Quad issue - unwanted distortion on clean setting

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frood

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I've been trying to troubleshoot this problem for a while and I'm running out of ideas. I've learnt a lot from this forum over the years, but can't seem to find a thread about this issue precisely.

For info, I use the Quad in three different ways: 1) straight into a focusrite interface, generally using the main outs and then adding an IR in the DAW. 2) in the loop of a digitech gsp1101 (guitar -> gsp input -> gsp send -> Quad input -> Quad main out -> gsp1101 return -> headphones). 3) straight into the fx return of an Orange TH30 combo.

The problem is a gritty, harsh sounding distortion on the clean channel (rhythm 1). It sounds like it's clipping somewhere and is very noticeable using a bridge humbucker (passive) and playing full chords in any way apart from super softly. If I play on the neck pickup, it's generally ok (unless I dig in), but tends to be a bit thin sounding. I suppose the distortion may be present on the other channels, but it's hard to tell. I have the output on 2 or lower, basically as low as I can get it without it getting too weak. master vol is about 4. In the 3rd setup described above the problem goes away, but it's very loud and not ideal for where I currently live!

I've swapped a bunch of the tubes in and out and that made no difference, so I'm thinking it has something to do with the output and/or how it's interacting with the gsp1101 and the interface. is there anything I can do to tame the output somehow? I'm sure it has not always been this way but my memory may be unreliable. I'm sure I used to be able to play it through the gsp1101 with headphones and the clean sounded truly clean.
 
pretty normal channel 1 settings, vol 7, treb 7, bass 2, mid 2, master 4. EQ is out (I use it on lead 1 only). It sounds clear as a bell when I play through a real power amp and cab. I can get some nice, natural breakup if I push the volume up to 10 and/or the treble up above 8. The nasty distortion (when I play through the gsp1101 or straight to the desk) is very different and it happens even with the volume lower than 7.
 
No, with the gsp1101 I use the main outs - the Quad sits in the loop of the gsp1101, which allows me to switch between the Quad and the modelled amps. I've tried all of the outputs into the desk (main outs, recording outs, fx send) and get the same problem. I'm wondering if I should use a DI box, but I don't know how that would work with the gsp1101.
 
My thought is that you are overloading loop of the gsp my quad is very loud I only have masters and outputs very low around one otherwise I overload the Axe fx inputs that I am using with it.
 
I'll do some experimenting with the master. I was playing it earlier today and it was sounding so good through the real power amp and cab, it's just frustrating I can't get something similar straight to the interface or in the headphones from the gsp1101, which is the main way I want to use it.

It's worth persisting with, I'm sure if that. The Quad just works beautifully for the sounds I want (well, not so much rhythm 2) and, touch wood, it's been pretty reliable. I'll keep trying.
 
Tried it earlier today with the master on 1-2 and the problem remains. I've tried recording using the XLR out from the gsp1101 straight to the desk and I'm still getting this horrible ice pick overload sounding distortion on R1. I'm guessing it's present on other channels too. I'm going to do some more experimenting tomorrow (there are a lot of variables!) but I'm really starting to think a DI box might be necessary.
 
Hi all,

I personally had the same symptom in a Studio Preamp :

In this Studio Preamp, the preamp stage's power supply filtering electrolytic capacitor was too old (dried out => very high ESR, it's internal resistance).
I found that it produced an unexpected "alternate", frequency dependant, audio path across all the stages . The signal was spread via the badly filtered power supply and the anode resistors. Specifically, the Lead Drive stage (high gain) was excited and produced some mild distortion which, in turn, was injected at a low but discernable (annoying!) level into the output stage.
Due to the frequency dependence of the signal leak, the distortion had a very trebly tone.
=> Note that it was not possible to cancel the extra distorted signal using the Lead Drive and Lead Master knobs.
I changed the electrolytic capacitor and that fixed the problem.

Another very common problem is the failure of one ore more ageing optical switches.
These switches have two parts in the same package : the led (= control) an the resistive part (= switch). Both can fail with age.
I had the led part failing in several switches in one of a my Quad Preamps, but a side effect of this failure mode is a messed-up, Xmas-tree like front panel and footswitch leds display, which you don't have.
But if the resistive part of the opto-switch start to deteriorate (=> isolation loss), the symptoms may be what you get.
AFAIK, opto-switches failure is quite common in our 30+ years old preamps !
In this case, and according to the schematics, I think that the Lead Drive and Lead Master knobs should have some effect, eventually totally cancelling the distorted signal.

(The circuits of the Quad and Studio preamps are extremely similar.)

My 2 cents...
Pierre.

Studio Preamp :


Quad Preamp :
 
Thanks pmx1, really appreciate the detail you put in your reply (six months ago...this forum moves slowly these days!). I tried my Quad out the other day, direct into my interface and a nice cab IR in reaper and it sounded great, no horrible layer of distortion on R1. I am using a new computer so I think I must have had the interface setup incorrectly somehow. Frustrating but to know why or what...but it works so I'm happy.

Can confirm that putting a 12AT7 in the first position is an absolute winner - it allows you more scope with the shared gain and treble controls for L1. Now R1 stays pretty **** clean with the gain up to 10. It doesn't noticeably affect L1 though keener ears might disagree.
 

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