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Pignose Hog 20

Started by highplainsdrifter, June 26, 2009, 11:41:04 PM

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diyfalk

#15
 :duh

That was the Prob...
A missing C 15 was it.
I put in a 1µF instead of .47µF that solved the Problem no more distortion.  cool!
Thank you all, haha.   :dbtu:

diyfalk



The Blue One is the New One!  :tu:

Ready to Rock...

J M Fahey

Very nice ... although it needs a shave  :lmao:
Please please we need a couple pictures from behind showing how it's mounted and how the batteries fit.
Thanks.

PS: obviously it was oscillating, and that .47uF to ground *was* necessary.
In fact , they skimped on the Zobel network, they needed one on each output tey used only one, hot to hot out.

Maybe it worked without, but on the edge of oscillation.

Glad you found it.

diyfalk


Sure some more pics.   8|

If you are interested in more let me know what you would like to see.
Maybe i will change the 1µF to .47µF if that is better?

I played now for an hour and there is just a little, little subtile distortion left.
I recognize that when playing more the Bass and darker Tones.
Maybe the batterys are a bit week.

By the way much better than before...


diyfalk

#19
What is also interesting is the schematic of the TDA7240A in the datasheet. (see below)

On the Output-Side are two C's and two R's (2,2 Ohm & 0,22µF).
I am asking myself, if that could be better by changing that in the Pignose...
It would be more symmetrically.

Any suggestens?

The C14 in Pignose schematic goes directly to GND.
There could be a R in series with that.

Roly

The CR networks on each side of the speaker are called Zobel networks and they are there to prevent instability/oscillation.

Who knows better about the IC, the manufacturer, or Pignose?

Me?  I'd follow the manufacturers datasheet.

Normally an amp would have only one Zobel network from the output to ground, but something that hasn't been mentioned here is that this particular chip is what is called a "bridge" amp, it is actually two output stages arranged internally to go in opposite directions.  When one drives its end of the speaker +ve the other one drives its end -ve.

This results in double the voltage swing across the speaker than would be available from a single amplifier, and is used where there is limited voltage available (particularly in cars, or as here with a battery supply) to get more power output from the same voltage (at the expense of twice the current, naturally - no free lunch).  Two outputs, two Zobel networks.

The actual values of the C and R in a Zobel are not usually critical and I've seen R's from 10r to 47r and C's from 0.1uF to 0.47uF, but if you stick to the datasheet values you can't go wrong.  One little point however is that these should go as directly as possible between each output pin and the chip ground pin to be effective, i.e. don't mount them on the speaker.

You have made my day - not a new chip, but a circuit design error.  Well done.  Enjoy.  :dbtu:
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

Agree, what Roly said, follow the datasheet, can't go wrong.
Thanks for posting the back pictures..
In this kind of "unusual" amps, besides the schematic itself, it's interesting to see how they solved the space and distribution problem.
I also find it weird that they used 2 x 6V batteries instead of a single 12V one, although clearly it evens out weight distribution.

diyfalk


That's soo cool, the distortion is gone, completely!  :dbtu:

A bridged amplifier inside the TDA-Chip and a missing  HF-cut.

I am out for work for a week now, but i am thinking to do a modifikation, that i have two inputs like my Vox Escort.

I would like to have in the front one input for my guitar and in the back a second input in paralel for FX or Playback.
So i would need a High Gain and a Low Gain Input.
I will look inside my Vox but if you have ideas or suggestens please let me know.

thanks so far. :tu:


J M Fahey

Interesting idea, that chassis cover/battery support plywood base.

geomiklas

The power supply fried on my Hog 20.  I found a second hand replacement which is rated 13.8vdc @ 1.7a ... So I tested the voltage and it registered at 17.8 volts.  I know that the 1.7a is sufficient as it is over 1500mA, but the higher voltage is my question.

galaxiex

Quote from: geomiklas on August 16, 2014, 04:21:24 PM
The power supply fried on my Hog 20.  I found a second hand replacement which is rated 13.8vdc @ 1.7a ... So I tested the voltage and it registered at 17.8 volts.  I know that the 1.7a is sufficient as it is over 1500mA, but the higher voltage is my question.

Unloaded (not connected) power supplies will measure higher voltage compared to when the supply is loaded.
Manufacturers usually rate the supply as it is found in a circuit with a load.
To measure higher voltage out of circuit is normal.
I'd say go ahead and use it, but you may want to wait for more experienced members answer.  :)
If it ain't broke I'll fix it until it is.

Roly

P = E I
13.8 * 1.7 = 23.5 watts

{assuming you don't have a junk bin well stocked with power resistors...}

The sort of thing I'd do if I wanted to be sure is take something like a car dome light bulb, 12 volt, low power, or 20W QI/downlight, and rig it up with a voltmeter and run it for a couple of seconds while you read the voltage under load.  Should put your mind at rest (...or not). 
If you say theory and practice don't agree you haven't applied enough theory.

g1

  Make sure it has the correct polarity at the connector.  This is critical as the wrong polarity will damage the amplifier.

galaxiex

#28
Bumping an old thread cuz I got one of these to repair.

The other guitar player in our band has one of these amps (He goes out busking sometimes)
and he complained about the "fizzy" note decay sound, except he called it a "hum/fuzz".

I actually remembered this thread and told him I could fix it.

As shown in post #19 above.
I added the two Zobel networks on each speaker output of the chip, pins 5 and 7.
.22 cap and 2.7 ohm R to ground as close to the chip as possible. (I didn't have a 2.2 ohm R)

Fixed!
All fizzyness gone!
I love this forum!
Makes me look like a hero!   :)   ..........but I'm really just a hack with a soldering iron.........  ::)

While I was in there I cleaned up some wiring and added a speaker jack
so he can unplug the stock internal speaker and plug in his 4X12 cab.
That should make some noise!

I already tried it with a single 12" Eminence Legend and it sounds great!
If it ain't broke I'll fix it until it is.

galaxiex

Here's some pix of the Zobel net I installed and the speaker jack.

The data sheet pdf for the TDA7240A has a handy chart on page 4
that tells you what the external parts do.
Very handy.  :)

I noticed that this amp has waaaay too much bass response for the 6" speaker and makes it "fart" on low notes.
So I reduced the cap on pin 1 of the chip down to 1uf. C13 10uf on the schematic.
Much better.

While I was in there I didn't like the "Squeal" distortion sound.
Way too "over the top" and harsh.
I removed the 4 1N914 clipping diodes D1,2,3,4 and put in 2 red LED's and 1 green and 1 yellow in their place.
Again, much better. The distortion sound is actually usable now.

I also put a 1uf film cap in place of the stock C10 2.2uf electro.
Again, less bass with that change.

re; the zobel network, The stock amp has R10 and C14, removed those since I put the 2R7 resistors and 220nf caps in.
Schematic shows C15 but it was never installed.

Here's the schematic etc for reference.
If it ain't broke I'll fix it until it is.