Another linear PSU for the Caiman - Some help needed

[HighFidelityGuy]

I've just finished building my first linear regulated power supply which I plan to use to power my Caiman. It's based around a 100VA toroidal transformer, a high quality bridge rectifier, 22000uF smoothing cap and a SuperTeddy (http://www.teddypardo.com/Articles/SuperTeddyReg.html) regulator board. Unfortunately I seem to have to have done something wrong. :doh:

I can successfully get whatever DC voltage I want out of the SuperTeddy and I've currently set this to 15V by measuring with my multimeter. However when I connected the PSU to the modified 7520 I use for testing it blew the fuse on the mains input. At first I assumed that the 1A fuse I'd used was too small but after some testing I realised that I have a 38V DC potential between the chassis ground and the DC ground. So when the DAC is connected via USB the PC/Laptop causes a short. I can successfully power the 7520 providing I don't connect the USB lead.

I'm guessing this is due to the fact that the two power transistors on the SuperTeddy are screwed to the chassis, causing the output smoothing cap to be connected to the chassis. I noticed that MartinT seemed to have his linear PSU wired up in the same way, so I'm not sure where I've gone wrong. I must have missed something. :(

Can anyone shed any light on what's going wrong?

I'm afraid I can't take any decent photos of my PSU as my wife's taken our camera on a trip to London. I'll try and take one on my iPhone but it will be crappy quality.

Thanks. :)

[Stratmangler]

Have you connected the mains ground to the PSU anywhere ?
Perhaps things should be floating earth.

[HighFidelityGuy]

Yep, mains ground is firmly attached to the chassis and that's how Martin seems to have his as you can see here:

http://www.mtc.me.uk/images/PSU%20Project%203.jpg

That is unless he's managed to isolate the transistors from the chassis somehow. :scratch:
I hope Martin doesn't mind me ripping off his photo. My iPhone's battery is dead as usual so I won't be able to post a photo of mine for a while.

There doesn't seem to be any info about how to mount the SuperTeddy on Teddy's website. I guess I could try emailing him.

Thanks.

[Stratmangler]

Try it without the tranny earth. Just a hunch.

[Puffin]

Have you insulated the TR transistors?

[HighFidelityGuy]

Have you insulated the TR transistors?

Nope. Should I?

[StanleyB]

I would isolate them just like the output transistors on a power amp. Don't forget the heatsink compound as well.

I have recommended the CB radio PSU from Maplin, then adjust it to give out 15V instead of its preset 13.8V. That output can then be fed into a Murata modded Caiman. The 12V and 5V output is clean as a whistle after that.

[HighFidelityGuy]

Ok thanks for the help. I'll sort out insulating the transistors. I'm pretty sure that will fix the problem.

[Puffin]

HFG. The transistors should be insulated with washers for the bolts that attach them and mica washers under the tranny's.

[StanleyB]

Like THIS (http://cgi.ebay.co.uk/TO-218-TO-3P-TRANSISTOR-INSULATING-KIT-Lot-20-/330434628528?cmd=ViewItem&pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item4cef704bb0)

[Puffin]

That'll do nicely;)

[HighFidelityGuy]

HFG. The transistors should be insulated with washers for the bolts that attach them and mica washers under the tranny's.

Ok thanks Puffin. I've found these on on the RS and Farnell sites but it looks like Maplin may do something similar, so I'll probably call in tomorrow and see what I can find. If not I'll order some.

I've just tried running the PSU for a couple of mins with the TeddyReg isolated from the chassis and everything works perfectly and the sound I heard through my headphones was excellent. It was difficult to stop listening but I was scared the transistors would over heat, so it's all switched off now until I source some insulated micas and washers.

Thanks for the help everyone. I'll now always remember that power transistors should be isolated. Every day's a school day. Luckily I didn't blow anything. :eek: I'm glad I used a low fuse rating.

:cool:

[HighFidelityGuy]

Like THIS (http://cgi.ebay.co.uk/TO-218-TO-3P-TRANSISTOR-INSULATING-KIT-Lot-20-/330434628528?cmd=ViewItem&pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item4cef704bb0)

Thanks Stan. :)

Here's a photo of my PSU in it's current unfinished state:

http://www.theartofsound.net/forum/[IMG]http://i2.photobucket.com/albums/y48/DTM2000/Hi-Fi/Beresford%20DACs/Caiman_PSU_01.jpghttp://i2.photobucket.com/albums/y48/DTM2000/Hi-Fi/Beresford%20DACs/Caiman_PSU_01-1.jpg

I can't wait to get it finished and connected up.
It's sitting on a copy of the Times folded in half for size comparison. Apologies about the quality.

[Puffin]

HFG. We all have to start somewhere. I have been at it for years, but I am still learning! I am glad it works ok. Maplin have what you need.

http://www.maplin.co.uk/Module.aspx?ModuleNo=30386

http://www.maplin.co.uk/Module.aspx?ModuleNo=30387

[HighFidelityGuy]

HFG. We all have to start somewhere. I have been at it for years, but I am still learning! I am glad it works ok. Maplin have what you need.

http://www.maplin.co.uk/Module.aspx?ModuleNo=30386

http://www.maplin.co.uk/Module.aspx?ModuleNo=30387

Thanks Puffin.
I've really enjoyed building this and doing other mods to my DACs. It's been really interesting and it's a great sense of achievement when you make something with your own hands. I can see my self getting into the DIY scene more in the future.

Cheers. :cool:

[HighFidelityGuy]

Just a quick update to say that I managed to get hold of the relevant insulating parts and got them fitted yesterday. The PSU now works perfectly and I've taken it to work today to start running it in with my 7520 that I use there for headphone duties. I also hooked it up to our PAT tester to double check it was safe and it passed with flying colours.

Unfortunately I'm not going to have much time to listen to it in my proper system at home as I'm going to Download festival on Wednesday, so I'm going to be busy packing my camping gear etc. At least it should be nicely run in by the time I get back.

Once I get back I'll do some comparisons between my new PSU and the official Caiman unit.

[Puffin]

Excellent. Wait to hear what you think.

[HighFidelityGuy]

Well my new PSU has been running in for a few weeks now, so I decided to do a bit of comparison between it and the standard Caiman 15V PSU. I basically just kept switching bewteen the two supplies and listenning to the same passage of music over and over again. The conclusion I came to was that my linear supply gave a slightly lower noise floor and therefore slightly darker and purer sound. When switching back to the standard supply, there was initially a sense that there was more detail in the background but I came to realise that this extra sound appeared to be extra harmonics on top of the signal which was slightly washing over everything. I didn't really detect any change in the bass region which supprised me as I was expecting the linear supply to tighten the bass up a bit. However I was mainly listening for subtle details in the trebble etc, so perhaps I didn't pay enough attention to the bass.

So my linear supply seems to prevent these extra harmonics and allows a purer signal to come through that's no doubt more faithful to the original recording and also slightly less tiring of the ears.

For the price I paid, which was over £100, I'd say that this mod doesn't have the best price to improvement ratio when compared to other mods available but I suspect using the modified CB radio supply that Stan tried will produce the same results for less money. The main reason I built the supply my self was that I've wanted to do that for a while and this seemed like a suitable application for such a project.

So overall I'm happy with the results and I've learned a lot in the process. So it's been a worthwhile and interesting project. :)

[Puffin]

HFG. Good to hear your efforts have been rewarded.