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Official M2 schematic
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Old 3rd June 2018, 09:48 PM   #2711
KevinHeem is offline KevinHeem  United States
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Official M2 schematic
Quote:
Originally Posted by BRN View Post
Would the M2 work with a tube preamp with 20db gain? Iím trying to decide between the M2 and F4.
What's the sensitivity of the speakers they will be driving?

I'm driving an M2 with a tube amp of about 20 dB of gain. This set up drives speakers with a sensitivity of 91 dB. It works great!

I've had an F4 as well...in fact I repurposed the chassis and power supply of the F4 and made it an M2.

ZM says we're not allowed to compare the two ...but I prefer the M2.

Last edited by KevinHeem; 3rd June 2018 at 10:00 PM.
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Old 3rd June 2018, 09:52 PM   #2712
Zen Mod is offline Zen Mod  Serbia
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it'll work, why not

though , in most cases 6,9 or 12db preamp gain is more than sufficient
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Old 4th June 2018, 12:37 AM   #2713
BRN is online now BRN  United States
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Thank you that helps with my decision. My speakers are single driver running full range at 89db. I do have 2 subs augmenting the lower frequencies.

Last edited by BRN; 4th June 2018 at 12:47 AM.
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Old 5th June 2018, 12:43 AM   #2714
BRN is online now BRN  United States
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Can I use the the AnTek 400VA 20 + 20 transformer? I’m going to use the DIYA power supply.
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Old 5th June 2018, 01:21 AM   #2715
KweeSong Lim is offline KweeSong Lim  Singapore
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Official M2 schematic
Quote:
Originally Posted by Mark Johnson View Post
That 600:20K transformer has a turns ratio of 1.0 :: 5.77 and so, following the arithmetic in this M2 thread, the transformer primary presents a load of (23500 / (6.77 * 6.77)) = 512 ohms to the dual JFET input stage. I.e. the input stage is driving a 512 ohm load (!) That's working the JFETs awfully doggone hard, especially at max input level.

I don't know about the Chinese transformer, but the two Edcor transformers I measured had a DC resistance of 44 ohms in their primary ("600 ohm") windings. 10 millivolts of DC offset from the JFET input stage, into a 44 ohm DC resistance, results in a DC magnetizing current of 0.23 milliamps in the primary (Ohm's Law). Enough DC current to make me, very uncomfortable.
Dear Mark, I did not measure the DC resistance but your observation is probably right as I reported earlier about the DC sensitivity. Let me make a measurement tonight and report.
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Old 6th June 2018, 01:15 AM   #2716
Eric is online now Eric  United States
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I've been working on my M2 for a little while now and still have a little bit to go. I've designed it in a "modular" fashion, with an outboard power supply that can also be used with my F4 and (not yet built) Aleph-J. I started with a 9x13" heatsink that I found at the flea market for $6. I had someone at the university product development lab cut it in half for me and I gave it light coat of black paint.

Because the sink wasn't wide enough to accommodate the circuit board with the mosfets placed in ideal locations for heat dissipation, I attached a 1/2" aluminum L-bracket and a 1/8" x 2" strip of aluminum bar to one side.

I used Tea-Bag's boards along with the Harris 9240, so I had to make R6 47k5 and R7 53k in order to achieve 0mV across speaker terminals. PSU rails are plus/minus 22.6v and I measure 0.594v across R13 and R14. With stock configuration, this works out to 1.26A bias or 58w per channel. The sinks provide a 20c rise over ambient temperature. I used some mica and goop to mount the mosfets, they get about 10c hotter than the sink itself.

I used a 20R multiturn pot in place of R3/R4. I used my most mis-matched jFets (10.66mA 2SK170 and 6.90mA 2SJ74) and the 20R allows me to balance the output of the buffer with precision. I slathered the jFets with some heatsink grease and wrapped them with heat shrink tube. For C2, I used a NOS Russian PIO 10uF MBGCh cap - thought it would "fit" better with the "tube" sort of flavor of this amp.

I used some steel banding with a bit of rubber padding to make a clamp to hold the local power supply caps (39,000uF each) to the sinks. This arrangement holds them tightly, but isolates them a bit from the heat of the sink. The outboard PSU will have another bank of caps and about 0R1 resistors, followed by the power umbilical, thus forming a CRC filter.

I'll use some figured maple for the front and top of the chassis. Just a bit more structural work to go then I'll move it into my main system for some listening.
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Old 6th June 2018, 01:41 AM   #2717
dewdrop is offline dewdrop  Singapore
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Quote:
Originally Posted by Eric View Post
I've been working on my M2 for a little while now and still have a little bit to go. I've designed it in a "modular" fashion, with an outboard power supply that can also be used with my F4 and (not yet built) Aleph-J. I started with a 9x13" heatsink that I found at the flea market for $6. I had someone at the university product development lab cut it in half for me and I gave it light coat of black paint.

Because the sink wasn't wide enough to accommodate the circuit board with the mosfets placed in ideal locations for heat dissipation, I attached a 1/2" aluminum L-bracket and a 1/8" x 2" strip of aluminum bar to one side.

I used Tea-Bag's boards along with the Harris 9240, so I had to make R6 47k5 and R7 53k in order to achieve 0mV across speaker terminals. PSU rails are plus/minus 22.6v and I measure 0.594v across R13 and R14. With stock configuration, this works out to 1.26A bias or 58w per channel. The sinks provide a 20c rise over ambient temperature. I used some mica and goop to mount the mosfets, they get about 10c hotter than the sink itself.

I used a 20R multiturn pot in place of R3/R4. I used my most mis-matched jFets (10.66mA 2SK170 and 6.90mA 2SJ74) and the 20R allows me to balance the output of the buffer with precision. I slathered the jFets with some heatsink grease and wrapped them with heat shrink tube. For C2, I used a NOS Russian PIO 10uF MBGCh cap - thought it would "fit" better with the "tube" sort of flavor of this amp.

I used some steel banding with a bit of rubber padding to make a clamp to hold the local power supply caps (39,000uF each) to the sinks. This arrangement holds them tightly, but isolates them a bit from the heat of the sink. The outboard PSU will have another bank of caps and about 0R1 resistors, followed by the power umbilical, thus forming a CRC filter.

I'll use some figured maple for the front and top of the chassis. Just a bit more structural work to go then I'll move it into my main system for some listening.
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Old 6th June 2018, 07:52 AM   #2718
Zen Mod is offline Zen Mod  Serbia
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just put something to bypass that PIO, for starters

I love them , but they tend to err on sirupy side ....... together with xformer , it can be too much ..... so , trying later some other type of cap could be wise
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Old 6th June 2018, 09:11 AM   #2719
Extreme_Boky is offline Extreme_Boky
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Quote:
Originally Posted by Mark Johnson View Post
That 600:20K transformer has a turns ratio of 1.0 :: 5.77 and so, following the arithmetic in this M2 thread, the transformer primary presents a load of (23500 / (6.77 * 6.77)) = 512 ohms to the dual JFET input stage. I.e. the input stage is driving a 512 ohm load (!) That's working the JFETs awfully doggone hard, especially at max input level.

I don't know about the Chinese transformer, but the two Edcor transformers I measured had a DC resistance of 44 ohms in their primary ("600 ohm") windings. 10 millivolts of DC offset from the JFET input stage, into a 44 ohm DC resistance, results in a DC magnetizing current of 0.23 milliamps in the primary (Ohm's Law). Enough DC current to make me, very uncomfortable.
I just checked the m2 schematics posted by Mr. Pass; it seems that, with a properly matched Q3/Q4, the common point between R3&R4 should be sitting at 0V DC with a symmetrical power supply; hence no current should be running through the primary. Any current running through the primary would disturb (deduct from) the quiescent current running through the Q3&Q4...
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Old 6th June 2018, 01:04 PM   #2720
Eric is online now Eric  United States
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Official M2 schematic
Quote:
Originally Posted by Zen Mod View Post
just put something to bypass that PIO, for starters

I love them , but they tend to err on sirupy side ....... together with xformer , it can be too much ..... so , trying later some other type of cap could be wise
This was my understanding as well, it is certainly easy enough to replace or bypass that big PIO cap. Just trying an experiment to see how different things sound. Need to finish the rest of the chassis so it becomes more portable.
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