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Old 23rd December 2012, 12:16 PM   #1
Tesla88 is offline Tesla88  Italy
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Arrow Good Soft Distortion

Hi Guys,
a friend ask me if I can build a sort of preamp to be used on voice recording to add some kind of "warmth" and "color" , the think all call "tube sound" but noone know what is talking about

A kind of soft distortion and soft clipping/natural compression would be ok , best iif most of 2nd harmonic distortion...

Any idea on what to do?

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Old 23rd December 2012, 12:20 PM   #2
Mooly is offline Mooly  United Kingdom
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Try a simple FET common source amplifier (such as a 2N3819 to experiment with) and an emitter follower buffer. Can all be made to work on a single battery supply if needed.
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Old 23rd December 2012, 12:25 PM   #3
Tesla88 is offline Tesla88  Italy
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Thanks but what about Do It with Tubes?
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Old 23rd December 2012, 12:31 PM   #4
Mooly is offline Mooly  United Kingdom
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Sorry, my mistake

Not really my thing, tubes, but the same principles apply. If you want more of a "tube sound" then try a simple common cathode configuration (that is to say a single stage running in isolation) with perhaps a cathode follower buffer stage added if needed.
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Old 23rd December 2012, 12:44 PM   #5
tinitus is offline tinitus  Europe
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since its a mic pre for recording voices, you will need lots of gain

I would suggest a mic pre with both solid state and tube stage
the small but important difference can be achieved by adjusting each gain stage seperately

but others might say a That chip is the best you can get (SY may chime in on that )
Dantimax (elektronik) - Preamps_-_poweramps (look up ThatMic)
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Old 23rd December 2012, 01:19 PM   #6
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Do it by putting random sized components to their places, do not measure or analyze anything and there it is.
The best sounding amplifier your friend ever has listened.

An other more professional way is to look for such tube data sheet,
where the manufacturer gives an application note about how to create a good tube sound .
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Old 23rd December 2012, 02:38 PM   #7
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Look up soft limiter circuits.
They round off the signal rather than hard clip it.
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Old 23rd December 2012, 03:04 PM   #8
rongon is offline rongon  United States
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Being a guitar player who's always loved old guitar amps, I think part of that 'tube sound' is the high frequency rolloff that comes from high-mu triodes like 12AX7's or 12AT7's working into too low of a target impedance. Try making a simple common-cathode 12AT7 stage with a 100k volume control to put in the signal chain as an effects box. The output impedance of this should be low enough not to be a complete disaster, but it will have some problems driving 100k target impedance of the next stage. The 12AT7 also yields a lot of second harmonic distortion, which might be what you're after.

Probably the best solution would be to find one of those old optical compressors that were so common in the 1960s, but are now collector's items. The combination of audio transformers with a 'soft' high frequency response and high-mu tubes can give you that 'tube sound' too.

Or, you know, I could be completely off base. Just an idea...
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Old 23rd December 2012, 03:12 PM   #9
chip647 is offline chip647  United States
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Part of the "warmth" is a limited treble range. If I were going to go for this sound I would use a single tube section of a 6SN7 (mu 20) or 6SL7 (mu 70) depending on how much gain. Just take the output signal from the plate with a 1uf cap. 250 volts B+ supply.

To dial in the "warmth" start out with a 1.5K cathode resistor (bypassed with a cap or not). Then start with a sizable plate resistor (30K for the 6SN7 or 150K for the 6SL7). Listen, then try doubling the resistor. It will do two things, one is that it will run the tube not very linear so you will get more harmonics and two it will increase the output impedance to loose some of the treble though the cable / amp interface.

Simple, easy and cheap. It will still sound good.
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Old 23rd December 2012, 03:28 PM   #10
jane is offline jane  Norway
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What about a triode with variable gm/ like ECC189 or a remote cutoff pentode?
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