Simple front-end tube mixer for DSD

[rjuly]

Hi All,

My first post on this new forum! Very pleased to see this tube subforum here with Ian - a breath of fresh air. Please be gentle - I am a rank amateur...

This will be an attempt to build a very simple three channel mixer based on Ian's design; mixing three mics into the inputs of a stereo DSD AD convertor. The idea is to use a stereo ribbon mic for the sides, along with a LDC mic to fill in the center. As this will be a sort of direct to (hard)disk type of thing, I'd like to keep the channel circuits as simple as possible, with only HPF in line and no fader, or pan - the mix bus being selactable as LCR with a switch. Level would be controlled by input gain. The source would be solo or small group acoustic music with the goal of capturing the sound as heard from a single point in the room.

Here is an initial block diagram:

My goal would be to keep things relatively simple, using Ian's twin Twin Line Amp V2 circuit for both the channels and summing amp.

Ian's design:
https://drive.google.com/drive/folders/ ... drive_link

- Richard

[mediatechnology]

Thanks for joining us Richard!

I'm not the tube guy here and can easily claim the rank amateur title.
We are lucky to have Ian onboard.
I'm going to sit back and learn.

Wayne

[rjuly]

Hi Wayne - Thanks very much!

Great to have Ian here in this environment. You can't buy what he gives away in return only for some common decency and respect. He has always been most generous to me.

- Richard

[Rusan]

"Great to have Ian here in this environment. You can't buy what he gives away in return only for some common decency and respect. He has always been most generous to me."

Hear hear! I don't wish to hijack this excellent thread topic, but I believe in giving honor where honor is due. Not only is Ian a walking encyclopedia + CAD program of All Things Electronics, he's an old-school gentle soul with a generous heart. Where his footprints are, there you'll also find those who have been enriched by his eminent knowledge and wisdom.

Welcome to TPADF, Richard! May your stay here be a profoundly profitable and pleasant one.

Cheers,
Rusan

[ruffrecords]

Hi Richard,

Your basic diagram looks good to me. When it comes to mixing you have two choices. You can use "traditional" voltage mixing where you feed each output to a bus via a resistor, Since each source should be low impedance, then the loss is equal to the number of inputs. In your case 3 inputs means a loss of 3 times so you need about 10dB of gain make up. The Twin Line Amp (TLA) also uses a 2:1 step down transformer so you will need another 6dB of gain.

A couple of things to be aware with the traditional method. First you need to make sure all the bus resistors are fed from a low source impedance even when not selected which means you need to switch them to 0V when deselected. One advantage of the traditional method is you can easily add a master level control. Simply wire a 10K pot across the bus and take the wiper to the TLA input. Of course you need to adjust the gain to take account of the extra 10K across the bus. However, if you use 47K bus resistors then a 10K pot is the same as adding five more 47K bus resistors so in your case the bus loss increases to 3 + 5 = 8 so now you need 18dB of gain make up plus the 6dB for the transformer = 24dB.

The other way to mix is using a virtual earth and the TLA can be configured to do this. The advantage of this method is you don't have to worry about bus resistors being grounded or fed from low impedance sources. You can in theory leave them floating when not selected but I don't think anyone actually does this because they can pick up noise. One downside of the virtual earth is that the gain with 47K resistors is unity (due to the internal 47K feedback resistor in the TLA) so you really need to find 6dB of extra gain to account for the output transformer. One way to do this is to halve the value of the bus feed resistors. In your simple case where you are feeding just a couple of buses this will work fine but in a more complex mixer where you have pan pots and several buses you need to make sure the total load seen by the TLA does not drop below 2K4. Lastly it is a little more tricky to add a master fader in the virtual earth method without adding an extra stage of amplification. You can do it by adding a pot to the secondary of the transformer.

Cheers

Ian

P.S. and thanks guys for all the lovely comments about how wonderful I am -- keep em coming

[rjuly]

Hi Ian,

Many thanks for your thoughts. My inclination is to go the traditional route. As my project is not really a mixing console in the usual sense, but rather a three channel mic pre that sums to a stereo output, I thought that I would dispense with a master level control. That would leave me with 16 dB to make up the full 60dB of output of the channel pre amps. Is that level makeup realized by amplifying the bus outputs using the TLA?

- Richard

[ruffrecords]

The TLA does indeed provide the necessary gain. Just fit it with 47K 25 turn trimpots and adjust to obtain the required gain.

You might want to think about the gain range you actually need and also the maximum gain required. You are correct that with a 1:10 mic input transformer the maximum gain is 60dB. Since the mic pre gain can be varied from 6dB to 40dB your input sensitivity will range from -60dBu to -26dBu for 0dBu output. It is possible on occasion that a particularly loud instrument causes the input to significantly exceed -26dBu. In that case it is common to include a switchable input 20dBpad before the mic input transformer's input. This increases the sensitivity range to -60dBu to -6dBu.

Conversely, there may be some situations where 60dB of gain is not enough. In this case a common approach is to also add an extra 10dB of gain to the mix bus TLA which means the input sensitivity range now shifts down to -70dBu to -16dBu. For the best of both worlds you could optionally fit a trim control with a 10dB range.

Cheers

Ian

[rjuly]

Yes, a pad in the mic pre circuit is smart, and a minor enough increase to cost/complexity. One question about components pre input transformer - Do the components need to be carefully matched to keep phase coherence across the balanced input, or does the overall value of the two componenets sum to create a single value across the primary?

- Richard

[ruffrecords]

The achievable level of CMRR does depend on how closely the components in the hot and cold arms are matched. 1% gets you 40dB and 0.1% gets you 60dB. However, I would not recommend spending money on close tolerance resistors. The important thing is that the values in each are are the same. The simplest and cheapest way to do this is simply by selecting pairs of the same value. I do this for 6K8 phantom power resistors and the 680 ohm resistors I use in my 20dB pads. You don't have to test many to get a few closely matched pairs.

Cheers

Ian

[rjuly]

In one of Ian's Tube Line driver schematics:

The variable gain is shown that atenuates the output of the 12AX7 tube using a switch to select a vaiety of values. The higest of these is 430R, Which when combined with the 20dB gain of a mic input transformer gives us the 60dB gain ceiling of the circuit.

My question is : Why the highest value is set by this resistor, rather than no resistor? Is it because the unattenuated gain of the tube can be adjusted with the resistor to be precisely 40dB?

Also, because I am designing this mixer to have no output fader for the channel, I thought to have a second stage of attenuation to act as a trim. This would be a second switch in series with the first, with smaller increments that might give me between zero and five dB of gain attenuation. How might I calculate the values for this trim switch?

- Richard