Poor Man' s Tube Mic Pre Channel Strip (Eurochannel)

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ruffrecords

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I am working on a new design of tube mic pre that I hope is going to become a standard building block for my mixers. Two identical amplifiers, a mic input transformer and 12V heater regulator fit onto a single Eurocard which connects to the outside world via a nice meaty 32 way connector that we talked about on another thread. Each amplifier can have its gain set from 6dB to 40dB. The first amplifier plus the input transformer give up to 60dB mic gain and I am thinking of including tracking for a 1pole 12 way PCB mounting Grayhill switch for gain setting. The second amplifier has enough gain make up for the poor man's or posh man's EQ. I am currently using 1mm pins directly onto the PCB for the tube sockets as this saves having to mount tubes at right angles. This means the card can be made into a module 2.4 inches wide by attaching a suitable front panel and you can get exactly six of them into a standard Euro sub-rack. Mic gain and a simple EQ should fit on a 3U high panel but I have checked that a 6U front panel can fit without compromising the mechanical  strength so that would give plenty of room for a three band poor man's EQ. At present I have  been able to leave a 30mm blank  space on the PCB immediately behind the front panel so there is room for panel mounted PCBs. I have a rough amplifier prototype working on the bench but the PCB layout is not yet complete and I need to thoroughly test this before freezing the PCB. So, at this stage I thought I would just mention what I am doing and put out a feeler to see if anyone might be interested in the PCB when it is done.

Cheers

ian
 

leadbreath

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jesus ian, hows about i just give u my blank chequebook and u keep them pcb's coming my way :D

ive been thinking about a tube channel strip for a while now and here it is...

 

erikb1971

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pfew! I am glad he's doing a mic pre first... don't need that one... and my hands are full with the posh men eq... hope to be ready by the time Ian presents his poor men's fairchild :)
 

0dbfs

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Yes this looks like a very cool channelstrip platform.

I have looked into the eurocard / subracks before as an I/O platform but they always seem so pricey.

Any specific sources for subrack components (rails, chassis, guides, etc)?

Thx,
jonathan
 

ruffrecords

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0dbfs said:
Yes this looks like a very cool channelstrip platform.

I have looked into the eurocard / subracks before as an I/O platform but they always seem so pricey.

Any specific sources for subrack components (rails, chassis, guides, etc)?

Thx,
jonathan

The cheapest are probably the ECO range by Rittal available from Farnell.

http://uk.farnell.com/rittal/3688116/subrack-eco-6u-175mm-84hp/dp/1198863

who also do the more expensive ones by Schroff.

Cheers

ian

 

brianvino

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there is an italian distribuitor very cheap www.Armadi-Rack.it
 

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brianvino

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http://www.armadi-rack.it/reparto/cat1-cestelli_subracks/cat2-subracks/rack.htm
 

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ruffrecords

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brianvino said:
there is an italian distribuitor very cheap www.Armadi-Rack.it

Thanks for the link. They seem to be similar prices to the Rittal ones. It is always goog to know there are alternatives sources of supply.

I see from the pictures you have been very busy. What you have already achieved is similar to what I plan to do. I notice your motherboard layout uses two row connectors but in the pictures you have three row connectors. Is there a reason for this?

Cheers

Ian
 

brianvino

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accessory price are very cheap.
i have made 2 project.
on 3 U use one conn row and on 6 U use two conn row
 

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ruffrecords

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brianvino said:
accessory price are very cheap.
i have made 2 project.
on 3 U use one conn row and on 6 U use two conn row

I think my question was poorly expressed.

I understand you have one connector on a 3U sub-rack and two connectors on a 6U sub-rack.

I was asking about the connectors themselves. On your motherboard the connectors are 32way and have two vertical columns of 16 pins. In the pictures, the connectors I see look like 48way or 96way types with three vertical columns of pins.

Cheers

Ian
 

ruffrecords

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I have finished the first PCB layout and sent it of to the manufacturer to make some prototypes which I will then build and test. I have made a general pin assignment for the 32 way connector which I'll publish later. I have also added a few extras for variations such as a Grayhill 1pole 12 way gain switch, line input, a relay to switch a HI-Z input straight to the grid of the first tube and separate outputs from each of the line amp stages so it could also be used as a passive mix gain make up amp. PCB layout pic attached.

Cheers

Ian
 

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brianvino

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ruffrecords said:
I was asking about the connectors themselves. On your motherboard the connectors are 32way and have two vertical columns of 16 pins. In the pictures, the connectors I see look like 48way or 96way types with three vertical columns of pins.
Cheers
Ian
OK
connectors are 32 + 32 + 32 pins max,  but 16 + 0 + 16 active only
 

ruffrecords

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brianvino said:
ruffrecords said:
I was asking about the connectors themselves. On your motherboard the connectors are 32way and have two vertical columns of 16 pins. In the pictures, the connectors I see look like 48way or 96way types with three vertical columns of pins.
Cheers
Ian
OK
connectors are 32 + 32 + 32 pins max,  but 16 + 0 + 16 active only

OK, I understand. Thanks.

Ian
 

ruffrecords

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I have built the first prototype and I am pleased to say it appears to be performing well. I made a couple of minor mistakes on the PCB layout but nothing earth shattering. Initial tests were encouraging.

Frequency response at 40dB gain into a 2k7 load was -1.5dB at 20Hz (mainly due to the 4u7 output cap) and then flat out to 100KHz.

At +26dBV into a 2K7 load (20Vrms) and 40dB gain, the distortion products were 2H @ -60dB, 3H @ -60dB, 4H @ -77dB and 5H @ -80dB, all others being to low to measure, so overall distortion is less than 0.2% and mostly second and third harmonic.  The distortion will be lower at lower levels and should be lower at lower gains. With a 2:1 transformer attached it should happily drive +20dBu into 600 ohms. (note to self; I wonder if I could use the Neve style 1:2 types backwards?).

Output noise at 40dB gain with the input shorted was 0,6mV on my Ferrograph test set which is about -62dBu. With a 1:10 transformer at the input this implies an EIN of around -122dBu - not bad but could be better.

The Sowter mic trannys arrived the other day so I have been able to test the other amplifier. With the input transformer fitted the measured gain using my Lindos test set  was 61dB and with the input shorted the output noise (quasi peak) was very close to -63dBu which gives an EIN of -124dBu.

Picture attached.

Cheers

Ian
 

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