C3g Aikido Phono

[Nick]

I would check the frequency response. 470u and 10r is at 33hz -3dB

[simon]

Good spot, I'd not even considered this. 2200uF would probably be better, starting to get quite large.

[simon]

I've done a great deal of prevaricating with this. I've been back and forth with single and parallel versions of PCBs for the Boozehound, even considered just building it point to point on perf board. Anyway I'm going to get a parallel version fabbed which I can use as a single first, then add a second.
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Boozehound MC Pre-Preamp Parallel2.png (189 KiB) Viewed 810 times

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I've drawn up the TubeCAD MC Pre-preamp too, this one's a stereo board because of the dual channel op amp used in the direct coupled version with DC servo.
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TubeCAD MC Pre-Preamp2.png (165.74 KiB) Viewed 810 times

[simon]

For power I was going to use 9V batteries, but I think I'll go with 4 no. 4R25 6V torch batteries. They should last for a very long time and allows me to use +12V and -12V for the TubeCAD, and +12V up to +24V for the Boozehound. I'm still thinking about how I'll implement this, but I'm thinking a box to put the 4 in, 2x2, with a switch to select +/-12V or 24V. I'm probably overthinking it and clipleads to get going will be fine!

A power connecter for the two rail supply is easy - a simple barrel connector will do. But any suggestions for a low voltage connector for 3 rails - +12V, 0V, -12V?

[simon]

I've also been playing with another all-in-one PCB for the phono itself in KiCAD, but I've done a few layouts and I'm not really very happy with any of them. And there's a risk I make it worse.

So I've drafted a smaller version of the RIAA filter board back in Eagle, hoping this might improve the small amount of hum that's going in to the Aikido stage. Nick picked up on the large pads I'd made so I could use different capacitors in the RIAA and suggested that they might be a bit of an aerial. Fingers crossed.

This is the new board:
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C3g phono rev 1 - RIAA.png (91.64 KiB) Viewed 809 times

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This is the existing:
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RIAA render final.png (116.56 KiB) Viewed 809 times

[simon]

The PCBs arrived yesterday so first job was to replace the RIAA filter board. Using my DMM on VAC (no point me trying to do it with a scope!) and it looks like there's 1mV less with the new board. The volume really does need to be all the way up now with no music playing to hear the hum. Nice.

Next job is to think about the parts for the MC pre-preamp boards. Or maybe ordering a new heater trafo from Toroidy to replace the current EI that buzzes/hums. Then I can box it up.

[simon]

A question for Nick. IIRC you mentioned when I popped round that you use ceramic caps to ground the input phonos - I used film as that's what I had to hand - are ceramics a better choice for this application? Does it make any difference?

[Nick]

Well, I may be wrong, but ceramic is generally used for RF as they have lower impedance than film for a given capacitance so better for HF.

[simon]

[simon]

Nuts. I bought a couple of these to switch the battery power on an off for the MC pre-preamps I'm playing with.

https://www.mouser.co.uk/ProductDetail/ ... 2F2g%3D%3D

Turns out they don't come with a locking nut... I didn't check the detail cos they always come with a locking nut, right? Wrong! According to the datasheet it's an M61 thread, whatever that it. M6 nuts don't fit. Google hasn't helped me so far, anyone know what a M61 thread is? Might avoid having to buy two more switches if I can find appropriate nuts.

[pre65]

It says bushing size 1/4 x 40 tpi, UNS I believe.

So, try these for size ?

https://www.ebay.co.uk/itm/204738273228 ... R5qWuLHcYw

https://www.gewinde-normen.de/en/unifie ... hread.html

[simon]

Brill, thanks Phil, I've been able to work it out with your help. £2.69 for two nuts hurts though!