MyRefC build guide

[westers151]

What VA transformer rating is judged to be good for the amps? I know 24V or 25V is good for 8 ohm loads, but what about the "grunt"? I've always assumed that 300VAC was about right for a non centre tapped dual secondary transformer, any more than this being wasted on a two monobloc chip amp.

Is 300VAC still about right, or is that a bit overkill?

[AndrewT]

a 100W amplifier can operate quite well with a transformer rated from 100VA to 200VA. any bigger becomes uneconomic.

A 60W Chipamp can use a 60VA to 120VA transformer.

Unfortunately a very low VA transformer has very high regulation.
I recommend that the transformer should be >=160VA.

A two channel amplifier of 60+60W would require 120VA to 240VA and add in that "avoid high regulation guidance" and you get 160VA to 240VA.

I don't know where 300VA came from. It is icing on the cake. An uneconomic way to improve an amplifier. But some builders do hear a sound quality benefit and consider the bigger transformer as offering good value for their budget.

I would consider a 300VA as good for a 100W + 100W two channel amplifier.

[jcon2]

To Clarify my experience with 47K, Resistors, R6, R9, and R20
Each mono channel has its own components and they are matched. I wouldn't dump out all the parts for both boards and now have 6 resistors and assume that they are all matched.
For the first board my values were approx
I believe that all of my 47K resistors were all like 46.8 kohm on my Fluke DMM. And so I felt they were all good.
The second packet all the 47K resistors measured 47.0 kohm.
With goal of .1% matching being 47000 +/ - of 47 ohm.
1% of 47,000. being 470 and .1% 47 ohm.
The 22K Resistors, R5,R8, R43. Two of them were the same 22.01 kohm and the the third was within 1% say 21,800
Anyway you get the idea. Two of them are nearly identical. If they all measure the same you are good. Just wanted to give people a heads up as Uriah went to the extra measure to match them and wanted everyone to give this thing he best shot for audio perfection.
Again Uriah many thanks for all your work.
JCon2

[AndrewT]

it's easy to bridge connect the 4 resistors (2off 22k and 2off 47k) onto a plugboard.
apply 10 or even 60Vdc from top to bottom, i.e. to the junction of the two 22k and the junction of the two 47k.
Now measure the mVdc difference across the bridge, i.e. black probe to one 22k+47k junction and red probe to the other 22k+47k junction.

Now swap out one 22k for the spare 22k. is the mVdc difference bigger or smaller. Keep the swapped resistor in place and now put back the original 22k to replace the one 22k that was not originally swapped.
Is this the lowest or highest mVdc difference.

Now put in the two 22k that gave the lowest difference.
check the mVdc again.
Now swap the two 47k across with each other. Is the mVdc difference bigger? If yes return the 47k to where they were.

You now have the 22k:47k ratios at their closest match.
The pair of 22k&47k on one side of the bridge stay together.
The other pair of 22k&47k also stay together.

You can see how these pairs fit into the MyRef schematic. Both 22k & 47k have a common junction. It's those common junctions that must be maintained in the Howland current pump. Doing that gives the highest impedance available from that set of 5 resistors.

[westers151]

Originally Posted by AndrewT

a 100W amplifier can operate quite well with a transformer rated from 100VA to 200VA. any bigger becomes uneconomic.

A 60W Chipamp can use a 60VA to 120VA transformer.

Unfortunately a very low VA transformer has very high regulation.
I recommend that the transformer should be >=160VA.

A two channel amplifier of 60+60W would require 120VA to 240VA and add in that "avoid high regulation guidance" and you get 160VA to 240VA.

I don't know where 300VA came from. It is icing on the cake. An uneconomic way to improve an amplifier. But some builders do hear a sound quality benefit and consider the bigger transformer as offering good value for their budget.

I would consider a 300VA as good for a 100W + 100W two channel amplifier.

Thanks Andrew.

Yes I don't know where 300VA came from, apart from me reading it on the forum.

I have a 300VA already, but it's in another amp right now. I think I'll get a 240VA transformer and perhaps do some testing between the two. I'll wager that I don't hear any differences

[AndrewT]

I'll match your wager and tell you, you will be hard pushed to measure any difference between 240VA and 300VA other than the charge to your debit card.

[regiregi22]

and size/weight differences

[earsandeyes]

Weather is detoriating so a chance to spend some time on the new arrivals.
Being one of the lucky ones with both PRP and Shinkoh resistors a final check and some questions:

-Positions are R10 (390) and R12 (3k3) and not R37?
-Alternatives are switches or soldering in and out. Would a switch detoriate the effect of such nice resistors?

Anyone alternative options for switching the resistors in and out?

Thanks

Michiel

[ClaveFremen]

Originally Posted by earsandeyes

Anyone alternative options for switching the resistors in and out?

SIL Sockets

[regiregi22]

mmmm.....you could solder a pair of copper wires, and in a quick soldering job, solder and de-solder the resistors to them. Or the SIL sockets Darios proposes.