Tube preamp for a TDA2040 chipamp - diyAudio
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Old 24th November 2010, 08:29 AM   #1
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Default Tube preamp for a TDA2040 chipamp

Hi, I'm thinking of building a tube preamplifier for a TDA2040 chipamp that I've built. As audio source I'll probably using an iPod or cumputersoundcard. Therefore I thouhgt that a low gain tube preamplifier should to the job. So then I found this one.

The gain i about 8 db (the page says) and it's supposed to drive a low impedance amplifier. So then I'm wondering, should a build a hihger gain tubepreamp instead, and is TDA2040 considered a low impedance amplifer? Been searching a lot for this, according to the data sheet it could be lowered by decreasing R1, but which value is considered low inpemadance?
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Old 24th November 2010, 08:45 AM   #2
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If the 2040 is built by the book you will not need any gain at all. Just add a pot at the input and thatīs it. If you still want a buffer to raise the input impedance a CF is all you need.

Stay away from the example you showed! In general stay away from Diyaudioprojects.

Last edited by revintage; 24th November 2010 at 08:49 AM.
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Old 24th November 2010, 10:27 AM   #3
DF96 is offline DF96  England
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With a compressed digital sound source, a tube preamp, and a chip amplifier you could combine into one system all the major types of audio distortion!

The gain of that circuit as claimed by the page is 8, not 8dB. Voltage gain of 8 is 18dB. The RCA book implies a gain of 12 (=21.5dB) (which seems more likely as the valve has a mu of 17 at normal bias). This is probably about twelve times too much. As with most unnecessary circuits, the main effect of this preamp is to add a little noise and distortion to the signal.

If you do decide to build it, ignore the advice about C4 needing to be a good quality capacitor. Probably the most critical capacitor is C3, but even that is not too critical in this application. RCA circuits tend to be a bit "bargain basement". This one makes the common mistake of connecting the grid straight to the pot wiper.
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Old 24th November 2010, 10:53 AM   #4
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Quote:
Originally Posted by DF96 View Post
If you do decide to build it, ignore the advice about C4 needing to be a good quality capacitor. Probably the most critical capacitor is C3, but even that is not too critical in this application. RCA circuits tend to be a bit "bargain basement". This one makes the common mistake of connecting the grid straight to the pot wiper.
Yeah, I thought so to. But I think it still is the best schematic I've found so far.

I know that a preamp for the TDA2040 really isn't necessary. The reason I want one is just to add a little of tube distortion to the sound. But a low gain tube preamp is what a should go for then?
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Old 24th November 2010, 12:24 PM   #5
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Quote:
Originally Posted by pyromaniac_ View Post
Yeah, I thought so to. But I think it still is the best schematic I've found so far.

I know that a preamp for the TDA2040 really isn't necessary. The reason I want one is just to add a little of tube distortion to the sound. But a low gain tube preamp is what a should go for then?
Donīt go for that one itīs lousy by todays standard. Go for a 6N6P, same mu as the not so very good 12AU7, or something else with acceptable linearity.

If you want something with lower gain than this, build a Anode-follower. Single tube, gain of your choice. So easy and still great.
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Old 24th November 2010, 12:28 PM   #6
00940 is online now 00940  Belgium
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Rather than using a very low gain preamp, I'd knock down the gain of the tda2040 as much as possible (as long as it is stable that is) and use the tube preamp to provide the voltage gain. Chip amps often sound better with lowish gain in my experience.

What's the input impedance of your tda2040 ?
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Old 24th November 2010, 12:37 PM   #7
DF96 is offline DF96  England
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Quote:
acceptable linearity
The OP wants some distortion!
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Old 24th November 2010, 12:43 PM   #8
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Quote:
Originally Posted by DF96 View Post
The OP wants some distortion!
, thats an interesting point of view.

Quote:
Chip amps often sound better with lowish gain in my experience.

What's the input impedance of your tda2040 ?
What would the reason be for a chipamp to sound better with even more GNFB, beats me?

For what I have seen inputZ is high but in the available applications mostly 22k (resistor on the input).
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Old 24th November 2010, 12:49 PM   #9
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Quote:
Originally Posted by 00940 View Post
Rather than using a very low gain preamp, I'd knock down the gain of the tda2040 as much as possible (as long as it is stable that is) and use the tube preamp to provide the voltage gain. Chip amps often sound better with lowish gain in my experience.

What's the input impedance of your tda2040 ?
Well there's the problem, I don't got clue. Been searching for it a lot. Don't laugh then I say that I think it's about 22k ohm, it might be totally wrong.

You have a good point though, maybe use a higher gain preamp and try to lower the gain of the chipamp instead. I've seem many DIY people who use high gain tube amp in front of there (solid state) poweramps, so maybe thats the way to go I don't know.

Found another tube preamp which makes sense to me, only that the gain is now 51 db, could probably be lowered a little by recalculating some resistor values and so, but maybe high gain preamp is the way to go anyway?
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Old 24th November 2010, 01:02 PM   #10
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the 12AX7 would be one of the last tubes I would use for a linestage... too much gain and it can't drive a low impedance load all that well either. In your case, you don't need gain and trust me - it's no fun to having _too_ much gain.

heck, just put a simple cathode follower in front of the amplifier or even use a FET. If you want to get complex, use a White Cathode Follower which (with the right tube) can drive just about any crazy load you will see at the line level.
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