A tube tracking console using ITB’s MkIII concepts

[rjuly]

Power supply mainboard assembled - that's HT and 48v:



and the ugly truth...



- Richard

 

[rjuly]

Making some progress with the power supply...



I am currently (no pun intended) wiring my grounding scheme, following the guidelines laid out in Ian's document entitled "Mixer Grounding 101" referenced from the DIY section of his customtubeconsoles.com website, found here:

https://drive.google.com/file/d/0B_...=sharing&resourcekey=0-RWZOzM7DCpk5RgwkKKZjCA
In that document Ian specifies that the 0v HT PS output should be connected (at the output) to the ground bolt (marked 'A'). Presumably the 12v is the same - 48v gets tied uptream as described elsewhere in the document.



I am wondering if it is the same thing to tie the 0v PS output to my bus bar, and then also tie the HT 0v pin of my DC power connector at a different point on the bus bar, or whether both conections should be made originating from the PS output as shown in the diagram.



..Likewise for the 0v connection of the 12v PS.

- Richard

...P.S. This project is kicking my A5$! Logistically challenging and a steep (for me) learning curve. It is very rewarding however. I have had some very long lead times for components and am still waiting for my HP amp transformers Sowter and my Sifam meters from Canford. I have most of the other stuff, and should hopefully be able to make some good progress building the monitor section once the power supply is finished.

 

[rjuly]

And now for something completely different...

Following up on your comments (Ian) about the HPF section. You said:

...The 1:10 mic input transformer usually is fitted with a 150K load which is reflected to the primary as 1500 ohms. So for a HPF all you need do is insert the right value capacitors in the hot and cold leads. For a 100Hz turnover, the total capacitance needs to be about 1uF which means you put about 2uF in each leg. 2.2uF in each leg will give a 96Hz -3dB point and 4.7uF in each will give a -3dB point at 45Hz. 11uF in each leg will give a -3dB point at 212Hz.

I take it this configuration is an example of the half section ("L") constant-k topology of this type:



I had wondered if it might be possible to implement the full "T" version using an inductor, as in this circuit from the schematic of a neve-style" eq clone:

... or whether this would impose too great of an attenuation for my single amplifier block design?

- Richard

 

[rjuly]

Power supply update:

48v dc out = ~50v vdc (good)
12v dc out = 11.58 vdc (good)
HT+ = 383vdc (hmmm...)

For the HT+ this seems a bit high - I expected something on the order of 330vdc. When I checked the transformer outs it came to 282 vac which is a fair bit higher than the 240 I specified when I ordered it. I am wondering if this will be workable...

- Richard

 

[ruffrecords]

In answer to your first PSU question, you do not need a bus bar. You should wire it exactly as in my diagram. So you wire the dc outputs direct from the PSU ouputs to the connector. You run a lead from the IEC connector ground pin straight to the chassis right next to the connector (the BOLT in my diagram). You take a single wire from the HT0V ouput of the PSU to the BOLT. If the PSU is external you also take a wire direct from the chassis pin of the dc power socket to the BOLT.

In answer to to383V HT output this typical for the unloaded output voltage. It is not a stabilised supply. You will need to adjust the series resistors for the actual load of the mixer.

Regarding the HPF on the mic input, the circuit you posted is such a filter but it is not really appropriate for fitting to the input of a mic pre. Simply adding a single series cap on the input will work but anything more complex is going to be very difficult to implement without compromising the noise performance of the mic pre.

Cheers

Ian

 

[rjuly]

So be it! Thanks for taking the time to help me.

- Richard

 

[rjuly]

Power supply re-jigged with IEC gnd pin, external chassis gnd, HT 0 and 12V 0 (but not 48 volt 0), going direct to BOLT :



Here is big ol' BOLT:




- Richard

 

[rjuly]

In preparation for beginning the prototyping for the mic channel pcb, I have been reading through the Mark 3 blog and the Mark 3 thread, to see what changes may have been made to Ian's original mic pre circuit design upon which the Mark 3 is based - the one described in the Part 2 section of the Design Philosopy page on your customtubeconsoles site, and (I am assuming) the same design shown in the EasyTubeMixer project as the EuroBoard - Mic Pre schmatic:


This circuit features an SRPP design which varies "the gain of the first stage at the same time as the NFB is changed such that the amount of NFB remains nearly constant and stability is never an issue".

I have read through the blog and from what I can see the changes have been primarily to the physical layout - the single channel 35mm width, vertical tube orientation, use of relays, use of headers for the front panel connections, moving the bus switching and solo circuit logic to a dedicated board, etc.. I wonder if I missed any significant changes mark 3 mic pre circuit design itself as shown on the upper half of this recent photo from the blog?:



From what I can see it looks basically the same as the original.

Ian - If there have been changes, is there a schmatic you might make available, or am I good to go with the 2012 drawing?

- Richard

 

[ruffrecords]

Very little has changed in the design of this amplifier.

1. Removal of the Hi-Z relay at the front because nobody wanted it.
2. The change to 250V HT has allowed higher value output capacitors to be used in the same space
3. Improvements in electrolytic capacitors means you can now buy a 22uF capacitor that will fit where the 10uF first stage HT decoupler fits.
4. The gain setting resistors and its coupling capacitor have been swapped. The capacitor is at the top and the resistors connect between it and gound. This is so the gain setting network can be 0V referenced. THe board includes a 1Meg resistor from the cap to ground so it will charge /discharge if no gain setting network is present. This change also means the junction of the cap and the 1Meg can be used a virtual earth mix point if the main input is shorted to 0V.
5. The change to 250V HT means you no longer have to elevate the heaters so one side of the heaters can be grounded at the power supply. This also makes it much safer to use the 12V heater supply for ancillary circuits like relays

Other than that the basic schematic is the same.

Cheers

Ian

 

[rjuly]

That’s great Ian - many thanks for outlining that for me.

- Richard

 

[ruffrecords]

Just to update my previous reply, the changes are all documented in the latest Twin Line Amp configuration document (attached).

Cheers

Ian

 

[rjuly]

I haven't been able to do much on this project lately as I have been moving house. In temporary digs in gloucestershire while waiting for a new house to built up in Arran. Hopefully can get some fabrication done over the next months with a less than ideal work space...

Thinking about getting power into the chassis. I was thinking to have the dc connection enter the chassis perpendicular to the planes of the daughter channel and monitor circuit boards, with the wires from the connector going directly from the side of the chassis, underneath and parallel to the power traces on the motherboard, connecting at the center for equal distrubution. If I understand it correctly this should give me the least interferance with those signals that travel at a relative 90 degree angle.

- Richard

 

[ruffrecords]

If they are all dc then they cannot interfere. I think this is something you do not need to worry about. I place the dc in connector on the rear panel and route the wires to the centre of the motherboard. Now, if you were using AC heaters it would be a different story.

Cheers

Ian

 

[rjuly]

Many thanks Ian - I sometimes overthink things... I don't think I have room for the power connector on the back at this point Here is the rear panel assembled for the first time today:


- Richard

 

[ruffrecords]

Yes, no room to spare there.

Cheers

Ian

 

[rjuly]

Looking now at the headphone outputs... I see from your HP amp design that you have placed a 32R resistor across the (presumably) 30-100 ohm secondary of the sowter 8665a. In using your design I am opting to have a switch that selects beween the three secondaries for different headphones. Would I then place a resistor across all three secondaries with proportionate values - ie a 100R and 250R for the other two, and all of that prior to the switch?

- Richard

 

[ruffrecords]

The 32 ohns shown in the schematic is meant to represent a load. It is not a resistor you need to fit. The headphone amp will drove 3V rms into a 32 ohm load which is 300mW. This is more than enough to deafen anyone and represents about +25dBm. For many modern headphones this is overkill and you can get away with using a standard TwinLine Amp with the Sowter transformer. If you definitely have 32 ohm phones and want to save a bit of money, Edcor used to do one of their WSM series with a 32 ohm output but cannot find it on the present web site. (you can see them in the picture of the prototype on my web site).

Cheers

Ian

 

[rjuly]

The 32 ohns shown in the schematic is meant to represent a load. It is not a resistor you need to fit...

Ah, I see. I had thought it was, having mistaken your bench photo from the ez mixer blog showing the the 8665 with a resistor across the secondary. I guess it was fitted for test purposes.

I have several pairs of heaphones ranging from 11ohm IEMs up to 600 ohm open back beasts. I'm definitely looking to take advantage of all three secondaries and as much headroom as I can muster. I will be using the 'improved' headphone amp circuit which looks to fit the bill nicely.

I understand that I don't need to fit any resistors across any of the three secondaries. Many thanks for the information

- Richard

 

[Stack]

Here is a rough mockup of a possible module layout. I have increased the dimensions to 110mm depth and 311.15mm tall (7RU). Components are much smaller in my imagination!

View attachment 83816
Not a great deal of space given that I will need to find room for an input and output transformer, and two insert transformers, plus the rest of the circuit components.

- Richard

I'd love building a hybrid tube & matched transistor recording console, without any doubt whatsoever ! the drums or at least the transient producers require as fast a slew rate I'd suspect as default to quick responses....I'd need at least a mix control between 'em so I could color that sound if required....As a drummer, I speak of the fastest replies from any electronics at all. For most others, I'd go with the softer and slower speeds, signal processing wise, as there's real value there in that the mid range build up levels are lower and more even to boot ! Rupert Neve's summing mixer with the machine control to automate the levels, lacking, seems a viable mixing option for in the box mixing. makes little difference to me howo it gets mixed, only that it's done with a minimum of any amplifier slowing or mucking it up too much....This idea really fascinates my brain in a very positive way.....keep up the good work, eh ?

 

[rjuly]

Making a bit of progress - fitted HP trafos, test fitting circuit boards and getting placement for the motherboard.





- Richard