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M2 inspired balanced output preamp idea
M2 inspired balanced output preamp idea
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Old 21st February 2019, 01:32 AM   #51
woofertester is offline woofertester  United States
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Originally Posted by indra1 View Post
I modified Scott Wurcer's model for the PC600/15k with your measurement and sim a driver idea.
Very neat. Thank you for this. I have your sim running. I have some Motorola MTP10N15 and some BD139 in my stash. I will have to give this a try.

It looks like a Zen line stage with current sources/mirror driving the transformer, yes?
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Old 21st February 2019, 01:35 AM   #52
woofertester is offline woofertester  United States
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Originally Posted by Mark Johnson View Post
Norwood's good for 200mA. Tucson is only spec'd to 35 mA. Rolling your own design using THT might select the OPA633 in DIP8, as the final output drive.
Thanks for this, Mark. I have the M2X input boards. I will have a look.
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Old 21st February 2019, 04:59 AM   #53
indra1 is offline indra1  Indonesia
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Originally Posted by woofertester View Post
... It looks like a Zen line stage with current sources/mirror driving the transformer, yes?
It is a simple balanced ccs loaded source follower. Have fun.
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Old 22nd February 2019, 12:51 PM   #54
woofertester is offline woofertester  United States
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Originally Posted by indra1 View Post
It is a simple balanced ccs loaded source follower. Have fun.
Thank you for the well wishes. I have sorted through a pile of qty 50 MTP10N15 and have found enough matched pairs to go forward. I measured Vgs at 51.9 mA.

The matched pairs span Vgs from 3.85 to 4.11 with two quads at 3.93 and 3.94.

A pic of the test rig for measuring Vgs is attached for amusement.
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Old 22nd February 2019, 01:13 PM   #55
Mark Johnson is offline Mark Johnson  United States
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M2 inspired balanced output preamp idea
How much DC current are you willing to push through the transformer primary? To avoid premature core saturation, to avoid permanent magnetization, and so on.

Thanks to Ohm's Law we can calculate it easily: Iprimary = Voffset / Rprimary

where Voffset is the offset voltage of the buffer amplifier, and Rprimary is the DC resistance of the primary. As I recall, Rprimary was about 250 ohms but you can measure your particular Edcor transformer's Rprimary with a DVM and find the exact number.

If you use a famous design procedure called r***** extraction to decide that you require Iprimary < 0.25 mA, then Ohm's Law says

Voffset < (0.25 mA) * (250 ohms)

Voffset < 62 millivolts

(Below 62 mV offset) is easy to do with ICs and no offset trimming (adjustment potentiometers).

(Below 62 mV offset) is possible to do with all-discrete circuits plus offset trimming.

(Below 62 mV offset) is possible to do with all-discrete circuits plus careful hand-matching of components, and no offset trimming.

It all depends on how much DC current you are willing to push through the transformer primary.
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Old 22nd February 2019, 02:49 PM   #56
woofertester is offline woofertester  United States
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Quote:
Originally Posted by Mark Johnson View Post
How much DC current are you willing to push through the transformer primary? To avoid premature core saturation, to avoid permanent magnetization, and so on.

Thanks to Ohm's Law we can calculate it easily: Iprimary = Voffset / Rprimary

where Voffset is the offset voltage of the buffer amplifier, and Rprimary is the DC resistance of the primary. As I recall, Rprimary was about 250 ohms but you can measure your particular Edcor transformer's Rprimary with a DVM and find the exact number.

If you use a famous design procedure called r***** extraction to decide that you require Iprimary < 0.25 mA, then Ohm's Law says

Voffset < (0.25 mA) * (250 ohms)

Voffset < 62 millivolts

(Below 62 mV offset) is easy to do with ICs and no offset trimming (adjustment potentiometers).

(Below 62 mV offset) is possible to do with all-discrete circuits plus offset trimming.

(Below 62 mV offset) is possible to do with all-discrete circuits plus careful hand-matching of components, and no offset trimming.

It all depends on how much DC current you are willing to push through the transformer primary.
Good point. Measure the primary resistance and make sure that DC current is low enough to not damage the transformer core.
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Old 22nd February 2019, 03:16 PM   #57
woofertester is offline woofertester  United States
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Default Indra1's driver Asymmetric rails

Hello Indra1

I have interrogated your driver circuit. Clipping with + / - 9V rails is quite asymmetric. I adjusted the negative rail until clipping was mostly symmetric. It looks like the negative rail wants to be approximately two times Vgs more than the positive rail for symmetric clipping.
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Old 22nd February 2019, 03:42 PM   #58
indra1 is offline indra1  Indonesia
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Yes Brian, clipping will be assymetric on symmetric rails. But it does not really matter for use up to 10Vpp you need to drive the Aleph 30 right?.
And use a 47 ohm dummy resistor instead of the transformer to first make sure DC offset is manageable as pointed out by Mark Johnson on your build.
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Old 22nd February 2019, 04:08 PM   #59
woofertester is offline woofertester  United States
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Originally Posted by indra1 View Post
Yes Brian, clipping will be assymetric on symmetric rails. But it does not really matter for use up to 10Vpp you need to drive the Aleph 30 right?.
And use a 47 ohm dummy resistor instead of the transformer to first make sure DC offset is manageable as pointed out by Mark Johnson on your build.
You are quite correct. Not necessary. I am just messing around having fun!

For sure I will use a dummy resistor initially.

Thanks and cheers. Have a great weekend.
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Old 23rd February 2019, 01:59 PM   #60
cubicincher is online now cubicincher  Germany
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Default answer to woofertester

Hello woofertester,


I have used those bufferboards mentioned in the beginning of this thread
to buffer the outputs of one of my active crossover projects (based on OPAmps). It behaved very well. But I wasn't measuring it. Only listening. Was a few months
ago.

But at the moment this project is sleeping. I made an PASS AXO and a M2X
inbetween......

Greets
Dirk
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