pre-amp ?'s

[shooter]

Got a non-working, no schematic mcmartin LX-40 as a Christmas gift.  Drew up a schematic from the point-point board, got it working.  Now I’m gutting most of it!  Gonna make a 2 channel pre-amp for my quadSE amp.  My questions are;

1.   what’s a “typical” pre-amp output signal level?  1vpp, 2?
2.   Does the audio cable out of pre into amp provide a common ground tween the two or are the grounds isolated?  The reason I ask, the mic/mixer had an isolated output rca jack.  Also there’s a switch to have the audio signal either come out the center pin, or the “shield” side with a 680r between center pin and shield.

I’m using the transistor circuits as one channel +OD and the other I was gonna put in just a 60’s style tube preamp section.

I’ve measured 3 cheapy guitar outputs and come up with a range of about 50 – 300mV depending on how hard I bang the strings.  Is that a reasonable working range for inputs to a pre-amp section?

Thanks for your help

dave

[PRR]

I'm not even sure if you want a guitar preamp or a hi-fi preamp?

What is a "quadSE"? Is that QUAD (a snazzy Hi-Fi brand) or a four-pack of something? What?

[stratele52]

I'm not even sure if you want a guitar preamp or a hi-fi preamp?

What is a "quadSE"? Is that QUAD (a snazzy Hi-Fi brand) or a four-pack of something? What?

Quad Single End ?

I read too many abbreviations written on many forums. Can't understand what people talking about

[stratele52]

 I’ve measured 3 cheapy guitar outputs and come up with a range of about 50 – 300mV depending on how hard I bang the strings.  Is that a reasonable working range for inputs to a pre-amp section?

Thanks for your help

dave

Yes if you want to use a guitar , for CD or record player it is only few MV , you may need a pre-pre amp.

Find and look on any Hi Fi preamp schematic to help you .

[HotBluePlates]

... for CD or record player it is only few MV , you may need a pre-pre amp. ...

CD players that aren't portable battery-powered devices have a hotter output than you think. I've plugged a typical CD player to the input of a power amp expecting 0.5v for full output power, and had to use the input volume control to turn down the signal level...

[shooter]

The recorded music is no problem, I can drive the amp straight in (getting 2.4v outta the sound card) with no pre and distort at about 9 on the volume.  When I plug in a guitar it's just way to quite for 20w amp.   So I figured since I have a preamp might as well get dual use from the amp, I can get the tone and vol there + be able to compress the el84's the way guitar folks like.  I wanted to make sure the pre was close to "standard" since I have seen schematics with pre-amp "out" jacks - no ac volt values tho.

I was hunting for a PT and found this one with weber, is this ok the way I have the rectifiers drawn?  I wasn't sure about ground off the bridges.  The lo-Volts will be for the transistors, the hi-Volts a 12??7 tube.  the sand side on paper is about 40mA total and guessing the tube side will be in the 3mA range.

anyway thanks
dave

[terminalgs]

The recorded music is no problem, I can drive the amp straight in (getting 2.4v outta the sound card) with no pre and distort at about 9 on the volume.  When I plug in a guitar it's just way to quite for 20w amp.   So I figured since I have a preamp might as well get dual use from the amp, I can get the tone and vol there + be able to compress the el84's the way guitar folks like.  I wanted to make sure the pre was close to "standard" since I have seen schematics with pre-amp "out" jacks - no ac volt values tho.

I'm still a little confused about what your amp is.  It sounds like a hifi type amp?

is 2.4v peak, peak-to-peak, or RMS?  Pro-Audio line level (+4dBu 3.4V peak to peak).   consumer level is a lot lower, -10dBV, which is .89V peak to peak). 

So, do yo think that with your sound card driving your amp, you are getting 20W output?  there must be some amount of pre-amp in your amp already.

No matter what, If you build a 12AX7 based preamp, you'll be able to get any guitar signal up to 2.4v RMS or otherwise.  You'll want/need an attenuator pot after the gain stage (or course).   If you need anything after that pot but before the output jack, it'll depend 100% on the input impedance of your amp.


If you have a schematic of that amp, I think you'll get better answers here....  otherwise, all anyone can do is make guesses.. (like I've done so far...) 

I was hunting for a PT and found this one with weber, is this ok the way I have the rectifiers drawn?  I wasn't sure about ground off the bridges.  The lo-Volts will be for the transistors, the hi-Volts a 12??7 tube.  the sand side on paper is about 40mA total and guessing the tube side will be in the 3mA range.

all you need is 150V to 250V B+.   Check out the PA522/PA211 transformer.  It has enough filament supply for 4 12AX7s plus a 12X4 rectifier.  See the dynaco PAS preamps for powersupplies for this transformer.  You could easily build the first stage or two of a champ using that power supply and transformer.

[shooter]

I'll check out those tranny refs. and all my ac(signal) is pk-pk scoped.  I do need 36ishvdc(loaded) for the sand part and anything from 100-300 for the glass.

My main concern's the interconnect tween pre-amps and amps, (hi-fi, guitar shouldn't matter much), and whether they share a common ground or an isolated ground?  whether there is a basic standard for signal out so somebody doesn't plug it into his guitar amp or hi-fi amp and blow all or some stuff up.

The build itself is a multi- tasking amp patterned off a hi-fi but without the $1,000 in "extra" iron!, I need music when I paint, I have everything from 128k napsterday to mastered recordings, i'm not picky .   The preamp is just an experimental piece for playing with tonal stuff and signal gain.  The amp "should" sound reasonable to both normal music ppl and guitar guys that play thru the pre and can have choices, clean, overdrive, '60's tube, (thinkin the v1 section from a Gibson ga16), with enough out to push the amp to get, well, guitar sounds!

I attached a non-updated schematic but the amp works good, except I have an undersized R in the pwr rail, correct size ordered.  I've ran the amp about 10hrs but the B+ keeps going down as the R keeps getting browner!  I tested 1channel of the pre  50mV gets things going, 2.4 starts squishing the positive lobe.  gutted the old parts, got new coming, still working out the tube side but, mostly someone else did it for me, I just gotta figure out who .  Once all the parts come in, I get the amp back up and re-tested, then I can start mating the two and testing signals.  Once i'm happy there, i'll bring in my guitar friends and get their opinions.  The music guys all loved it.

[terminalgs]

I attached a non-updated schematic

an updated schematic is best.  what's missing?

My main concern's the interconnect tween pre-amps and amps, (hi-fi, guitar shouldn't matter much), and whether they share a common ground or an isolated ground?  whether there is a basic standard for signal out so somebody doesn't plug it into his guitar amp or hi-fi amp and blow all or some stuff up.

if it is a separate box,  with its own power supply, then you can add a ground lift (google 'tube wizard grounding' ).

Another option is to add another tube to your existing amp.  1) skip  transformer+enclosure costs,  2) dont sweat  ground lifts, 3) you already have the power supply. There are creative things you can do to allow the  current amp to exist as well as new preamp+current amp.

BTW, the resistor that determines your amp's input impedance doesn't have a specified value  (grid leak on the first stage).

your amp:
Looks like 1/2 a push pull amp with a cathodyne PI with the lower cathode take-off not used.  I'm not sure that second stage really does anything (other that shorten 6SN7 tube life).  if you are happy to take a soldering iron to it, you can cut that 6SN7 stage out  and use it for a gain stage in front of the first 6SN7 section (for guitar). or stick a 6SL7 in there and double the gain.  or both....

also,, AS-IS, you don't have much gain in that amp  for any signal you plug in  to it.., a 6SN7 is Mu=12, and with a 50K Ra, I'd suspect your gain is in the area of 6X ??  or lower? 

do you have two of these?  or do you listen to a lot of mono?

[shooter]

I believe the preamp box I got for Christmas had a raised ground, they have a 150R tween center-tap and ground.  I was guessing for noise isolation.  they are 2 separate boxes. once i get the P.S. up I will be playing with gain on the 6sn7, the el84' don't seem to need much drive.   I finally asked google the right question and found a good link for my questions;
 http://www.ovnilab.com/articles/preamp.shtml

I only have one currently although I use the 4ohm output tap, parallel my 8ohm speakers and feed stereo into the amp, eyes closed, correct spacing n more than one person thought it was stereo.  Like I said, it's for music, but I know enough guitar guys that like new toys to try, mostly studio work and home play stuff.  I learn from them n this site how to take a truly crappy scope signal and understand why they jump up n down saying WOW, NICE!!!

thanks for all the help!!