Passive Mixer

[drsnaut]

Hello people,

I am making some plans for a simple mixer to accommodate an easy listening setup for 2 record players.
With this setup I want to be able to play one after the other record without the 'silence' of switching records (taking the needle off etc.) to
keep the listening sessions more fluid and uninterrupted. While I'm at it, I'd add a mono/stereo switch for the moments I'd play a mono record
(for people who are wondering, mono records sound better in mono).
This mini-mixer will consist of 2 volume pots (one for the left record player, one for the right), a mono/stereo switch and a direct source connection
for a 3rd record player. This will be fed into the phono stage I have on my amplifier.
I would like to keep the signal as clean as possible, not running it through big capacitors, opamps etc. If that's possible is of course the question I'll hope
to find out with this.

Possible issue 1:
Since I have a good tubed phono stage, I'd like to keep this mixer as passive as possible. I'm not sure if a small signal which an element picks
up lets itself be passively attenuated. I have seen phonostages with a 100k Log input so I can't figure out why it shouldn't, but still I'd like it confirmed.
Then, what would be a good potentiometer value? 25K, 50K 100K?
The element I have is an MM Goldring Elektra (load resistance 47k, load capacitance 150-400pF so I guess I have to get a 50K pot for it?).

Possible issue 2:
The merging of signals. which I made grey on the schematic since I am not sure if I'm able to just connect the signals and be done with this.
I have seen a tubecad issue on mixers that separatesthe channels using a tube. The Tube CAD Journal: Vacuum tube mixers
I am just wondering, is this necessary, can I go without? What would be the consequences of either choice?

If there would be just very little crosstalk, I could make a 'signal lift' before the volume pot to completely silence the other channel?

I'm curious to see reactions on this, thank you very much!

 

[Tubetec]

Signal level out of the cartridge is very small and weak , trying to mix passively is going to mean more signal loss and more noise . Better lift the signal level before trying to mix it .
You need a cross fader which is two stereo channel inputs and depending on position , to the left input one to the right input two and in between a mix of both . A mono record will play back just fine over a stereo set up , its only if you wanted to 'mono' a stereo record you'd need the switching arrangement . A cartridge output is designed to see a particular load and have a specific equalisation curve applied , things just probably wont work as planned with the passive arrangement up front your thinking of . A simple a/b source switch would be one way to do it but you wont have any way of auditioning or setting the levels . Small DJ mixers have all the facilities built in , including a seperate head phone cue mix where you can audition the second record , set levels etc before you crossfade it into the main mix outputs . I'd say figure out a way of taking your good turntable signal out of the preamp line level , send that to a line level input on a DJ mixer , then connect the other turntables to the phono inputs . Something like a Vestax PMC 02/03/06 are fairly good quality and not too expensive. Im a long time out of hifi and DJ equipment sales at this stage , so probably many more options out there too now.

 

[JohnRoberts]

Back in the 70s I published a DJ mixer kit, but it didn't use vacuum tubes.

JR

 

[drsnaut]

Hmm, I was suspecting something like this. I'd keep a look out on a cheap Vestax instead of opening a can of worms. Thanks for the advice!