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Good Soft Distortion

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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 :rolleyes:

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?

:hohoho:
 
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Sorry, my mistake :D

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.
 
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 :D .
 
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...
 
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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Joined 2011
Thanks but what about Do It with Tubes? :)

I assume this is for recording studio use? If so, the parts selection and construction technique are pretty demanding, if you are up to the task, you can check out Gyraf's DIY projects: Gyraf Audio - G9 DIY.

And if your friend just want something "tubey" to fool around with at home, there are many soundcards with tube preamps built-in for decent prices. Not trying to talk you out of the project, but just a reminder that there is a huge difference in requirement for pro vs amateur use... ;)

Jaz
 
My friend is a studio engineer with an SSL analog mizer, i built some stuff for this studio like a pair of 1176 compressors , a cabinet simulator for guitar , DI boxes , etc
So he's not a fool who likes having nice looking preamp with a tube lighted by some blue leds :D

Really don't know what he wants this time , he gave me some vocal tracks for test saying try to give some warm and color...

I started from a single stage form 6CG7 with a 47k load on the plate and a trimpot from K to Ground to obtain a well balanced distortion , only tested with sinewave.

I read carefully all of you stated , please go on with suggestions , i really appreciate!

And sorry for my bad english...
 
Ex-Moderator
Joined 2011
My friend is a studio engineer with an SSL analog mizer, i built some stuff for this studio like a pair of 1176 compressors , a cabinet simulator for guitar , DI boxes , etc
So he's not a fool who likes having nice looking preamp with a tube lighted by some blue leds :D

Really don't know what he wants this time , he gave me some vocal tracks for test saying try to give some warm and color...

I started from a single stage form 6CG7 with a 47k load on the plate and a trimpot from K to Ground to obtain a well balanced distortion , only tested with sinewave.

I read carefully all of you stated , please go on with suggestions , i really appreciate!

And sorry for my bad english...

Since your friend is a real pro, please take a look at the Gyraf's site for more information, even if you don't end up using their design, you can still get some ideas for your build, good luck with your project, keep us posted on how it turns out.

Jaz
 
I looked at the gyraf , intresting stuff :cool:

good luck with your project, keep us posted on how it turns out.

Sure , i hope to do something in these days , but i'm goig to build a pair of 1176 compressor again these days . must be ready for january :eek:

Anyway once they're finished i'll try to do something on this project.

Please keep posting intresting ideas/schematic

Have a nice day a merry xmas :hohoho:
 
Pick a tube of your choice that is known for good neutral sound capability. Build a clean buffer stage with a cathode follower to give good drive ability. Now add a coupling cap (.22uF 450V) to a 50K pot and an additional 5K resistor to ground from the anode of the first stage. As you decrease the resistive load on the anode, second harmonic distortion will go up.

For instance, I'm testing a 6CB6 (Triode strapped) with a current source load and fully bypassed cathode. At 200V on the anode, and 1.5mA through the tube I get second harmonic less than -80dB (0.078% thd) and third below -90dB. Not fantastic, but a good gain stage.

If I add a 2.2uF cap in series with a 10K resistor to ground, the second harmonic goes to -32dB and third only increases to about -72dB.

Voila! Tube sound. 3.6% second harmonic THD.

The downside is as one increases the anode load to increase second harmonic distortion, one also increased the load on the stage and gain decreases. Thus, you need a gain control between stages to allow you to compensate for the decreased gain as you increase distortion.
 
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