[Curt]

How do you figure you can get 50 Watts at class A with a bias current of 30 mA?

Not sure exactly, they don't like to give all the internal details of IC designs. Don't think we can just go by Iq here though.

It's an IC for mass production consumer products and high quesient currents are not really welcomed in those types of products.

I think they play a game and effect the bias to change via an internal current source and op-amp bootstrap, They did publish this blurb:

"This large-signal, high-power buffer must be capable of handling extremely high current and voltage levels while maintaining acceptably low harmonic distortion and good behaviour over frequency response; moreover, an accurate control of quiescent current is required.

A local linearizing feedback, provided by differential amplifier A, is used to fullfil the above requirements, allowing a simple and effective quiescent current setting."

I spoke with one of the designers (when designing the MB-100) and he told me the bias was set inside the IC and could not be changed in any way externaly. He said the load effected the class A performance. He also mentioned a newer IC they had completed to overcome other bias issues but we believed the chip I wanted to use (1st one) sounded better.

So, not being able to calculate where the change is I usually just listen for an increased THD+N. This is my best guess.

[JohnR]

Um, the quote you've given says nothing about Class A...

To be in Class A both output devices have to be conducting all the time. I think you'll find that the real "Class A" output of the TDA7294 is on the order of a few milliwatts at most.

[peranders]

Hantra, do you have any info about the Aloia amp?

[Curt]

Um, the quote you've given says nothing about Class A...

To be in Class A both output devices have to be conducting all the time. I think you'll find that the real "Class A" output of the TDA7294 is on the order of a few milliwatts at most.

Like you said, I know what class A is...

What I showed you translates to: the bias is dynamic and adjusts itself during actual operation.

I'm not sure a few milliwatts would be worth the IC Mfg to call it class AB.

[JohnR]

Like you said, I know what class A is...

Good. So suppose you fed a sine wave into a TDA7294 such that 50W continous output power was produced into an 8 ohm load. (The chip is still operating in Class A, right?) How much heat is it dissipating?

[Curt]

55-60W dissapated as heat, it's about 45% efficent.

[JohnR]

Let me get this straight. You are saying that an amplifier operating in Class A is 45% efficient?

[Hantra]

Hantra, do you have any info about the Aloia amp?

Well, not that much.  It is the best solid state I have heard aside from Halcro.  It's a 30 watter with a 50+ pound power supply seperated from the main amplifier chassis.  As far as specs, I am not sure.  It's Italian, and they don't have very many reviews in English on the amp.  I had the inductive power supply version.

Curt:

I am positive that the amps I heard were bone stock.  My friend talked to Wayne, and he tried to tell him it was a LOT beter with his mods, but the disappointment factor was high b/c of the extraordinarily long wait, so he didn't want to sink more into the amps b/c they didn't sound good to begin with. . .

B

[Curt]

Let me get this straight. You are saying that an amplifier operating in Class A is 45% efficient?

Just the output stage, we were talking about how hot the TDA package + heatsink got.

The complete amplifier's efficency is different:
Amp Eff = OPS Eff x PSU Eff
(some are as low as 25%)

This thread is about IC output stages not complete amplifier design  

[JohnR]

OK fine, so you are saying that a Class A output stage is 45% efficient? You can even do the math to prove it, right?

[Curt]

OK fine, so you are saying that a Class A output stage is 45% efficient? You can even do the math to prove it, right?

What's with the short one liners? Is this a discussion or you just looking for a fight?

[JohnR]

Err.... what? I asked a simple enough question. Which you have avoided twice now, are you going to try for three?

The question is this: what is the heat dissipated by an amplifier producing 50W into the load, if it's running in Class A? The purpose of this question is to show you that your claim that the TDA7294 operates in class A at 50W output is bogus.

[Curt]

The question is this: what is the heat dissipated by an amplifier producing 50W into the load, if it's running in Class A? The purpose of this question is to show you that your claim that the TDA7294 operates in class A at 50W output is bogus.

Now that's a good question, you should have asked it that way before.

There are two ways to approach this, I will follow your lead first:

Running 50W class A the heat dissipation would be a maximum of 150W, my rule of thumb for continuous power is 3X the audio output power.

The TDA package can dissipate 50W plus what the large heat sink adds to that (maybe double?).

But, even with both added together listening to a sine wave play into a resistor doesn't look good for 50W class A using a TDA.

If that's where you were going I agree with you.

But, using another approach, Music peaks (much like listening to music not a continuous load test) it could be different.

I think you know my thoughts on this (IEC) so I won't explain all over again, but lets just assume a power window with 60W music peaks the rest of the music falls at the levels it belongs at. This is 60W music peaks but the average power is much lower than a continuous power test.

This is easier on the TDA and a realistic listening situation.

If the before mentioned current source op-amp bias control (from the TDA data sheet) really works dynamically during operation, trying to keep the distortion down by trying to maintain class A operation, this would explain the very low THD up to medium listening levels. IMO this is quite possible.

The TDA case and heat sink combination could more than likely handle the heat required for the average music peak power non-continuous case.

This is what I was thinking about, this is an IC that performs a little different than discrete transistor amplifier designs.

There are some unknowns inside the IC so I simply judge it by its sound, watts out, current out and THD+N at different levels.

It's difficult to apply and equation to an unknown, it's easier to look at the input and just see what comes out.

[randytsuch]

cjr/gonefishin,
Thanks for the replies.

I already check out the chip amp section of diyaudio,
there is another forum here,
http://pub134.ezboard.com/ffakeidsfrm1
on DIY chip amps.

I have also asked Scot how much he charges for his kits, his prices seem reasonable, especially for how much Hantra is raving about it.

Randy

[Hantra]

especially for how much Hantra is raving about it.

Randy:

Stay tuned for my full review.  I am not going to do one until they are fully broken in.  I have finally heard what I have been told would happen with Black Gates on break-in.  

[randytsuch]

Randy:

Stay tuned for my full review.  I am not going to do one until they are fully broken in.  I have finally heard what I have been told would happen with Black Gates on break-in.  

Hantra,
I am still curious as to how much Scott is charging for the kit, have already asked him for pricing.
And, if I have to wait for breakin, it's 2-3 weeks to really make sure BG's are broken in.  Besides, I am already in the planning stages to make a gain clone, now I just need to decide if I do a complete DIY, or if I should buy Scott's kit to make it, or both  .  I already have the case and transformers, and was getting ready to place an order with digikey for most of the rest of the parts.

Randy

[Hantra]

Randy:

If you already have most of the parts, I am sure you could get a kit cheaper with less parts.  

As far as the boards, the pricing is on the site.  Not sure if you'll get a better deal through Digikey.  I like sourcing everything at the same place if possible.  Then again, I am too gunshy to do DIY b/c I blow things up. . .    Plus, I like having the expertise from someone who has already made mistakes that I may make, or can diagnose them.  That's why I go for the assembled version. . .

I think Scott is still tweaking around with the design, and he may not finalize pricing until then, but for now, the finished product is $500 per pair for the monoblocks built. . .

B

[randytsuch]

Randy:

If you already have most of the parts, I am sure you could get a kit cheaper with less parts.  

As far as the boards, the pricing is on the site.  Not sure if you'll get a better deal through Digikey.  I like sourcing everything at the same place if possible.  Then again, I am too gunshy to do DIY b/c I blow things up. . .    Plus, I like having the expertise from someone who has already made mistakes that I may make, or can diagnose them.  That's why I go for the assembled version. . .
 ...

Hi Hantra,
I saw the pricing on his site, but it is vague as to what the kit includes.  For the price, I am pretty sure it does not include the enclosure, and I was also wondering if Scott sells them, and for how much.
How do the enclosures look?  Does he have one for the PS, and another for the chip?  Looks like that was what he was planning to do.

Randy

[gonefishin]

So, let me understand this. . .

Is it that you think you can better the 300SE, so you are going to build one?  Just asking b/c I have never heard one.  What don't you like about it ?

B

   hantra...beats me if it'll sound better    I hope so.


   Actually, I'm not to the point where John is...I do not (and should not) design my own tube amps.  I can read schematics, solder and I'm fortunate with having people near me and people on-line (like here) that are much more knowledgeable than myself.

     I guess it all comes down to...I got the bug  


     Take care>>>>

[Hantra]

How do the enclosures look? Does he have one for the PS, and another for the chip? Looks like that was what he was planning to do.

The enclosure is as good as any I have seen for cheap.  They are very nice, heavy gauge metal.  I am thinking about having mine chromed just to match my Kora pre-amp.  How vein is that!?  

Anyhow, there is only one enclosure per monoblock.  The power supply doesn't affect the amp assembly since it's only like 2" cubed.  Scott says even if you shielded it, or put it in a different box, it wouldn't make a difference.  The thing already has an amazingly low S/N, and a startlingly black background.