audio-talk

That sounds like use a X8G then. That said I might revisit with all 1210 parts as even 1206 are a bit skinny.

I've just checked the parts, which was worth doing. C2, 6n2, is only available in 1210 as C0G 1%. It is available as 1206 1% in X8G or 5% C0G. Better to resize the pad for 1210 or is X8G adequate here?

R1, 1k92, isn't available, only 1k91. I imagine the extra 10 ohms doesn't make much difference ...

TBF I haven't fully scoped the parts, I looked at a couple of the caps and they were available in 1206 C0G 1%. I should probably do that, though if they're not available in 1% C0G there's probably not much choice in iRIAA anyway.
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I've added two more output GND pads as I'll probably just use two ...

I suppose SMT will be a tighter layout, but presumably it will largely be dependant on the accuracy of the parts used?

A 45 output valve is enough for me, a 300B wouldn't need to be fully driven to be more than loud enough. My "compromise" of efficient drivers helps here.

I think that was a reply to me rather than you Paul.

Okay so it might need a bit more voltage input that 1V, but everything's a compromise.

I haven't looked at any design so there may be more going on that I haven't considered.

Getting enough voltage swing is a problem with the d3a. Or at least I think that was the case.

It's so long since I've used 300B I had to look at the datasheet again. For the higher HT op of 350V the grid voltage is -74V. I seem to remember getting a gain of around 70 in triode, so close enough ...

Nick wrote: Fri Jan 10, 2025 9:33 pm Well, its old news, but I remember that high gm pentodes did a very good job of driving 300b's.

If I ever did 300B again I think I'd have to try a D3a

That's a good idea, thanks.

I'll probably buy a few of each component to match. It'll only be as accurate as my DMM likely isn't, but at least I can match the channels.

Okay, I'm not buying or selling but this seemed the best place to put this. Now I doubt anyone's interested in those funny old black disk things but if anyone would like a inverse RIAA filter I'm going to send gerbers off for a PCB based on the Hagerman iRIAA2 filter.

www.hagtech.com/pdf/iriaa2.pdf ...

Bienvenue JM.

This is a slightly different version to Dave's that I looked at last year. But yes, the electrolytic caps will be at least 23 years old so it's sensible to replace them all rather than just some. Dave's C1-C4 were okay but these clearly aren't. The amp was a bit of a pig to work on so ...

How good does the surface finish need to be before the first primer coat? Spotless, or do minor scratches fill okay?

A lot of messing then...

I've never used spray paint - how easy is it to get a good finish? And does the ally need priming first?