John Curl's Blowtorch preamplifier

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X.G.

X.G.

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Joined 2003
beppe61 said:


Dear Sir,

very interesting post of yours.
So the classic LM317 is not that bad.
Could you give me some hints about the topology you use with it?
I am very interested in +/- 30V dual supply.
Thank you so much.
And of course my sincere congratulations for your excellent creations !

Kind regards,

beppe
Click to expand...

hi,beppe

my reg PS is based on my frined's design which showed below
 

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gerhard

gerhard

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john curl

john curl

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Joined 2003
Gerhard, these devices would work, but there are better devices in general. These devices are made for RF. They usually have low capacitance (good), and moderate Gm (OK), but they have low voltage breakdown and probably an even lower initial leakage threshold. It is better to find parts similar to these, except that they trade off higher voltage for lower Gm. These usually make a better current source.
 
X.G.

X.G.

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john curl said:
It is an interesting schematic, SG, but it could be made quieter with the addition of a few parts. Buffering the TL431 would lower the noise contribution of the regulator. This could be done with a simple RC filter, after each TL431.
Click to expand...

thanks for your advice,Mr.John.I would try...

I had tried to lower the noise of output voltage:add a 220U EL cap paralelled with the TL431,add a 22U EL cap paralelled with the NFB resistor of TL431,add a RC network of 1U film cap and 10 ohm resistor paralled with the output of PS,and add a BJT buffer in correct amp.each modify give me a lillte improve of the sound.

the 'buffer' what you said is refer to the 'simple RC filter' or a active buffer? thanks again.

X.G.
 
john curl

john curl

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X.G. I would put a 1K resistor just after the TL431 output, then the cap. The problem here is: The TL431 makes 100+ nV/rt Hz noise and has a very low output impedance. Putting a cap DIRECTLY across the output will NOT do very much, if anything useful. The cap must have something to work against to be a good RC filter. Now, 1K and 220 uF for example, gives a rolloff starting just below 1 Hz, so at 100 Hz, you have about 20 dB lower noise from the source, and at 1KHz, you have about 40dB or 100 times lower noise. For a line amp, this would be OK. A moving coil pre-preamp should use a completely different design and reference, because the bass frequencies can be boosted as much as 80 dB, or 10,000 times. In this case, the 1/f noise that would remain, would be annoying during measurement and even listening.
 
Ultima Thule

Ultima Thule

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Joined 2004
The TL431 seem to be kind of a "favorite" judging from the many past posts in this thread, but still I see many design examples where all it's qualities is not fully utilized, why not then just using a low noise BJT as a Vbe multiplayer as a passive reference for a shunt transistor?

I mean if we have a big el-lyte giving the output supply voltage a varation for a given load in the range of 1-0,1 Hz... the ultimate precision of the supply voltage level can't be that important, say 15 V with +-0,5 V, yes/no?

Cheers Michael
 
john curl

john curl

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Michael, I am only addressing a specific schematic that was posted here. The solution is fairly easy, and it is a good example of an ongoing oversight.
This oversight is trying to bypass a voltage source with just a capacitor. It just doesn't work very well at audio frequencies. Yet, people, both pro and amateur, will try to bypass a zener, or a TL431, or LED's without thinking it through. Of course, it is best to start with a low noise source, if it is practical. When it is impractical, or all that you can get is some component, then knowing how to apply an effective low pass filter is useful. We can go even further with active cap multipliers, or even an active low pass filter, but that is a lot more work than a simple RC time constant added after the voltage source.
 
john curl

john curl

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Joined 2003
I received an e-mail from a group of contributors to this thread, and I was sent a schematic, that is, in principle, an accurate description of the CTC line stage. It is OK to publish this schematic on this website, as it is only a generic description, which I stole from Charles Hansen, anyway. This might make things more clear, as to what I was trying to achieve with the Blowtorch preamp. Thanks Charles.
 
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