Mark 3 Vacuum Tube Mixer

[rjuly]

On your v1 backplane board I see that the DC pins of the multiconnector have connections for headers, similar to the ins and outs. I had thought that the DC might be on a bus, that would connect to all the connector on the backplane with a single connection to the power supply. What is the thinking behind having them accessable separately?

-Richard

 

[ruffrecords]

I was confused at first until I realised your photo is of the board upside down. There is a utility supply bused across the backplane. It is the unlabeled 2 pin Molex just below the OUT2 connector at the opposite end of the board. That is the one that should be labeled DC. The ones that are currently labelled DC are intended to be used as a fader return.

I would be worth you while reading this blog post which describes the derivation of the pin out:

The Ins and the Outs

From this you will see that not all the pins are assigned. Some are left deliberately unassigned to allow for different backplane PCBs for different mixer types.

Cheers

Ian

 

[rjuly]

Hi Ian,

Yes that makes sense - the fader return being that connection. As for the buses, there are six. I take it that the one labelled 5/6 is the one you referred to.



For my own purposes I can use 1/2 for the main stereo bus, 3/4 for the solo bus, and 5/6 for the dc power. Nice design of the backplane!

- Richard

 

[ruffrecords]

Hi Ian,

Yes that makes sense - the fader return being that connection. As for the buses, there are six. I take it that the one labelled 5/6 is the one you referred to.

View attachment 96463

For my own purposes I can use 1/2 for the main stereo bus, 3/4 for the solo bus, and 5/6 for the dc power. Nice design of the backplane!

- Richard

I did not explain my self well enough. If you look at the end of the motherboard opposite the one in your photo you will see an unmarked 2 way Molex just above the OUT2 connector. This is the DC bus. It is quite separate from 5/6. So you have a dc bus as well as six signal buses.

You should see that all the power is in the centre part of the board. It starts with the 48V, followed by the HT and heaters and lastly the auxiliary dc bus.

Cheers

Ian

 

[Gavin Pursinger]

Thx for sharing Ian..
I bet it sounds clean and open. The musicians you record with this MUST bring their in tune 'A' game MOJO...523.9 hz

 

[rjuly]

I did not explain my self well enough. If you look at the end of the motherboard opposite the one in your photo you will see an unmarked 2 way Molex just above the OUT2 connector. This is the DC bus. It is quite separate from 5/6. So you have a dc bus as well as six signal buses.

You should see that all the power is in the centre part of the board. It starts with the 48V, followed by the HT and heaters and lastly the auxiliary dc bus.

Cheers

Ian

Yes indeed... I have reviewed the details on your blog, and I am now clear. Many thanks for that.

- Richard

 

[rjuly]

The power supply is also under development. For heaters I no longer use home built linear supplies; I use MeanWell SMPS. They are smaller and electrically quieter than any linear supply. For small mixers using up to 100mA of HT I use the International Power IHB250-0.1 available from Mouser. The HT supply for the |Mark 3 is still under development. The basic plan is to use a Meanwell 300V/500mA SMPS which is then regulated down to 250V.

I am building a small power supply to use for my prototyping setup and would like to be able to power three channels of your standard 3 triode circuit. I thought to use something like the Meanwell SMPS for the HT (the International Power IHB250-0.1 seems unavailable) , and the 12v heaters and relays and leds. Could I use a similar pre-built supply for the 48v, or is it better to build one?

- Richard

 

[ruffrecords]

HI Richard,

For three of the three triode circuit you will need no more than 50mA of HT. Unfortunately I am not aware of a Meanwell low current HT supply or are you planning on using a dc/dc convertor?

The 12V heaters and LEDs can certainly be run off a Meanwell SMPS; just remember to rate it for at least twice the expected nominal heater current so it can cope with the heater inrush current. You will need something like a 3A type. Something like this would do:

IRM-45-12ST | Mean Well Encapsulated, Switching Power Supply, 12V dc, 3.8A, 45.6W | RS

although my favourites are their LRS series.

Meanwell also do some compact 48V SMPS that are fine for phantom power with a little extra filtering and a load to make sure they regulate properly. There was a thread here some time ago all about suitable SMPS for phantom power and how to use them.

There have been some odd problems with availability of some power supplies from places like Mouser. I think it may due to some CE marking problems which ought not to affect us any more.

Cheers

Ian

 

[rjuly]

Hi Ian,

For three of the three triode circuit you will need no more than 50mA of HT. Unfortunately I am not aware of a Meanwell low current HT supply or are you planning on using a dc/dc convertor?

I guess that was wisful thinking on my part given that I couldn't locate the IHB250-0.1

There have been some odd problems with availability of some power supplies from places like Mouser. I think it may due to some CE marking problems which ought not to affect us any more.

What I am seeing that purchase (presumably in the UK) is restricted due to 'government regulations', on Mouser as well as other distrubutors. If you know of a source, I would be grateful for a pointer. Alternatively, if there is another low current linear supply available - I haven't been able to find one yet. I may be reduced to asking a friend in the U.S to aquire one for me.

The 12V heaters and LEDs can certainly be run off a Meanwell SMPS; just remember to rate it for at least twice the expected nominal heater current so it can cope with the heater inrush current. You will need something like a 3A type. Something like this would do:

IRM-45-12ST | Mean Well Encapsulated, Switching Power Supply, 12V dc, 3.8A, 45.6W | RS

although my favourites are their LRS series.

Ah yes - the dreaded inrush current - thank you for that tip.

Meanwell also do some compact 48V SMPS that are fine for phantom power with a little extra filtering and a load to make sure they regulate properly. There was a thread here some time ago all about suitable SMPS for phantom power and how to use them.

I will have a look for that thread - that seems perfect. Once again, many thanks for your help.

Cheers,

Richard

 

[Brian Roth]

Maybe I should order one or two "just in case":

https://www.mouser.com/ProductDetail/International-Power/IHB250-0.1?qs=0xCm9DOQnC5apaiylMDitA==
Bri

 

[rjuly]

Maybe I should order one or two "just in case":

https://www.mouser.com/ProductDetail/International-Power/IHB250-0.1?qs=0xCm9DOQnC5apaiylMDitA==
Bri

I think they are available - they just won't sell them in to me here in the UK! :-(

- RIchard

 

[ruffrecords]

There is another thread i started some time ago all about HT supplies for small tube projects:

HT Transformers for Vacuum Tube Preamps

The most straightforward approach is to use a mains transformer followed by a rectifier and some CRCRC smoothing. My HT250 PCB is ideal for this and the thread will give you a range of transformer options.

Cheers

Ian

 

[rjuly]

...My HT250 PCB is ideal for this ...

Cheers

Ian

SOLD! Can you put me down for an PCB, Plz?

- Richard

 

[ruffrecords]

SOLD! Can you put me down for an PCB, Plz?

- Richard

PM me with your address and I will pop one in the post to you.

Cheers

Ian

 

[rjuly]

Hi Ian,

I was looking at your schematic for the phantom power circuit. Had a look through your blogs as well, but couldn't find the voltage rating for the 4700uf capacitor. Is it 400v? Those puppies are not cheap! Looks like I will need to buy two as I noticed that I had somehow mis-specified the one for my larger console phantom power supply build as a 470uf 400V cap.

- Richard

 

[rjuly]

Hi Ian,

I was looking at your schematic for the phantom power circuit. Had a look through your blogs as well, but couldn't find the voltage rating for the 4700uf capacitor. Is it 400v? Those puppies are not cheap! Looks like I will need to buy two as I noticed that I had somehow mis-specified the one for my larger console phantom power supply build as a 470uf 400V cap.

- Richard

I see in your Lunchbox schematic it calls for an 100v 2200 capacitor, so I guess 400v 4700 is unlikely. Would the lunchbox version be better suited to my 3-4 channel project?

- Richard

 

[ruffrecords]

I see in your Lunchbox schematic it calls for an 100v 2200 capacitor, so I guess 400v 4700 is unlikely. Would the lunchbox version be better suited to my 3-4 channel project?

- Richard

2200uF should be plenty. With three channels, even with the inputs shorted to chassis the current draw will be less than 50mA which would cause less the 300mV peak to peak ripple at the smoothing capacitor which the regulator will have no trouble removing. IIRC I used 330uF for the phantom supply in the Classic Solo design which is a single mic input. So for three channels, 1000uF should be quite sufficient.

Edit: which stacks up because the Lunchbox design is rated for 6 channels.

Cheers

Ian

 

[rjuly]

2200uF should be plenty. With three channels, even with the inputs shorted to chassis the current draw will be less than 50mA which would cause less the 300mV peak to peak ripple at the smoothing capacitor which the regulator will have no trouble removing. IIRC I used 330uF for the phantom supply in the Classic Solo design which is a single mic input. So for three channels, 1000uF should be quite sufficient.

Edit: which stacks up because the Lunchbox design is rated for 6 channels.

Cheers

Ian

Hi Ian,

So, just to make sure I'm clear... That will be 2200uf at 400v for the 3-4 channel version, and 4700uf at 400v for the 9 channel version using the 350 HT supply I built last year. I accidently put a 470uf/400v cap in there, but luckily I haven't had a chance to use it yet. I need to get the right cap in there!

 

[ruffrecords]

Hi Ian,

So, just to make sure I'm clear... That will be 2200uf at 400v for the 3-4 channel version, and 4700uf at 400v for the 9 channel version using the 350 HT supply I built last year. I accidently put a 470uf/400v cap in there, but luckily I haven't had a chance to use it yet. I need to get the right cap in there!

OK, this is 48V phantom power we are talking about. This supply needs its rectifier to be fed from its own separate 50VAC transformer. The voltage rating of the caps should be 100V not 400V. You only need 400V rating caps for the HT supply. (And I am not sure anybody actually makes a 2200uF 400V rated cap).

To sum up you need three supplies:

A) 250V HT which the HT250 will give you when fed from a suitable mains transformer
B) 12V dc for the heaters which a Meanwell smps can supply
C) 48V phantom supply which can either be a linear regulator type frod from 50VAC or a small smps.

Cheers

Ian

 

[rjuly]

Hi Ian,

Thanks for that clarification.

- Richard