Input buffer for LuDEF, SissySIT and similar amplifiers

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#101
Sorry, “Swiss cheese” version was made only after your question and was not available from the beginning. Gerber files attached here and at first post.

Swiss cheese.png
 

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#104
It will work perfect as headphone amplifier or preamplifier. If potentiometer is connected at input, C5 should be reduced to 33 – 47 pF.
Gain can be set by R12/R11 ratio (post #29).
However, OPA551F are now in limited supply. You can find them only at Texas Instruments store, Win Source (Hong Kong based)and some other less known suppliers: OPA551F availability
 
#106
I've been looking into using this as a preamp too, actually one using a transformer, like in ZM's thread https://www.diyaudio.com/community/threads/whats-wrong-with-the-kiss-boy.293169/ and also found the OPA551 hard to find. Last night I found https://www.digikey.com/en/products/detail/texas-instruments/OPA551FA-500/1572793 which appear to be exactly the same as the OPA551FA/500G3listed on the BOM in post #1, except for a different finish ont h leads. (Whatever that means...) So I bought 4, thinking they should work out just fine, and hoping I haven't wasted $30. In any event, they still have 329 of them, it seems, so I hope the info is helpful.

Of course now I understand I need to buy two 33-47 pF capacitors... Why do I always find ONE more thing I need just after finishing a digikey order? :headbash:

Best

Nigel
 
#108
Hi tombo,
Could you advise a low noise power supply for this buffer? For now I'm leaning towards the original boards from AMB Sigma22, or something similar...

I'm planning a dual mono version (2x toroid, 2x psu, ...)

Thank you for any tips 😏
 
#109
Hi Nigel,
any OPA551FA/something is OK. Nice find and very helpful information on OPA551FA as Octopart search didn’t find them this morning. Seems that they arrived today.
No need to worry about C5 value. It will be perfectly fine without any C5 or with any small value like 10 pF and up to 33 pF with 25 K potentiometer.
 
#110
Hi HRDSTL,
This buffer doesn’t need low noise power supply. It has its own super regulators on board and basic CRC supply is OK. I will prepare an adequate CLC with Schaffner common mode filter, but it is not scheduled to be very soon.

In the meantime, use anything decent available that provides +-18 V DC minimum. Max. is +-60 V, but that high input voltage will require optimal PCB ventilation for adequate cooling.

Edit: don’t waste money on dual mono, please. Reason is the same as explained above. Each channel has it own super regulator, so two channels can be supplied from the common source still maintaining perfect isolation between channels.
 
#111
tombo56 said:
Hi Nigel,
any OPA551FA/something is OK. Nice find and very helpful information on OPA551FA as Octopart search didn’t find them this morning. Seems that they arrived today.
No need to worry about C5 value. It will be perfectly fine without any C5 or with any small value like 10 pF and up to 33 pF with 25 K potentiometer.
Click to expand...
Glad to be helpful, even if only in a small way, and thanks for confirming the part is OK.

Out of curiosity, what controls the C5 value here? What would happen using a larger pot, such as 50k or even 100k? Smaller C needed? What would happen if you used the original value of 220pF for C5 with a pot on the input?

As far as PSU goes, I'm planning to try the Good Gemini supply from ZM's Iron Pre design. Would be lower voltage (+/- 15V) as originally designed, which I understand from earlier in the thread would be fine with slight changes in R values, but it seems the schematic can also be adjusted to provide different voltages.

Best

Nigel
 
#112
C5 with R3 serves as an input RC filter, limiting possible RF (MW, LW bands) entry into buffer. As buffer has minimum 2 MHz bandwidth, it could amplify radio waves caught by cabling. It is just reasonable precaution but not mandatory.

As potentiometer will put much larger resistance value in front of C5, input filter will start to limit response at much lower frequency. For instance, with 10 K (25 K potentiometer close to 12 o’clock) low pass point would be at 72 KHz. It’s OK but unnecessary low.

As for the Gemini shunt supply, keep that fine piece for the some circuit that will benefit from it. It will be wasted here (super regulators thing). 🙂
If required, buffer can be supplied by +-12V minimum with 4 resistors change. For 15 V it is enough to change R1,R2,R7,R8 to 4K7.
 
#113
tombo56 said:
C5 with R3 serves as an input RC filter, limiting possible RF (MW, LW bands) entry into buffer. As buffer has minimum 2 MHz bandwidth, it could amplify radio waves caught by cabling. It is just reasonable precaution but not mandatory.

As potentiometer will put much larger resistance value in front of C5, input filter will start to limit response at much lower frequency. For instance, with 10 K (25 K potentiometer close to 12 o’clock) low pass point would be at 72 KHz. It’s OK but unnecessary low.

As for the Gemini shunt supply, keep that fine piece for the some circuit that will benefit from it. It will be wasted here (super regulators thing). 🙂
If required, buffer can be supplied by +-12V minimum with 4 resistors change. For 15 V it is enough to change R1,R2,R7,R8 to 4K7.
Click to expand...
Thanks for the excellent explanation. I think I understand.

The idea with the Good Gemini was really in two parts; firstly, I already have the Iron Pre built and running with the Good Gemini in it, so I was intending to put these buffers in to compare them with the 2SK170/2SJ74 buffer in ZM's schematic. Not a fair A/B if I change the power supply.

Also, since until yesterday I couldn't find any OPA551 I had previously planned to try your suggestion from post #70, of connecting the supply from the Good Gemini at C13/14. Now I've found the OPA551 this is sort of irrelevant, but I may try it anyway, to see how it works out. I can add the OPA551 later, of course. I take your larger point, though, that there is no need to regulate before the super regulators...

Best

Nigel
 
#114
Your plan is well set, but Gemini regulators could be the obstacle for proper results.
If this is the correct Good Gemini shunt schematic, those regulators are designed to provide 30 – 40 mA to load, with only 55 mA CCS current.
Good Gemini
In case of connecting buffer supply directly to C13/C14 points, I’m not sure if Gemini regulators will like having 330 uF connected in parallel.
Quiescent (idle) current of buffer is around 20 mA without OPA551 and 30 mA with them. Not much will remain to “kick” the VAS transformer. Anyway, you can test as planned but be ready to look for other PS.
 
#115
tombo56 said:
Your plan is well set, but Gemini regulators could be the obstacle for proper results.
If this is the correct Good Gemini shunt schematic, those regulators are designed to provide 30 – 40 mA to load, with only 55 mA CCS current.
Good Gemini
In case of connecting buffer supply directly to C13/C14 points, I’m not sure if Gemini regulators will like having 330 uF connected in parallel.
Quiescent (idle) current of buffer is around 20 mA without OPA551 and 30 mA with them. Not much will remain to “kick” the VAS transformer. Anyway, you can test as planned but be ready to look for other PS.
Click to expand...
OK, thanks. Looks like things may not work out as well as I hoped, but no harm in seeing what happens. And I can always put together a CRC or CLC to try the buffers out with a different PSU, I guess. Should be fun.
 
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