[550914]

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[kevin gilmore]

you are going to put a serious strain on the power supply if you do this. may eventually damage the switching transformer.

 

[AudioCats]

problem with tl783 is that as the voltage goes up more and more power is wasted in the control resistor. And those zeners won't save the regulator from blowing up as
they will turn to dust just as quick. Best not to use over 150v. Although we both know someone that uses them at 500v.

much better is kerry's grhv-mini regulators. much more energy efficient, 1 module that bolts to a heatsink and is the size of a to-264. And rated up to 500v.

running the transformer on the 100v taps long term is a very bad idea. eventually the internal transformer temperature fuse will open, and then when you short
it out, the transformer will eventually smoke. i have a 717 here that did that. The result after shorting out the fuse was the high voltage windings shorted to each other.

more and more people are doing stuff like this, may be time to get new transformers quoted that fit into the existing space. Better would be a toroid turned sideways.

the zener string is there only to help during the power on.

the grhv module sounds interesting. Thanks for the heads up. SiC devices are kind of hard to get these days though.

The transformer in the 313 seems to be a super overkill for the amp. It barely gets warm in my non-regulated 313 (set to 120v for 120v input). The transformers in 007t and 727 are much hotter.

 

[AudioCats]

Would it makes sense to raise the bias current as well on my SRM-252S by replacing the D4 red led with a TL431 set at 2.5v?

My amp is vented on the top and I have a 12v 1.8A ifi PSU but I'm wondering if a 2.5v reference could be too much for this amp or not.
Atm I have a GREEN led instead of D4.

(Attached the schematic for reference)

the transistors in output stage will not handle the heat if using a 2.5v reference at D4.

in my 212, I added small copper TO220 heatsinks and tried it with LM385. The sound got a bit better but with the top open the transistor body was 72C, it will get way too hot once the top cover is installed. I don't see a easy way to pipe the heat directly to the amp body.

(The main difference between LM385 and TL431 is the minimum shunt current spec. 1mA for TL431, 10uA for LM385.)

 

[AudioCats]

2.5v precision reference comparisons
(313 stage-b, driving sr007)

to avoid:
LM4040 (cold, with "mid-range burn")
LT1004 (the sound is fine, but doesn't justify the $7 price; this is a dual reference with a 7v and 2.5v in one body


-- LT1634 is the most transparent but very upfront (on stage), sharp and clear sounding , I don't like it for long listening
-- TL431 and LM385 are similar. TL431 has wider sound stage, more layback sounding; LM385 is more upfront, warmer and more engaging.

I'd recommand trying the TL431 first, if there is still irritation in the sound then change to LM385.

 

[550914]

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[AudioCats]

4040 L brackets from amazon, $16 for a 10 pack.

Kind of tricky to install, but do-able. The holes in the bracket are super large, I just used normal screws with large washers to mount it to the aluminum heatsink plate.
(I will clean up the thermal grease later...)

Put the amp back together (cover and panels installed, all screws installed) and run it --> after 15hrs, the temperature of output transistor (measured at the mounting screw head) was 56C, transformer body was 45C. ambient temp ~20c. (Stage-b, with 2.5v reference at D4. Total power consumption was 18w/27va.)

 

[AudioCats]

Yes it seems wiser not to try 2.5v with the 252S.
I've replaced the red led with a green one so the current is still a bit raised, it's better than nothing

LM385 has an adjustable version, it can do 1.25v~5v by adding two resistors. This version has lower spec (3% initial accuracy, 150ppm/c) than the fixed voltage ones.

I have ordered a few to try out in my 212. I will set it to 1.6v and compare the sound with the stock red led.

 

[AudioCats]

(SRM-212) Using precision voltage reference to replace the red LED in the current buffer (output) stage. The red LED is D4 in the srm-212.

Precision voltage reference used was LM385-adj; Added 100k as top resistor, 33k as bottom resistor ==> 1.65v, close enough to the red LED's 1.6v.

comparison was done using a pair of sr303.

** LM385-adj @ 1.65v: more upfront, more "live", easier to spot little details, bass details ok, stronger bass slam than LED, slightly boomy.
** Red LED 1.6v: sound not as full, somewhat simplified and harder; more mids-centric; much lighter bass, more "one-note".

overall, similar kind of improvements as experienced in 313 mods, but to a lesser degree. In the 313, installing the LM385 improves transparency, everything was better. In the 212 the improvement is not quite as obvious, but still definitely there.

 

[AudioCats]

(SRM-212) Aluminum strips as heatsinks for the output stage

bought some 6" aluminum rulers off amazon, $6 for a 4-pack, they are 16mm wide and 1.4mm thick.
https://www.amazon.com/POP-up-Alumi...p+aluminum+scale+6+inch&qid=1633805060&sr=8-2

As rulers they are not very high quality. But they work fine as aluminum strip heat sinks. Hole spacing 44mm.

Adding these strips increases surface area by 3~4x.

 

[AudioCats]

(Srm-212) effects of different reference voltages at D4

This was done with aluminum strip heatsink installed. DO NOT TRY voltage higher than 1.7v without heat sinks.

using LM385-adj.
Compared 1.67v (100k/33k) and 1.84v (100k/47k).
-- The sound with 1.84v was more full and realistic. Not by much, but definitely there.



power consumption and temperature:
1.6v (stock LED) at D4: 0.37A power current draw (regulated 12v); voltage rails 202v (only measured one; assuming the other rail is the same)
1.84v at D4: 0.41A / 12v; rail at 196v.

with 1.84v at D4, case top cover installed, all screws installed, using 12v regulated supply, ran overnight:
=> 62c at output transistor body(mounting screw).

(with internal heat sinks installed) I think 1.8~1.9v is about as high as it should be pushed,

 

[550914]

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[AudioCats]

....
I'm gonna order a pair of LM385 to do the same on my 252S right away
....

LM285 is the higher spec version of LM385.

LM285= -40 ~ +85c
LM385= 0 ~ +70c

the LM185 is much much more expensive. it will handle +125c but I suppose such capability is not needed in an audio device.

I'd recommend using LM285-adj. It is about $1 more.

also, the resistor feeding D4 (R25 in the 252) is 820k stock. I am using 390k in my 212. If you want to keep the 820k stock value, then the top/bottom resistors in LM285 circuit need to be higher than what I showed above (otherwise these two resistors will drain so much current the LM285 won'd do much). maybe 330k/100k for ~1.6v.

 

[550914]

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[AudioCats]

formula from the data sheet:
Vout=1.24*(Rbottom + Rtop)/Rtop


1.8v Vref with stock emitter resistor (750 ohms) will set the CCS bias to about 1.6mA. I ended up changing the emitter resistors (R23/R24) to 1.2k and just use a fixed voltage 2.5v LM385. it gives similar result.