seeking opinions...

[mich]

hi all.
im trying to put together a solid state amp-like a polytone with components that ive got..here is my idea:
take the input stage+tone circuit from the polytone schematic, send the signal to the stage center revereb pcb(simple driver-receiver that i got), and send that to the 3886 amp pcb -

http://www.generalguitargadgets.com/pdf/ggg_lm3886_amp.pdf

, and then power the whole thing with one 38V@3A power transformer......

here is what i have

any sugestions or predictions on how well it will function will be great.....
thanks

 

[Gus]

The IC power amps don't always sound good with bass. I believe it is because of the output protection being activated more with bass than guitar playing. The polytone I was in years ago had power transitors and a LF391 part of the power amp IC.

That IC can sound ok with home HiFi because CDs have been mastered and have limited output voltage range.

Build it and try it.

 

[mich]

thank for the advise gus, my intention is to use the amp for guitar so no problem with bass ...but do you think that my aproach with the power supply (powering 3 circuits) schem looks o.k?

 

[mich]

o.k, i made some changes,does this looks better?

schem with changes

 

[PRR]

Is there some way you can post schematics smaller? BMP is an uncompressed format; GIF format will crunch line drawings MUCH smaller. Less strain on Kev's dial-up connection. Also your image is 3000 pixels wide, so on my 800-pixel screen I could only see a few parts at a time.

> power the whole thing with one 38V@3A power transformer......

Needs to be a center-tapped transformer. Otherwise your "ground" won't be half-way between the supply rails.

That means you need three terminals on the input connection:

C37 C38 are redundant.

In general, I am not too keen on 4558 chips in good audio, but it may be the tone you want.

 

[mich]

sorry for the big image...ill convert to gif or jpg.

Needs to be a center-tapped transformer. Otherwise your "ground" won't be half-way between the supply rails.

that means that there is no such thing as "virtual ground"? i think i read something about getting a dual supply from one source-dividing the voltage like that.....ill search for it....


In general, I am not too keen on 4558 chips in good audio, but it may be the tone you want.

yes, the polytone schematic uses this chip,and thats the tone im after...

thanks for helping guru PRR..... :grin:

 

[PRR]

> that means that there is no such thing as "virtual ground"?

Not at the many-Watt level.

You see it a lot in low-level stages, and even in headphone amps, where power is so small you can afford a little more power in a circuit that "forces" a halfway point between the rails. But in a loudspeaker amp we can't afford that much waste.

 

[Kev]

[quote author="PRR"]... Less strain on Kev's dial-up connection.

... C37 C38 are redundant. [/quote]

orrr ... thanks bloke .. :thumb:
nice to be remembered

c37 38 ... mmm
if you were to relocate the regulators over to the preamp board than the caps would still have value. Depending how far away the board is. Still no harm keeping them.
Use a three pin connector and then you can chose to have the ground or lift he ground on the pre-board

LM833 is a cool chip for these jobs.


Why have the power amp on the end of the 10 ohms resitors ?
Is this specifically for the chip amp ?

 

[VacuumVoodoo]

Hi,

For guitar i would change R1 & R2 to about 56-68kohm which will give more variation in pick-up loading between the two inputs.
Also junction between C7 & C8 needs to be connected to the wiper of the Bass pot.
I'm not sure about capacitor values in the EQ (had no time to do a simulation, i might do it for you after the New Year).
What mideq center frequency are you aiming at ?

Alex

 

[PRR]

> nice to be remembered

Well, just as an example of the many people who don't have the bandwidth to download 2,000 white dots, when a GIF compression would losslessly reduce a line-art image to a few percent of original size.

> c37 38 ... mmm ...if you were to relocate the regulators over to the preamp board then the caps would still have value.

Good point.

> LM833 is a cool chip for these jobs.

Agree in general, but guitar-amp tone is a funny thing.

> Why have the power amp on the end of the 10 ohms resitors? Is this specifically for the chip amp?

I don't know. It reduces power on test-tones. However with the 4,700uFd caps, guitar transients may pass at very high power with lower level in the sustain. This is similar to a vacuum-rectifier guitar amp, and this large drop from peak to sustained power does seem to be an important effect in a guitar amp.

 

[Kev]

This is similar to a vacuum-rectifier guitar amp, and this large drop from peak to sustained power does seem to be an important effect in a guitar amp.

yep guitar amp
sorry not thinking
people call this sag or sagg ... something like that. :roll:

PA amp and low imp power supplies and outputs ... old habits for me.

As for guit tone, you can never tell what people are going to like ... on paper one stomp box should sound like crap but doesn't ... :shock:
and then another with a great schem and layout and parts ends up with no zip.
you get that

 

[mich]

great that this topic kicked in again...

first off all, my initial idea was to build this amp without any costs(using only parts that i have :grin: ,and since PRR mentioned that the transformer
that i have wont do the job (at the high watt level), i found another solution...-this tranny was from a piece of hi fi stereo that i found in the street-it has 2 AN7149N power chips,and from some info i found on this chip,it gives 5-6 watts per side.anyway,i left the transformer where it was,fixed the amp section (of the stereo hi fi)-and it workes great!
i injected 1kh sine wave to the input-after conecting out to a 12" speaker and man-lots of power!
so i designed the pcb for the polytone preamp and tone control circuit,including power supply for the preamp and reverb (on the same board) - using the "virtual ground" aproach....
so now im stuffing the board....if all turns out o.k-ill be happy to share with anyone who might want such a thing.....(polytone preamp+standart +/-15v supply). :thumb:

alex:
i just copyed the input stage and tone circuit from the polytone schematic,using its values-i know the fender amps use 68k resistors at the input,but i want to find out some why's about this polytone amp....

Also junction between C7 & C8 needs to be connected to the wiper of the Bass pot.
-absolutly right!i missed this one when copying......thanks :thumb:

 

[mich]

finished.just waiting for the reverb tank.thank you bo,for sending the polytone schematics. :thumb:

i used a dead reverb tank as chassis-worked great.
sounds really nice for jazz stuff....really nice...

:thumb: :thumb: :thumb: for helping....