[ruffrecords]

There has been a lot of interest in my 'Classic' tube mic pre design that forms part of the EZTube Mixer and Lunch Box projects. I started the Lunch Box project because building a tube mixer is a pretty big undertaking (ask Holger!). It occurred to me that many people might like an even easier introduction to the world of DIY tube mic pres so I decided to look at a single board version of the Classic pre using AC heaters and with on board HT and phantom power supplies. One PCB to build that would fit into a readily available enclosure. Add an output transformer, a mains transformer, a front panel and some sockets and you are in business.

So I have created a PCB layout which is 150mm by 100mm which includes the preamp and power supplies. This will fit lengthways in a Hammond 1402FV case with the output transformer beside the PCB and the mains toroid at the rear. XLRs could go at the front or the rear - both panels are aluminium and easy to work. The four toggle PCB can be used for phantom, pad, mic/line and phase switching or you can roll your own. I have even left a link between the mic transformer secondary and the pre input stage so you can fit a DI input if you wish. I pan to build one for myself to use in my little studio so let me know if there is any interest.

Cheers

Ian

Edit: Final project documentation in the classic solo folder: http://www.customtubeconsoles.com/diy

[maq3396]

Wow!

Do you ever stop working?

Cheers
Mac

[bernbrue]

Hi Ian,
I would love to have two or three of these plus the 4 toggle pcb. Thanks a lot.
Bernd

[ruffrecords]

Wow!

Do you ever stop working?

Cheers
Mac

I know what you mean but for me it is not work. I retired quite a few years ago which has given me the time to do what I always really wanted to do - design tube audio stuff. So Basically it makes me a happy bunny.

Cheers

Ian

[ruffrecords]

Hi Ian,
I would love to have two or three of these plus the 4 toggle pcb. Thanks a lot.
Bernd

Thanks Bernd,  I just ordered an initial batch of 5 prototype boards. They should be here in a couple of weeks or so. I have just contacted Aaron at Don-Audio about a custom mains transformers and I expect to have one of those in my hands in a few weeks also. I already have the Hammond case so with luck I will have a prototype running in about  a month.

Cheers

Ian

[bernbrue]

Hi Ian,
Thanks for the info. Take your time, I'm not in a hurry.
Best
Bernd

[Edward]

Hi,

If possible, I'd like two Classic Solo PCBs with the 4 toggle PCB.

Thanks!


/Edward

[erikb1971]

wow... a classic Ruffrecords  pre amp even doable for me.. I am in for one plus toggle pcb!

[anjing]

Good day Ian,
  I'll take one witout the toggles and i'll make a Ruff Di out of it!

Regards,

Pierre

[prh]

Ian, this looks like a wonderful project.  I would be interested in 2 each preamp and toggle boards.

Cheers, Paul Hunt

[Winetree]

I'd be in for 4 sets ( P.S.B and toggle )

[craig]

Hi Ian
Put me on the list for two when ready

Thanks

[ruffrecords]

Crikey, we're well into double figures already!! I got an email saying the prototype PCBs have been shipped. It's down to Hong Kong Post now. I have ordered the custom mains transformer from Don-Audio.

Cheers

Ian

[ruffrecords]

Prototype PCBs arrived yesterday. I will build one up while I wait for the custom mains toroid.

Cheers

Ian

[ruffrecords]

Just about finished building the first prototype PCB. Couple of small errors need correcting but otherwise looking good. Pic attached.

Cheers

Ian

[bernbrue]

Hi Ian,
Are there no tube sockets provided or is it just for the proto? Looks very good! Tell us when you are ready to ship the boards and what you get for them.
Best
Bernd

[ruffrecords]

Hi Ian,
Are there no tube sockets provided or is it just for the proto? Looks very good! Tell us when you are ready to ship the boards and what you get for them.
Best
Bernd

I don't use normal tube sockets because they add too much height. I am using the same technique as I use in the EZTube Mixer boards. I use 1mm gold pins that solder directly into the PCB. This make a very low profile socket that takes up very little space:

http://uk.farnell.com/harwin/h3161-05/socket-pcb-gold-knurled-1mm/dp/149319?ost=149319

Cheers

Ian

[ruffrecords]

I have now conducted some initial tests on the prototype PCB using a CInemag CMMI-10-PCA mic input transformer (1:10) and a Canford VTB2291 output transformer wired 2K4:600. The results are encouraging.

Maximum gain is 70dB with a 600 ohm load (1.5dB more unloaded) which is plenty even for ribbon mics.

With gain set to 60dB, the output noise is -60dBu which implies the EIN is about -120dBu - not bad for a tube mic pre - there is a little hum in the output which appears to be due to the ac wires from the power transformer travelling across the PCB. I should be able to imrove the routing and eliminate the hum when the proper transformers arrives.

With the level control fully anti-clockwise, the output noise is -80dBu with a 600 ohm load - again a satisfactory result.

Distortion at +20dBu into 600 ohms is 1% at 1KHz. This is a lot higher than the Eurochannel design but not unexpected and allows you to create plenty of 'tube sound' by over driving if you wish. At 0dBu the distortion into 600 ohms was close to 0.1%. I did not measure it but the unloaded distortion should be a lot lower.

Nest steps are to fit the main PCB  into the box, build a 4 toggle PCB and attach it to the front panel and connect it to the main PCB and retest.

Pic of bench test set up attached.

Cheers

Ian

[ruffrecords]

Taking a hint form all the hum problems I had with the lunch box, I got Aaron at Don-Audio to make me a custom mains toroid with extra screening for this project. It arrived at the beginning of this week so I thought I would first try it out and repeat the noise tests before assembling things into the Hammond enclosure. I first assembled the main PCB, the new mains transformer and the Carnhill output transformer on the bench and repeated the test I did earlier. The results were essentially the same with one exception; when I turned the gain right down to zero there was a small amount of hum in the output noise which was between -80dBu and -85dBu. I discovered this got worse as the mains transformer was brought closer to the Carnhill output transformer and my moving it away and rotating it through 90 degrees the hum disappeared and the noise dropped to close to -85dBu.

This is interesting because the screening of the mains toroid ought to be sufficient to stop this. I am now wondering if the small residual hum I heard in the lunch box test were due to the same cause - toroid to Carnhill coupling. One way to test this is to connect the output of the lunch box module nearest the mains transformer to a near and a far Carnhill output transformer and compare the hum. I'll keep you posted.

Cheers

Ian

[Holger]

Ian, I can confirm your findings. The Carnhills are a bit picky to 50 Hz. I've used the Sowter 2k4/600 for my latest tube summing amplifier and I'm happy with the results. IIRC the noise was at -90dBu or better.
I also have similar issues with the Krässemaschine, when I put some active speakers on the top I have a noticeable mains hum in the stereo bus.