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Been kinda quiet on the build front while waiting for parts, so I finished the front plate. I rounded the corners, sanded, and drilled two holes for the power switches. Switches will be flush mounted push buttons with an LED ring - one for each channel. I'll post final images once the last shipment arrives.
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Yep, using the last of my Formby's low gloss Tung Oil Finish, which, ironically I don't think has any actual tung oil in it at all... Formby's is now gone and MinWax seems to be selling a similar product (guess they purchased Formby's?). I don't know if the MinWax formulation is the same or not.
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Spent the past day and a half doing final wiring on my SissySIT amp. I always seem to underestimate the amount of time it takes to get all of the wiring in order. The back panel is a piece of walnut that I split down the middle, joined edge to edge, and added some ribs for strength.
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Here are some additional inside shots and a few of the closed chassis. I put the power switches in the walnut stripe to hide them a little. First power up was smooth and uneventful. I spent a little while dialing in bias for the input and output stages and zeroing the offset. Like all of these heat monsters, the numbers in my DMMs all seem to dance around a bit. I've learned from my Aleph-X days that ballpark is good enough once fully warmed. Set bias to about 1A85 and resulting sink temp seems to be near 42-44c. Haven't had time to really listen yet, have an event to go to next.
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Thanks, Anand. Tomorrow I'll connect it to a few pair of speaker to see how it sounds. At a minimum, I'll connect it to my FHXL (single driver) speaker and likely the giant Calpamos speakers as well. I'm very curious to hear it after all of this time. I think I purchased the kit from ZM in 2019 or 2018. I think I've had the sinks even longer...
I'm a little surprised at the final overall dimensions: 480mm wide, 420mm deep, and 200mm tall. I was trying to lay things out so they are clean and neat. I also want to measure the PSU ripple and see it on my scope. I expect it should be pretty clean. The only part of the psu shared by each channel is the power cord.
I'm a little surprised at the final overall dimensions: 480mm wide, 420mm deep, and 200mm tall. I was trying to lay things out so they are clean and neat. I also want to measure the PSU ripple and see it on my scope. I expect it should be pretty clean. The only part of the psu shared by each channel is the power cord.
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Wow! Just measured PSU ripple and speaker output with the inputs shorted. Pos and Neg rails on both channels as well as speaker terminals ALL measure 0.00mVAC! 
Color me impressed! I didn’t think 33k uF caps would be enough to produce a zero ripple PSU.
Can’t wait to push some music through it after work tomorrow!
Color me impressed! I didn’t think 33k uF caps would be enough to produce a zero ripple PSU.Can’t wait to push some music through it after work tomorrow!
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Thanks, Dan! That was my first thought about the PSU ripple as well - I was expecting to see something in the single digits of mV ripple given that I only have 33kuF before and after each choke. The chokes are Hammond 159ZJ, 10mH, 5A, 0R160 Ohms. I'll have to recheck ripple tonight, something seems fishy to me as well. I also sat and played with bias and offset adjustments last night. When I made ripple measurements, the amp had only been on for a few minutes. It seems that the bias wants to creep upward as the amp warms up. After about three or four hours last night of tweaking, putting the lid back on, letting it sit for 10 mins or so, and watching the meters, the bias was creeping just north of 2A0. I was seeing about 25c thermal rise on the sinks opposite of the transistor mounting sites. I think it started somewhere closer to 1A7 when it was cold, so I want to see where it starts off tonight at first power up and see where things are/go.
Front end bias seems to hold pretty stable at 20-21mA, but the offset seems to wander +/- 3 or 4mV or so on a pretty continuous and somewhat random basis. Output stage bias seems to climb at is warms and speaker offset also seems to dance around by +/- 20mV or so. I discovered that merely putting my hand on the sink creates an instant drop in bias (voltage dropped across the PSU choke by about 10-15mV). It pops back up again as I remove my hand. I'm guessing it's a capacitance thing more than my hand changing the sink temperature because of how rapid the reaction is.
I only had the opportunity to listen using my ipad as a source last night (yep, way less than ideal) on my big Avro open baffle speakers (dual 12" woofers, dual 8" midranges, single tweeter). Two observations are that this amp produces plenty of bass (a tough trick with OB speakers) and the next thing that hit me was "spaciousness" or "depth." The OBs tend to excel at this, but the SissySIT made that remarkably clear. The other remarkable attribute aligns with the zero ripple thing: this amp is ABSOLUTELY SILENT. With no signal present, there is no hum, no rush, no hiss, NOTHING at all with my ear pressed against the tweeter. Yes, the amp was powered on and yes, I did have an input signal connected
Very impressive, Zen!
After a few measurements tonight as it transitions from cold to warm to satisfy my OCD, I'll put it into the family room system and drive my big Calpamos speakers with a more proper signal.
Front end bias seems to hold pretty stable at 20-21mA, but the offset seems to wander +/- 3 or 4mV or so on a pretty continuous and somewhat random basis. Output stage bias seems to climb at is warms and speaker offset also seems to dance around by +/- 20mV or so. I discovered that merely putting my hand on the sink creates an instant drop in bias (voltage dropped across the PSU choke by about 10-15mV). It pops back up again as I remove my hand. I'm guessing it's a capacitance thing more than my hand changing the sink temperature because of how rapid the reaction is.
I only had the opportunity to listen using my ipad as a source last night (yep, way less than ideal) on my big Avro open baffle speakers (dual 12" woofers, dual 8" midranges, single tweeter). Two observations are that this amp produces plenty of bass (a tough trick with OB speakers) and the next thing that hit me was "spaciousness" or "depth." The OBs tend to excel at this, but the SissySIT made that remarkably clear. The other remarkable attribute aligns with the zero ripple thing: this amp is ABSOLUTELY SILENT. With no signal present, there is no hum, no rush, no hiss, NOTHING at all with my ear pressed against the tweeter
. Yes, the amp was powered on and yes, I did have an input signal connected Very impressive, Zen!
After a few measurements tonight as it transitions from cold to warm to satisfy my OCD, I'll put it into the family room system and drive my big Calpamos speakers with a more proper signal.
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Holy Crap!!! What a treat this amp is! I'm running some flac files through my pre, into the SissySIT, then to my Calpamos speakers (giant Tony Gee design). Wow! Voices float in holographic space in front of and above the speakers. Depth is tremendous! It's quite an enveloping presentation. Love the overtones of a single SIT running "on its own." Some instruments and notes just seem to hang in space and linger there for just the right amount of time. I detected a hint of this last night while driving the Avro speakers from my iPad, but it's much more apparent with better source material. Vocalists and single instruments are excellent, as is more complex and heavy stuff. No issues at all with pushing a set of 15" drivers around! They have an impedance dip to about 6R near 80Hz, other than that, they have a pretty easy impedance curve. Highly reminiscent of the Sony VFet2 that was just in the playback chain, but with "more."
The family is starting to get annoyed at the prolonged volume level...
Take a bow, Zen
The family is starting to get annoyed at the prolonged volume level...
Take a bow, Zen
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There are few interesting asymmetries in the behavior of the two channels. In the left channel, the positive rail exhibits no measurable ripple, while the negative rail exhibits about 2.7mV of ripple. It is exactly the opposite in the right channel. The dc offset in the left channel spikes to about -200mV within the first 30 seconds, then settles to about -25mV by the 5 min mark, then bounces around the zero mark, going pos and neg by about 20mV or so. The right channel maintains about -100mV offset for about the first 30 mins, then settles closer to zero after about an hour. Given that even 200mV of DC offset into an 8R load is less than 0.005w of dissipation, it’s not worth worrying about. Both channels run on 23.8vDC rails.
The giant chassis absolutely dwarfs the 3U chassis for the Sony VFet amp.
The giant chassis absolutely dwarfs the 3U chassis for the Sony VFet amp.
