Feeler: EZ Tube Lunch Box

Hi Sven,

Transformer size is determined by the total VA it has to supply. Here is your specification with the required VA added:

I: HT -> 240VDC (300mA)      72VA
II HTR -> 14VDC (2A)                28VA
II: 48V -> 50VAC (20mA) <-?  1VA
IV: Utility -> 14DVC (>= 1A)    14VA

Which totals  115VA. Before going any further we need to check these figures especially as the total VA is dominated by the HT. In fact, each PCB draws about 15mA. Since we can have a maximum of four PCBS the total HT draw will be no less than 60mA. The other thing is that the nominal HT voltage is 300V. So the HT VA becomes about 18VA. This reduces the total to about  61VA. These figures are not quite right because we need to take account of rectifier efficiency but if we allow 80VA then we shall have a reasonable margin for error.

A typical 80VA toroid transformer is just under 90mm in diameter and just under 45mm thick.. Since we have 2 modules width to fit the PSU into I think the PCB will easily fit into one width and the transformer into the other.

The one thing that concerns me is where to fit the utility supply. I certainly will not fit on the Eurocard PCB. Maybe we can fit it on its won little PCB beside the transformer

I have to admit I am puzzled why you think you need so much power from the utility supply. What are you thinking it will power?

Cheers

Ian

Hey Ian,

Before going any further we need to check these figures especially as the total VA is dominated by the HT.

Click to expand...

How? I would like to start asking some people/companies for a custom transformer with the specs to see where we are in terms of size.

The one thing that concerns me is where to fit the utility supply. I certainly will not fit on the Eurocard PCB. Maybe we can fit it on its won little PCB beside the transformer

Click to expand...

Good idea! I think building a small Utility PCB's shouldn't be a problem. I can start with a 1st iteration tomorrow and update my 3D model.

I have to admit I am puzzled why you think you need so much power from the utility supply. What are you thinking it will power?

Click to expand...

Christmas is just around the corner!

My simple math was based on some LED's (typically 20mA) and Relays (around 10mA based on some formulas i found to get the rated current from the rated voltage and the coil resistance of a typically relay).

Let's start an simple example:

(per channel)
Status Led for Mic/Line, 48V, Polarity, Pad = 4 x 20mA
Led (lamps are much higher) for vu meter = 1 x 20 mA
Some switching relays pre/post, insert whatever let's say = 4x 10mA

= 110mA

x 4 channels = 440mA
+ headroom x2 = 880mA

Maybe it is based on a personal preference, but in total the cost for physical switches is most of the time higher as a simple relay, a few components and a nice status led. Take a normal RAFI switch -> around 14€ with a led. And the feature set for a channel in my head not ends here. If i want grouping options inside a channel. Another 4 indicator leds are required.

Does it make sense?

Cheers, Sven

"Do not put off till tomorrow what you can do today."



There really isn't anything more simple or?
It's a 30x50mm pcb.

Cheers, Sven

dipfrik said:

Does it make sense?

Cheers, Sven

Click to expand...

I understand your arithmetic. What I do not understand is how you are going to squeeze a VU meter, four relays, four LEDS and the extra switche(es) into a 3U module along with my PCB??

Cheers

Ian

dipfrik said:

There really isn't anything more simple or?
It's a 30x50mm pcb.

Cheers, Sven

Click to expand...

Looks good to me. You will probably need to add a heatsink for the 7812.  Does the utility supply have to be regulated??

Cheers

Ian

I understand your arithmetic. What I do not understand is how you are going to squeeze a VU meter, four relays, four LEDS and the extra switche(es) into a 3U module along with my PCB??

Click to expand...

I thought it would be comfortably not to think about the power of a the psu module if you want e.g using not 4 additional modules with each 14HP width but also building only 2 modules again both with 28HP width exactly like the psu module. Or a diffferent setup like your Mark 3 sub rack. Or something like that:

------------------------------------

6U  6U  6U  6U  (patchbay)

  (4x 14HP)          --------------

                                    3U PSU (28HP)

------------------------------------

In my mind there a lot of different setups possible without re-thinking the psu module.

Looks good to me. You will probably need to add a heatsink for the 7812.  Does the utility supply have to be regulated??

Click to expand...

I never built an unregulated one.
For the heatsink this one should be fine.



Cheers, Sven

dipfrik said:

In my mind there a lot of different setups possible without re-thinking the psu module.


Cheers, Sven

Click to expand...

You are absolutely right. For this reason I have increased the HT supply current to 60mA. This will power 4 x 6U modules plus a TLA under the power supply and a couple of VU meters - four channel 2 bus mixer in a 6U rack with PSU - wonderful!!!

I have asked Terry (toroidman) of Canterbury Windings to prepare a quotation for a custom transformer. The specification I gave him is:

Primary 0-115, 0-115 @ 50/60Hz for series/parallel operation
Electrostatic Screen
Sec 1 240V @ 60mA
Sec 2 50V @ 60mA
Sec 3 14V @ 3A
Sec 4 14V @ 1A
GOSS band

He has already replied with his quote for:

Power rating: 74VA
Primary: 2 x 0-115V @ 50/60Hz for series/parallel operation
Electrostatic screen
Secondaries: 0-240V @ 60mA rms + 0-50V @ 60mA rms + 0-14V @ 3A rms + 0-14V @ 1A rms
GOSS band
Regulation: approx 8% on 60mA sec’s & approx 12% on 14V sec’s
Estimated dimensions:  93mm diameter away from leadouts x 53mm high
Mounting: M6 clearance hole in a potted centre
Leadouts: approx 200mm long flexible (stranded)
Audio grade construction

I won't publish his prices but he gave me 1-ff, 5-off and 10-off prices. Maybe you would like to get a quote from your own favourite supplier and we can compare prices by PM.

Cheers

Ian



Diameter preferably not more than 90mm

Thickness preferably not more than 55mm

I won't publish his prices but he gave me 1-ff, 5-off and 10-off prices. Maybe you would like to get a quote from your own favourite supplier and we can compare prices by PM.

Click to expand...

I sent out an email asking for a quote here in germany.

You are absolutely right. For this reason I have increased the HT supply current to 60mA. This will power 4 x 6U modules plus a TLA under the power supply and a couple of VU meters - four channel 2 bus mixer in a 6U rack with PSU - wonderful!!!

Click to expand...

Yeah! This would be marvellous!!
Short question: How you increased the HT supply current to 60mA?

Cheers, Sven

Hey Ian,

we talked about the idea of self contained 3U/14HP modules in previously posts and i started to think this one step further based on the Helio Type 69 EQ.

My initial assumptions are based on the following ideas/goals:

1. Input transformer 10k/10k (Sowter 3675)
2. Output transformer 2k4/600 (Carnhill or Sowter)
3. Gain make up based on the 2nd Stage of the amp card (2 tubes)
4. 2x Inductor for MID/LOW (VTB9042/VTB9043)

...next ones are based on sufficient space for switches/pots on the frontpanel

5. 3 vertical mini pcbs's which are holding the caps for lows/mids/highs
6. Grayhill switches are on "adapter" pcb's which are connected to the pcb over idc connectors with ribbon cables

I put everything together in a 1st 1:1 3D model.

What do you think?

Cheers, Sven

dipfrik said:

Short question: How you increased the HT supply current to 60mA?

Cheers, Sven

Click to expand...

I just realised our transformer is not specified correctly. Here is a link to my document on HT supply design:

http://www.ianbell.ukfsn.org/EzTubeMixer/docs/power/HTdesign.pdf

Basically I think this means we need to multiply all the transformer currents by 1.61. I will check this out and make a new specification. Transformers can be a PITA.

Getting 60mA HT current is just a matter of changing the transformer specification. The HT supply is a very simple CRCRCRC circuit. There is no regulation. It is the same circuit as used in the HT350 PCB but the capacitors are reduced to 220uF. Two things determine the ripple on the HT supply: the current draw and the size of the capacitors. On the HT 350 we have 470uF capacitors. In the lunch box PSU we have 220uF capacitors with 220R series resistors . I have simulated the lunch box PSU with a 60mA load and there is 20uV rms ripple at the output which is close to -92dBu. The EZTube output stage rejects this by about 35dB so it will be below -120dBu at the output. We could probably draw a lot more current from the HT supply before we had to worry about ripple.

Cheers

Ian

Basically I think this means we need to multiply all the transformer currents by 1.61. I will check this out and make a new specification. Transformers can be a PITA.

Click to expand...

no problem - I agree - PITA!
Hopefully we can stay once again in the physical dimensions.

Getting 60mA HT current is just a matter of changing the transformer specification. The HT supply is a very simple CRCRCRC circuit. There is no regulation. It is the same circuit as used in the HT350 PCB but the capacitors are reduced to 220uF. Two things determine the ripple on the HT supply: the current draw and the size of the capacitors. On the HT 350 we have 470uF capacitors. In the lunch box PSU we have 220uF capacitors with 220R series resistors . I have simulated the lunch box PSU with a 60mA load and there is 20uV rms ripple at the output which is close to -92dBu. The EZTube output stage rejects this by about 35dB so it will be below -120dBu at the output. We could probably draw a lot more current from the HT supply before we had to worry about ripple.

Click to expand...

Getting the 60mA HT current by changing the transformer specs makes sense.
The rest of your explanation reached definitely  my actual level of knowledge -  i'am lost but will start some research to dive deeper into psu design principles

By "simulated" means with software like Spice or?

Cheers, Sven

dipfrik said:

Getting the 60mA HT current by changing the transformer specs makes sense.
The rest of your explanation reached definitely  my actual level of knowledge -  i'am lost but will start some research to dive deeper into psu design principles

By "simulated" means with software like Spice or?

Cheers, Sven

Click to expand...

Yes, I use LTspice. It is free and works well.

I have been looking again at the heater power supply design. When I first decided to have it outside the lunch box, one reason was the heat dissipated in the heater regulator raised its heatsink to what I considered to be too high a temperature. Part of this is due to the voltage dropped across the regulator chip. For the LM338 it needs to be about 5V so when we take 2 amps for heaters we dissipate 2 x 5 = 10 watts in the heatsink. If we could lower the voltage across the regulator chip we could reduce the dissipation and also the transformer secondary voltage. I have had a look at the LT1084 low drop out regulator. This is guaranteed to regulate with just 1.5 volts across it. If we allow it 2V and another 2V drop in the rectifiers this means the peak ac input voltage needs to be 12.6 + 2 + 2 = 16.6V. If we used a 12VAC secondary we would get 12 x 1.414 = 16.97 volts. This would allow us to raise the transformer current without it getting much bigger. I need to check the figures and there also seems to be some confusion about the availability of this part and also of its packaging. I might just buy one from Farnell just to see what it actually looks like. I don't want to design in a part that people cannot buy!!

Cheers

Ian

Cheers

Ian

Working with only Carnhill Transformers is much more challenging

10k/10k is a VTB 9071
2k4/600 is the new one based on the VTB 1847



Cheers, Sven

@Sven,

In your first post about an EQ module with built in amp you said a 12HP module. I think you mean 14HP. The tubes will not fit in 12HP. I think the output transformer is too big. There will also be a couple of big electrolytics in the amp somewhere.

Cheers

Ian

Oh sorry, i did this mistake two times over the last posts.
I updated the posts 3U/14HP OR 3U/28HP.

The 3D model was based on the correct dimensions.

So you think it is not possible? Not possible at all in 3U/14HP?
I would give it a try.

Cheers, Sven

dipfrik said:

So you think it is not possible? Not possible at all in 3U/14HP?
I would give it a try.

Cheers, Sven

Click to expand...

It needs further thought. You might be able to fit in all the components but that does not mean you will be able to track it.

Let's start with the two tubes. They get hot so they should not be so close together or vertically one above the other. Let us put one near the top and one near the bottom of the module. Then put the other amplifier components between them. There is a big electrolytic for the output cap, another for the gain setting, a couple of orange drops (one at the input and one between stages) that will fill this space.

Next let's look at the switches. Here is the 3U Helios EQ PCB layout:

[img}http://www.ianbell.ukfsn.org/EzTubeMixer/docs/EzTubeMixer/Helios69EQ/3UHelios69PCB.png[/img]

There are a lot of connections between the switches and the capacitors so it makes sense to keep them close together.  There are only 6 connectorins to the inductors and another four to the pots and switches. You could cut off the inductors and make a PCB about 50mm deep with the three switches and all the RC components plus a single  12 way IDC connector to carry the inductor,pot and switch signals to the main board. You could then fit this PCB above the main PCB. Directly underneath this PCB, on the main PCB, you could fit the two inductors and the two pots. You also need to find room near the front for the gain trim pot so it can be adjusted from the front panel.

In the space left between the tubes and inductors there should be plenty of room for the input transformer. The only thing I am not sure about is the output transformer but personally I see no reason why this should not be fitted to the rear panel along with the input/output connectors.

Cheers

Ian

I have recalculated the power supply transformer requirements with an LDO regulator for the heaters as follows:

Assuming we want to be able to power a maximum of 6 modules:

1. Heater supply: 12V at 0.45 x 6 = 2.7 amps.  A 22000uF smoothing cap will have about 1.2V peak to peak ripple. Allowing 1.8V for diode drops, 1.5V for regulator drop means we  need 16.5V peak from the transformer. A 12VAC winding will provide 17V and give us a little margin. Transformer current Iac  is Idc x 1.6 -= 2.7 x 1.6 = 4.32 amps

Transformer spec: 12VAC at 4.3 amps ( about 52VA)

2. HT supply. For 6 modules we need a maximum of 90mA (15mA x 6) at 300V. We lose about 40V in the CRCRCRC smooting network so we need about 340V ant the input. This means the peak transformer voltage is 340 x 0.71 = 240VAC. Transformer current Iac is Idc x 1.6 = 90ma x 1.6 = 144mA.

Transformer spec 240VAC at 140mA (about 35VA)

3. Phantom power. I usually use a 50VAC winding for this to get about 70V input to the regulator. Assume a max of 6 mics at 7mA each = 42mA.  Iac = Idc x 1.6 = 42 x 1.6 = 67mA (say 70mA)

Transformer spec: 50VAC at 70mA  (about 3.5VA)

Total VA = 52 + 35 +3.5 = 90.5 so it looks like it will need a 100VA transformer. Typical 100VA toroids seem to be about 91mm diameter and 46mm tall.

If you add the utility winding and also us a LDO regulator then you need another 12VAC winding at 1.6 amps which adds 19.2VA for a grand total of nearly 110VA.

I have looked at the data sheets of some example toroids at different VA ratings. Typically:


We can probably fit the 91mm or 92mm diameter toroids in but the 95mm might be just too  big for the Fischer cassette. I have measured the HB type cassette and it is exactly 96mm tall inside so we have very little room for error.

I then looked around my workshop for real toroid examples. I found a 50VA one and a 60VA one. They fit into a 14HP Fischer cassette easily. I eventually found a 100VA one and I was pleased to find this fitted also without much trouble. It seems the diameter specified includes the wires which are all one one side so the diameter is less if you rotate the transformer a bit and it fits easily.

I also checked that the big 22000uF capacitor and the 512AB heatsink will also fit into a 14HP module. SO, with a 28HP module we should have room for the transformer and the PCB.

I will send the new transformer specification to toroidman for a quotation.

Cheers

Ian

Hey Ian,

great! I will send the updated sepcs to my guy too.

Thanks, Sven

I think the main missing link is a self contained EQ module with its own input transformer and gain make up amplifier and (possibly) output transformer.

Click to expand...

After a few nights of moving boxes in 3D in a 3U module (14HP) with a one eurocard deep pcb (100mm x 160mm), i'am struggeling still with the spacing. The goal was putting input and output transfomer inside the module. With Carnhill transformers there is a lot of space gone, and we need to put a few things on "min pcbs" which can be added vertically.

Ian had some good thoughts, but again i struggled with the available space. So i thought about the options again and was going back to my original placement solely based on sowter transfomer. But this was kind of deluding myself, because there is no 2K4/600 for the output available (10k/10k exists with the 3293 or even better specs the 3575)

Why not ask Sowter?

I'am happy to say, that Brian Sowter himself responded very quickly and told me that it is no problem to build one with these specs and it would be in the the same size with pcb mounting pins as the input transformer. (pricing like the 4383 around 52 GBP ex. VAT)

So i ordered two of them now, and will process creating a "self contained" eq module eurocard pcb with one amp stage from the amp card (no 2nd 6V heater supply because we are not using the buffer amp design fom Ian) and input/output transfomers  based on two sowter transformers.

I will keep you updated with the process in the next days.

Cheers, Sven

@Sven,

Did you check with Brian Sowter what the maximum output level of the 2K4:600 output transformer would be?

I have made a simple sketch of the side of the cassette holding the toroid. I have assumed it is 110VA and is 95mm in diameter. I have had another quote from my transformer man and he says it is tight but  it should just fit into 95mm. I have also added in  the mains input IEC connector, an on/off switch and a fuse. For higher heater currents the ordinary bridge rectifer gets very hot and it does not have a heatsink. So I have shown a dotted outline of where a 25A bridge rectifier could be bolted to the cassette so it acts as a heat sink. I cannot find room for your auxiliary power  regulator PCB.

I have received a LD1084V from Farnell. It is in a TO-220 package,. I will connect it to the 12V supply I am building and test it out with 6 channels of heater load.

Cheers

Ia