Doug Self Preamp from Linear Audio #5

[Carl_Huff]

The part in the circle is a 20uf tantalum cap that was requested by users that had oscillating 12 volt regulators.

 

[tomlang]

I used a 22uf tantulum but have not powered it up yet. It IS polarized AFAICT so be sure and orient it correctly.

 

[SheldonSm]

Ok thanks. Kind of what I thought but wasn't sure what value. I'm assuming that I can use 22uf 16volt or higher. I'll do my best to orient correctly.

 

[starcat]

The R values can be seen in #246 as well, 47k and 10k.
That oval thing is not a resistor and wasn't present on earlier versions either.

 

[aniko71]

I just received the PCBs and I started to solder the components. But in the files that I got I have two different PSU schematics regarding some component values. I attach both of them. Please advise which are the correct values for R5, R6, R9, R10 resistors.

 

[Carl_Huff]

Both work but PSU2 is most correct. Jan Didden suggested the values in PSU2 as an improvement. Thanks Jan.

 

[aniko71]

Both work but PSU2 is most correct. Jan Didden suggested the values in PSU2 as an improvement. Thanks Jan.

Thank you very much for your quick response.

 

[diyralf]

Both work but PSU2 is most correct. Jan Didden suggested the values in PSU2 as an improvement. Thanks Jan.

Is this PSU better than a LM317 supply?

 

[Carl_Huff]

I think it is, but I invite other to disagree, altho a LM317/ LN337 combo can be very good as well.

 

[diyralf]

It seems rather artificially overengineered to me. It also doesn't have short circuits or temperature handling. If it had less noise, okay.

 

[Carl_Huff]

It seems rather artificially overengineered to me. It also doesn't have short circuits or temperature handling. If it had less noise, okay.

If you know how to improve the PSU, I invite you to document your revised circuit on this thread. If your change is worthy members will quickly embrace it and I will integrate it to the PCB.

 

[Hicoco]

Both work but PSU2 is most correct. Jan Didden suggested the values in PSU2 as an improvement. Thanks Jan.

see jan comments on PSU1 design oversight Post # 1404

 

[Frabor]

Hi,

new to this thread and looking at the information. I cannot find the part number of the rca connectors on the BOM from dropbox. tks.

 

[Carl_Huff]

That's because I supply those when you by the PCB kit.

 

[starcat]

I saw them somewhere at Mouser as well: https://www.mouser.at/ProductDetail/490-RCJ-412323

 

[Carl_Huff]

Those at Mouser are dimensioned different than the ones we provide but should work fine if you are doing your own PCB layout.

 

[starcat]

I see, thanks Carl!

 

[Carl_Huff]

If anyone is looking for more of the RCA quad connector blocks that we ship with the kit, we have loads on hand.

 

[aniko71]

Can anyone tell me if I can use CK1054 Rotary switch instead of CK1059 that is mentioned in BOM?

 

[fvzeppelin]

Inspired by the discussion here about an IR controllable version of this preamp, I have started again to think of possible solutions. I have had many ideas which I often discussed with @Carl_Huff (thank you, Carl, for your time - I wouldn't have come this far without your help). Most of the ideas were quickly discarded, and some made it to a design/routing phase. Finally, I implemented a concept which allowed me to re-use the preamp PCB and the +-15V supply PCB:

1. I designed a PCB to hold MCP41HV51 digipots which fits exactly on the original preamp PCB as a replacement of the mechanical pots. Luckily, I had one of these PCBs already at hand from an earlier try
2. The IO PCB needed a few modifications:
   • separate grounds for audio/relays
   • low side relay switching with an ULN2003
   • updated 12V supply
   • for my convenience: selectable relay supply (onboard/external) and selectable output delay control (onboard/external)
3. As microcontroller I am finally using an Adafruit Grand Central M4 Express which can be programmed using the Arduino IDE and many of the Arduino libraries can be used, too. Originally, I had used an Arduino Mega 2560 which caused a lot of headache to me as it turned out to be the source of hizz in my setup.
4. I added a small RGB OLED (small enough to fit into a 1U case) and a rotary encoder + switch.
5. The digital stuff is powered by a small 5V SMPS.

Some impressions:

Front:

Digipots:

Modified IO board:

Everything combined:

Example splash screen

So far, I haven't done measurements yet but the preamp sounds good to me (=without audible noise/distortions) which is my primary goal. Nevertheless, there are things left to be done which I will follow up step by step:

• Implement the IR control which I had honestly forgotten when designing the front panel
• Implement using the big standby switch
• Cable cleanup
• Implement NVM storage of settings
• ...

I have a readily designed zero crossing detection circuit and all the parts for it at hand. But after listening tests, I am no longer planning to implement measures against zipper noise as there just isn't any that I could hear.

Any hints/suggestions/questions are always welcome!