Help required on Logitech Z-2300 toroidal transformer

[RishiGuru]

Originally Posted by RishiGuru

Guys, I am sorry to reply so late, but right now I am out stationed due to official work and thus cannot post, but hope to be back in home this week.

I have deferred buying the new toroid since I will be out stationed, and have given the Z-2300 toroid to the local winding shop. When I get back I will see it they could get it right, and if not, buy a brand new.

Today I got this GREAT NEWS. I repeatedly made a dozen requests to know the VA rating of this toroid in their "Online Inquiry" & "Contact" section of their website.

My request to Ten Pao -->

Kindly inform me about be the VA rating of this toroidal transformer

Model No: TOG433028F0
Input: AC 230V/50Hz
Output: AC 20.1V x 2

This transformer is used by my Logitech Z-2300 and had gone dead. Since my Z-2300 is outside warranty & thus no support from Logitech, I need to buy one from the market. So, I need the VA rating of this toroidal transformer. I have repeatedly asked for help in the "Online Inquiry" section of you website with no reply.

Kindly please help me.

Thanking You,

RishiGuru
Country : India

Their answer after a dozen request :

Dear RishiGuru,

After confirming with our engineer, the VA rating for this model is 150.9, and our company didn't set any retail place in India, so please contact with your Logitech Z-2300 supplier.

Best Regards
Sam Su
Marketing Department
TEN PAO INTERNATIONAL LTD.
Tel: 86-752-2312309
Fax: 86-752-2312813
Skype: sam_su85
Email: (E-Mail Removed)
MSN: (E-Mail Removed)
website: Ten Pao International Ltd

From: RishiGuru
Sent: Sunday, December 12, 2010 9:39 PM
To: (E-Mail Removed)
Subject: 線上查詢

----------------------------------------------------------------------------------------------------

The toroid is 150.9VA
So, it seems 150.9 VA --> 2 x 20.1 V, 3.75A

It seems that 200W claimed output is to much for the 150.9 VA transformer after all.

Also what I learned from TechGurus like you, in order to produce Z-2300 clamied 200W RMS one needs a transformer that is capable of to deliver a minimum of 1.27 times the claimed watt.

Since Watt = Volts X Ampere = VA
200W requires = 200 X 1.27 = 254 watt or 254VA = ~ 255VA transformer as a minimum requirement.

Since here we have 150.9VA.
Maximum output of the system = 150.9/ 1.27 = 119W =~ 120W.

Am I right or wrong. Kindly help me.

Also since Z-2300 uses Class-AB amplifiers generally having an efficiency of at around 50%, that real power that is sent to the speakers is = 120/2 = 60W only.

Man I cannot believe what I am writing.

[Minion]

Hi , 119w is the power what the transformer supplies but that isn"t the output power of the amp .....

Class A/B Amplifiers aren"t very effecient , they are maybe 50% effecient at best which would mean that an amplifier useing a 120w Transformer might be able to do 60w total ......(I use 120Va transformers for a single LM3886 at 50w)

So it is possible that this isn"t the right transformer , it is also possible that they grossly over state their specs or they use PMPO to overstate their specs ......

[RishiGuru]

Originally Posted by Minion

Hi , 119w is the power what the transformer supplies but that isn"t the output power of the amp .....

Class A/B Amplifiers aren"t very effecient , they are maybe 50% effecient at best which would mean that an amplifier useing a 120w Transformer might be able to do 60w total ......(I use 120Va transformers for a single LM3886 at 50w)

So it is possible that this isn"t the right transformer , it is also possible that they grossly over state their specs or they use PMPO to overstate their specs ......

I gave a call to the local repair shop where I send my toroid, they replied me that the internal fuse was blown. I never new their is a fuse plug in there. They also told me that the primary & secondary winding are perfectly OK. I have told them to repair that and make it ready by Thursday. I will be home by Thursday and try to fit in my Z-2300. You guys be with me. I still need all your help. But, I am so happy that I do not need to buy a new one.

And since now I know that the transformer is severely underpowered do you think from now on I should never go beyond 50% of the volume so that the internal fuse do not blow again?

I love a lot of bass heavy songs. And I know this bass requires a lot of power from the transformer in order to push the 8 inch subwoofer. Am I right?

[Minion]

Some transformers have an internal fuse , it shouldn"t blow unless there is a lot of current being pulled , for instance if there was a short in a winding or on the amp board , though there should be seperate fuses on the amp board .....

I don"t think that logitech would make an amp that blew when used at max volume so there is a chance that it will blow again , maybe because of a fault in the design or just a fault in a component ......

[RishiGuru]

Originally Posted by Minion

Some transformers have an internal fuse , it shouldn"t blow unless there is a lot of current being pulled , for instance if there was a short in a winding or on the amp board , though there should be seperate fuses on the amp board .....

I don"t think that logitech would make an amp that blew when used at max volume so there is a chance that it will blow again , maybe because of a fault in the design or just a fault in a component ......

Minion, I do have a question. I downloaded the TDA7295 (80W) & TDA7296 (60W) spec sheets from the web.

The power transformer is rated to have an output of two times 20.1VAC when it has its rated load. So its resistance has already dropped the open circuit voltage and its peak will be 1.414 times higher which is 28.4V peak. The single rectifier drops it to 27.4V and the ripple drops it to 26VDC.

Now looking at the output power vs supply voltage graphs I found out that both this power amps at 26VDC (toroid secondary output) produce 38W @ 0.5% THD. Its only when we increase the supply voltage, that TDA7295 is capable of delivering more power.

But, since we know that Z-2300 toroid can do 26VDC,

1) Applying 2 X TDA7295 for the sub woofer produces 38 X 2 = 76W @ 0.5% THD @ 26VDC
2) Applying 2 X TDA7296 will produce the same power which is 38 X 2 = 76W @ 0.5% THD @ 26VDC

So from a logical point, applying two TDA7295 is a waste of money at 26VDC since both produce same power. Am I right?. Or do TDA7295 have a slight advantage at 26VDC? What do think think about it?

[Minion]

From looking at the Datasheets they seem allmost like the exact same chip accept one has a bit higher voltage rating and a bit higher output , they seem to put out the same output at a given voltage ......

The Data sheet also calls the output wattage at "60W Music Power" which means it can put out that much power for a very short time but has a lower RMS wattage .....

[RishiGuru]

Originally Posted by Minion

From looking at the Datasheets they seem allmost like the exact same chip accept one has a bit higher voltage rating and a bit higher output , they seem to put out the same output at a given voltage ......

The Data sheet also calls the output wattage at "60W Music Power" which means it can put out that much power for a very short time but has a lower RMS wattage .....

Minion, I also learnt from other forums that bridging two amplifiers almost doubles the output voltage swing and almost doubles the current in the speaker.

Now, from the graphs of TDA7296 data sheet I have found that TDA7296 produces 38W @ 0.5% THD @ 26 VDC. [Helped by a AudioGuru]

So the output power is (almost 4 times) a little more than 3 times as much as with a single amplifier (120W instead of 38W).

So, let me clarify
1) Two TDA7296 bridged together will provide a little more than three times the power of a single TDA7296.

Since TDA7296 produces 38W @ 0.5% THD @ 26 VDC,
2 X TDA7296 = 38 X 3 = over 114W =~ 120W.

Will the THD of the bridged TDA7296 amplifier be 0.5% at 120W?

2) Also TDA7296 power dissipation (heating) at 38W @ 0.5% THD @ 26 VDC shows to be 16W so its efficiency is 70% which is pretty good for a class-AB amplifier. The power dissipation increases when the output power is reduced to half.

Again since the 150.9VA transformer can supply only about 180VA when overloaded so the total output power of the four TDA7296 amplifiers will be 70% of that at max volume which is approx (180/100) * 70 = 126W

Since Logitech stated 200W --> 120W(Sub) + 80W (Satellites)
Ratio of power distribution :: 3:2

So, the sub gets (126/5) * 3 =~ 76W while each satellites get 25W

Now, 76W to the bridged TDA7296 which is 76/2 = 38W for each of the two amps. [Note: Here why don't we do 76/3 = 25W ???]

3) What is the max output & efficiency of the TDA7295 at the clipping point @ 8 ohms @ 0.5 THD @ 26 VDC? I cannot find it from the graphs of the spec sheet.

4) Looking at the fact that the amp has 4 X TDA7296 each capable of handling a max of 5 ampere which brings a total of 20A peak.

Now the transformer is rated 150.9 --> 20.1V X 2, 3.75A

Dividing the system current to the four TDA7296 = 3.75/4 = 0.94A for each TDA.

Also voltage applied to each TDA7296 = +/-20.1 = 40.2V

Since Watt = Volts X Ampere = 40.2 X 0.94A = 37.79 = 38W

This exactly matches with what the graph said "TDA7296 produces 38W @ 0.5% THD @ 8 ohms @ 26 VDC".

What is your say on this? Did Logitech calibrated the transformer to reach this goal of 38W?

5) Lastly, if I replace all this 60W TDA7296 chips with 100W TDA7294 chips, will it harm the other components of the amplifier? Or rather will the amplifier be more robust ?

I know I have asked a lot of questions, but at the same time I know you are the one who can help me out? I will love if anybody answers this questions.

Thanking you, RishiGuru

[Cobra2]

Checked mine, trafo seems to be in ~ 250VA size.

Arne K

[RishiGuru]

Originally Posted by Cobra2

Checked mine, trafo seems to be in ~ 250VA size.

Arne K

Cobra2, have you used new capacitors in the Z-2300? The two big capacitors on the left side of your amp photo are ripple filters, I also have those. But the pair of capacitors on the right side of the amp, well I do not have those.

My amp picture is provided below:


You have a pair of capacitors where my yellow wire passes. What are the use of those? What is the farad value & voltage capacity of those?

It will be very helpfull, if provides any kind of performance advantage

[Cobra2]

A couple of 1000 uF extra... the power-rails (+/-) are from the jumpered lines, gnd(0V) is the "perforated" part.

Arne K