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Salas hotrodded blue DCB1 build
Salas hotrodded blue DCB1 build
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Old 6th January 2020, 12:53 AM   #5851
nashbap is offline nashbap  United States
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Salas hotrodded blue DCB1 build
Quote:
Originally Posted by Salas View Post
Its not crucial to have exactly same idss pairs in channels. They can be say 9mA in L and 8mA in R. But if much different, THD measurements will noticeably change between channels. Especially at high signal levels.

I tested a pair of 10ma Idss (one self biasing the other) and a pair of 6 ma Idss at 0.5v input 1khz signal using my Focusrite. I got THD of .00079 for both Idss pairs. . Perhaps THD is much higher at higher signal levels which I cannot check properly with my equipment.


However, I also did an additional test. I used a 8ma as follower with no source degeneration and a 9ma selfbiased with source degeneration of 3.5r to give me a near zero DC offset. At 0.5v, THD was .00068. So far these are the lowest readings I have got. H2 was a bit higher than H3 without any degeneration in the first two cases and and lower than H3 in the third case with the 3.5r degeneration. I tested twice but still wonder whether the results are plausible.


Thanks. nash
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Old 20th January 2020, 12:43 AM   #5852
gadut is offline gadut  Indonesia
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Salas hotrodded blue DCB1 build
@Salas

with more inventory, dcb1 should drive 4 stages : Minidsp with 10k input, LXmini kit with 25k and 2 FW clone with 100k input. my calculation shows total paralel input 6.2k

i assume dcb1 should be able to drive this load, but for sure i need your confirmation.
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Old 20th January 2020, 01:03 AM   #5853
Salas is online now Salas  Greece
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As a resistance value yes it can but the many interconnects will also add significant capacitance that demands current the more HF the signal goes. Buffer all of them outputs with 100R in-line resistors and try results. I would better use the DCG3 high constant current biased (Class A SE) output stage preamp in this case (but it has gain too).
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Old 22nd January 2020, 07:34 AM   #5854
gadut is offline gadut  Indonesia
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Salas hotrodded blue DCB1 build
i already have amb a20 as headamp but i think that extra gain will jeopardize my gain structure as i usually put my potentiometer around 10.00 - 13.00

i will consider using tkd selector 2R6 which might suit my need
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Old 26th April 2020, 11:10 PM   #5855
twitchie is offline twitchie  Canada
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Salas hotrodded blue DCB1 build
I had built up a DCB1 (Mezmerize) black GB board hoping to use the shunt psu for a test of other things and spent an evening measuring LEDs. I had a quad of matched 2SK170 and picked out a pair of N and P mosfets from a batch I bought from Tarasque long time ago - measured and matched, although they are not matched N-P (matched NN and matched PP) but not that far off based on the stickers (4040/4040 and 4086/4087).

I couldn't figure out why I had such an imbalance between + rail and - so I merely swapped out red LED for green on one and red LED for blue on the other and left it with about 0.1V (100mV) diff.

Now I know, thanks to 6L6 that blue LED are no-no as a vref so out it comes and so does the green one on the other side. I put in 2 new LEDs on both sides (5 each side) and I get a full 1.0V difference between the + and - rails (10.7 and -9.7).

Now when I measure the Vdrop from ground to LED legs and do the math, I see a consistent 2.0x V across the + side LEDs and 1.8x V across the - side LEDs. I found this disturbing because the LEDs all came from the same bag, same batch that I had gone through so much trouble to measure and match.

When I measure Vdrop across a single LED in circuit, I see the same thing. MMeter shows 2.0V on the + side LED and 1.8V across the - side. Completely puzzled at this point because I fully expect to see 1.8V through an LED, not 2.0V.

Just measuring the PSU at this point. BJT (550 and 560) are C-grade hFE (550CTA and 560CTA) if that matters.

This is my first DCB1 so I wasn't sure how close the rails should be but I suspect after reading some of this thread that 1.0V is far beyond normal.

Anyone have ideas on what I should start replacing or measuring?
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Old 26th April 2020, 11:26 PM   #5856
Salas is online now Salas  Greece
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Mezmerize classic has inbuilt different levels of current sourcing between Vrefs. Not your fault. That's by design. It was to tweak its sound for the better when running low PSU bias sans sinks. Blue Hypno that was for hot-rod, and the Mez 2018 "ten years after" reissue after nobody was planing to run a Mez naked anymore, do not have that tweak. This is the Hypno thread so you wouldn't meet significant rails imbalance references in the posts anyway.

If you want it perfectly symmetrical (especially suitable when also using it as a general supply) do the "leg trick". That will set it to modern mode. See point 6 in the following link: Mezmerize B1 Buffer Preamp
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Old 27th April 2020, 01:30 AM   #5857
twitchie is offline twitchie  Canada
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Ah, so I'm not crazy after all? What an evil trick

Thanks so much for the reply Salas, and thank you for the design as well!

I have Hypno boards from the same era (around 2008 I think). Will they also have the same built-in vref difference?
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Old 27th April 2020, 01:31 AM   #5858
Salas is online now Salas  Greece
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The blue edition Hypno never had it. Because never meant to include a run lukewarm without sinks mode. For 2008 let me check.
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Old 27th April 2020, 01:46 AM   #5859
Salas is online now Salas  Greece
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Do your legacy boards look like this one? If yes, they do have the sneaky trap. Specific JFET to restore symmetry by off board legs joint is also shown. Ignore the rest of notes on the pic, they were about other requests.
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File Type: jpg HypnoReg.jpg (245.2 KB, 88 views)
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Old 27th April 2020, 03:25 AM   #5860
twitchie is offline twitchie  Canada
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Thanks Salas! It does look like that's the version I have. I will build the 2008 Mezmerize the way it was intended and then use the Hypno board for shunt psu and modify the leg as directed.

20200426_231939.jpg

20200426_231952.jpg

The small heatsinks are temporary as are the 20R resistors. When it's ready to be installed properly I will mount to chassis and replace with parallel 18R resistors.

One question - am I right that the Alps pot goes in with the shaft in the middle of the board and not sticking out? It seems to be meant that way based on the silkscreen. If I mount it facing the wrong way it will mean the pot turns the wrong way or will it still work that way?

Last edited by twitchie; 27th April 2020 at 03:29 AM.
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