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Super simple single stage tube preamp
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Author:  mwhouston [ 23 Jul 2011, 04:28 ]
Post subject:  Re: Super simple single stage tube preamp

I have been posting my mistakes with the hope that others will not make them. In this buildl I failed to properly solder the output from the preamp to the pot. Result was no left channel. Easy fixed. BUT the big mistake was when I drilled the hole for the tube socket it got a little off center. When I drilled the hole for the tube in the case I offset the hole the wrong way.

So I had to drill a second tube hole but the mounting holes were no good anymore. So I filled with the dummy mounting holes with chromed nuts and mounted the cct. brd. on sticky stand-offs. The wrong hole now wears a "The Black" brass plate. The diecast cases I use retail for $34 so I had to use it. Don't tell anyone I stuffed-up.

I used this (The BlacK) preamp for my Opera day with the Melbourne Audio Club. Only one person arrived. He loved the preamp and has heard all my changes in gear from 2 years ago.He thinks this is the very best my system has sounded. The only peice which has changed since the last time he was here is the preamp. He really loved it. He has one of my BoZ.

I tested The Black:
At full volume: @ 1KHz 1.98V in and 15.94V out.
Frequency response <10hz to 56Khz into 22K @ 80% volume (this has fallen sharply)
Distortion similar to the MKII

The base plate is lined with bitumised heavy Al foil. Some air holes at the bottom.
Attachment:
TheBlackBottomss.jpg
Attachment:
The Blackss.jpg

Author:  Suncalc [ 23 Jul 2011, 10:57 ]
Post subject:  Re: Super simple single stage tube preamp

This statement by Mark got me to thinking...
mwhouston wrote:
Frequency response <10hz to 56Khz into 22K @ 80% volume (this has fallen sharply)
I think that sometimes we don't get a very good idea of what is happening with output impedance in some of our line stages. This stage is a good example. The 12AU7 stage has a grid circuit output impedance of 6450Ω without the 100k volume pot on the output.

Now putting the volume pot in this position has several real advantages. The first is that the noise figure of the stage is significantly increased by putting this large resistor after the first gain stage. Hence a much quieter stage. The second is that it gives us some flexibility with what we want to do with the next stage w.r.t. headroom and total voltage swing. However, the real question is this: What is that volume pot doing to the preamp output impedance as we change the volume setting? The total output impedance in conjunction with the Miller capacitance of the next stage is what is going to set the high frequency rolloff. So let's investigate.

The output impedance is a fairly simple matter to calculate from the equivalent plate circuit model. If we look at it from a fractional resistance standpoint the output impedance looks like this.
Attachment:
Out Z linear.png
And if we are driving a next stage with a 1000Ω grid stopper and a miller capacitance of 100µµf then the high frequency rolloff looks like this.
Attachment:
Upper F linear.png

Unfortunately these plots don't give a lot of insight because in audio circuits we use audio (or log) taper pots. For these pots the 10% fractional resistance point is typically at the 50% shaft setting. So what happens to the plots when we change the x-axis scale to %shaft rotation on an audio taper pot? Well, they look like this.
Attachment:
Out Z log.png
And the high frequency rolloff looks like this.
Attachment:
Upper F log.png

Now we can begin to see why Mark made the comment above about the falloff being greatest at around 80% volume. That's where the output impedance is peaking and driving a 22kΩ input with a 26kΩ output impedance is going to produce some significant loss.

My only point in all of this is that, like all the other aspects of circuit topology, the placement of controls can have a significant effect on the circuit operation and sometimes single numbers, be they peak or average, don't always tell the whole story.

Questions or comments?

Author:  mwhouston [ 23 Jul 2011, 19:09 ]
Post subject:  Re: Super simple single stage tube preamp

Matt: Thanks for the explanation. Still 56K is still good upper response it just didn't look that way when compared to the other two. But gain is massive and that's what I needed.

The preamp (as stated) got some excellent comments from Qian. After Qian left I played it for another 4 or 5 hours. I know it is very good. It sounds very good. 56K top end or not. How will the Universal (4S Universe) compare?

Author:  Suncalc [ 24 Jul 2011, 10:49 ]
Post subject:  Re: Super simple single stage tube preamp

mwhouston wrote:
How will the Universal (4S Universe) compare?
Hmmm... interesting question. My guess is that the universal with a 12AU7 will be very close. Perhaps just very slightly cooler due to the distortion number. As I said before, I think that the 12AY7 is in the best operational configuration in the universal design. From the 12AY7 I expect great things; even better than the 12AU7 in the MK III.

The others I think will just be a matter of preference. The 12AT7 should be warm and mellow while I expect the 12AX7 will be very cool and tight. I would expect better bass from the 12AT7 than from the 12AX7.

I can't wait to hear the results of your tube rolling. :thumbsup:

Author:  mwhouston [ 28 Jul 2011, 18:39 ]
Post subject:  Re: Super simple single stage tube preamp

Matt (and others). On a another thread, some 6 months back I built a tube compressor which never worked. From what I understand the compressor has variable gain which diminishes as the signal through it increases over a threshold. The first of these preamps built to the original cct. maybe a candidate for a compressor. If we redesinged the preamp, based on the orginal cct. to have very large anounts of NFB both from the cathode resistor (which may need to be large in value) and the NFB cct. from the plate to the grid would not this act as a compressor. I realise this will not have a threshond but we can work on that.

Author:  Suncalc [ 28 Jul 2011, 20:31 ]
Post subject:  Re: Super simple single stage tube preamp

mwhouston wrote:
... would not this act as a compressor.
Actually, I don't believe so. The point being that regardless of how the stage was configured from a feedback standpoint, the configuration would produce a static amplification factor. The entire idea behind a compressor is that the amplification level is dependent on the magnitude of the smoothed (i.e. time averaged) input signal. Sorry. :(

I believe that the detector and controlled stage is still the proper approach. However, to get a logarithmic compression as required for audio reproduction it think that a variable-µ tube control stage may be required. This is the approach taken in most tube receivers for the AGC control (which is essentially a compressor function). I'll go look at some vacuum tube AGC circuits and see if anything jumps out at me.

Author:  mwhouston [ 28 Jul 2011, 20:47 ]
Post subject:  Re: Super simple single stage tube preamp

Matt; Off thread-Which brand of Kentucky bourbon do you drink? I wonder if I can get it here??

Author:  Suncalc [ 28 Jul 2011, 22:23 ]
Post subject:  Re: Super simple single stage tube preamp

Quote:
Matt; Off thread-Which brand of Kentucky bourbon do you drink? I wonder if I can get it here??
Well, my old standby is Maker's Mark, very smooth and easy drinking. But when listening to the best of new amplifiers, nothing can beat two fingers of Woodford Reserve. For my money, the finest Kentucky Bourbon ever crafted.

Every amp sounds better with some Woodford in the glass! :wine:

Author:  mwhouston [ 29 Jul 2011, 03:13 ]
Post subject:  Re: Super simple single stage tube preamp

Matt: Still off thread-Woodfords is AU$70 here. I bought bottle, nice bottle. It is as sour as heck! Give me a Scotch to wash it down. It may grow on me??!!??

Author:  rparsh [ 31 Jul 2011, 18:41 ]
Post subject:  Re: Super simple single stage tube preamp

Are you aware of Austraulian Pink Floyd? They kick ass here in the states on public tv. I watched and recorded them and they are the best! I looked at my old original PF CD and it was dated 1969. WoW! I always loved to play it wide ass open at 60WPS RMS and was never arrested for too loud music. Floyd used to rule!

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