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Old 31st May 2013, 04:48 PM   #11
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Originally Posted by billbo View Post
I was asked the question in regards to the biasing of the tube in the thread Simple low voltage tube preamp stage which made me revisit the design of the cct.. In essence the bias takes the average voltage that is across the filament (approx 0.6 volts). In real tests it showed to be about 0.1 to 0.3 of a volt. I considered this is too low and would pose a problem driving the tube positive. This is not a real problem if you can drive high current from the source but it is not recommended for audio. I did more research and redesigned the cct. to have more grid bias. (about 1 volt). The revised cct. has a 100 Ohm resistor which will give 1 volt bias and raised the supply voltage to 30 volts. This has greatly reduced distortion and has made the cct. more stable.
Once again sorry for the hand draw schematic.

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Billy D...
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What diodes are used in the string in the filament section? I will have to look for that fet too. That is a new one for me.
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Old 2nd June 2013, 12:09 AM   #12
billbo is offline billbo  Australia
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The diodes are basically there as a safeguard to protect the filaments. If anything happens to the supply voltage the diodes will conduct and clamp the voltage at around 2.4 volts which will not harm the tube filaments. any small rectifier diodes will do. I used the small signal diodes 1N4148. As far as the fet is concerned, you do not have to use the fet I used. Use a good fet which has a voltage handling of at least the supply voltage (30 volts) and handle at least around 20mA. I just happen to have these fets and that is why I used them. They are overkill for the job since they are a 250 volt 310mA device. Don't stress on these components. Just use what you have or can get.

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Old 2nd June 2013, 01:36 AM   #13
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Originally Posted by billbo View Post
No I am not planning to put any post filtering at all.
Depending on what your amp is following the DAC you might well find improvement in implementing a passve anti-imaging filter. That's because the DAC's image frequencies increase IMD. Valve amps tend to have lower IMD so if you're using a SS amp I'd guess the improvement will be more marked - it certainly was in my case. My soundstage opened up enormously when I introduced such a filter.

I have some filter schematics up on my blog - click the number in the margin to the left of this post.
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Old 5th June 2013, 05:45 PM   #14
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i also particularly don't need the roll-off compensation cuz my pi4 speakers start rolling off at 14k ...
I suppose that's one way of looking at it ... but another way is now you've got double the rolloff! It's even worse!!1!!

Thorsten said it and I'll repeat it: you have to fix the NOS sinc rolloff. Even if you're old, even if you don't have tweets, even if ....

If you don't, you're just listening to rolled off digital ... and haven't really heard NOS ...

And your 4Pi's don't roll off at 14k ...

Pi Speakers - four Pi loudspeaker performance data

PS You should get Wayne's new horn, he's perfected it sooo much it doesn't need the top-octave compensation anymore!!!
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Old 6th June 2013, 05:37 AM   #15
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Hi AudioLapDance,

can you tell me what values should I change according to the schematic in your post #5 so that I'll obtain the same +4dB gain at 20khz but with an 100 ohm resistor in the equation as the I/V (rather than with that 47 ohm posted there)? thanks
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Old 6th June 2013, 11:27 AM   #16
billbo is offline billbo  Australia
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For the sake of sinplicity, build the circuit and try it out. I have insralled it as is. To me it sounds better than my other dacs and thats what counts. At least I have a dac that sounds different so I have diversity. For the novice at heart it is a good project even as a preamp.. This is a DIY forum where beginners have subscribed and can appreciate simple modifications.

Last edited by billbo; 6th June 2013 at 11:34 AM.
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Old 7th June 2013, 05:01 AM   #17
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Originally Posted by AudioLapDance View Post
I suppose that's one way of looking at it ... but another way is now you've got double the rolloff! It's even worse!!1!!

Thorsten said it and I'll repeat it: you have to fix the NOS sinc rolloff. Even if you're old, even if you don't have tweets, even if ....

If you don't, you're just listening to rolled off digital ... and haven't really heard NOS ...

And your 4Pi's don't roll off at 14k ...

Pi Speakers - four Pi loudspeaker performance data

PS You should get Wayne's new horn, he's perfected it sooo much it doesn't need the top-octave compensation anymore!!!
you're absolutely right. i had a bit of a head smack moment there. i still haven't finished the speakers yet so we'll see how much i want the compensation afterwards.

what horn are you talking about?
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Old 7th June 2013, 06:20 PM   #18
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Originally Posted by luxury54 View Post
Hi AudioLapDance,
can you tell me what values should I change according to the schematic in your post #5 so that I'll obtain the same +4dB gain at 20khz but with an 100 ohm resistor in the equation as the I/V (rather than with that 47 ohm posted there)? thanks
Yikes, looks like I overstepped here billbo, sorry. Taking another look at our circuits ... they are very different. And I think I can help you out:

You mention your 4x1543 DAC but what wasn't included was the DAC output circuit. I'm guessing it's a regular IV resistor (guess 400 ohms) scaled (4x 1543 Iout) to get about 1-2Vrms output.

This voltage is then 'buffered' (and amplified a bit) by your tube circuit.

My circuit is more of an IV convertor rather just a buffer for voltage.

Sorry for the confusion.
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Old 11th June 2013, 12:44 AM   #19
billbo is offline billbo  Australia
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All O.K. mate. This is what the forum is all about. The DAC is actually 8xTDA1543's. I/V resitor is about 2.2k/8 = 275Ohms (I used 270 ohms) and the VRef resistor is 1.5k/8 = 187.5 ( I used 180 ohms). I will post some pics and more specs on the respose of the output.

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Billy D...
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Old 11th June 2013, 03:03 AM   #20
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Cheers, mate!

So, to add sin c compensation to your DAC you'd have to scale the peaking circuit (.15mH, 330nF, 27 ohm) to your IV resistor (270 ohm instead of T's 47 ohm)

Or you could include the compensation in a 2nd order filter with your x2 tube circuit. Instead of a proper smooth roll-off (q=.7), you create a peak @ 20k (q=1.0?) that adds a few dB to compensate the sin c roll-off! Elegant! (I think CarlosFM and T tried it ... there's a TINA op-amp circuit around somewhere ...)

But you'd have to 'nail-down' your circuit a bit more (no adj gain, etc...)
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