DIY rotary DJ mixer

[efinque]

The riaa module is fixed gain. In practice you may need more output voltage (gain) to have a proper SNR in the whole circuit.

+40db for line in? This is way too much gain for typical line in!
Please define what YOU call line (if possible reference level, if you don't know which kind of gear you'll be using).

I know this from a fact that tape channels (such as the one in my PM-90) used to be a lot more quiet than modern line level channels. The gain could be boosted or attenuated with jumpers or by replacing a component in the circuit.

The transformer are not wired as they should and you do NEED an active stage to drive them.

They're wired like this (unbalanced-to-balanced)

 

[KrIVIUM2323]

Are you going to use your tape deck to mix?
More acurate question, what source are you using for your line input?

 

[efinque]

Are you going to use your tape deck to mix?

No.

But as for gain staging the EQ module comes with opamps which could help a bit in the output section.

I *could* wire the line gain (=upper) pots to replace the trimmers.. I don't know if doing so would give too much control over the gain although there isn't much to begin with.

EDIT : I don't know if feeding the preamps with higher voltages gives more gain... I've only tested them with 12VDC regulated power supply so far (if the transformer frying incident doesn't count) but as a downside it could also introduce more noise.. I need to find a compromise.

 

[KrIVIUM2323]

So... if you are going to use typical 'line' semi pro source, 20 db gain is more than ok.
And now we go back to where to put this gain stage in the signal chain.
There is a reason why it is placed post riaa and switchable between line and riaa: it serve to compensate between output difference between line and riaa.

A typical consumer da output(some digital deck player) will have something like 2volts max out.
Compare to the max 0.77volts of riaa... you'll need this gain stage to compensate between both of them.

Do you still follow me?

Back to your output transformer: there is no relation between the way it drive (the line out balanced) or is driven (unbalanced) and the need for a driver stage before it.

There is need for a low output impedance to drive a line and your potentiometer does not provide it.
There is no way out : you need an opamp (or other gain stage) to drive the output transformer.

You talk about an eq... which wasn t planned at first. Where do you want to place it in the signal chain?
You seems not to care about but this does change many things about headroom and reference level at which the circuits are working. And you should have planned this first (as it ll determine SNR, headroom,as well as the characteristic of the output transformer...and so the rail voltage you ll need in the psu too).

I don t follow your comment about the eq circuit and the gain stagging. Here too i think you don t really understand principle of snr, headroom and such... but i may proven to be wrong.

The rail voltage determine the max output voltage your circuit will have and the headroom you ll have in the preceeding stage (eq, buffer, gain stage, preamp,...). If a psu is well designed there is no noise issue so no compromisd to be made (given you know what to design for). I would target at the more headroom possible so the more voltage opamp allow.

 

[efinque]

There is need for a low output impedance to drive a line and your potentiometer does not provide it.
There is no way out : you need an opamp (or other gain stage) to drive the output transformer.

I believe the opamps should be in the summing instead to reduce crosstalk but that'd make it an active module in the device and that puts additional stress to the PSU.

You talk about an eq... which wasn t planned at first. Where do you want to place it in the signal chain?

The most logical place would be after the summing stage to get it in the booth outputs too... although I'm a little worried pulling two separate outputs from it (the booth/zone outs are unbalanced)

 

[efinque]

Btw I was getting very strange readings from the Akai PSU... something like 38VDC (unloaded)

 

[KrIVIUM2323]

I don t understand your concern about stress with the psu. Once circuits (all of them in your desk) are determined you ll know what you need and what to design your psu for.

Before that you'll need to 'prototype' each module individually and check them one by one to be sure they all run ok. So you ll need something to test them before going to a final design.
For riaa or other modules your wallwart should be enough but i m pretty sure that you ll need a dual power supply at one time (i suspect your headphone amp will need that but without a list of modules you plan to use...).

I advised about that earlier in this thread, and this was not just to say it there was a reason about that (and now you face it).

Back to your passive summing bus: it is named passive yes but it ll need some make up gain to recover the passive loss. You could use this gain stage to drive the transformers but it may not have enough 'juice' to drive them. It ll depend of what you target as reference for output level and load it ll see but here too you have not defined any of your needs...
Anyway what you did in your latest experiment does not mean anything as it doesn t include your planned make up gain stage...

About your eq, ok if you place them post summing bus make up amplifier you ll have to worry much about headroom than driving multiple output stage.

About driving multiple output: opamps are relatively easy load (you can make input impedance high if you use fet opamp like tl072) and to have two 20k input z in parallel makes a total load of 10k... real easy for most opamp to drive. Add two more and you are still at a 5k load still easy for an opamp to drive.

Now the eq: the real concern is headroom/SNR (signal/noise ratio, the noisefloor). Typical eq board will give you up to 12/15db gain. You ll probably never use it this much but who knows...
So to have headroom you ll need something like 12db margin in the circuit to be confortable. 12db of boost and 12 db of headroom and you'll have a need of +24dbu output, which mean your max voltage post summing amp is around 0dbu...(and rails of +/-15volts minimum). This is low output for (post make up gain)summing bus in my view* and noise may become an issue (you compromise SNR as you must not forget you already need headroom in the chanel and you ll have attenuation in the fader -one more source of possible noise) and this can be worst if the eq circuit is not pristine clean... You could argue you can have an other gain stage to recover but you ll introduce more noise and distortion along the way...

This is the reason why eq are in chanels in modern mixers almost never in main output, and the reason why the chanel have huge headroom too (re read the A&H V6 spec)...

*: i must admit if it was mine i would use vca for fader/crossfader and this solution give different topology including make up gain with maybe lower noise. In fact something close to the GSSL signal path (this could use the same audio signal path, just use a different CV to drive the vca from a fader/crossfader, more or less the same as a SSL4000 desk).

 

[KrIVIUM2323]

Ok, i had a look about the Bozak mixer (partial) schematic i've found:

http://www.wavemusic.com/community/attachment.php?s=16caa27342f1db031019bc520a31586b&attachmentid=894&d=1147750819

http://www.wavemusic.com/community/attachment.php?s=16caa27342f1db031019bc520a31586b&attachmentid=895&d=1147750859

Are they the one you get inspiration from?
They miss all the active gain stage though...

 

[efinque]

Ok, i had a look about the Bozak mixer (partial) schematic i've found:

http://www.wavemusic.com/community/attachment.php?s=16caa27342f1db031019bc520a31586b&attachmentid=894&d=1147750819

http://www.wavemusic.com/community/attachment.php?s=16caa27342f1db031019bc520a31586b&attachmentid=895&d=1147750859

Are they the one you get inspiration from?
They miss all the active gain stage though...

They're found here as zip files...

 

[kooma]

What modules you are going to use here?
could you please list module numbers etc.

Remi has given you some exellent advices; I would also first test each module indevidually, then start thinking how to combine them and last calculate how much power you need/what kind of psu.
after all that is done start building the "final"version.(that you will likely end up modding after you get it working,
or that how it usually goes with my builds

 

[efinque]

The current PSU works like a charm (the one I pulled from the Akai tape deck) when tested on a single module. I think I could build a linear supply from kit and power that with a transformer.

It's only that the left channel is dead.. it could be due to a number of reasons but I suspect that I burnt an opamp (there's some traces of burnt plastic on the pins)

EDIT : but it sounds really impressive.. a bit distorted maybe but in a good way, think of it in terms of warmth. I don't know if it has anything to do with the Lundahls.

EDIT 2 : it also could be a short in the passive summing... who knows. Lots of troubleshooting..  :-\

 

[efinque]

Here's the build so far... I call it Avenue.

The RIAA/phono preamps aren't functional as of now.. I put a line signal through one of them and it came out the way it should (=the way line signals go through RIAA preamps..) I guess but my turntables didn't.

EDIT : I could replace the RIAA's with line preamps and use externals instead.. it's also missing the headphone amp which is an important feature in a DJ mixer (and I've yet to test multiple channels)

EDIT 2 : I think it's better to pull the PFL's from the pots too instead of straight from the preamps...

 

[efinque]

I think I broke my multimeter...

 

[efinque]

The monitoring section isn't fully functional just yet.. I might have to source another headphone amp module because the one I have now could be fubar (I screwed up desoldering a capacitor and the coating plus some of the copper peeled off.. )

I did put some jumper wires to fix the circuit but I didn't get a signal through, only some crackle from moving the input wires (the module has a LED which lights up though so it can't be completely trashed)

 

[efinque]

Added an unbalanced RCA booth out... it's not as loud as the master outs though, something like a few dB difference I think. (edit : they're direct from the summing bus through a potentiometer, which to my surprise is quite noiseless.. it's actually quieter than the main outs)

Originally I was supposed to make the booth outs 6,3mm TRS jacks but for some reason I didn't (it's getting really hard to cram all the extra goodies in there..)

EDIT : I've also been thinking of a preamp post-summing... although I think I'll wait what the EQ module does to the outputs.

 

[efinque]

Also, would I need a preamplifier after the phono cards?

EDIT : I just tested the configuration but I blew a fuse with all the wire spaghetti.

 

[efinque]

The gain pots are now straight from the line preamplifier trimmers.. I should've figured this out some odd 10 years ago  :-[

EDIT : the trimmers are still intact in case the gain staging needs to be fine-tuned later on.

EDIT 2 : replaced the trimmers (they're 1kOhm actually) with the 100k logs.. I guess it's better than nothing. I think 10k would suffice... (I did source some 1k stereo log pots though)

EDIT 3 : The headphone amp seems to be working despite the desoldering incident... I've yet to wire the damn thing though along with the cue selector switch. Fun fact about the headphone amp ; it's actually a slightly modified hearing-aid device that's designed to operate with 3x AA batteries..

 

[efinque]

Here's the most recent build pic with the cue selector switch and the headphone amp/monitoring in place (you can't see it because it's behind the front panel..)

EDIT : would I also need to power supply to the transformers?

 

[efinque]

Here's a front shot...

 

[efinque]

Added a power on LED to the front panel (from the headphone amp)

I thought I broke the hp board at first because to me the LED that came in the kit seemed too small so I put a bigger one in there (I couldn't desolder it; it's still there although it's not working because the copper peeled off) with jumper wires but it was the wrong way around, flipped it and now it works to some extent.

EDIT : I also tried powering the input modules in parallel but I didn't notice a difference (I'm using a 12V/700mA DC power supply for the preamps and a 4,5V/190mA DC for the headphone amp)

EDIT 2 : with this much desoldering, tinkering with the power on etc and testing it should be near vandal proof. I remember that in an interview Alex Rosner told he tested the Rosie mixer by pouring a bottle of Coke in the circuitry.