Cambridge Audio A4 amp and LM1875 PCB ;o)

[Geoffroy]

Good evening to some, good afternoon to others and good morning !

I bought yesterday (still waiting for the package obviously ;o)) a LM1875 PCB kit (stereo) with the "power" distribution board. (you already see I don't really know what I am talking about....).

I always wanted to build an amplifier, starting small I thought would be a good idea and I understood that with this kit, some patience, a casing and a proper power supply things might work out and I would learn a lot.

Looking for a case I found someone who wanted to give a broken Cambridge Audio A4 integrated amplifier. I picked it up today and it turns out that it seems (from the smell and the black spots) that only the amplifying section of the device blew up (the transistors).

From a simple amplifier my hopes are now raised to "maybe" building an integrated amplifier by just substituting the amplifier stage. However my knowledge is down to 0 on a scale of whatever and I need enlightment. Where can I find details about this amplifier? (tried CA website to no avail). Are there rules to follow between preamp stage and amp stage?

The above mentioned info would be a good start for me to understand whether I remmove it all from the case or keep it in.

Any additional wisdom would be gladly received.........as I am sure it will be needed along the way......

Kind regards

Geoffroy

[mikesnowdon]

When I needed a service manual for my CA amp I just emailed CA and they sent me one with all the schematics. First you need to sign up for the customer care centre then aks for a manual. Its a bit like a forum, you ask a question and it goes ona board and a technical advisor will reply....

I would start by posting some good quality internal shots of the Cambridge amp. From there you could check the voltage of the transformer either by reading the label or using your multimeter (not on the 240v input wires ) Find out which wires go to the powerstage, they will go via the bridge rectifier and then through some largish caps. These will be the largest caps in there probably. From there you can trace it to the powerstages.

Chances are you might need a new transformer for the chipamp, if the voltage is too high for your LM1875. Other than that its a case of finding out if everything else in the amp still works ok. From there it should be quite easy to substitute the power amp stages. Start with some photos and find a schematic of the Cambridge or a service manual.

Good Luck!

[lanchile]

Well,I would use the chassis and if you want to use the transformer check the secondary voltage and see if it is the voltage recommended for the IC in this case LM1875.I use LM3875 and I use 22vac on the secondaries and after the bridge rectifier I get around 32vdc good enough for 6/8 ohms speakers and it can even drive 4 ohms too but, I use 8 ohms only.

[Geoffroy]

Ok, step 1 ask Cambridge step 2 get a multimetre (yes I am just starting....). Then check power output and transformer. It kind of tells me that step 1 is to wait for the chipamp kit first

[Geoffroy]

Thank you again for your precious comments above !!

Cambridge Audio told me they lost the manual and schematics etc... for the A4, I don't really believe this . It just means I have to look very closely at each board to see what is going on

I now have played around a bit with the amplifier and it took me a while to understand the "big picture" of how this baby works. The bottom line is that the rectifier bridges and the power amp stages are built on one single board (big picture I said ) . It therefore seems easy to remove the board and replace them by the lm1875 pcb's I prepared.

My only worry is the energy supply. Indeed, the transformer is rated as:

240 V 50Hz
30V 0V -30V at 2A I suppose it means 120 VA (which seems low).

Looking at the "National" webpage it seems the lm1875 can deliver >30 Watts with +/- 30V and 8 ohms load (I presume they mean speakers in my case??) with 10% THD.

Is the info above telling me that I can fit the lm1875 rectifier bridge and power amps pcb's (I got the kit from chipamp as said above) in the amplifier safely? Will I be limited in terms of gain meaning that to get a proper sound i would need to avoid cranking the potentiometer too high....? ( I have the feeling the above question is wrong as the current going to the amplifier willbe the same hence the THD will remain at 10%...).

Last but not least.... I had a look at the transformer and only three wires are coming out while when I look at pictures from toroidal transformers with 2No secondary leads (4 wires). My interpretation is that usually each secondary lead has a "ground" wire while in this case it is one "ground" which really does not change anything please advise if I am wrong...before I burn the house down testing whether it works

Any help with the above would help a lot I can post pictures if anyone needs it to help!

Kind regards

Geoffroy

[pacificblue]

Originally posted by Geoffroy
Cambridge Audio told me they lost the manual and schematics etc... for the A4

That's a good one.

Originally posted by Geoffroy
30V 0V -30V at 2A I suppose it means 120 VA (which seems low).

120 VA is enough for 120 W output power. So, if the amplifier is a nominal 2*60 W type or less, the transformer has the right size.

Originally posted by Geoffroy
Looking at the "National" webpage it seems the lm1875 can deliver >30 Watts with +/- 30V and 8 ohms load (I presume they mean speakers in my case??) with 10% THD.

Is the info above telling me that I can fit the lm1875 rectifier bridge and power amps pcb's (I got the kit from chipamp as said above) in the amplifier safely?

On the contrary. After rectification and smoothing the voltage will be higher than the nominal transformer voltage. As a rough estimate multiply the transformer voltage by 1,41.

Anything above 2*20 V or 20-0-20 will be difficult to heatsink.

Originally posted by Geoffroy
Will I be limited in terms of gain meaning that to get a proper sound i would need to avoid cranking the potentiometer too high....?

You should choose the gain so that your weakest source will lead to nominal output with the potentiometer at its rightmost limit. E. g. your source is a CD player with 2 V output and your amplifier has an output swing of 20 V, use a gain of 10. If your source is a tuner with 0,3 V output use a gain of 20/0,3 = 67. But respect the limits. The ICs from the Overture series usually work with gains between 10 and 50.

Originally posted by Geoffroy
the current going to the amplifier willbe the same hence the THD will remain at 10%...).

The THD level is a function of the output power. Look at page 3 in the datasheet. And the output power is a function of the volume potentiometer's position, gain, load and input level.

Originally posted by Geoffroy
Last but not least.... I had a look at the transformer and only three wires are coming out while when I look at pictures from toroidal transformers with 2No secondary leads (4 wires). My interpretation is that usually each secondary lead has a "ground" wire while in this case it is one "ground"

Correct. The center-tapped transformer you have needs one rectifier bridge to provide a split power supply. A transformer with dual secondaries can be used with a single bridge, but should be used with a separate bridge per secondary winding. Other than that one will work as good as the other.

[Geoffroy]

Thank you very much Pacific Blue for your replies, I still am a bit lost on the following:

I don't know what is the required power for the amplifier board.

The transformer proposed by the chipamp website is this one which seems to have a higher power rating than mine.

What does it mean for my setup?
After all my amplifier has a higher voltage but a lower "amperage" (don t know how to call this in english sorry ). I would have though it does not matter as a transformer supplies power regardless of the voltage and that the voltage is defined only as the maximum (in absolute value) power supplied at a certain "amperage" but I seem to be wrong here.......

Thank you for your help!

Kind regards

Geoffroy

[pacificblue]

Originally posted by Geoffroy
I don't know what is the required power for the amplifier board.

The higher the supply voltage, the higher the output power and the bigger the required transformer. The minimum rating would be a transformer with the same amount of VA as the total output power of the amplifier. It is usually good to have a bigger transformer. Up to three times the amplifier output can have a positive effect on the sonic performance. But the advantage is not big and your speakers may not be up to reveal it. On top of that there is the regulation issue. Regulation describes how much the voltage rises above its nominal value, with no load. The smaller a transformer, the worse is the regulation. Therefore it is always good to avoid transformers with less than 100 VA. From there on upward regulation is usually less than 10 %.

Originally posted by Geoffroy
What does it mean for my setup?
After all my amplifier has a higher voltage but a lower "amperage" (don t know how to call this in english sorry ). I would have though it does not matter as a transformer supplies power regardless of the voltage and that the voltage is defined only as the maximum (in absolute value) power supplied at a certain "amperage" but I seem to be wrong here.......

With a 30-0-30 V transformer the heat dissipation of the LM1875 is too big. The small surface of the LM1875 cannot transmit so much heat to the heatsink. For your setup that means you need a transformer with lower voltage 2*18 V or 18-0-18 is reasonable. That gives you ±24 V rails. They will sag under load to ~±21,5 V, which leads to ~19 W per channel with 8 Ohm speakers. If you want to use 4 Ohm speakers, aim for a 2*15 V or 15-0-15 V transformer.

[Geoffroy]

Originally posted by pacificblue

With a 30-0-30 V transformer the heat dissipation of the LM1875 is too big. The small surface of the LM1875 cannot transmit so much heat to the heatsink. For your setup that means you need a transformer with lower voltage 2*18 V or 18-0-18 is reasonable. That gives you ±24 V rails. They will sag under load to ~±21,5 V, which leads to ~19 W per channel with 8 Ohm speakers. If you want to use 4 Ohm speakers, aim for a 2*15 V or 15-0-15 V transformer.

Ok, this is quite a clear comment, however I am trying to understand roughly the "concepts" behind this. Please tell me if I am wrong....

If I look at the National pdf brochure/data sheet, there is a set of curves showing that the lm1875 can operate with a supply voltage of +- 30V (page 3 of the pdf). The output load per transistor is about 33 W (or VA I guess) which gives a total power output (at 8 ohms indeed) of 66W. This means that the heatsink would need to dissipate the remaining energy? (hence wasting energy and overheating anyway)

It means I got to find a set of transistor/a kit which will have a power output of about 120 W total to avoid overheating and staying in nice operating conditions right? (or buy a new transformer...)

Last but not least, the way diode bridges are designed depends on the caracteristics of the transformer being used? It would mean that on chipamp kits and the likes sold online they assume that we would have a transformer of a reasonable power rating............

Last and least, what is the link (equation) between current and voltage in the transformer output?

Thank you again for your help.

Kind regards

Geoffroy

[mikesnowdon]

Hi Geoffroy.

I don't know as much as Pacific or Andrew but I might be able to help you a bit.

The term VA stands for Voltage X Amps. The voltage output of the transformer multiplied by the amperage it can deliver will give you the VA rating (also referred to as 'wattage'). This will help you to understand why Pacific is recommending a certain VA rating for this amp.

Take the transformer AC output voltage and multiply by roughly 1.414 to get your 'DC' voltage (voltage after the bridge rectifier). This is what your chips will see.

Take a look at the graphs on the national datsheet. You should see one called something like 'output power v supply voltage. This will show why Pacific mentions a different voltage for 4 and 8 ohm speakers. (Although the chip can operate at a higher voltage than Pacific recommends you will have trouble getting rid of the excess heat. Also there is distortion to consider. Chips will have an optimum supply voltage where the distortion is at its lowest. See the graphs.

The power supply you have with your kit is designed to work with it. I think you just need to get a suitable transformer and your set.

Hope that helps.

PS: the biggest barrier to learning is the misunderstanding of its terms or the words used to describe it. If you feel your getting bogged look back over the material you have been reading and find any words you didnt fully understand. Use Wikipedia or simmilar to find explanations and definitions. Its a nice trick

Do well!