Cambridge Audio A4 amp and LM1875 PCB ;o)

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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
 
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 :eek: ) 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!
 
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.:smash:
 
Thank you again for your precious comments above :D !!

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

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 :rolleyes: ) . 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....? :)dodgy: 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 :hot:

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

Kind regards

Geoffroy
 
Geoffroy said:
Cambridge Audio told me they lost the manual and schematics etc... for the A4
That's a good one. :smash:

Geoffroy said:
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.

Geoffroy said:
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.

Geoffroy said:
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.

Geoffroy said:
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.

Geoffroy said:
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.
 
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
 
Geoffroy said:
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 %.

Geoffroy said:
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.
 
pacificblue said:

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
 
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! :)
 
Geoffroy said:
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)
The amplifier's power transistors form a voltage divider with the loudspeaker. That means voltage drops across the transistors, and as soon as there is a music signal, there is also a current flowing through them. Voltage times current is power, and that power is converted into heat. A music signal is dynamic, so the power dissipation is changing all the time. The formula to determine the maximum power to dissipate is on page 6 in the datsheet.

Geoffroy said:
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...)
You are right to assume that the dissipated power also has to come from the transformer. However a music signal is not always drawing the maximum current, transformers can deliver more than their nominal current and peaks are buffered by the smoothing capacitors, so that a transformer rated at the amplifiers output power is usually sufficient.

Geoffroy said:
Last but not least, the way diode bridges are designed depends on the caracteristics of the transformer being used?
It is important to select the rectifiers with sufficient current rating. Other than that the bridges are the same. You need at least one, but it can have a few advantages to use two, if you have a dual secondaries transformer.

Geoffroy said:
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.
Yes, they assume that. And they even assume that you have to know, how to choose, and that you know what you get, if they recommend one. E. g. if they sell you a 68 W IC with a transformer that turns it into a 20 or 30 W amplifier.

Geoffroy said:
Last and least, what is the link (equation) between current and voltage in the transformer output?
VA = Volt * Ampere
Stated Volts are given at nominal load, will be higher by the regulation at no load and lower at overload.
Stated Amperes are the nominal load and are rated at a specified temperature. That means transformers have to be derated for higher temperatures, but that is rarely an issue in audio applications, if you don't put the transformer into a hermetically sealed enclosure. Higher current can be drawn at increased harmonic distortion that can pass through the power supply and affect the sonic performance of the amplifier. Another reason to use a bigger transformer than necessary.
 
mikesnowdon said:
Also there is distortion to consider. Chips will have an optimum supply voltage where the distortion is at its lowest.
Actually the THD changes with the supply voltage and the frequency so it is not that simple.
If you look into the LM3886's datasheet you will see that e. g. for 8 Ohm the lowest THD+N for 1 kHz is lower with the higher voltage, while at 20 kHz it is lower with the lower voltage. The distortion curve wanders with higher supply voltage towards higher output power. So the point of lowest distortion will be at a higher output power, while the distortion at lower power increases. Therefore some of the curves are cut off on the left side.;)
A good reason to choose the supply voltage, and thus the output power according to the sound pressure level you mostly listen at.
 
I had a long read through the information about the lm1875 (true reading the datasheet helps, after all that what it is for......Thx to all for mentioning it :D ) and the conclusion is my transformer will not supply a suitable voltage for the advised operating conditions so on that aspect of things new power supply required and I understand why.

Now if I want to use the transformer I have I will need it to power a more powerful kit/amplifier. I checked the lm3886 but even then with something like +/- 43 Vdc per amplifier pcb it seems too much (Have read the national data sheet hopefully in the right way) even though the output power seems to fit. Any idea about which other "easy to build" solid state amplifier kits could do with such a supply? I had a look at the very thorough Decibel Dungeon page on PSU's and it seems that there is no "chip amp" which could be powered by this.... (+-30 Vac at 2 A as written on the transfo, I checked the voltage with a multimetre but not the current so it might not be the real output current).

Finally, for my cambridge A4 "dead" integrated amp I was hoping to re-use the preamp stage in order to have an integrated amplifier. I now know more or less how the PCB's are laid-out inside but not the real details. Is there anything I should be looking for/testing before using the integrated preamp as a source for two new amplifier channels? Could it limit my choices? Would this preamp be a "passive" preamp not using power from the supply?

As said before I could not get hold of a schematic but can post pictures.

So many questions but the day I find proper answers will hopefully mean something nice is on the way :D

Thank you all again

Geoffroy
 
Geoffroy said:
I checked the lm3886 but even then with something like +/- 43 Vdc per amplifier pcb it seems too much
If you take into account that the rail voltage sags under load to around 37-38 V, you can use the LM3886 with that transformer. National uses 2*30 V transformers in their BPA-200 application. You can only use that voltage with 8 Ohm speakers and it will take serious heatsinking.
For a two channel amplifier that transformer limits you to 8 Ohm speakers anyhow.

Geoffroy said:
Any idea about which other "easy to build" solid state amplifier kits could do with such a supply?
There is a Solid State Forum here, where you could look. Here is a good one, but shipping cost may be an issue. SymAsym is also a good amplifier that could run from that voltage. No commercial kits I know of, but there are group buys for PCBs every now and then either here or on other sites and it will work on veroboard as well.
Or you could search for STK hybrid ICs. They are nearly as easy to implement as chipamps, are available with much higher output power, but don't have quite the same sonic quality.

Geoffroy said:
I checked the voltage with a multimetre but not the current so it might not be the real output current).
If it is written on the transformer, you can usually rely on it.

Geoffroy said:
Finally, for my cambridge A4 "dead" integrated amp I was hoping to re-use the preamp stage in order to have an integrated amplifier. I now know more or less how the PCB's are laid-out inside but not the real details. Is there anything I should be looking for/testing before using the integrated preamp as a source for two new amplifier channels? Could it limit my choices? Would this preamp be a "passive" preamp not using power from the supply?
If it is no passive preamp, i. e. only an input selector and a potentiometer, you can always convert it into one. Given that it has bass and treble control, it is probably an active preamp.
 
Geoffroy.

You could still use you LM1875 kit and PSU. Add voltage regulators to bring the +/-43v down to something more suitable. These have other advantages too, especially if you use a low noise one like the Teddy Pardo 'PowerReg' Have a read up on regulated supplies first, especially in power amp applications.

With this solution you could have a better sounding chipamp. ;)
 
A simple IC regulator like LT1083 or LM338 will do the job and as an additional benefit..you'll have a higher quality psu with low ripple ie. ( if you design the pcb correctly...keep the spacing and location of critical components correct ).You can have a look at the regulated psu for chipamp sections...or look at datasheets for LM338 ( this is pretty exhaustive and has plenty of application schematics...)http://www.national.com/mpf/LM/LM338.html
 
Well I actually bought a 225 VA +/- 18V transformer and will try to finish my first "simple" project with this.

Then I will try other configurations..... Part of this weekend will be devoted to seeing how I can keep the active pre-amp part running and see if I need to change some parameters of the amp boards for that.....

:D
 
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