Raising sensitivity of input gain

[Gregor]

Greetings. I'm using a modeler pedal with a stereo amp. The amp's input is designed for line level inputs. However, even with the modeler cranked the amp doesn't get REALLY loud. It's rated at 100 watts (RMS) per side but my
Cyber twin with 65 watts per is easily twice as loud. Are there some ways to modify the input sensitivity so I can get more gain? I know I can run a mixer in front to boost the signal but I was hoping to avoid another box in the signal chain. I'm willing to modify the input electronics if necessary unless someone knows of some inline transformers that will boost my signal...
Any help would be greatly appreciated.
Modeler: Digitech RP 355
Amp: Groove Tubes SFX Spacestation

[eschertron]

Hi Gregor, and welcome to the forum!

I hope you will find MEF both fun and informative

I didn't see any info on the SFX Spacestation, just the MkII version which apparently has some input tweakability added. Does your unit have a pad switch or a "+10/-10" selector of any kind? If it does, there may be room to play there. If not, there simply may not be the input gain available that you need.

I did not see any schematics for this unit during my brief search. If you have access to schems, there are many brains here that can offer advice. Depending on what the SFX looks like inside (and your confidence level for building stuff!) adding a circuit board that sits inside the SFX with the gain you need might be an option.

[g1]

The power amp spec. is -10 to +10dbu. The RP output spec. is max. +10dbu.
So it sounds like there is a problem. You should be able to get full power with no mods. or extra gear.

[Gregor]

The only way the RP puts out anything close to +10 dBu is if you insert the compressor circuit and crank its gain/level. However, this also changes the sound, substantially at times. Be great if I could get close to +10 without the compressor. I thought I would try using the XLR outputs with line matching transformers on the input, because they have a higher level (see the specs). Perhaps that will give more level. Or use the headphone jack... Any thoughts?

[J M Fahey]

It's rated at 100 watts (RMS) per side but my Cyber twin with 65 watts per is easily twice as loud.

I believe you, and I do NOT think you have a sensitivity problem

The comparison is very unfair:
* the CyberTwin has 2 x 65W amplifiers , or 130W total, which I'm certain can be overdriven if you set the amp loud enough, driving 2x12"very efficient (around 99 dB/m) Celestion Guitar speakers

* the Groove Tubes SFX Spacestation has one 100W amp driving one HiFi/PA single 8"speaker, which I would be surprised if it reached 92dB sensitivity, would rather expect some 90dB or so.
The "other" 100W amp is not firing forward but driving a sideways mounted, open baffle 8" speakers, which rather than contributing to the main sound, must "bend" the main wave by "kicking it sideways in the rear end".

I imagine it works like having a motorcycle going forwatd and being steered by hitting the back wheel sideways with a rubber mallet.
You will need considerable power to steer it but it's "useless" to pull the bike forward or backwards.
Only way I can think it can produce a stereo field from a single speaker: bending the audio which emanates from it.

To boot, given the expected uses, I'm quite certain it has a compressor/limiter which does not allow it to clip, not even try, so it must stop before 100W , probably 80 or 90W .
Never clipping also means its smooth waveform cuts much thess than the squarey one the Cybertwin can produce.

So in a nutshell we are comparing a 130W guitar amp driving 2 very efficient (99db) 12" guitar speakers to a Hi Fi box, limited, with a 100W amp driving an 8" HiFi/PA type speaker with probable 90/92 dB sensitivity.

No surprise that the Fender sounds much louder.

[Steve A.]

I would try plugging in a different device just to see how it works. I didn't look up the specs... is this line level? If there is an impedance mismatch I have gotten great results using an inline audio transformer.

Steve A.

[Jazz P Bass]

A decent 'Boost' pedal may be needed to drive the amp to full power.

[Gregor]

Yes, JM, you're definitely right on a number of counts. The mid-side thing this amp does is great for enhancing the stereo field but doesn't really increase loudness (which would only be 6dB at best ie double power =3dB + another speaker= 3dB = 6dB total gain). It does increase the sense of size as in huge and fat but not necessarily loud, if that makes any sense. I did replace the front speaker with a Jensen Neo 100 with a sensitivity around 99 dB. Unless I go to a 4 x12" cab with sensitivity of 105dB I don't think it'll ever get seriously loud and I was going for small size. Ha, size matters!
I'm sure you're right about the amp's limiter too--being designed for no distortion rather than to drive at high levels.
Perhaps I'm looking for the "unobtainium": a stealth mini-amp that looks like a practice amp but then blows the competition away,,, SMFT.

[Gregor]

Hey folks,
I found a solution: My unit is an "SFX 100" which means it has a mic preamp and pad as well as the normal line level inputs. Guitar Center had GT make this version for the coffee house musician. So anyway, I run an XLR with 20dB attenuation to the mic input for the front channel and the usual cable for the side and WOW, now I have some serious volume. I can now believe this amp puts out 100 watts/side, and all in a very small package. I guess I CAN have my cake and eat it too! At any rate, this is my "sleeper" amp; a kind of whimpy looking "sports car" that'll blow away a lot of the competition. One minor problem: if I push it too hard it gets all compressed and mushy, but I think that's somewhat normal when you overdrive the input.
Thanks for all the help. I'm still open to other suggestions...

[Gregor]

I realized that the signal going to the "side" channel isn't strong enough. Can you recommend a decent clean boost pedal or processor?

[Jazz P Bass]

They don't get any 'cheaper' than this.

$40.30

Electro-Harmonix Nano LPB-1 Power Booster Guitar Effects Pedal - Rogue Guitar Shop

[Gregor]

I know these work well with a guitar output. But remember I'm wanting to boost the "line level" output from my Digitech modeler into the the Spacestation amplifier. Would it work equally well for this or is there some other...?

[eschertron]

I know these work well with a guitar output. But remember I'm wanting to boost the "line level" output from my Digitech modeler into the the Spacestation amplifier. Would it work equally well for this or is there some other...?

A couple this to consider:
You are running the 'main' channel via a mic level input, correct? My suspicion is that the Digitech's output is a lot closer to guitar level than line level. Have you tried the Digitech's two outputs to A/B compare their levels? Maybe the line level is simply not working right.

The Rogue website doesn't link to any technical docs, but you can web search to find the specs on the booster. Just knowing if it runs on 9vdc (or 12, or 18...) will help estimate the max possible signal out. Also the specs should give you a dB gain number which will also help you evaluate its fitness for the task. A seat-of-the-pants rule I use is that 20dB gain (volume) = 10X increase in amplitude.

If you can guesstimate the amount of gain you need, then choosing the right tool is a matter of looking at the specs.