effects loop issue

[daz]

I put a passive loop in my 6V6 amp so i could use a Boss 1/2 rack size DSP. Master pot out>send jack>DSP>return jack>PI input cap. So heres the issue....no matter where i set the amp's master i then set the input level on the DSP at the max i can without clipping. It has a clip meter on it. Then i have a global out level on the DSP i intended to use as my overall master volume since i can't touch th master once the DSP's input level is set. The patch is set to 100% volume and the DSP's overal output goes to 200%. Even set to 200% with the input meter at max the amp doesn't get anywhere near as loud as needed to play out. Use the amp w/o the DPS and it';s loud as F. Oh, and theres a -20/+4 switch on the back i keep at +4 or else the input will OD at very tiny master levels.

This would seem to be a DSP issue but them why would a rack processor not allow at the very least unity gain from input to output. I have gone thru the patch and global functions of the DSP and anything i could find that would limit output levels is full up. Any ideas?

[loudthud]

So after the levels are set to satisfy the DSP, what happens if you unplug the DSP and substitute a path cord? Is it plenty loud?

[Gregg]

It's all a matter of input/output levels.
As you noticed if you set your send level to avoid DSP clipping it's not enough to drive the PI and the amp to full power. You may need up to 3 Volts of signal at the PI input in order to get a full drive and if your DSP clips at -10dB or even at 0dB that's not enough.
You may also have output impedance issues (too high) considering you're taking the Send signal from your Master pot.

[daz]

So after the levels are set to satisfy the DSP, what happens if you unplug the DSP and substitute a path cord? Is it plenty loud?

Yep, loud as hell.

[daz]

It's all a matter of input/output levels.
As you noticed if you set your send level to avoid DSP clipping it's not enough to drive the PI and the amp to full power. You may need up to 3 Volts of signal at the PI input in order to get a full drive and if your DSP clips at -10dB or even at 0dB that's not enough.
You may also have output impedance issues (too high) considering you're taking the Send signal from your Master pot.

Itls designed to be both instrument and line level so i would think it should be plenty.

[KevinOConnor]

Hi Guys

The effects unit is likely designed to have unity gain from input to output, as most effects and processors are set up. So, there may not be much benefit gain-wise setting the unit for -20dB or for +4dB. The benefit will be that in the +4db mode, it is unlikely that you will distort the input to the effect.

A passive loop cannot be made to properly accommodate -20dB pedals and effects while still providing full output from the amp. You would have to modify the PA's feedback loop if it has one, so its gain will be 10x higher. Given the low open-loop gain of most tube PAs, that means simply opening the NFB loop when a pedal is in the effects loop.

An issue that might be affecting things is the input impedance of the effects unit. This might load the MV and reduce signal level, making the overall problem worse. The best fix is a proper active loop with adjustable return gain, such as the ones shown in the Effects Loops chapter of TUT. You can build a loop using opamps or discretes or tubes and all will work well. You should avoid anything like the so-called "lossless loop kits" which are quite dreadful - many design flaws and not lossless at at all.

Have fun

[Steve A.]

daz:

A "Boss 1/2 rack size DSP." Would that happen to be an SE50 or SE70? I had both of those back in the 90's...

As KO'C pointed out a passive FX loop is a bit of a kludge... They may or may not work properly with a particular piece of gear and when they do work there is often a loss of volume. Even with those restrictions I like to add them to amp builds because they are so simple to do.

The easiest fix for your problem might be to run the output from your FX processor through a clean boost pedal or a small mixer to bring up the signal level. I usually can't run my amps at full volume anyway so that hasn't been a problem for me.

Steve A.

P.S. If you already have the voltages required by an op-amp in your guitar amp it is really easy to add a boost to your FX return. Put a variable resistor in the feedback loop and at 0 ohms you usually have unity gain (no boost) which increases as you add resistance.

I like modifying Fender SS amps from the 90's (before they added those darned digital FX!) because they usually have an op-amp stage between the preamp and power amp which can be converted into a completely transparent master volume control by adding a variable resistor to the feedback loop.

[daz]

I guess i'll just it's just not gonna happen. The NFB trick won't work, as i have a push/pull pot on the presence and the wire for NFB going to the speaker jack is on it. When i pull it i have no NFB and the difference in volume while substantial is no where near enough added volume to be stage worthy. I think i'll just get a small 20 watt SS amp to use as effects return and run it from the speaker DI and set the DSP to 100% wet mix. The tone will be better too since no need to run the amp's signal thru a DSP, plus a littl stereo ambience. Used to run like that when i was gigging but gave it up when i became an old mans who just jams with friends now and then.

[KevinOConnor]

Hi Guys

There are other things you can do to increase the post-loop gain.

The most obvious is to add another gain stage. if using a tube, even a low-mu 12AU7 provides a voltage gain of 12 or so in typical circuits. Its mu is 20 and you can tap this fully with a modification that you could also use on the existing tube stage to bump its gain. If you replace the plate load resistor with a current source, you will get maximum gain from the tube. You said you were getting a gian of 78 from the existing stage, and presumably this is a 12AX7. Bumping this to 100 is only a 2dB increase - audible but not impressive.

If you don't have supplies for opamps and no space for another tube, you can add a discrete mosfet stage with fixed or variable gain. keep in mind that an added gain of just 2x multiplies the net boost from 78 to 156, definitely something you can hear. using a high-voltage mosfet the extra stage can be powered from the tube plate supply. Just decouple it so you do not run into oscillation. Use a TO-220 package mosfet not one of those LN-150s in a TO-98.The mosfet is not going to run hot or even warm but those LNs are not all they are cracked up to be.

Have fun

[Steve A.]

Hi Guys

There are other things you can do to increase the post-loop gain.

The most obvious is to add another gain stage.

If another tube was to be added I think I would go with one of the tried-and-true buffered FX loops on the internet or in your books.

BTW thanks for bringing up MOSFETs!

Steve A.

[KevinOConnor]

Hi Guys

Steve, I'm always coughing up something... mosfets... bits of wire... cat hair...

The mosfet gain stage is wired like a concertina spllitter with divider bias for the gate. A cap in series with a pot from the mosfet source to ground provides adjustable gain. Signal is taken from the drain through a coupling cap. Depending on the pot value, you might be able to go down to unity gain but this often results in a dead part of the sweep. You can also just use a fixed R to ground instead of the pot for fixed gain, or switchable Rs for precise gain steps.

I used this in my Aurora amps and a tube version in all kinds of things.

Have fun