start thinking.
What resolution does your DMM have? Probably 1999 count giving 1 part in 2000 resolution.
Put your two resistors in series. Attach a voltage supply so that the voltage across each resistor is held @ ~195.0 mV.
Compare the voltage across each resistor. You will see how well they are matched to X parts in 1950. That's a resolution down to 0.051%
If you have a 3999 count or 19999 count DMM then you can improve on that.
Next you have a Wheatstone Bridge.
That will get you down to <0.01% since you have two 22k and two 47k.
Set them up as a bridge and compare the ratios. It's the ratio accuracy that this circuit depends on.
I can easily see the effect of that 0r5 (output resistor in the modiofied Howland) in the bridge that I tested. That's down to 0.002% of the 22k.
But I still await answers to my two questions http://www.diyaudio.com/forums/group-buys/160768-myref_c-ultimate-bom-39.html#post2200751
Guys
With the resistors that are matched you might have to do what jcon did and test the three of them of the same value to see which are matched. Once I got them matched I put them in the resistor bag not remembering that there was another of the same value in there. Hardly any of them were more than 100R different from each other. Almost all fell within 60R and my matches are way less than that. So even if you put the unmatched in with the matched you would still have far better than .1% required for a Howland Current Pump and if my recollection serves this is actually an Improved Howland Current Pump which requires even less precision. The .01% is a luxury not needed but easily achieved.
I realized my mistake in time for a decent percentage of the kits and many of you got matched resistors in a separate bag. For those of you who didnt just use your DMM to determine which 2 of the 3 are closest and if you cant determine with the DMM rest assured they are still better than .1% by probably .05-.08% and will be still as good as .01% in this application.
Andrew I would answer your questions if I could.
Uriah
With the resistors that are matched you might have to do what jcon did and test the three of them of the same value to see which are matched. Once I got them matched I put them in the resistor bag not remembering that there was another of the same value in there. Hardly any of them were more than 100R different from each other. Almost all fell within 60R and my matches are way less than that. So even if you put the unmatched in with the matched you would still have far better than .1% required for a Howland Current Pump and if my recollection serves this is actually an Improved Howland Current Pump which requires even less precision. The .01% is a luxury not needed but easily achieved.
I realized my mistake in time for a decent percentage of the kits and many of you got matched resistors in a separate bag. For those of you who didnt just use your DMM to determine which 2 of the 3 are closest and if you cant determine with the DMM rest assured they are still better than .1% by probably .05-.08% and will be still as good as .01% in this application.
Andrew I would answer your questions if I could.
Uriah
Thanks Uriah, I may look at testing them, but as they're all so closely matched then I don't really see the point as they're already all within spec.
A clarification question, to you all, regarding the bridge area on the boards. There are a lot of holes in that area which I think are to allow the following options:
1. Individual rectifiers that are supplied with the kit. That's obvious.
2. Allow for a single bridge chip as per the original build - I'm pretty sure that's the four holes in the B1 area.
3. AC1 and AC2 are the holes for the spade connectors that have the two secondary wires connected to them.
4. PGND - Again the two smaller holes are for the spade connector.
That leaves three larger holes in that general area; One is next to PGND, one is next to AC1 and the other next to AC2. I'm pretty confident these holes are to allow the secondary transformer wires, and the PGND wire to be directly soldered to the board and thus eliminating the spade connectors.
Is my analysis correct?
A clarification question, to you all, regarding the bridge area on the boards. There are a lot of holes in that area which I think are to allow the following options:
1. Individual rectifiers that are supplied with the kit. That's obvious.
2. Allow for a single bridge chip as per the original build - I'm pretty sure that's the four holes in the B1 area.
3. AC1 and AC2 are the holes for the spade connectors that have the two secondary wires connected to them.
4. PGND - Again the two smaller holes are for the spade connector.
That leaves three larger holes in that general area; One is next to PGND, one is next to AC1 and the other next to AC2. I'm pretty confident these holes are to allow the secondary transformer wires, and the PGND wire to be directly soldered to the board and thus eliminating the spade connectors.
Is my analysis correct?
post766 from BOM thread
Just avoid the jcon warning as confirmed by Uriah.
Thanks Andrew and to be honest I was going just go ahead and build it.
I'm sure the amp may not as optimal as it might be in a perfect world, but my hearing isn't perfect, my room isn't perfect and the recordings I listen to aren't perfect, so any improvement is likely to lost due to the other factors. There comes a point when you've got to say "that'll do", and then get on with it.
I'm sure the amp may not as optimal as it might be in a perfect world, but my hearing isn't perfect, my room isn't perfect and the recordings I listen to aren't perfect, so any improvement is likely to lost due to the other factors. There comes a point when you've got to say "that'll do", and then get on with it.
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?
Is 300VAC still about right, or is that a bit overkill?
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.
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.
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
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
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.
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.
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
PRP versus Shinkoh
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
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
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