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Aidan135711 1st August 2008 08:17 PM

Need drawings
 
Hello everyone
I need schematics for a guitar practice amp.It is a Sound City SC10.I think it is a 10W amp.
When it came to me the TDA2030 power amp IC was smoked and the power transformer too.It was a center tapped thing with 2 diodes as full wave rect.
IC was replaced with TDA2030A(I think it can handle higher supplies) but I don't know what the secondary voltage of transformer should be.I have tried several but bad distortion sets in after volume level 2.This with a sine wave test tone @200mV p-p.Even worse with guitar plugged in.
I have tried several starting from about 12V and now the transformer I have in gives smoothed supply of 25VDC ripple 200mV p-p(maybe this ripple is too high?),the maximum I can give it,but the problem persists.
I have searched a lot for drawings but can't find any.
Has anybody out there got them?

Inch-o 6th August 2008 01:36 PM

Re: Need drawings
 
2 Attachment(s)
Quote:

Originally posted by Aidan135711
Hello everyone
I need schematics for a guitar practice amp.It is a Sound City SC10.I think it is a 10W amp.
When it came to me the TDA2030 power amp IC was smoked and the power transformer too.It was a center tapped thing with 2 diodes as full wave rect.
IC was replaced with TDA2030A(I think it can handle higher supplies) but I don't know what the secondary voltage of transformer should be.I have tried several but bad distortion sets in after volume level 2.This with a sine wave test tone @200mV p-p.Even worse with guitar plugged in.
I have tried several starting from about 12V and now the transformer I have in gives smoothed supply of 25VDC ripple 200mV p-p(maybe this ripple is too high?),the maximum I can give it,but the problem persists.
I have searched a lot for drawings but can't find any.
Has anybody out there got them?

Hi Aidan,

You can approximate the secondary voltage of the transformer, by checking the voltage of the main power supply electrolytic capacitors.
Let's say they are some 35Vdc... (you shouldn't really trespass this)
If you have a two diodes rectifier, then the rectified voltage of such a PS is calculated as follows:

Upeak(capacitor)=Ueff(transformer secondary)*SQURT2

(SQURT2= square root of 2)

So Ueff(transformer secondary)=Upeak(capacitor)/SQURT2

35V/1.414=24.75Vac

So a centre tapped transformer with a secondary of 2*24Vac should do perfectly
All of the above is valid if your power supply is as per the attached picture.

kikikaka 6th August 2008 06:39 PM

2 Attachment(s)
forget that power supply with only 2 diodes...

Aidan135711 6th August 2008 06:55 PM

2 Attachment(s)
Hi Inch-o
Thanks for your reply.Very useful information,however I forgot to mention that my PSU is single supply.Does this calculation still apply?
Also I had a closer look at the datasheet for the TDA2030A and the typical circuit(figure 12) can have up to 44v single supply,typically 36V.Maybe I am not giving it enough volts?
The biggest transformer in my junk-pile only gives about 18v either side of the centre tap.Looks like I will just have to buy one.

Inch-o 7th August 2008 08:46 AM

Hi,

No if you have the PSU as per your picture, then the calculations are a bit different:

Uout(capacitor)=Ueff(Half transformer secondary)/PI

In fact the same capacitor is being charged every half cycle by just one of the halves of the transformer secondary.

In any case, you should also take in accaount the maximum permissible supply voltage to the TDA2030.

Cheers,
Inch-o

Inch-o 7th August 2008 08:50 AM

Quote:

Originally posted by kikikaka
forget that power supply with only 2 diodes...

What you draw, its a classical full wave rectifier. Manufacturers of "low cost" apparatusses often decide to spare on everything in a circuit, to cut down costs....
So that is why I supposed it was a PSU like the one from my picture.
I didn0t know the amp has a single ended PSU.


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