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Signal after treble pot - impedance?


 
3/15/2000 7:51 PM
lion
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Signal after treble pot - impedance?
I’m planning to make a effect loop in one of my DIY amps.  
 
I want to make it a parallel loop only – keeping the dry signal in the tube domaine – and mixing in the wet (digital delayed) signal.  
 
The preamp is pretty much standard, with a CF driven tone stack – into the PI.  
 
I intend to tap the send signal off the treble pot wiper - and pad it down a little through something like a 100k send-level pot.  
 
For the return signal I’m planning on an extra tube stage, to bring the level back up, and from that into the unused lower part of the PI (using the PI inputs as a mixer like some Matchless - and others, I think!)  
 
But I need some advise regarding the send signal. What would be the impedance after the treble control followed by a 100k send level pot?  
 
(There was some debate here regarding this a long time ago, but I didn’t save the thread – and I can’t remember the conclusion)  
 
I’m going for the best S/N ratio, so I’m considering whether it would be better to add a CF stage for the send signal, or I could do without it? - and use it in the recovery stage instead – running 2 triodes in parallel for a little less noise?  
 
The phase of the dry/wet signal should be allright (with or without the CF driver) – with the added recovery stage and mixing the wet signal into the other half of the PI – right?  
 
The usual expert advise, comments and help otherwise from this board will be highly appreciated.  
 
Take care  
Lion  
 
3/15/2000 8:53 PM
Gil Ayan
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quote:
"The preamp is pretty much standard, with a CF driven tone stack – into the PI.  
 
I intend to tap the send signal off the treble pot wiper - and pad it down a little through something like a 100k send-level pot."
 
 
Well, 100K is a little load, such that you might load down the output of the TS too much. One thing you could so is go from the wiper of the treble to like a 220K resistor and then into a 500K pot os so. That will pad the signal down without being a such a small load. If you find you lose highs this way, use a brightness cap around the 500K pot -- something like 120pF should do fine.  
 
quote:
"  
For the return signal I’m planning on an extra tube stage, to bring the level back up"
 
 
That sounds good, a lot of amps do that.  
 
quote:
"But I need some advise regarding the send signal. What would be the impedance after the treble control followed by a 100k send level pot? "
 
 
Well, it would be low enough, look at it this way: it would definitely be smaller than whatever is on the ground side of the 100K pot (which is, at maximum, 100K). :)  
 
 
quote:
"I’m going for the best S/N ratio, so I’m considering whether it would be better to add a CF stage for the send signal, or I could do without it?"
 
 
That would not be ideal from a S/N standpoint -- think of it like this: adding any component at all will always decrease your S/N, period. But on the other hand, the CF would ensure that the TS is not loaded down, so the dry signal you tap off it would be "pure," whereas not using the CF would result in your dry signal being affected the minute you plug your box into the FX loop's send jack.  
 
quote:
" - and use it in the recovery stage instead – running 2 triodes in parallel for a little less noise?"
 
 
Well, the CF should be on the SEND stage... then then use a gain recovery stage on the way back into the P.I. And that return stage should be bandpass, so I would use a big (47uF) bypass cap and a big coupling cap after it. You want the wet signal to be the same as the dry, in therms of brightness, etc.  
 
quote:
"The phase of the dry/wet signal should be allright (with or without the CF driver) – with the added recovery stage and mixing the wet signal into the other half of the PI – right?"
 
 
You can never tell. You see, the FX box has its phase shift, which would vary from box to box, etc. And if you add a delay... isn't the phase shift is obvious? :)) Really, the way to run a paralle FX loop is to only return a wet signal, and at that point you live with any phase cancellations. But if you bring in a wet + dry signal, it will interfere with the dry signal that was not routed to the box.  
 
Gil
 
 
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3/16/2000 10:15 AM
lion
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Hi Gil – thank you for the reply  
 
>Well, 100K is a little load, such that you might load down the output of the TS too much. One thing you could so is go from the wiper of the treble to like a 220K resistor and then into a 500K pot or so. That will pad the signal down without being such a small load. If you find you lose highs this way, use a brightness cap around the 500K pot -- something like 120pF should do fine.<  
 
OK – I get it now.  
 
>That would not be ideal from a S/N standpoint -- think of it like this: adding any component at all will always decrease your S/N, period. But on the other hand, the CF would ensure that the TS is not loaded down, so the dry signal you tap off it would be "pure," whereas not using the CF would result in your dry signal being affected the minute you plug your box into the FX loop's send jack.<  
 
I guess it means that I’ll have to give it a try – and take it from there, deciding whether to use a CF or not.  
 
>Well, the CF should be on the SEND stage... then then use a gain recovery stage on the way back into the P.I. And that return stage should be bandpass, so I would use a big (47uF) bypass cap and a big coupling cap after it. You want the wet signal to be the same as the dry, in therms of brightness, etc.<  
 
Sorry, I can see now that I wasn’t very clear about this.  
What I meant to say was, instead of using one half of a extra tube for a CF in the send stage - and the other half in the recovery stage, I thought it would be better – if I could do without the CF that is – to use both halfes of the added tube in a parallel config in the recovery stage - with the benefit of a little less noise here.  
 
>You can never tell. You see, the FX box has its phase shift, which would vary from box to box, etc. And if you add a delay... isn't the phase shift is obvious? :)) Really, the way to run a paralle FX loop is to only return a wet signal, and at that point you live with any phase cancellations. But if you bring in a wet + dry signal, it will interfere with the dry signal that was not routed to the box.<  
 
No, it was my intention to bring in the wet signal only – and I understand about the phaseshifts in effect processors and added effects in general.  
Maybe it’s trivial and theoretical, but I was thinking more about the phase relations inside the amp – in the dry signal path vs the loop path. Going through the recovery stage the signal in the loop would be 180° out of phase with the dry signal – which would be just right for mixing in through the lower input on the PI. The *second* input on the PI being 180° out of phase with the *normal* input – or am I missing something here.  
Anyway, this is probably a long shot and to no practical use at all.  
 
Thank you for the advise.  
Regards  
Lion  
 
BTW – wasn’t it you that build the ****leator unit and posted something re this a while back?  
Judging from the schem it looks pretty advanced with a lot of components. What do you think about it – is it worth the hassle – and do you think it would make a difference in my amp compared to the minimal loop I’m considering.  
 
3/16/2000 10:58 AM
Dietmar
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Engl has done a well fitted parallel fx loop on their Savages. If you like, I will send you the schem.  
It is done by a complete triode system (one 12AX7) and send level is lowered by one system and return level is refreshed by the other system.
 
3/17/2000 2:09 AM
Carl Z
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Dietmar;  
 
Any chance you could send one this direction as well? Sounds a little different than the usual setup. I'd be interested to take a peek as well.  
 
Regards;  
Carl Z  
Summit" target="_blank">http://www.geocities.com/summitamps">Summit Amps
 
3/19/2000 11:09 AM
lion
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Hi Dietmar!  
 
Sorry for the late reply - I've been out of town  
for a couple of days.  
 
Thanks for your kind offer - I would very much like  
to have a look at the Engl schem!  
 
Regards  
lion
 
3/17/2000 9:38 PM
dpcoyle
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You could have a noisy time trying to drive a solid state low Z device with the high Z source of the common cathode topology if you don't have the c.f.  
Dan
 
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