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 on: Today at 07:37:00 AM 
Started by Manfred - Last post by tarahall
Agree on LM3886 opinion, but I guess I found the reason: LM3886 is simpler to use and it boasts 50 or 60W RMS ,
Now TDA7293/4 claim 100W RMS which simply is asking too much from the exact same package.

Thanks to y'all for your explanations.
If I want a 60W "RMS" into 8 ohms output stage which of these chips would be the most suitable:- LM3886, TDA7293 or TDA7294.


 on: September 20, 2017, 10:58:55 PM 
Started by Manfred - Last post by J M Fahey
JM can you please clarify this. When data sheets (and I quote the LM3886 here) says things like:

 "68W cont-avg-output power into 4 ohms" or "38W cont-avg-output power into 8 ohms"

what does the term "cont-avg-output power" mean?
is that their way of saying 68w RMS or 38 RMS?
They are speaking with the Dictionary in hand, which is fine with me.
cont-avg-output power
is what average people on the street (and thousands of brohures, magazine articles, user manuals and probably even some books) calls RMS power.

Strictly speaking there is not such a thing as RMS powr, but RMS voltage or current.

That said, what RMS power actually means (or should) is "power calculated using RMS voltge, sustained during a reasonable time"

personally if if I check an amp puts out 20V RMS, when fed some classic test tone, usually 1kHz or 400/440Hz , into a 4 ohms resistive load, without visible clipping and for at least 10 minutes, would prefer at least 1 hour, then *I* wonīt argue at ll if you say it is a "100W RMS amplifier".

Thereīs been tons of arguments lately arguing gramatics while the real problem lies in the monstrous fantasy claims posted everywhere using undefined, pure fantasy numbers.

Arguing against a time tested, **easily repeatable and consistent** system which has been used for decades and does not require hard to find equipment , difficult Math and on which most agree (or heve agreed since forever until tye latest Revolution)  sounds stupid and dangerous to me, because dissing and putting out of the way a stable and repeatable test procedure, anything else goes.

FWIW the FTC itself has dropped **officially** any pretense of measuring or norms enforcement.

 on: September 20, 2017, 12:03:36 AM 
Started by Manfred - Last post by Enzo
Just for marketing, somethimes they come up with some sort of spec where for one glorious nano-second the thing can be measured as putting out a thousand watts.  COntinuous output power means it can actually produce it.

A real world example.  I had an upright piano fall over on its back in our truck.  it pinned the toe of my boot.  Damned thing weighed 900 some pounds.  I could get my fingers under the edge.  I was able to jerk up on it just enough to free my boot.  Now I cannot lift 900 pounds, and certainly can't hold it up.  But I can apparently for just an instant peak my lift at 900 pounds.  SO in one sense, i can lift 900 pounds, but in any real sense I cannot.  Just so continuous versus peak power coming from an amplifier.

 on: September 19, 2017, 09:49:14 PM 
Started by Manfred - Last post by phatt
 (cont-avg-output power) = Continuous Output Power.

So a sine wave input would be "Continuous" but music is unlikely to ever be Continuous.

Regarding your Q of RMS output others here will know more,,
I just look at these as a ~40Watt chip.
Also remember that the whole power thing is governed by the power supplies ability to deliver that power so a larger transformer is the key if you want max Wattage. (no free power 8|)
Then you will need a lot of heat sink if you intend to run them hard.

 on: September 19, 2017, 08:45:52 PM 
Started by Manfred - Last post by tarahall
All these chipamps are "powerful Op Amps" , just varying in supply and power output, same design rules apply to all.

JM can you please clarify this. When data sheets (and I quote the LM3886 here) says things like:

 "68W cont-avg-output power into 4 ohms" or "38W cont-avg-output power into 8 ohms"

what does the term "cont-avg-output power" mean?
is that their way of saying 68w RMS or 38 RMS?

TIA Gavan.

 on: September 19, 2017, 08:39:21 PM 
Started by tpb03 - Last post by tarahall
That said, it *should* work like a plain TDA2050 single supply datasheet example, so "just trust the datashet, not the seller".

The data sheet clearly states that the TDA2050 is a 35W IC so the seller claim of 120w is obviously seller promo "BS".

 on: September 19, 2017, 02:03:31 AM 
Started by Alexius II - Last post by tpb03
Just a quick question re resistors to set Vd & Vs for voltage biasing in the schematic. If I want to run at 12V or 19V (I have these power supplies), should I just use the calculators on the ROG Fetzer Valve revisited page after working out the Vp and Idss for my MPF102? The values in the schematic are twice that of the average values on the ROG page.

 on: September 17, 2017, 08:22:52 PM 
Started by tpb03 - Last post by tpb03
Thanks J M.

I will treat it as per the datasheet single supply. I also have a TDA2030 board that is claimed 15W 6-12V single supply. As I have the TDA2030 board already, I will test it at 12V and then higher just playing music from my phone.

I have been searching for a preamp and found this schematic on the forum (post

It is Runoffgroove's Fetzer Valve with a buffer. This should give me a good approximation of the Champ tone going into the power amp board. Its also setup for 18V, although I did have a look through my laptop supply collection and the nearest is 19V. I'll test them all with my multi-meter to work out true voltages as from experience laptop supplies and wall-worts can be +/- 2V either way.

I would like to modify the preamp for two inputs like the 5F1. I'll report back when I proto this, but I think I will need to change the 4.7K input resister to 2 resisters running in parallel like the champ input. I can then solder the 1 Meg resister from active to ground on the input jack rather than on the board.

As for speakers, I have an 8" 8 Ohm guitar speaker from an old Park G10.

I'll report back here to share my experience as I am bound to need to help along the way!


 on: September 16, 2017, 03:27:00 PM 
Started by ilyaa - Last post by J M Fahey
Dear ILYAA: donīt know where are you from.

If USA, and within warranty period, send it back, period, and get a real JTM45 or build one out of a Ceriatone kit, a suitable prebuilt cabinet and your choice of speakers , no need to make everything from zero up.

If in  Country with no *real*  access to Factory resources and where getting a full refund is a problem (such as here in Argentina), then it may be worthwhile trying to reform it, but it wonīt be a standard repair job , it will be reengineering , and definitely expensive. (not in parts but in applied knowledge).

Donīt understand why Fender Authorized Service could not repair it or worst case get a replacement unit from Factory.

 on: September 16, 2017, 02:34:23 PM 
Started by ilyaa - Last post by g1
A common failure with power scaling amps is that the power Fet shorts, leaving the amp stuck at full power all the time.
Too bad that does not seem to be the situation here, as that seems to be something the owner would accept in this case.
You are kind of stuck either drawing out the schematic, or finding it somehow.
I'm sure it is possible to bypass the power scaling like JM metioned.
Was the unit repaired or replaced last time (serial # would be different)?
If it's on the 'do not repair' list, they will just replace the unit, and there may not be a schematic.
If it's not on the DNR list, there must be a schematic somewhere, even if it's only available to authorized repair centers. (hint hint if you know of anybody  ;) )

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