audio-talk

Andrew wrote: ↑Mon May 18, 2020 5:11 pm

pre65 wrote: ↑Sun May 17, 2020 8:58 pm Can you remind me Andrew, do your boards have a trimmer for fine tuning the output ?

Yes, but I also keep saying trimmer pots are rubbish, and not to use them. Sadly, I am often ignored However, if it's a prototype then I'll excuse you.

I would add that putting the trimmer on the board is easy however getting it off again, not so much ... well for me anyway.

Nick wrote: ↑Mon Dec 28, 2020 6:18 pm I built one up for a 300b, for 5v out a few weeks ago, that needed 8.5v AC in before there was no dropping out on the scope on the low points of the input AC, 8.5v = 12v for no load, so that would put the drop at 7.5v, but that was with a 30va transformer feeding it.

Did you just feed in raw AC into the regs, or was it rectified and smoother to some extent ?
My impression is that you need about 4 or 4.2V headroom from the low points of whatever you feed into the regulators, in order for it to work.

No, I was trying to find what transformer I needed to actually use. Used the low forward voltage diodes on the board and the caps, so no extra smoothing. Connected output to 300b, input to the secondary of a 12v (it was a 120va transformer now I remember), then fed the primary with a variac. Scope across the 300b filament and adjusted the voltage so it wasn't dropping out on the low parts of the rectified ripple.

So yes, I agree with your impression as I would expect.

izzy wizzy wrote: ↑Tue Dec 29, 2020 10:34 am

Andrew wrote: ↑Mon May 18, 2020 5:11 pm

pre65 wrote: ↑Sun May 17, 2020 8:58 pm Can you remind me Andrew, do your boards have a trimmer for fine tuning the output ?

Yes, but I also keep saying trimmer pots are rubbish, and not to use them. Sadly, I am often ignored However, if it's a prototype then I'll excuse you.

I would add that putting the trimmer on the board is easy however getting it off again, not so much ... well for me anyway.

I bought an excellent solder sucker made by Engineer (SS-02) which really helps. I have a few tools made by the same company and all are excellent.

I recently bought a bench power supply and last night I tried using it to feed the input to one of Andrews excellent DC boards. Partly because it makes my bench less messy than transformers and wires and partly because I wanted to set the voltage going in. I found that a 6.3v output drawing 0.315A required the input voltage to be set at 11v.

When I build my phono stage it will be a 2-box design with PSU and stereo amp. I'm considering putting transformers for the heaters in the PSU with pre-regs and feeding DC through the umbilical. Couple of questions for my learned friends...

1. Any downside to feeding the boards with pre-reg mostly DC supply instead of AC ?
2. Any downside to placing pre-regs the other side of an umbilical in a separate box? I remember Andrew advising to keep the boards themselves fairly close to the valves for DHT valves as the output is the cathode, but the pre-regs can be distant right?
3. Can the boards share inputs from the same pre-reg, subject to current draw constraints?

1. No, other than the bridge and caps at the front are unneeded
2. No, maybe two ceramic caps on the inputs to the reg to chassis to get rid of any rf picked up in the cable.
3. As long as the output are not connected.

Nick wrote: ↑Tue Jul 13, 2021 8:51 am 1. No, other than the bridge and caps at the front are unneeded
2. No, maybe two ceramic caps on the inputs to the reg to chassis to get rid of any rf picked up in the cable.
3. As long as the output are not connected.

Thanks Nick

RhythMick wrote: ↑Tue Jul 13, 2021 8:16 am I found that a 6.3v output drawing 0.315A required the input voltage to be set at 11v.

If course as Nick says with a pre-reg I don't need the diodes at the front of the board and thus I have 2 diode drops.

As the DC boards are built and ready I'll just accommodate that extra drop in the supply. If I were building fresh (I have some spare boards not built) I guess it would be worth not including the diodes ? Bridge wires across from the rectifier outputs to the caps.

Good to see you chaps are still having fun with these

Andrew wrote: ↑Sun Jul 18, 2021 9:21 pm Good to see you chaps are still having fun with these

Absolutely. I think they are brilliant.

In my builds using these boards I've always mounted the boards so that the fins point upwards in normal playing use, so that the heat rises away from the transistors. The disadvantage of that is that the terminal blocks are inaccessible especially when the amp is upside down being worked on. I'd even considered mounting the blocks on the reverse side on the boards in my next build.

Of course I realise I'm confusing conduction and convection and I guess both matter but perhaps convection is less important than I'd thought. Is it ok to mount the board so that the transistors are at the top of the heatsink in normal use?

What does everyone else do? Am I making life hard for myself unnecessarily?

It's difficult to design for every eventuality, and I have seen heat damage to some of the PCBs in hot chassis, but I haven't yet had reports of dead regs. Some of these must be close to 9 years of service, I don't think I'd disappointed with that, nothing lasts for ever.

Andrew wrote: ↑Thu Aug 05, 2021 9:31 pm It's difficult to design for every eventuality, and I have seen heat damage to some of the PCBs in hot chassis, but I haven't yet had reports of dead regs. Some of these must be close to 9 years of service, I don't think I'd disappointed with that, nothing lasts for ever.

Sorry Andrew I must have not expressed my question properly. I certainly wasn't questioning the design of the board and I hope you didn't take it that way.

I was simply asking if it is ok to install the small boards with onboard heatsinks upside down. I guess the heat is taken away from the transistor through the heatsink by conduction and as the air around the fins warms up natural convection will cause that warm air to rise. If the board is upside down that warm air will rise towards the PCB.

Until now I've installed the boards so that in normal use the warm air rises away from the PCB, but it's certainly less convenient. I just wanted to ask what others do and whether it mattered.

I think with this next build I'm going to do it the easy way and monitor the temperature, if necessary I can install a small fan.

It's probably capacitor life that'll suffer first.

IslandPink wrote: ↑Thu Aug 05, 2021 10:32 pm It's probably capacitor life that'll suffer first.

Interesting. So do most people install them PCB down ?