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PostPosted: 05 Feb 2015, 00:36 
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allenb wrote:
mwhouston wrote:
The original design for this preamp uses capacitive coupling to drive into high impedance loads around 250K ohms. Not sure you could use a OPT without a re-design.


My son has a couple of vintage Southwestern Technical Products Universal Tiger mono block amps from the 70's that are supposed to have around 25K ohms input impedance and he want's me to build him the 4S. Is there a practical way to configure this preamp to handle an input impedance this low?

Allen

You either need a preamp cathode follower type tube preamp or but a Pass B1 backend after the 4S stage.

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PostPosted: 05 Feb 2015, 19:54 
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allenb wrote:
My son has a couple of vintage Southwestern Technical Products Universal Tiger mono block amps from the 70's that are supposed to have around 25K ohms input impedance and he want's me to build him the 4S. Is there a practical way to configure this preamp to handle an input impedance this low?
If you move the volume control to the input and then use a 12AU7 with bypassed cathode, the output impedance is around 11.5kΩ. This is not ideal, but it should drive you son's amp ok. I also have a 12AU7 design with an output impedance of 6.8kΩ. It's a little warm (but some people really like this as it's the traditional "tube sound") and it will drive the amp just find.

I realize that these aren't "universal" solutions, but your son shouldn't really need the gain of the higher µ tubes. Let me know if you want a schematic.

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PostPosted: 06 Feb 2015, 00:13 
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Actually, the vintage "warm" tube sound is what he's after so yes, I'd love to see your schematic of the 12AU7 with 6.8K ohm output impedance.

I don't think his feelings will be hurt not being able to try out alternate tubes.

I'm having a tough time finding "proof" that his amps are in fact 25K ohm input impedance so just in case it's much higher, what should be a practical low limit for for an amps input impedance to be able to be driven by the 4S Universal?

Thanks!

Allen


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PostPosted: 06 Feb 2015, 15:04 
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Hi Matt, i had the time last week to work on the problem, before i became father.
so I had done the drop resistor in the PS and replace the bad cap, but the plate and bias voltage were still off, so i have tried one off my used chineese 6n9p i got in my tube stash and with 275v B+ and the anodes voltage are 141 and 145V, the bias are -1.9xV. when i will have some spare time, i will check with signal gen and scope the signal path.

Thanks Matt for your help!


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PostPosted: 07 Feb 2015, 13:36 
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Allen;

I may have spoken a little too fast. :blush:
Suncalc wrote:
allenb wrote:
My son has a couple of vintage Southwestern Technical Products Universal Tiger mono block amps from the 70's that are supposed to have around 25K ohms input impedance and he want's me to build him the 4S. Is there a practical way to configure this preamp to handle an input impedance this low?
If you move the volume control to the input and then use a 12AU7 with bypassed cathode, the output impedance is around 11.5kΩ. This is not ideal, but it should drive you son's amp ok. I also have a 12AU7 design with an output impedance of 6.8kΩ. It's a little warm (but some people really like this as it's the traditional "tube sound") and it will drive the amp just find.

I realize that these aren't "universal" solutions, but your son shouldn't really need the gain of the higher µ tubes. Let me know if you want a schematic.
Upon further consideration I realized I had not accounted for the AC load line rotation of the 4S driving a low impedance. I think I agree with Mark that a buffer stage will be required. I recommend keeping it an all tube design and approaching it thus.
Attachment:
2 Stage.jpg
This design has several good things going for it. First it preserves the "Universal" nature of the design so rolling different tubes is no problem. The second is that the Cathode Follower buffer can handle anything thrown at it with less than 0.5% distortion. In most cases any additional distortion will be impossible to detect. The stage has a very low output impedance (<600Ω) so it can drive any input impedance above about 2kΩ. This means almost all audio equipment with single ended inputs.

The only hitch is that you'll have to choose the coupling capacitor based on the lowest input impedance you intend to drive. For a 5.3Hz rolloff and a 25kΩ input impedance, the value of Cc should be about 1.3µf or so. A 1.5µf would work fine. You can look at the notes and calculate a value as required. This approach does add one 12AU7 tube to the build, but it also makes the preamp much more versatile.

Take a look and let me know what you think.


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PostPosted: 07 Feb 2015, 19:04 
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I'm glad to hear I can maintain the universal aspect by adding the cathode follower stage. So, I'm assuming I won't need to make any changes to the power supply circuit with the additional tube?

Is the part of the sketch showing the double throw switch with the cathode bypass cap correct? Looks like in one position it would be cap only without the 1K resistor.

Thanks for all your help in accommodating my request for help!

Allen


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PostPosted: 07 Feb 2015, 19:24 
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allenb wrote:
Is the part of the sketch showing the double throw switch with the cathode bypass cap correct? Looks like in one position it would be cap only without the 1K resistor.
Yes it is correct, if a little unclear. The switch shown is one side of a DPDT switch used to switch the bypass capacitors in or out. In the right hand position, the cap is switched in. In the left hand position, the cap is switched out and shorted with a resistor to bleed off the residual biasing charge. This is to prevent the bias point from jumping, due to a pre-charged capacitor, when is the switch is thrown back in with power on. This prevents a "thump" from running through the system.

This is only an issue if you decided to include a set of switchable bypass capacitors. In most instances it would not be included. Given that this is for driving a line level input, I would leave out the bypass capacitors all together as the additional gain is not required.

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PostPosted: 07 Feb 2015, 20:31 
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Matt, I don't like the pot in the plate path. To me that means DC is constantly flowing through the pot. A cheap pot will not last long here before it goes noisy.

What is wrong with a more standard Rp load resistor, coupling cap, the pot then onto the next stage? Why have such a noise item in such a critical position?


Not being critical just curious?

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PostPosted: 07 Feb 2015, 22:22 
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I'm with Matt here. Leave out the bypass cap. A hint of local NFB is good right here. Never GNFB.

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PostPosted: 07 Feb 2015, 22:37 
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mwhouston wrote:
Matt, I don't like the pot in the plate path. To me that means DC is constantly flowing through the pot. A cheap pot will not last long here before it goes noisy.

What is wrong with a more standard Rp load resistor, coupling cap, the pot then onto the next stage? Why have such a noise item in such a critical position?
Well, there are three ways to insert the volume control into this circuit (without inserting an additional coupling cap). The first is the way I have drawn above. Yes this does promote a small DC current through the potentiometer. However, the current is small and a good quality pot should not become noisy in this configuration. If this is a concern, a switched attenuator with metal film resistors should be used.

The second approach is to do as you say and wire it like this:
Attachment:
#2.jpg
I don't like this arrangement as it presents a wildly varying impedance to the gain stage. I would not do this unless there were no other option available.

The third approach is to simply put the volume control out front. Like this:
Attachment:
#3.jpg
This is the simplest solution. You do lose about 20dB in noise figure, but if a quality potentiometer (or switched attenuator) is used, it should not matter.

The figure in the post above (with a potentiometer as the gain stage load) came directly out of my design notebook without modification. When doing my personal designs I often make assumptions which I wouldn't necessarily apply to something I post on line. I don't really have a concern with that configuration based on my normal use of a PEC sealed or Alps potentiometer or a switched attenuator. But if anyone has a concern, then they should use configuration #3 with the volume control out front.


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