Chipamp as preamp?

[lumanauw]

I wanted my system to be simple. I have only one supply, +/-35V.
I need gain preamp. I can built it by opamp, but it will need another supply, +/-15.
So, I think about building gain preamp using chipamp, like TDA7294 so I dont have to provide another supply.
Has anyone built chipamp as gainpreamp? Or it is not suitable at all for small signal gain preamp?

[HDTVman]

Why not use 7815/7915 regulators from main PS to give you the voltage you want for preamp power supply? I know some of the others may try to give you reasons not to do this but the big boys have been doing it for years so why can't you? After all we are only talking 4 to 6 parts. 2 regulators, 2 output caps and maybe 2 bypass caps. Seams simple enough to me.

Later BZ

[GregGC]

Or, why don't you just set the gain you need and no preamp needed at all. If you don't know, the gain is set by the two resistors in the NFB.

How much gain do you need?

/Greg

[djQUAN]

.............or you can add the additional +/-15V supplies using only dropping resistors and zeners and caps, much like rod's P27 preamp.

[lumanauw]

No, no, I do not intended to use opamp at all. Infact I wanted to use chipamp for opamp use. Why? In another thread (MOX thread) , someone built active Xover using diyopamp, the supply is more than +/-30V (opamp uses +/-15V). I think why dont we use chipamp as opamp?

[djQUAN]

because power opamps are more expemsive, has less SQ compared th most opamps, preamps handle only small signals and are usually stable at unity gain.

[boholm]

. . . but still there isn't a technical reason not to do it.

One can always discuss the normal hifi/audio quality issues, but simply and technically there shouldn't be anything against it. You must however look in the technical specs of the opamp to see, if it can handle, whatever you wish it to do. Diagrams, please . . .

[lumanauw]

My candidate for this is LM1875, because it is small chipamp, only TO-220 size. Just download the datasheet, I found some thing that maybe a problem if I use it as opamp.
In Xover, the opamp is only used as buffer, the -input is connected to output.

1. The LM1875 datasheet said that it is stable for gain 10 or more. Is this mean this LM chip cannot be used as buffer (-input tied to output)?

2. The datasheet also said about possibly ringing with capacitive load. In Xover, it is always capacitive load

3. The bias current is 70mA. Hmmm.... If the supply is +/-25V, then it will dissipate 3.5W? At first I think I can use chipamp without heatsink, if only used as opamp. But this dissipation certainly needs heatsink.

I just look at the LM1875 datasheet for a glance and has found this 3 obstacle if I wanted to use it as opamp.

It is not possible to use chipamp as opamp?

[moamps]

Originally posted by lumanauw
It is not possible to use chipamp as opamp?

Hi,
Using chipamp as opamp hasn't any sense. IMHO.

Regards

[carlmart]

Originally posted by lumanauw

I just look at the LM1875 datasheet for a glance and has found this 3 obstacle if I wanted to use it as opamp.

It is not possible to use chipamp as opamp?

It is possible, but no advisable.

A preamp's chip should have several capabilities that no power amp chip has: it should work in class A (if possible), it should have a large bandwidth gain and it should have low distortion.

If you look again, the MOX articles are based on a discrete op-amp, which should even be better than a monolithic chip.

You can use the +/-35v you have and filter/lower it to a voltage you can feed into +/-18v or +/-24v regulators, to power your preamp ICs.

Or you can go discrete and power it higher.


Carlos