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Old 24th November 2010, 03:29 PM   #11
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Quote:
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?
You can reduce the gain of the TDA2040 by increasing R2. Put a 10 k variable resistor to find what you like. The bigger the resistance, the lower the gain (and bigger NFB).

The input impedance of the TDA2040 is practically the value of R1.
You can put there 100 k if you want.

As a preamp, since you do not need much gain, but want to have distortion, put a 12AX7 stage with unbypassed 10 k cathode -
and 100 k anode resistor. Then the gain will be some 13 dB only (assuming the R1 is 100k). And surely some amount of distortion .
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Old 5th February 2011, 04:44 PM   #12
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Ok, let's see if I've got this right. What artosalo suggests is that I increase the input resistance of the TDA2040 amp to 100k (R1). And then use a single 12AX7 stage like the one a linked earlier only that a changes the values of R10 (anode resistor) and R9 (cathode resistor) to 100k and 10k.

I now calculated the gain of the preamp circuit to be 8,5 db. artsalo wtote that it would be about 13 db, so what have I missed? The input impedance of the TDA2040 does not affect the gain, or does it?

Or maybe he didn't refer to the circuit mentioned earlier at all, just a single 12AX7 stage. I'm really confused, please help.
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Old 5th February 2011, 05:26 PM   #13
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I now calculated the gain of the preamp circuit to be 8,5 db. artsalo wtote that it would be about 13 db, so what have I missed? The input impedance of the TDA2040 does not affect the gain, or does it?
Below is what I calculated. The gain is 13 dB. There are also the equations so that you can check again.

But what is your problem ?


Click the image to open in full size.
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Old 5th February 2011, 06:26 PM   #14
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My problem is that I'm stupid. You're calculations seems right, don't know why I didn't got it right.

However like your idea to use a low gain 12AX7 stage to get som distortion, so I quickly throw this schematic together. Haven't calculated the value of the capacitors yet, but I guess it should be something like 0,1uF for C1 and C2 and 3-5uF for C3. All 400V. Donšt sure if this could work, feels to simple some how. What do you think?
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Old 5th February 2011, 06:39 PM   #15
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Leave the last 12AX7 half (cathode follower) away and connect the signal directly from C2 to TDA2040. Then the gain will be 13 dB, assuming R1 is changed to 100 kohms. Then C2 should be min. 220nF.

By the way, the schematic you made has such an error that there is no bias arrangement for the cathode follower.
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Old 5th February 2011, 07:03 PM   #16
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Quote:
Originally Posted by artosalo View Post
Leave the last 12AX7 half (cathode follower) away and connect the signal directly from C2 to TDA2040. Then the gain will be 13 dB, assuming R1 is changed to 100 kohms. Then C2 should be min. 220nF.

By the way, the schematic you made has such an error that there is no bias arrangement for the cathode follower.
R1 should be 100k, I know. And yeah, I missed a resistor at the schematic. But maybe one stage is enough however. Think I'll build it only with one stage, connecting the TDA2040 to C2 and then I'll see how it sounds.
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Old 5th February 2011, 07:10 PM   #17
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The chip amp will become a nice RF oscillator if you provide too much negative feedback (reduce gain). The datasheet recommends gain be 24db minimum.

The 22K resistor on the input pin to ground permits bias current to flow for the input stage. Without the bias resistor the amp falls silent or spits and crackles if the input signal is strong. If you change this value much, I'd be doing some testing.

My recommendation is to leave the TDA2040 components close to the datasheet values and add a divider network or pot and attenuate any gain from the tube stage.

I'd use a "poor" tube or really louse up the circuit to get some distortion or your just going to have a pointless stage. A well designed tube stage is going to be low distortion anyway.
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Old 5th February 2011, 11:46 PM   #18
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Quote:
The chip amp will become a nice RF oscillator if you provide too much negative feedback (reduce gain). The datasheet recommends gain be 24db minimum.
Correct. Then R3 should be 12 k, but not less. Then the gain of TDA2040 is 25 dB.


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Without the bias resistor the amp falls silent or spits and crackles if the input signal is strong.
Absolutely so, but 100 kohms should be fine.

When R3 is 12 kohms, the sensitivity (to full output) of TDA2040 is some 550 mV.
And after adding the 12AX7 (with suggested circuit) the overall sensitivity is some 120 mV. Therefore you should place the volume pot just at the input of 12AX7.
I quess you typically use line level source. There would be also a good place for fixed attenuator.
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Old 6th February 2011, 04:27 AM   #19
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I'm not sure if this is what you want, if you're looking to add some distortion that sounds like a poor tube circuit. But a 12B4 is a very linear low gain tube with a low Rp. This is a very good sounding circuit. Gain of about 6-6.5.

Use a 10M45S as a plate load; 9-11 1.7V red LEDs in the cathode circuit and adjust the 10M45S for about 130V on the plate. Should be around 15-20mA.

For an output cap, use the minimum size that gets you the LF response you need. PPFX Multicaps sound very nice here. Of course V-Caps are much better and 10x the price.

Stuart
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Old 6th February 2011, 06:11 AM   #20
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I was actually thinking of putting an passive attenuator or at least a volume control in front of the 12AX7. Lowering the gain of the chipamp to 24 db sounds like an good idea.

For tube I was thinking of using one half of an 12AX7 for each ch, so I only need to use on tube then. Can't think of why it wouldn't be a good idea. I also got some really old 12AX7 tubes, maybe the sounds worse (but in a good way) than newer ones.

Quote:
Originally Posted by sepolansky View Post
I'm not sure if this is what you want, if you're looking to add some distortion that sounds like a poor tube circuit. But a 12B4 is a very linear low gain tube with a low Rp. This is a very good sounding circuit. Gain of about 6-6.5.

Use a 10M45S as a plate load; 9-11 1.7V red LEDs in the cathode circuit and adjust the 10M45S for about 130V on the plate. Should be around 15-20mA.

For an output cap, use the minimum size that gets you the LF response you need. PPFX Multicaps sound very nice here. Of course V-Caps are much better and 10x the price.

Stuart
Sounds interesting. Doesn't really get it though, got any schematic, exemples or so?
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