(Building Idea) Amp project

[Rock_on]

So, I'm tired of using my practice amp... I forgot what kind of IC is used in there, i tried changing caps but it just sound too bassy for me.

anyway,

So i saw this Craig Anderton, yeah i think so, practice amp build. It says TDA2003 can be used as a substitute. And that's available here in our area (i think so, again). It's plug and play kind of an amp, but i want something special about it so when I'm done making my Fetzer Valve I'll put it in there as my preamp.

So here's my question:

Should i lower the power amp section's gain? And just let the Fetzer Valve boost my signal to what the power amp need? My reason for this is to avoid clipping, which would sound bad 'coz this one is a solid state device. From what I always see on the internet, power amp should always sound clean -- Cleaner than the cleanest and just let the dirt pedals do their jobs.

I saw that it takes 50mV input signal, i'll just write it if it means anything.
And this one also, on the same site, i saw someone who said adding diode to the input to avoid clipping... just like what i'm trying to achieve, but isn't my idea better? i dunno, O_o

[GibsonGM]

You can lower the poweramp's gain (if accessible)
You can lower your preamp's gain
You can drop some signal when you couple the pre to the power amp, too.

Personally, yup, I'd rather lower the signal 'naturally' and by planning rather than clip it with a diode, which is NOT clean! 

50mV is sensitive, seems like pretty low headroom to me.    I think you'll be ok if you decide what comes before (your "pre" stuff), and just couple it to the poweramp thru some kind of RC network to lower the signal without letting it get too muddy.

BTW, some power amps ARE meant to clip, quite hard, and are found mainly in the tube family )   But yeah, SS doesn't usually give nice results when you push too hard. 

[Seljer]

Many power amp chips are designed to function at a specific gain level and may act funny if the design guidelines aren't followed, you have to check the datasheets. So basically you tend to leave the actual amp circuitry alone and you're left with slapping volume control in front of the power amp or just adjusting the preamp. The amps design with the preamp will probably have a volume control anyway so you don't have to do anything   , if the preamp is clipping the poweramp -> turn it down .

If you want any volume for it you're going to have to let it get dirty one way or another. If you're not a fan of the chip's distortion you can attempt to soften it up a bit by making it clip in a controlled way in front of the poweramp (i.e. two reverse parallel diodes going to ground in front of it, and then set the overall gain levels so that the diode's clip the input just before it gets large enough to clip to power rails on the output). So you're not avoiding clipping per se, just changing it's source.

The only way to get it to stay clean no matter what is to add a lot of watts to it.


Also, keep in mind that the bane of many small amps it the speaker, cheap little amp = cheap little speaker.

[Rock_on]

Should i adjust my preamp output to nearly clipping the input of power amp then just back of a little using a trimmer?

then put another pot before (or after, idk) the trimmer for a volume control?

would that make sense? I'll use this as a level control, then another pot at the input side of fetzer valve as gain control... or am i doing it reversely?

then another pot again at the end of the power amp as master control?

then tone stack (please suggest, i want tmb/bmt control not a one-knob like BMP) after the preamp?


what do you think guys?

[GibsonGM]

I'd just be inclined to use the Fetzer as-is, with the output pot being your power amp input pot.  You may need to tweak things because you ARE going to clip the power amp input with the Fetzer!  I'd find a way to control its gain with a pot...or scale the output with a voltage divider BEFORE the output pot...

Yes, you can find out where you want to limit your input level to the power amp, and make sure there's always that much resistance in series (or use a divider as stated above).  A trimmer is good for this to dial in exactly, but can always be replaced with a fixed resistor if you like.

I don't think I'd worry much about an actual gain control for so simple a circuit, but if you want more control you could always add one to the front of the Fetzer and call it such...altho turning down the input to the Fetzer will make the amp more noisy (hiss).  Maybe hack a gain control into the Fetzer...

[GibsonGM]

PS: another option, if you want a tone stack, is to use something like the Fetzer for a preamp, put in the TMB stack, then a recovery stage >> power amp.     Not sure what the Fetzer impedances would do to a TMB, tho, so I might just opt for a buffer.

That would give you >>input buffer>>>TMB>>>Fetzer>>>power amp.

It's not a bad idea...See the ROG  "Tone mender" for some ideas on how it could be done - you could almost slam that right in the middle of what you're doing...

[Gus]

Sometimes the best improvement for an amp is a different speaker(s) or to try it with an external cabinet.

[Seljer]

if it really is 50mV input on the power amp you probably could run a passive tonestack after the fetzer and still have volume to spare after the loss

[Rock_on]

Okay i'll just go with a passive tone stack.

for setting the fetzer output not clipping the power amp input, how should i wire the trimmer? in series or like a normal vol control?

i mean, 1 and 2 tied together going to the fetzer output then lug 3 going to the power amp input?

or

lug 1 to fetzer output, lug 2 to input of power amp, lug 3 ground?


Fetzer Valve Deluxe version has an 1m input control... ROG won't put it there if it would just add some noise isn't?

and what would be the advatage of doing this

>>input buffer>>>TMB>>>Fetzer>>>power amp

than this?

Fetzer for a preamp, put in the TMB stack, then a recovery stage >> power amp

just asking, that may change my mind and just go with that configuration