Patchwork 6: Power Supply Upgrade

[MJL21193]

Here it is:

Picture 1184.jpg Picture 1052.jpg

6 channels of my Patchwork amps. This is used exclusively to actively drive my main speakers - 3-ways. It has been in daily use since I finished it.
Though I am perfectly happy with its performance, I want to improve the power supply. Mainly, this is just whimsy - the existing power supply is more than adequate but I can't leave well enough alone.
The Plan:
I will change the main power transformer from the current 700VA, 40-0-40 toroid to a ~600VA EI 46-0-46. I may (depending on space within the existing chassis) regulate this supply. This will supply the output stages of the amp modules.
I will add two 12VA 48-0 transformers to create a highly filtered and regulated supply for the front end of each amp module.
At this time, I'm not sure how I want to filter the front end supply. Cap multiplier or passively, CLC perhaps.
Create a new softstart circuit. I already have a softstart in the amp, but I can improve upon the original design.

Thoughts, comments, concerns welcome.

[Bigun]

If it means messing up that gorgeous chasis - don't do it !

On the other hand, if it means you'll have a redundant 700VA 40-0-40 you can send to me, go for it

I like the split supply for front-end and back-end. I've no experience with cap mux but a CLC has little to go wrong, using a common mode choke for the L to keep size and cost down.

[MJL21193]

Originally Posted by Bigun

On the other hand, if it means you'll have a redundant 700VA 40-0-40 you can send to me, go for it
.

I won't be doing anything with the outside of the chassis and I'm afraid I'll need to re-purpose that transformer Gareth...next amp project needs it.

This is the preliminary stage - the planning stage. I'm not actually going to do much yet but I would like some input.

The amp as it is today:

NPX_01193.JPG

needs a dusting! (one of the reasons for upgrading it - to clean it! ).

[Bigun]

I think it looks superb, I wouldn't mess with it if I were you -just build a new project.

[MJL21193]

Playing it safe is no fun Gareth.
I have had THIS project on my schedule for quite a while now and it's nearly time to start. I just have to finish up another mega-project first - This One.
I have simulations to run, transformers to test.
One question for anyone out there that might know: is there any problem completely enclosing an EI transformer? I would like to make a steel box that covers it. I thought I read somewhere that this can be a problem.

[MJL21193]

I ran a series of load tests on the proposed new power transformer for the output stages. I initially estimated the transformer to be in the 600VA range and it looks like I'm off by a bit.
The series output voltage with no load is 93VAC. I worked my way down to 13.7ohms of nichrome wire and I have 86VAC at that load. That is 6.28A at less than 10% regulation - not bad and should be suitable for this part of the project.

[Bigun]

Originally Posted by MJL21193

I just have to finish up another mega-project first - This One.

Wow, that's what I call a project.

[AndrewT]

I liked your original amp a lot.
Shame to muck about with it.

I loved the aesthetics.

What about two 3channel versions? The extra space would allow the room for the HV transformer and the HV PSU.

I am not keen on replacing a 8.75Aac low regulation transformer with a 6.28Aac higher regulation transformer.

BTW,
How do you check that your 13r7 nichrome wire is still 13r7 when passing 6.28Aac?

[MJL21193]

Originally Posted by AndrewT

I loved the aesthetics.

Thanks Andrew,
The outer appearance will remain unchanged, just guts will get 'improved'.

Originally Posted by AndrewT

What about two 3channel versions? The extra space would allow the room for the HV transformer and the HV PSU.

I need 6 additional channels in my next project (centre channel - 2, rear surrounds - 2 each) and if it were possible, I'd put those in that case too! I'd really like to reduce the quantity of individual components.

Originally Posted by AndrewT

I am not keen on replacing a 8.75Aac low regulation transformer with a 6.28Aac higher regulation transformer.

BTW,
How do you check that your 13r7 nichrome wire is still 13r7 when passing 6.28Aac?

Regarding the transformer, I ran one more load test last night - 10 ohm load and the voltage dropped to 83.4. Allowing for a rise in resistance with temp, say a worst case 1000*C rise will increase resistance to ~11.4 ohms. That gives me 7.3A at ~10% regulation and I believe this is more than adequate for this application.
TBH, I don't think the transformer that is in place now has seen anywheres near it's potential, as for actual loading. Over-speced, current wise but not having a high enough voltage (and its size) is the reason I want to pull it.

[AndrewT]

Originally Posted by MJL21193

the voltage dropped to 83.4. Allowing for a rise in resistance with temp, say a worst case 1000*C rise will increase resistance to ~11.4 ohms. That gives me 7.3A at ~10% regulation

Your measurements are showing 11.5% regulation at 600VA (7.3Aac).
I would expect the toroid to be <4% at 700VA (8.75Aac).
I still don't like it. I think you are going to see 4times as much voltage variation coming from the EI transformer vs the toroid transformer.