Joe Davisson's new Guitar amp

[Joe]

Been a while since I've been on...

Never finished my build, mainly couldn't get the preamp right. Still working on the power amp design though (fixed the VI limiter for starters).

Might explore using an output cap, only to prevent the slight bias shifts (which can occur with different loads or possibly extreme cold/hot) from putting DC onto the speaker. I don't believe it would ever be enough to matter, but cabinets are expensive, and maybe it's "just not right."

[joecool85]

Good to see you back Joe.  I was just wondering today if I should email you to see where you were at with this.

Maybe you should try a different preamp, may I suggest the K20-X?

http://www.ssguitar.com/index.php?topic=1885.msg12904#msg12904

I'm working on making it a 9v battery based pedal but have stalled on the project.  As it is right now it works perfectly on +/- 6v and up.

[Davelectro]

Been a while since I've been on...

Never finished my build, mainly couldn't get the preamp right.

Hi Joe,

What was the problem with your preamp?

[askwho69]

HELLO ALL!

is there a layout on this? can can i ask quick question? how much watts is this sorry cant really thorough my child keeps roaming around need to watch and type  :trouble

[joecool85]

Last I remember, something like 30watts were expected from this amp.

[askwho69]

ohhthis is nice but i need something like 70 - 100 watts any ideas for chip tda7294?

[joecool85]

ohhthis is nice but i need something like 70 - 100 watts any ideas for chip tda7294?

Any reason you need so much wattage?  Also, are you looking for 70-100w RMS or 70-100w PEAK (aka "Musical")?

A LM3886 will give you 50w RMS/100w PEAK at 8ohm.  They are also rated for 64w RMS/128w PEAK at 4ohm but many have complained of the chip being thermally unstable at that point.

[askwho69]

well the reason isit has different tone for me and also gain . . .  i dont know really but higher wattage for me is somewhat good to my ear LM3886? how about the TDA7293-94? is not good?



A2

[bry melvin]

I Don't think most of the forum is fond of TDA7294 it has an awkward pin out for perf board and a nuisance for designing circuit board. I use some but you need good heat sink designs.

I also get circuit boards from hong kong. I don;t try and make them. I use them for repairs of hopelessly fried power amp pcb's when I repair others' stuff.

As for watts there is minimal difference of sound level from the 30 watts you can get out of a TDA2050 ( you need a LOT of heat sink to do this at 4ohms) the 60 or so from a 3886 or 729X. They are all just fine for home or studio/small gigs... From there I jump to 300-600 wats for gigs.

to double the sound level of a 60 watt amp you really need about 600 watts. (x10)

My stage rig...to keep clean at big gig levels I use two Carvin 200SX plus extensions powered by Peavey 160 watt ss power amps 2 vintage JCM800s for blues/dirty a peavey heritage (130 watts) and an Ampeg g212 (125) for steel and a Guild thunderbird 40 Watt (tube  1964) for some vintage sounds miked to the 1200 Watt PA system.

At home and studio I use a modded Epi valve jr ( about 3-5 watts) Fender FM15 and 25R (modded in the gain channel)(and through an old sound city 2X10 with eminence alnico speakers) for miked sounds and a Peavey (tube)TGRaxx and KMD SS preamps for DI.

There is a BIG difference btween gig and studio amps. TOO much power amp at home and studio at low level and your speakers are not putting out the sound they should be. OR you could have  low power speakers in your amps and you'll blow them if the amp is cranked when you play out. You CAN use something like a pair of eminence alnico 10s with the high powered amp in the home setting and get the 30-60watts worth of speakers in their sweet spot...BUT crank the power too high and you just got a 200 dollar junk pile.

My experience tells me a 100 watt amp is of minimal use at home and in most studio settings. Most lower power amps have crappy speakers. Put a low power amp into a godd speaker and you'll get a surprise.

Here's some examples May with little 10 watt amps  Clapton with a Champ (electric version Layla) With all the amps I have around I spend mopost of my work day playing through a Frontman 15 and a valve jr!

[askwho69]

Wow bry melvin you really stack me some info there! i thought "before" in recording 100 watts is the best! wow now i know! speaker  - 30 watts - good preamp and tonemender will be good home and studio Lm3886 is my goal

by the way i play metal so 10 watts still good for heavy riffs in recording?

[bry melvin]

To get that kind of gain here in our studio...or in between that and bluesy I often use: guitar active pickups---->pedal of choice---->peavey TJraxx preamp or a KMD SS recording preamp----->Modded Valve JR to mike OR  Crate GX130C preamp ----DI to mixer board or to a homade TDA2050 power amp.    We have better luck using DI with heavy distortion...If not and miked we run a 20 second or so leader (quiet instruments...just hum and  hiss) at the beginning of the track to sample and then filter out later. I have used high powered amps in the studio....but in an "isolation" booth. To get mikes to tolerate high volume is very difficult. I almost never record with a high powered amp unless it's live and outdoors.

[J M Fahey]

LM3886? how about the TDA7293-94? is not good?

Agree and add: LM3886 is powerful enough for pubs and such, has good pinout and you can find usable layouts on the Net, or draw your own, or buy ready made PCBs (Chipamp.com)
TDA729x has *stupid* pinout, must have been designed by a drunk on a Friday night, needs to have pins connected, which have 3 or 4 *other* pins in the middle connected to other components, its heatsink must be *over* the PCB and can't be on one side, etc.
Why didn't they join said pins *inside* the chip?
Stupid.
It forces you to use a double-sided board, and even then the design is klunky, or go the Marshall route: mount it on its own miniboard (still double face) and connect it to the real world through a very unreliable 15 pin connector and ribbon cable.
Have I already said it?: stupid.
And you can only pull 70W out of it (in the Guitar world); use them for 100W (you can) and you'll replace them every 4/6 months.
EDIT:

by the way i play metal so 10 watts still good for heavy riffs in recording?

Your microphone does not know about watts, its output level will be adjusted at the recording console to the level needed in the mix.
Mor than power, go for a *good* speaker cabinet, because its response *does* get recorded.
A "big" cabinet will give "big" sound.

[askwho69]

Wow again! thanks for very informative and critics about 729X:D i  <3) this forum

[joecool85]

I didn't know about the TDA729x series chips either, always learning something new here.

[J M Fahey]

Just as an exercise on a long cold boring Winter night, draw the TDA7294 pinout on a sheet of paper, put needed components around and try to solve the maze, penciling lines from pin to pin without crossing them.
After getting tired, redraw using a blue and a red pencil meaning top and bottom layer, it won't be enough either.
Don't worry about drawing a "nice" PCB, just solve the connectivity paths.
You'll soon say: "why oh why they didn't set such and such pins side by side?" or even worse: "why didn't they connect those points *inside* and use only 1 external pin for them?"
And when you solve it, you'll see that inevitably some tracks must pass on the flange side of the chip, making it impossible to bolt it to a regular heatsink or the backpanel.
Or even worse, you'll need to mount your heatsink *on* the board.
Not a good idea to mount heavy parts on the board, specially on a guitar amp.
Google TDA7294 images and see for yourself.
None of that happens with older but well designed LM3886.