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screamer 3rd February 2009 04:42 PM

TDA1514 based amplifier giving low power
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hello everybody , I am trying to repair a guitar amplifier based on TDA1514a , the amplifier on paper has a stereo power output of 40w +40w at 4ohms , but this amplifier is giving around 25w per channel before clipping, the supply rails are at +/- 20vdc ,the main filter capacitors are4700uf and 25volt rated and are OK , the shematic is below attached , I check everything on the amp , and it seems that everything is OK , I changed one channel TDA1514 and the power output is the same ,
Can anybody say me that if it is normal so less power from this TDA ??, the data sheet specifies up to 50watt

Thank you
Best regards

screamer 4th February 2009 07:10 PM

In this amp the TDA1514 works in a rather different circuit that sugested on Philips Data Sheet aplicattion circuit , the values and configuration of many of the circuits asociated with the chip are rather diferent , bootstrap circuit or SOAR and Mute/standby are rather diferent ,

screamer 6th February 2009 10:52 AM

Nobody has used this Chip?
I only want to know if it is normal so less power from this chip on this circuit , perhaps something must be wrong on the amp ,.
I am a quite sure that the amp don't have any problem , but I need to verify it , with the opinion of someone that has work on any amp working on similar specs with this chip.

Thank you.

pacificblue 6th February 2009 11:30 AM

According to the datasheet you need 27,5 V rails for 50 W. 20 V must therefore give around 25 W. P = U / R. Your amplifier is okay.

screamer 6th February 2009 01:37 PM

Thank you , but the load is 4 ohms.

If I am correct:

I am not sure if this is valid for these amps , because at 27.5 volt per rail , doing this gives around 100w at 4ohms.

The power supply sag at max power is 1to 2 volt , so total 18-19 volts per rail.

pacificblue 7th February 2009 07:54 AM

It is nearly valid. Replace the rail voltage in the formula by the sagged rail voltage minus the losses across the transistors and through ripple on the rails. They should be around 3-5 V.

25 W at 4 Ohm would be 10 V eff -> ~14 V peak. 18 - 4 brings you just there.

For comparison. National's Overture Design Guide predicts 24,48 W with 4 Ohm load for 20 V rails with the LM1876. Not too far off.

screamer 9th February 2009 09:28 AM

Thank you , I didnt know how to figure out , the amount of losses that it can have .


AndrewT 9th February 2009 12:21 PM

what's the purpose of 10uF (C2) in series with 220nF (C3)?

Why do they attenuate through the first inverting opamp,
then apply gain through the second opamp (switchable between inverting and non-inverting),
then attenuate with 10k (R23) & 3k3 (R24),
then apply gain through the chipamp?
Seems nonsensical.

Is the 120k (R28) and 220nF (C15) at the input there to prevent electric shock through the guitar strings?

pacificblue 9th February 2009 03:47 PM

Their motto is "transistor distortions sound bad, while tube distortions sound good", so they try to make transistor amps that sound like distorted tube amps. Too much gain is supposed to bring each individual stage into distortion. Then they have to attenuate again, because they also don't want too much distortion.
The 10 F is probably also there to create a certain type of distortion. Must be the secret of the typical Marshall sound.

The crossover for the tweeter is also very special. To think that Marshall is one of the most renowned and respected companies in that sector...

screamer 9th February 2009 04:51 PM

This amp isn't a typical marshall guitar amp , is to amplify acoustics guitar or electroacustic , The purpose of the amp is to amplify like "HI-FI" without any distortion , a large dinamics instrumet signal , the schematic ,it is only the power amp , the first Op-amp and the IC3 circuit work as a compressor limitter to avoid distortion from the large peaks on he input signal , the switch on the second opamp is for Phase- out of phase output , to avoid cancelations when mixing with other Miced instruments.
The compresor is not very well designed , in my humble opinion , it not takes the output stage on the peak detector circuit so the results are a little poor , other design options that you coment ,I think that , dont have any especific pupose , simply , the people that desing this come from other circuit topologies (hibrid valve and opamp circuits) this things are usual ,on most of their designs, I know the most Marshall desings , but this amp is quite rare ,
The tweeter is a piezo , that is conected on paralell with one channel , it only has a 10ohm for protection if it shorts.
Thats all .

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