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PostPosted: 08 Jun 2018, 22:22 
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Joined: 28 Dec 2010, 22:07
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I am wondering something and I'd like to see how objectionable it is among tube aficionados.

Say I use a low-powered sweep tube (12BQ6, etc) or even a run-of-the-mill 6V6GT. Now say I use something like a 6AB4 or other triode gain stage to boost the voltage and allow the room for something like 20 dB of feedback (reducing voltage gain by 10). Now, let us say I install it in a box with one speaker and use a summing circuit to add L and R channels from a stereo output. That means both channels will be audible but there won't be any separation. The idea would be a single speaker and maybe a single box. That would allow me to have an acceptable output from a tube amplifier without having to build two amplifiers or lose the information in the left or right channel.

Is this something that is commonly done? I know we all like to build two channel amplifiers.

Ed

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PostPosted: 09 Jun 2018, 16:46 
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Hi Ed, I'm not sure what you are trying to do. Do you already have the stereo source? What is the output from it? If so there are lots of ways to get a mono signal from it. Why so much NFB? If you don't need that much gain why make so much, less is easier to get. How much output do you need?

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PostPosted: 09 Jun 2018, 19:11 
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gofar99 wrote:
Hi Ed, I'm not sure what you are trying to do.

Make a small, single channel amplifier driving a single speaker. Take input from a two-channel source and mix them so content from either channel is not lost.

gofar99 wrote:
Do you already have the stereo source? What is the output from it?

Yes, in the general sense that almost all audio output these days has a left and right channel. The output would be line level, which seems to be roughly 0.3v RMS at its highest level. I'd want the gain to be such that peaks would be loud and I could turn the volume down on songs that are recorded near the maximum volume level.

gofar99 wrote:
If so there are lots of ways to get a mono signal from it. Why so much NFB? If you don't need that much gain why make so much, less is easier to get.

To make the mono channel, I'd use a 470 ohm resistor from the left and right channel to the top of a 100k volume pot. That would result in mixing of each channel without much attenuation.

The NFB would be mainly to get around the restrictions of the output transformer and output tube. Using a beam pentode in single ended configuration, I might get as low as a few percent distortion, but NFB could reduce that by a lot. It would also require quite a lot of gain to lead to that improvement. Let's just say I use a 12AU7 with both stages used for an overall gain of 100, then NFB it down for a total amp gain of 10 or 20.

gofar99 wrote:
How much output do you need?

Let's say 3 watts. Let's also say 3 watts would only happen at peaks that are supposed to be louder, and average output would be closer to 500 mW or so. That way, I can have some semblance of dynamic range. Like, say I'm going to use a 12DQ6B with a plate/screen voltage of 180v.

Ed

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PostPosted: 10 Jun 2018, 10:41 
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Simple and very effective mixers can be generated using twin triodes with common plate loads or summing signals after separate plate loads. Attached is an example of the former from the Radiotron Designers Handbook (4th Edition) and the latter is generally implemented using either a mid ยต twin triode (like the 12AU7) or a dedicated mixer tube like the 6J6. Once the signal is mixed, jut continue with a single channel amp design as normal.
Attachment:
Simple Mixer.png


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