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Super simple single stage tube preamp
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=3266
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Author:  nodz [ 11 Sep 2012, 22:19 ]
Post subject:  Re: Super simple single stage tube preamp

Was going to build the universal with the 12AX7

Author:  mwhouston [ 12 Sep 2012, 08:03 ]
Post subject:  Re: Super simple single stage tube preamp

nodz wrote:
Was going to build the universal with the 12AX7

As you know Matt designed it to accommodate all 12A?? tubes. But each time I found the12AU7 the best. I have all tubes and able to plug and play but I always come back to the 12AU7.

Author:  nodz [ 12 Sep 2012, 15:15 ]
Post subject:  Re: Super simple single stage tube preamp

I read through the whole thread and came to the same conclusion, that the 12AU7 seemed to give the best results. I just happen to have about 10 x 12AX7 on hand, largely from old guitar amp projects. So I guess my background is I came from a hobby where a little bit more gain and some more distortion didn't really matter...haha...Also, I am new to the audiophile hobby, so I am still very much learning what sounds good to me and what doesn't, so at the moment, I guess, the more things I try, the more I learn, the better my experience will be. I appreciate your input and I am certainly not ignoring your experience, I will certainly try an AU7 but probably will also try the AT7 just to see what the difference in sound is. I also have extremely varied taste in music, at one end of the spectrum I was brought up on punk music (I hear some people groan and laugh - that's not music) but I also have a reasonable opera collection (CDs and LPs), so I want to try different things, to see what brings out the best (or worst) in these different genres.
Cheers.
Chris

Author:  mwhouston [ 12 Sep 2012, 17:54 ]
Post subject:  Re: Super simple single stage tube preamp

nodz wrote:
I read through the whole thread and came to the same conclusion, that the 12AU7 seemed to give the best results. I just happen to have about 10 x 12AX7 on hand, largely from old guitar amp projects. So I guess my background is I came from a hobby where a little bit more gain and some more distortion didn't really matter...haha...Also, I am new to the audiophile hobby, so I am still very much learning what sounds good to me and what doesn't, so at the moment, I guess, the more things I try, the more I learn, the better my experience will be. I appreciate your input and I am certainly not ignoring your experience, I will certainly try an AU7 but probably will also try the AT7 just to see what the difference in sound is. I also have extremely varied taste in music, at one end of the spectrum I was brought up on punk music (I hear some people groan and laugh - that's not music) but I also have a reasonable opera collection (CDs and LPs), so I want to try different things, to see what brings out the best (or worst) in these different genres.
Cheers.
Chris

Opera and classical will work well with the preamp.

Author:  nodz [ 12 Sep 2012, 19:25 ]
Post subject:  Re: Super simple single stage tube preamp

Was thinking of separating the ps from the preamp, so that I can use the ps for other preamp projects

Author:  etet [ 12 Sep 2012, 21:02 ]
Post subject:  Re: Super simple single stage tube preamp

Very interesting project, I have read through several times and I must be missing something (I miss a lot... :confused: ...)Why is a linear pot being used for the volume control vs a log? Thanks in advance.

Author:  mwhouston [ 13 Sep 2012, 05:19 ]
Post subject:  Re: Super simple single stage tube preamp

etet wrote:
Very interesting project, I have read through several times and I must be missing something (I miss a lot... :confused: ...)Why is a linear pot being used for the volume control vs a log? Thanks in advance.

Without reminding myself of every 4S post I have to say the post is possibly mine. As you know you can use a log or linear pot provided it is the right value. All that happens is that the volume/attenuation "characteristics" take on a different role. A log or linear role is not critical and I have found an attenuator working with log or linear characteristics really is no big deal.

Author:  etet [ 13 Sep 2012, 18:04 ]
Post subject:  Re: Super simple single stage tube preamp

mwhouston - OK, thanks for the reply. I've never tried a linear for volume...I'll give it a try.

Author:  Suncalc [ 13 Sep 2012, 22:05 ]
Post subject:  Re: Super simple single stage tube preamp

etet wrote:
Why is a linear pot being used for the volume control vs a log?
First, I always specify a log A potentiometer for a volume control. Is there somewhere in the thread where it said to use a linear taper pot for the control?

In any event, now for a little more detailed explanation. The human ear is a logarithmic detector. This means that as absolute sound pressure level increases exponentially, the human ear will register it as a linear progression. This is why the standard unit of measure for sound is an exponential scale is decibels (dB) relative to a reference. So the issue for audio circuit design becomes how do I get an exponential transfer characteristic in the voltage realm from a linear progressing on the control shaft position? The answer is to use a log taper potentiometer.

Consider these plots for a linear taper potentiometer wired as shown in the 4S circuit.
Attachment:
Linear Pot.jpg
The first plot on the left is a fractional voltage out as a function of shaft position. As expected, the voltage transfer characteristic is linear. However, the second graph shows how this transfer characteristic would be heard by the human ear. In this case the major volume change is bunched up at one end of the shaft positions. In effect almost all the "volume control" function occurs on only half of the shaft rotation; the later half resulting in very little change.

Now consider the same plots for a logarithmic taper potentiometer.
Attachment:
Log Pot.jpg
Here you can see that the plot on the left for the voltage transfer characteristic has the vast majority of the voltage change taking place on the latter part of the shaft rotation; the earlier part contributing almost none of the adjustment range. However, now look at the dB scale. The human ear will hear this voltage transfer characteristic as a perfectly linear progression from very soft to full volume. Here the "volume control" function is evenly distributed across the entire range shaft positions.

This is the slightly longer explanation as to why one should always use a logarithmic transfer characteristic for volume control. I hope this help's to clear things up a bit.

Author:  nodz [ 13 Sep 2012, 23:36 ]
Post subject:  Re: Super simple single stage tube preamp

Bits and pieces have arrived. Got to order the valve sockets but I can get cracking on the power supply.

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