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4S Universal Tube Preamp Project (12A*7)
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=4628
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Author:  Les [ 01 Jul 2014, 13:54 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

jccarter wrote:
I am currently assembling the parts to this, and I can't find the internal shield/card, or I don't know what to call it when I search for it. Any suggestions?

It's a piece of metal (aluminum), nothing more. You can probably find some in your local hardware store. :up:

Author:  Suncalc [ 19 Oct 2014, 16:47 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Well, I have another update to the tube list for the 4S Universal. This is a special case because you can only use this tube if your heater supply is capable of putting out a full 600mA into the filament. If so, the 12BH7 tube will expand your tube rolling capabilities. This tube is halfway between the 12AU7 and the 12AV7 from a gain perspective and is very low distortion. I don't have any on hand but if someone tries it, be sure to let us know how it sounds.

Here is the new tube performance list just for reference.
Attachment:
20141019 4S Tube List.png

Gio, Could we get this new table added to the project page?

Author:  mybestboss [ 04 Dec 2014, 23:36 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

If I want to use 12AT7, How should I change anything in this circuit?

Author:  Suncalc [ 05 Dec 2014, 00:41 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

The reason that it is "Universal" is because you don't need to change ANYTHING! Just build the circuit and roll different tubes to your heart's content.

Author:  booangler [ 07 Dec 2014, 10:02 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Matt,

I built a Bottlehead Quickie where it was suggested that I replace the plate load resistors with a choke. I was wondering if that could be done with the S4? What would you expect he outcome to be?

Best,

Alan

Author:  Suncalc [ 07 Dec 2014, 11:06 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

The primary reasons for choke loading are to lower the load loss (thus making the power supply much less expensive) and to significantly lower the output impedance of the stage. Choke loading was commonly used, in the 1910s and into the early 1920s when the science of transformer manufacture was in it's infancy, to drive loud speakers. The fact that it immediately fell out of favor when reliable transformer manufacture was perfected should teach an important lesson.

There are a few things to consider when doing this. First, the DCR of the choke is small and therefore so is the load loss. As such, the bias point will be shifted up to very close to the B+ voltage. The 1.2kΩ biasing resistor will force very high bias voltages and currents. Either the B+ will need to be lowered significantly, or the biasing resistor will need to be increased significantly. In either case, you'll need to pick a target tube to use and stick with it. The "universality" of the design will be gone.

Also, all the strictures of transformer coupled design still hold. The greatest of these is that when the load of a stage is purely reactive, and the load reactance is small as compared to the reactance of Cpk and Cgp in parallel (as is virtually always the case with small triodes at audio frequencies), the input impedance of the stage has a negative conductive component which, if proper phase margin is not maintained, will lead to oscillation. Because it is not possible to guarantee phase margin if one or both ends of the stage are externally connected (i.e. we don't necessarily know to what they will be connected) it is not particularly advisable to use the choke-capacitance coupling method in standalone stages.

There has been much talk of choke loading from some of the high end manufacturers trying to sell it as something new. However, in comparison to both transformer and RC coupling, it is inferior in performance. Circuits have to be carefully designed to make it work and due to PS coupling, the risk of "motor boating" and oscillation are both high.

In general, I'm not much of a fan of choke loading. Most designers threw it away 100 years ago and I am reluctant to revive it in my designs.

Author:  UJF [ 11 Dec 2014, 09:18 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

I like the 4S concept. Is it possible to add a treble and bass control and what might be a schematic for that addition.
Thank you

Author:  mwhouston [ 11 Dec 2014, 18:27 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Sometimes a simple tone control is better. Or a tilt control.

Author:  Suncalc [ 13 Dec 2014, 13:42 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

UJF wrote:
Is it possible to add a treble and bass control and what might be a schematic for that addition.
There are some complications for this approach. The issue is that the tone stack really needs to be buffered from the outside connections to ensure proper operation. That's doable but you end up with a buffer, followed by a tone stack, followed by the amplifier stage.

I have a design upon which I'm working to do just this, but it's not yet quite ready for publication. Check back in February of so.

Author:  Jujuman [ 23 Dec 2014, 17:36 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Hello All;
Great project and I am looking forward to building this soon...... What would be the best (cleanest sounding) way to change the RCA OUTPUTS to balanced signal for XLR inputs on my amp?

Thanks in advance for all your help.

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