Feeler: EZ Tube Lunch Box

[ruffrecords]

Too heavy/big? Will it fit?

Cheers, Sven

It weighs 1.4Kgm  It is 95mm in diameter and 45mm thick:

Cheers

Ian

 

[dipfrik]

Where in the schematic do you use the Solen capacitors? I think it is not necessary to use a film capacitor to couple to the optos. You can use an electrolytic.

It is the big 10uF/600V in series before the output which goes into the output transformer for balancing.

Kubi wrote about this on his D-AOC mod page under http://audio.kubarth.com/daoc/ at bullet point (6), but i didn't tested this yet.

Cheers, Sven

 

[ruffrecords]

Where in the schematic do you use the Solen capacitors? I think it is not necessary to use a film capacitor to couple to the optos. You can use an electrolytic.

It is the big 10uF/600V in series before the output which goes into the output transformer for balancing.

Kubi wrote about this on his D-AOC mod page under http://audio.kubarth.com/daoc/ at bullet point (6), but i didn't tested this yet.

Cheers, Sven

OK. There is no need for it to be 600V - 400V should be plenty but I agree with kubi, a 10uF electrolytic will be just as good. I have never seen any real improvement by adding a smaller film cap in parallel but even so you will save a lot of space.

Cheers

ian

 

[dipfrik]

A short update to the Lunchbox PSU and the custom power transformer.
Works!

Oh, and don't get confused about the eurocard pcb on the photo.
This is a pcb i designed and ordered with the last batch of other pcb prototypes. This one includes the utility dc part.

So my additional mini utility psu board is not really needed. Everything can be fitted on one pcb.

Cheers, Sven

 

[ruffrecords]

That is a very interesting board, Sven. which supplies do the two heatsinks serve?

Cheers

Ian

 

[dipfrik]

Hey Ian,

it is based on your original schematic and architecture.
I didn't even reduced the size of the 48V smoothing cap as well as deleted the heatsink for the 48V supply. Nor did i exchanged the LM338 with an LD1048V.

The only thing i did was squeeze, squeeze and squeeze to get extra space for the 12V utility vdc parts.

Here a look at the actual pcb design for this 1st iteration.

Cheers, Sven

 

[ruffrecords]

I appreciate the effort you have made to squeeze all the power supplies onto a Eurocard. However, there are one or two aspects of the layout that I am not 100% happy with.

1. The main heater 0V track from the regulator output to the output connector is a small track with several vias in it. Either this track should be much thicker and the vias replaced by links or the connector should be moved closer or the track should be replaced by an insulated wire connection.

2, The 12V track  of the 12V heater supply from the rectifier to the smoothing capacitor and then to the output connector is also too thin. This and the 0V track can carry as much as 3 amps.

3. The heat sinks are oriented horizontally. This means the air will not flow easily around them so they will not dissipate the heat as well as they should. Ideally they should be oriented vertically.

As a rule you should make the 0V and +ve tracks of any supply as fat as possible and use wire links or zero ohm resistors rather than vias.

Cheers

Ian

 

[dipfrik]

Hey Ian,

thanks for the input.
I really appreciate it!

Let me try a next iteration.

Cheers, Sven

 

[ruffrecords]

One thing that might help is if you put the 12V heater supply on the right hand side so it is near the output connectors, then have the HT supply to the left of that and then the 12V utility and lastly the phantom power on the far left.

One other small thing, the very first resistor in the HT smoothing chain needs to be rated at 5W. It does not get particularly hot but it has to withstand the initial inrush current when the supply is first switched on and I have seen smaller resistors burn out because of this.

Cheers

Ian

 

[ruffrecords]

The 28HP Fischer cassette for the power supply arrived this morning. I am pleased to say, even with the PCB moved in a slot for clearance purposes, it and the mains transformer fit in side with ease:

Here is a pic of the lunch box with four regular modules and the 28HP power supply module fitted:

and one with the channel modules removed:

After Christmas I will build the 28HP PSU and test it.

Cheers

Ian

 

[zamproject]

Hello Ian

I follow this since beginning, also the EZtubemixer tread
Bravo!!!
I think your eurocard and km6 design choices is excellent, way more professional and robust than 500 series which mechanics look like a toy to me...
Hope I can join this in 2015 with a little tube mixer, or a rack with EZcomp ?!?

Zam

 

[ddt]

Hi Sven,

A couple of observations on the layout. Not that I feel much of an authority on PCB design.

- Are you sure you want to run that high voltage trace to the 220R/7W resistor in between the pins of the high voltage connector? You could re-route it along the top of the diode bridge.

- I agree with Ian on the trace thickness, there's no reason not to have massive traces from input connectors to the diode bridge to the first smoothing cap at least.

- I've heard an opinion that caps accross transformer secondaries only serve to resonate with the transformer, if I was doing this I'd probably make provision for a small value, high power resistor after the diode bridge instead, as the first thing the diodes see instead of the large filter caps.

- I haven't really checked this on your design, but I really like if the traces to electrolytic caps run to the pads on the bottom side of a PCB, it makes repair in case of lifted through plating much easier.

Cheers,
Dan

 

[ruffrecords]

Well, the Xmas holiday is over all too soon and I have been forced to spend some time in my workshop!!

I have been playing around with the Lunch Box power supply module. Drilled the hole for the custom mains transformer and fitted in in the box. As expected, a regular M6 transformer  bolt head fouls the sub-rack when you try to insert the module so I have ordered some countersunk ones. Also fitted the 25 amp rectifier for the heaters to the same half of the module. Next up will be mains socket, fuse and switch.

I have also been playing around with the NFB EQ to see if I can come up with a simple two pot version that gives top boost/cut on one control and bottom boost/cut control on the other. Results so far look encouraging - it just need a dual pot for each control. This means it should be possible to fit a mic pre, rotary fader, 2 band EQ and maybe even a pan pot into a single 3U module. More details when I have them all worked out.

Cheers

Ian

 

[ruffrecords]

Finished simulating the two pot NFB EQ. It seems to work OK. Attached is a schematic. The two 47K resistors R1 and R2, along with the 100uF/250V electrolytic C1 are existing parts of the second amplifier on the Eurochannel card. R1 and R2 determine the minimum gain which is 6dB. The EQ sits between the bottom of C1 and 0V. It can be switched in and out using S1.

The EQ is a variation on the simple Pultec EQ I discussed earlier. In this case the hi cut and boost have been combined into a single control (R3A and R3B) as have the lo cut and boost (R4A and R4B) both of which are dual 50K log pots. The nature of the EQ is such that both cut and boost are applied together. With the pots in the centre position and equal to 5K each, the boost and cut are equal and opposite and the response is very nearly flat (there is about a 0.5dB bump around 1KHz. The EQ IN/OUT switch S1 completely disconnects the EQ so when switched to out the response is flat. As each pot is turned clockwise from the centre position, the resistance increases and more cut than boost is applied. Similarly, as each pot is turned anti-clockwise more boost than cut is applied. Unfortunately this is the opposite of the way conventional cut/boost controls operate where turning clockwise applies boost. To fix this requires a pair of dual 50K REV LOG pots which are even harder to find than single rev log pots. A simple way round his is to apply the pot legend to the right of the pot instead of below it. The centre position is therefore with the pot knob pointing horizontally to the right. The pointer then move upwards for boost and downwards for cut. I'll draw apicture later to make this clearer.

The EQ uses equal value capacitors and inductor for hi and lo bands. The two inductors are both 1H. Rather than use an expensive Carnhill or Sowter I have ordered a couple of WIlco 1H inductors from Banzaimusic (thanks to Forth Monkey for finding these). I have no idea how good they are which is why I have purchased a pair. They cost 13.90 Euros each.

Cheers

Ian

 

[ruffrecords]

Here is a sim plot of the EQ response.

Cheers

Ian

 

[Holger]

Very nice idea.

 

[ruffrecords]

Thanks Holger. I have not yet worked out how to get LTspice to plot the hi and lo responses independently of each other as the previous graph shows, so here are the lo and hi responses separately:

https://www.dropbox.com/s/9h72loa5mbk0cgl/allalone.png?dl=0

Cheers

Ian

 

[ruffrecords]

I think I mentioned before how the 'Classic' board might be useful for the Lunch Box, especially as one board could be pressed into service as a pair of mic pres. I think I gave a basic descrition of the circuit before but never posted the schematic. Here it is:

As you can see, each amplifer is very much the same as the poor man's tube gain make up stage and you can configure each stage as a mu follower or as an SRPP as you wish. The board also includes the relay for an instrument input and places for a couple of mic input transformers and the usual phantom power decoupling circuit. There are no built in controls. These are all external to the board which gives it a lot of flexibility.

You can use both stages with a level control in between to make my original 'classic' mic pre design. Configured as mu followers, each stage has about 30dB of gain so with the 20dB in the transformer there is up to 80dB gain available - more than enough for ribbon mics. If you want a transformer balanced output then it is probably best to make the second stage an SRPP for better drive capability and follow it with the usual Carnhill 2400:600 transformer.

Cheers

Ian

 

[ruffrecords]

What prompted me to post the schematic of the 'Classic' was the arrival in the post this morning of some little PCBs to use with it. This is a new version of the 'four toggles' PCB. The original one was wider because it included the 48V decoupling circuit. However, this is already on the Classic PCB so you don't need it on the four toggles PCB.  Here's the schematic:

it has the usual phantom on/off switch followed by a 20dB pad in the mic input. Unlike the EZTubeMixer, where the next switch is the phase switch, in the four toggles board it is the mic line switch followed by the phase switch which allows you to change the phase of both the mic and the line input.

Removing the phantom decoupling  makes the board smaller so it is easier to fit two of them side by side in a standard 28HP wide module. I had 10 of these made at Iteadstudio.com for the princely sum of $15. Not only was I able to make the board smaller but I was able to squeeze two of them side by side into the 50mm by 50mm standard cheap PCB service board size offered by Iteadstudio. I even had enough room to include a dotted line so you can saw them in two if you want - so 20 PCBs for $15 - not bad. Here's a pic of the PCBs, one with toggles in place:

Cheers

Ian

 

[ruffrecords]

Here's a quick front panel layout I created of the Classic with the two knob EQ and four toggle board described earlier.

Cheers

Ian