Peavey 5150 pre-amp out vs effects send out; guitar input; impedance and dbu

[toebens]

hi,
disclamer - i'm a newb and i'm having several (very) stupid questions

Little funny story upfront: I must admit that i blew my soundblaster gold back when i was really young: I connected the speaker out of my solid state 80watts amp to the line in of the soundblaster. The amps preamp out or headphones jack was just too silent - no matter where i connected it to - line in or mic in. Therefor i tried it - blewing it :zombie:

ok now i read that a lot of people in here that own a peavey 5150 prefer the effects send out instead of the pre-amp. I wondered why this is and looked up the manual:

http://www.peavey.com/assets/literature/manuals/80302108.pdf

The manual contains a diagram. There it shows that there is a - just another - "tube stages" after the effects send / return section. The preamp out comes after that additional tube stage.

1) Can you tell me what tube stage this is? Is this maybe one of the 12AX7 preamp tubes? (i.e. 4x 12AX7 until efx send out and then 1x 12AX7 before preamp out)

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I'm still a newb in regard to all this. Yesterday, about 2 decades after that soundblaster gold story, i then finally read about line level, mic in, impedance, high and low impedance and dbu levels... but i'm still having some questions.

Back to the peavey manual - there is also a spec page that lists:

Effects send:
Load impedance 47K ohms or greater
Nominal output: -10dBV, 300mV RMS (this should be about -8 dbu; http://www.sengpielaudio.com/calculator-db-volt.htm)

Preamp output:
Load impedance 47K ohms or greater
Nominal output: +10dBV, 3V RMS (this should be about +12 dbu)

The phone type input jack on my todays audio interface (UA-101) for high impedance (HiZ) lists: -36 to +4 dBu

2) Would this mean that i can only connect the effects send out of the peavey to my HiZ input? (Because the preamps output dbu would be to high (thereby causing distortion)!?)

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I also checked impedance of some pickups:
EMG 81:
RMS Output Voltage: 1.25 (should be about +4dbu)
Peak Output Voltage: 1.75 (should be about +7dbu)
Output Impedance (Kohm): 10

Seymour Duncan Distortion TB-6 Bridge:
Output: ?
DC Resistance: 16.6 k (is this the output impedance?)

Peavey:
High gain input:
impedance: very high Z, 470K ohms

Low gain input (-6 dB pad):
impetence: high Z, 44K ohms
all levels are increased by +6db

3) Why is it having two inputs at all?
Why the -6dB pad? Only to increase it by +6dB again???
Is there any advise what input to use?
(I have used the high gain input all the time)
Wouldn't the pickups impedance better fit the impedance of the low gain input?

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4) Often times there is a huge difference between x out and x in impedance. Like
a) above for the pickups out ~10-20K ohms to amps high gain guitar input
470K or
b) the effects send out 47K ohms vs the effects return 470K of the peavey
Why is the input impedance 10x times or greater than its output impedance?

many thanks in advance for making me :Smokin: understand it

 

[JakeAC5253]

The tube stage is a 12ax7 acting as a Cathode Follower, which is acting as an impedance converter from high to low. Preamp out = high impedance, FX Send = low impedance. I'll read the rest of your post and comment later, little busy now, but thought I'd answer that since I read it off the bat.

 

[TheWinterSnow]

1) It is just a recovery gain stage to bring the levels back up from pedal (around +4dB) back up to something the power amp can use. The Preamp out is after this gain stage and sounds better as it is designed to be used with outboard gear or direct recording.
2) Maximum power is +10dB but is adjustable with the post gain knob.
3) The low input is a -6dB pad, there is no extra gain. In the schematic I can't see where all levels are being increased by 6dB, it just the low input has a voltage divider reducing the signal and the high input does not.
4) That is how it works you want a device to work properly, its output needs to be loaded at least 10 times greater than the input impedance (load) that it will be going into.

 

[toebens]

Thanks upfront.

Both preamp out as wall as effect send are having the same impedance - 47K ohms. Only its "Nominal output" differs.

 

[TheWinterSnow]

That is because the gain stage after the FX loop is giving you some extra power.

 

[toebens]

i was posting before i saw your post thewintersnow.

2) do you mean that you can control the preamp outs +12dbu with the peavey's post-gain control on the front? if so why is it labeled post-gain?

could i also use the "sens 2" control on my edirol UA-101 input instead?

i would have connected the effects send or preamps out with 47k (47000ohm) to the high impedance (HiZ) on my UA-101 but according to your answer to 4) this would be wrong!? so i need to connect it (the out 47000ohm) to a low impedance input (an input with about 47000/10=470ohm), correct!?? what input can this be on my UA-101? the left or right phone input jack on the front (without having HiZ button pressed) or any of the inputs on the back!?

that stuff with the impedance in combination with the db(u, v, rms, whatever) is confusing me a lot

EDIT: wait now i'm completely fucked up in my head: can it be that you typed it the other way around???

EDIT2: oh shit. but for the pickups output and the input jack on the amp it is the other way around:
pickup out ~15000ohm
to input jack (high gain) 470000ohm.
heeeeeeelp. i need a break now.

 

[TheWinterSnow]

yes the post gain controls the volume of both the FX loop and the preamp out. It is called a post gain because the control is physically after the preamp in between the preamp and power amp

The gain control on your interface will not work, you need to bring the level down at the amp, keeping the gain on the interface as low as possible to reduce noise

The input impedance needs to be at least 10 times higher than the device that it is connected to. So for example if a pickup's output impedance is 10K, then the amp's input impedance needs to be at least 100K.

 

[toebens]

I get this for the pickups - guitar input example - 10k pickup out to greater guitar input (high gain) 470k. (10k < 470k; output < input)

but for the preamp out or effects sends i don't get it really. it states in the manual: "Load impedance 47K ohms or greater".
that would mean it needs to be connected to an input that is (10x) greater than this. so this would be 47k * 10 ~ 470k.

On wikipedia it says: "The impedance of a line input is typically around 10 k&#937;" - so this would not be enough.

ok, this would mean that i have to connect it to the HiZ input on my UA-101 again ("Hi-Z inputs (Z being the symbol for impedance) which have an impedance of 470 k&#937; to over 1 M&#937;" says wiki)

but i doubt that this is correct because i read here in the forum that everyone says "connect the preamp out to the line in".

can you please make me understand it?

 

[JakeAC5253]

Well you are rightfully confused about this because this is a topic that could go either way really. For all intents and purposes, the output of a metal amps' preamp stage like the 5150 is still instrument level, it hasn't really been amplified, only clipped and distorted until there is no headroom left, kind of like a Tubescreamer pedal, but more involved. Even though this is true, its signal characteristics are closer to a line level signal, which would be a signal at -10dBu. What this means is that you can run the preamp distorted signal into a device which has an input impedance lower than ideal for instrument level signal and it won't lose much signal fidelity. You could easily run this through the line level input on your interface. I do believe that wikipedia is inaccurate in its statement about average line level input z, most of the circuits that I've seen have line level input impedances between 47k and 220k, 100k is pretty standard for a line level input z. After all that, I have had best results using the FX Send of my 5150 straight into a Direct Box. Sometimes knowing the theory doesn't help you get to the best sounding end result

 

[toebens]

well i'm glad that i got it right then.

after doing my post above and searching for posts that stated "connect preamp out or effects send out to your line ine" i could not find one anymore.
only this one http://www.ultimatemetal.com/forum/backline/839024-6505-impulses.html#post10452381 by you - thewintersnow - stating "As for the GX, the preamp out is a high impedance relatively and you would realistically need a DI into a line in OR go form the preamp out straight into an instrument in"

JakeAC5253 can you please explain what you mean by:

Even though this is true, its signal characteristics are closer to a line level signal, which would be a signal at -10dBu. What this means is that you can run the preamp distorted signal into a device which has an input impedance lower than ideal for instrument level signal and it won't lose much signal fidelity

do you just mean its db/volumne?

as both Effects send and preamp out are having the same impedance: Load impedance 47K ohms or greater. but the
effects sends is having:

Nominal output: -10dBV, 300mV RMS (this should be about -8 dbu; http://www.sengpielaudio.com/calculator-db-volt.htm)

while Preamp output:

Nominal output: +10dBV, 3V RMS (this should be about +12 dbu)

audio interface (UA-101) for high impedance (HiZ) lists: -36 to +4 dBu

i suppose the preamp out would be too loud for the HiZ input (probably adding distortion; (maybe resulting in clipping) or whatever there happens - i'm no expert yet *g)
therefor the effects send (-8dbu) should better fit that volume range (-36 to +4 dbu) and probably sound better. maybe this is why you also think that it sounds better with the effects send?

the inputs on my units back can be switched between:
-nominal input level +4dBu; maximum input level +16dBu and
-nominal input level -10dBu; maximum input level +9dBu and
the first option could fit that volume range as well however they are still not HiZ inputs.

so the theoretical best option should be effects send to HiZ input on front right!?

could i try the preamp out or effects send on the non-HiZ inputs as well (ignoring the impedance) without destroying anything? i don't want to brake another soundcard
when is it getting critical? at what impedance difference or voltage difference? i mean there was a impedance mismatch between my 80watts speaker out and the soundblaster gold line in, too *g

 

[TheWinterSnow]

Yes, i said that the best option would be to either run the FX/Preamp Out throught a DI and Mic Pre combo into the line level interface or just plug the FX/Preamp Out into the instrument input. Both the instrument and line in are both line level, the instrument just has a higher impedance.

And again, you have the post gain knob, it controls the volume of both the FX loop and the preamp out. The preamp out was again designed for direct recording which is why I recommend it over the FX send. +12dB will not fry a +4dB interface, just clip it, and if it clips, turn down the post gain on the amp.

The setup is simple. Amp->Preamp Out->Instrument In and you are done. Set the instrument input gain on the interface relatively low and use the post gain on the amp to get a usable signal, peaking somewhere around -18dBFS on your interface and you are done, nothing more to think of its that simple.

 

[JakeAC5253]

JakeAC5253 can you please explain what you mean by:



do you just mean its db/volumne?

No, it has to do with the signals' own impedance, that's why I said it was a characteristic of the signal itself, signal impedance goes beyond discrete component values.

These aren't exactly light topics. I applaud your eagerness to step into this, but if you really want to gain an understanding of this type of stuff then you will need to spend the time and research and just try to fit everything together. This may take years, but this kind of topic is just not something that can be taught (actually taught) within the context of a conversation. You can obviously create other topics on the subject when you formulate new questions and I am glad to teach what I know as I am sure others are as well, no one is telling you not to. Your original question has been answered satisfactorily by the both of us posting, but realize that you just won't gain the understanding that you desire within the context of one internet thread. You should reflect on what's been said so far and just start making music You aren't going to damage anything so long as you keep a speaker load of correct impedance plugged into the speaker out of the amp head.

+12dB will not fry a +4dB interface

You keep saying +12dB and +4dB, what do you mean by this? I hope you aren't referring to dBu, because that's way off, preamp output is more like -10dBu and would never even approach +4dBu which is speaker level audio. Are you saying that the signal at the preamp out is +12dB relative to the signal strength at the input? Just saying +12dB without an inferred reference point is really not saying anything constructive.

 

[TheWinterSnow]

Let me explain it seems the whole dB thing can be really confusing and I manage to see a lot of experts even get it wrong. Yes I meant dBu. A +4dBu signal is about 1.737Vp or 1.228Vrms. Now in the Peavey manual they list the preamp out of having a maximum voltage of 3Vrms (or +10dBV) which is the same as about 12dBu when plugged into a professional line level device of 600 ohms.

As for mic preamps, taking a quick look at the 2-610, it is rated to have a maximum power rating of +20dBm, which as a measurement of power, means that the equivalent voltage is +10dBu, not -10dBu.

Last time I checked, 3Vrms (+12dBu) was not speaker level unless you are talking about a half watt amp/speaker setup, +4dBu would be equivalent to a 90mW amp, definitely not speaker level by any means.

 

[toebens]

you guys - thanks for your help!