX.

Gil

[Bob-I]

Thx for the insight Gil. I haden't thought about the limit on headroom on 9V effects.

[d95err]

My thought is to break the ckt just AFTER the master volumes

I notice that lot's of designs put the loop after the MV. However, it doesn't make sense to me. This means you have to adjust the send level of the loop (or the input/output level on effect if it's a passive loop) everytime you change the master setting. All in all, two or three knobs have to be changed in order to set the overall volume.

If you put the loop after the MV, it becomes a true *master* volume, and the loop levels can be set once and for all.

Maybe I'm missing something. What benefits would there be to put the loop after the MV?

[Bob-I]

My thought is to break the ckt just AFTER the master volumes

I notice that lot's of designs put the loop after the MV. However, it doesn't make sense to me. This means you have to adjust the send level of the loop (or the input/output level on effect if it's a passive loop) everytime you change the master setting. All in all, two or three knobs have to be changed in order to set the overall volume.

I've never had a problem with that. The drive into the loop changes, but if all is set correctly it doesn't need to be readjusted.

If you put the loop after the MV, it becomes a true *master* volume, and the loop levels can be set once and for all.

Maybe I'm missing something. What benefits would there be to put the loop after the MV?

The concern is the loading on the output of the gain stage. If you don't have a constant (the MV) then the loading may change.

[d95err]

The concern is the loading on the output of the gain stage. If you don't have a constant (the MV) then the loading may change.

If the loop is buffered, the load should stay constant.

For the multi-loop design, I think the easiest way would be to put one transparent tube buffered loop in the amp (before the MV). This would take care of taking signal levels down to typical solid state levels and back up again. This loop does not need switching.

Then you can build an external solid state loop switching system with one or more loops that can be synchronized with the channel switching on the amp.