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4S Universal tube preamp
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=4087
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Author:  wdecho [ 22 Oct 2015, 07:57 ]
Post subject:  Re: 4S Universal tube preamp

Matt, what is the output impedance of this preamp as in the original circuit like I built?

Author:  Suncalc [ 22 Oct 2015, 15:50 ]
Post subject:  Re: 4S Universal tube preamp

The peak output impedance is listed on the project page in tables 1 and 2. However, the output impedance varies with potentiometer setting. This is the price we pay for lower noise figure. The typical characteristic looks like this.
Attachment:
Screen Shot 2015-10-22 at 3.44.30 PM.png

This is assuming a driver stage output impedance of 11.5kΩ without the volume control.

Author:  wdecho [ 22 Oct 2015, 17:00 ]
Post subject:  Re: 4S Universal tube preamp

Suncalc wrote:
The peak output impedance is listed on the project page in tables 1 and 2. However, the output impedance varies with potentiometer setting. This is the price we pay for lower noise figure. The typical characteristic looks like this.
Attachment:
Screen Shot 2015-10-22 at 3.44.30 PM.png

This is assuming a driver stage output impedance of 11.5kΩ without the volume control.


Thanks Matt, I missed the output impedance in the chart, or rather I have not looked at it since building my preamp of which I am immensely enjoying. The reason for the question is Nelson Pass has a new amplifier for diy using a circuit that mimics a V-fet transistor of which has curves like a triode tube with a similar sound that puts out 50 watts. I am debating building it and he states that it needs a preamplifier with an output impedance of 100 or less which makes this preamp not usable.

Author:  mwhouston [ 22 Oct 2015, 17:22 ]
Post subject:  Re: 4S Universal tube preamp

Most SS amps are low input impedance. What you can do is build a Pass B1 buffer to go between you 4S preamp an SS power amp. I've built a few 4S preamps with a B1 output stage which sounded really excellent. Both sold on first listen to those with SS power amps.

Author:  wdecho [ 22 Oct 2015, 18:16 ]
Post subject:  Re: 4S Universal tube preamp

I already have a B1, in fact I have built 2 and yes I thought that might work but I was hoping it would not be needed. I am old school thinking, less parts sound better. The circuit Nelson designed does not have a buffer circuit much as most of his amplifiers do so I was thinking a buffer would be needed. If you have not seen the latest Burning amp festival with Nelson Pass check it out here. A Class A SE 1 stage 50 watt amplifer with a triode like tube sound.

https://www.youtube.com/watch?v=wa_k0abLpAU

Author:  PCR [ 30 Jan 2016, 03:50 ]
Post subject:  Re: 4S Universal tube preamp

Mark thank you for your ideas and support for such a handy and doable preamplifier project :) . I am collecting parts for building one 4S universal preamplifier for 12A?7 with your ss rectified version. This is my first DIY build. I am confused about the voltage of the 0.1 uF caps which are just before heater of 12A?7 and B+ 250 V HT. And by the way what does HT means here? You wrote on the project page that given schematics are for reference, are that schematics exactly complete for a build?
Thank you in advance.
Image

Author:  mwhouston [ 02 Feb 2016, 09:00 ]
Post subject:  Re: 4S Universal tube preamp

PCR wrote:
Mark thank you for your ideas and support for such a handy and doable preamplifier project :) . I am collecting parts for building one 4S universal preamplifier for 12A?7 with your ss rectified version. This is my first DIY build. I am confused about the voltage of the 0.1 uF caps which are just before heater of 12A?7 and B+ 250 V HT. And by the way what does HT means here? You wrote on the project page that given schematics are for reference, are that schematics exactly complete for a build?
Thank you in advance.
Image

Tweaks on my tweaks, but you are in good hands. You have accepted the step down tranni as a step up. Now we have 240v for HT - High tension or B+ or plate voltage.

Across the heaters of your tubes place a 50V 1uf cap. Balances any AC differential.

The schematics are complete.

Author:  PCR [ 17 Feb 2016, 15:11 ]
Post subject:  Re: 4S Universal tube preamp

Thank you for answer. What are the watt values of 24 VAC Wall Wart and PT1?

Author:  mwhouston [ 17 Feb 2016, 17:51 ]
Post subject:  Re: 4S Universal tube preamp

PCR wrote:
Thank you for answer. What are the watt values of 24 VAC Wall Wart and PT1?



24V AC @ 1A. The step up\down tranni 24V @ 150mA.

Author:  PCR [ 18 Feb 2016, 05:45 ]
Post subject:  Re: 4S Universal tube preamp

I have to 24V AC @ 1A tranni. Going to use one of them as 24 V AC Wall Wart and one of them as PT1. Can it be a problem above 150 mA of PT1?

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