Mark 3 Vacuum Tube Mixer

[ruffrecords]

Part 5 just posted:

http://mark3vtm.blogspot.co.uk/

Cheers

Ian

 

[anjing]

Really nice stuff Ian!

Pierre

 

[bockaudio]

Why the decision to use the same xfmr for line in and mic in?

 

[ruffrecords]

Why the decision to use the same xfmr for line in and mic in?

Principally cost but also space. Good quality input transformers cost upwards of £50. The EZTube boards are designed to fit onto a standard Eurocard which is 160mm by 100mm. There is not enough room to fit two input transformers and everything else a mic pre needs on a board that size. For the Mark 3 there is more space available so it might be possoble to include a separate line input transforme. In fact, on the prototype 6U board I have included an extra unassigned input transformer which could perhaps be used for this

There is little to choose between one or two transformers in performance terms. The single transformer version starts with a33dB pad, adds 20dB gain via the transformer and then  13dB in the mic amp. Since the mic amp has 6dB gain minium, a standard 10K:10K line input transformer would need a 6dB pad followed by 6dB of gain. The noise performance of the amp is virtually identical in both cases.

Cheers

Ian

 

[beatnik]

what are the cons of having a single in transformer for both line and mic sources?

 

[ruffrecords]

what are the cons of having a single in transformer for both line and mic sources?

Few if any. With the current design, the mic preamp has gains of 40,37,34,31,28,25.22,19,16,13,10 and 7dB. At the line input you have a 33dB attenuator at the front followed the 20dB gain from the input transformer so the level at the preamp input is at -13dBu for a 0dBu line input. So for a nominal line input, the mic pre is only asked for 13dB of gain. From this point, the gain can be reduced by 6dB to cater for hotter inputs and increased in 3dB steps to cater for lower level inputs like -10dBu consumer sources. The normal compromise with this method is in noise performance because, as a rule, all attenuators worsen noise. However, in this case, the limiting noise source is the output noise of the mic preamp which, as I said earlier, is around -80dBu until the preamp gain reaches about 20dB when the input noise begins to dominate. So, bottom line, noise performance is not affected.

The 33dB attenuator is designed to look like a 150 ohm source to the microphone transformer. To do this, the attenuator consists of two 3.3K resistors and one 150ohm resistor. This means the input impedance of the line input is about 6.7K which is a bit less than the standard 10K bridging impedance but is fine for the vast majority of sources. If you were really concerned about that you could use two 4.3K resistors and a 200 ohm resistor which gives 32.8dB attenuation and raises the input impedance to about 8.8K.

The level reaching the mic transformer is around -30dBu which is well within its capabilities so distortion is not going to be an issue. You might think that a 1:1 line input transformer (10K:10K)would have a better frequency response than a 1:10 mic input transformer (150:15K). However, they have very similar secondary impedances, hence similar inductances, hence similar HF and LF losses.

Cheers

Ian

 

[beatnik]

interesting explanation. thanks Ian

ah and btw, the new mixer project looks amazing! thanks for sharing

 

[ruffrecords]

Just posted a small update:

http://mark3vtm.blogspot.co.uk/

Cheers

Ian

 

[ruffrecords]

Another post to the blog:

http://mark3vtm.blogspot.co.uk/2014/05/insert-options.html

Cheers

Ian

 

[dipfrik]

Hey Ian,

can you share some more information about the status of 6U EZTubeMixer PCB? Maybe a new directory on your server?
Is it your plan to make them also available over the white market in the future?

Thanks, Sven

 

[dipfrik]

one more

Quote from your Mark 3 Blog:

I then contacted Colin at Audio Mainteneance to see if there was any possibility of obtaining a VTB2291 in a PCB mounting format similar to the VTB1847. It turns out that this is no problem and Colin was able to send me a prototype to try out.  Not only is it no problem, but because it is just a slight tweak to the process of manufacturing an existing transformer, there is no price increase either.

Is this official?

Thanks, Sven

 

[ruffrecords]

one more

Quote from your Mark 3 Blog:

I then contacted Colin at Audio Mainteneance to see if there was any possibility of obtaining a VTB2291 in a PCB mounting format similar to the VTB1847. It turns out that this is no problem and Colin was able to send me a prototype to try out.  Not only is it no problem, but because it is just a slight tweak to the process of manufacturing an existing transformer, there is no price increase either.

Is this official?

Thanks, Sven

Yes, it is official. Colin even gave me the Carnhill number which I wrote down somewhere but cannot find right now. When I find it I will post it here.

Cheers

Ian

 

[dipfrik]

Great!

 

[ruffrecords]

New blog entry:

http://mark3vtm.blogspot.co.uk/

Cheers

Ian

 

[dipfrik]

...interesting right most 3U module on the top row on the rack picture

Cheers, Sven

 

[anjing]

Hi Ian!  Great stuff! Did i just see a two knob ezcomp in the test subrack?

Regards!

Pierre

 

[ruffrecords]

The mystery module at the right of the 3U rack is a prototype EZTube compressor. I posted about this a little while back but there was no response so I have not domne any more work on it.

http://groupdiy.com/index.php?topic=44550.msg731329#msg731329

Cheers

Ian

 

[guitardick]

Wow, Ian that is some very good work.  What a monster of a project!  You Rock!

 

[ruffrecords]

Hey Ian,

can you share some more information about the status of 6U EZTubeMixer PCB? Maybe a new directory on your server?
Is it your plan to make them also available over the white market in the future?

Thanks, Sven

I somehow missed this post, Sven. Yes, I will create a directory for the Mark3 and fill it with info about the new PCBs etc as it becomes available.

Cheers

Ian

 

[dipfrik]

Great!

Thanks, Sven