Preamps in a stompbox

[phector2004]

Hi everyone,

I was going to build a franken-amp by putting a preamp in front of a 5W@8Ω Tube+IC Hi-Fi kit I got off Ebay, but laziness in terms of modifying the Hi-Fi's power supply to feed the preamp has got me down a different route: Pop that preamp into a stompbox, so that I can "module"-ize them, or even replace with a common preamp pedal ("English Channel" or JCM800 emulator, etc)

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So here are my questions for you:

1. Can a guitar signal be fed into a Hi-Fi amp? I have very little knowledge of impedance (AC resistance, adding in the effects of C's and L's?), and I feel like impedance will play a role in this/give me a headache...

2. Would I need to modify the preamp I'm working on (derivative of a JC-120 preamp) or any other preamp circuits I might choose to put in the path to properly 'condition' the signal (maybe buffers/followers etc?)


Got all the parts, now I'm just waiting for a big block of free time (next week!) to sink my hands into this. Hope somebody can help

[zambo]

I have fed a solid state 3 watt , 7 watt , split signal after singel volume knob stereo set of 7 watts with a valve caster as the pre amp and it worked just fine. Im not the sharpest tool in the shed but i didint have any magic smoke whining or horrible hissing problems. None of the afore mentioned amps had an ot and the 3 watt is capable of fiting in an altoids can. Quit handy really. The stereo 7 watts powered two 4x12 cabs at 8 ohms each and it was quite loud and pleasing to the ears. Hope that helps, Zambo

[phector2004]

nice!

Good to hear that!

Original plan was to build it, plug guitar in, cross fingers and hit a chord, but I've had bad experiences trying to record through a mic input, so I'm worried it might be a similar case where it's designed for 'Hi-Fi' signal

I'm tempted to procrastinate and build it tonight  
But I can't

[davidallancole]

Its completely fine using a hi-fi amp for guitar as long as your not using hi-fi speakers.  When using guitar speakers, it will clean out the harsh highs where as the hi-fi speakers will not.  I played a Metal Zone 2 for many years through a Pioneer 50 watt stereo amp.  I used a 15" Peavey pa speaker, which couldn't reproduce the harsh high frequencies and it sounded awesome.

Most solid state power sections in the solid state guitar amps are a hi-fi amplifier. 

[zambo]

and i think the 1 meg resistor from tip to ground at the input sets the impedence for guitar anyway doesnt it? I couildnt swear to it, but i think thats what makes the high impedence input. Any smart guys have an answer on this? Schematics point to yes........

[davidallancole]

Its a combination of the 1 meg resistor and the impedance of the input device be it tube, jfet, bjt, op-amp.  With tubes, jfets and some op-amps, the input impedance of the device is high enough that the 1 meg resistor does set the impedance.

[petemoore]

  Amplification is very like other amp.
  Transistor is an amp, and like the amplifier in a box which drives a speaker it expects:
  An input which is above 's' and below 'x' in 'force' [amplitude].
  An output which restricts current flow to some extent, if the reactive resistance [speaker impedance] is too low, overcurrent will cause heat, heat causes the observer to turn it off in time or else.
  A power supply which is sufficient to...supply a constant voltage and 'enough' current.
  :Depending on how it is set up [peripheral circuit sets currents, gain, etc.
  You can experiment with chips, a transistor...try an LM386.
  The 386 is an amplifier chip, so like any amplifier, it gets too hot [possibly outgassing it's magic blue smoke] if you run too much current through it [don't burn your thumb by pressing on it, just glance across it].
  Also like any amp, it very ineffeciently uses DC power. The more power you make available to supply it with, the more power you get out of it [don't overcurrent though].
  It's all about operating ranges, and not exceeding or exceeding them, for chips and SS...don't exceed them [especially on the output amplifier], or exceed them greatly [such as is done with stompboxes], for tubes...exceeding them is similar to stompboxes [commonly done; tube/stompbox effects exceed at least the setup for minimum distortion].
  So...they all expect a certain amount of input potential. Current drawn by the input can be made extremely small [start with Fet or tube], or so it doesn't matter [use a voltage follower to increase current fed to input].
  Start with a small load [16ohm or more speaker], large voltage supply [don't exceed a component rating], and a smallish input voltage.
  Ways to find the limit:
  *Drop the supply voltage and the output amount drops, the rails are closer to the signal swing...swing the input voltage too far now and it will clip as a rail is hit.
  *Increase the input voltage [use gain stage/booster] so the swing of input voltage increases enough to make the output hit the rail [amp clips].
  *decrease output impedance [use a 2ohm speaker] to see how fast the heat comes, you may only have a few seconds though...trust me it'll get hot super fast if...exceeding the minimum ohmage by 'so much.
  ...These* are all interrelated and are part of the operation/setup of all amplification devices [from the basic transistor on a breadboard to the amplifier unit].
  Bass [lower frequencies] = requires more current.
  The speaker effeciency [db rating] makes the amplification transduction [speaker turns AC into sound pressures = air 'waves'] much louder or too quiet. A higher Db rating is the easy way to get greater air pressure differences [loudness] with same amplifier pressure.
  So hifi SS amp...read typical manual [don't exceed anything] if you hear clipping turn it down [hopefully before something pops, better to never clip 'em], that said, any amp can be pressed past it's rating...just boost the input, lower the output impedance or power supply voltage...it'll clip, you'll find the parameter...why I suggested the LM386 and a 9v supply, the clipping of such a small potential [you can connect a 9v to most any speaker for a very short while or longer, depending on how long it takes the heater/speaker coil to heat past crusting or melting point]...so it's cheep [use a socket so you can try another if you release the magic smoke from the first one], won't blow your speaker, and you can play with the supply and input voltage...an 8ohm speaker or whatever load you want [the chips don't cost too much]. The exact same things happen with any amplifier, the 386 is weak enough to be 'innocuous' to other equipment, and it's easy to run it outside it's non-distortion/clipping ratings with just small '''auuughhhhggghss...oh nO'', poppin' in another chip usually just illicits a short, quick 'agh', and soon the realization that running it within parameters that don't make it overcurrent are preferrable. The distortion may too prove 'preferrable' but when you get into a bigger amplifier it can become enough to prove extremely irritating [the sound is ugly of SS output clipping, and can blow the speaker]...figureing out how to run the LM386 clean [signal swing doesn't try to exceed power supply potential] transfers these understood user requirements to any amplifier.

[phector2004]

thanks for the advice!

The kit schematic shows a 1M resistor right after the 68n input cap... might change the value of that cap if it sounds nasal!
The output cap is this giant 4.7millifarad can

Pete, I'll look out for clipping, as I want it to be clean (until I boost the tube a bit to clip!). It's got a giant heatsink strapped onto the LM1875, so I doubt heat will be a problem. The amp is a kit, so it's designed to work. I was more interested in making a preamp!

4 more days and I get the iron out 

[ViolenceOnTheRadio]

I think the "laziest" solution is get yourself a cheap 9 volt power supply off Ebay or if you're like me, you have plenty of spares.
Crack it open, wire it into the same enclosure of your tube amp and simply connect the AC leads to the appropriate leads of the tube amps power interface. Now you have the power supply for your preamp rigged to turn on when you fire on the amp.

Probably a pretty common methodology for "boutique" style amps.

[phector2004]

Problem is, it's a submini pentode... kit came with a 24VDC@1A wall wart, so I'd need to make a voltage divider to bring it down to 9V so that I don't fry my MAX1044 to bring my preamp to 36V for the Jfets to be in their optimal setting, all the while taking into account the current draw of the amp IC (Its supposed to put out ~20W, realistically the kit maker states 5W into an 8ohm load)

I like your wall wart idea though. When I start playing more with amps, I'll try it out! I remember seeing a certain amp (Ibanez, maybe?) with a built in mosfet booster or tube screamer or something. Could probably mod a future one the 'lazy' way.