[steinchen]

while populating the Rev.C board some questions have been coming up to me. Since I haven't been able to find the answers by the documentation or by the search function I consider it a good idea to start a construction thread like the ones for PPA v2 and M³

1) R67 = RLED and should be (depending on the LED used) about 4k, right ?
2) the spec for C27 / C28 is missing. I can't find that small polypropylens, even styroflex types are starting at 47pF, polys much bigger. Should I go for the 47pF styro or pick ceramic caps ?
3) R65 ? R66 ?
4) how to wire a volume control pot to the amp ?
5) under what circumstances should I consider to populate the optional 20k pots for R63 / R64 ?

 

[phobus]

Quote:

Originally Posted by steinchen 1) R67 = RLED and should be (depending on the LED used) about 4k, right ? 2) the spec for C27 / C28 is missing. I can't find that small polypropylens, even styroflex types are starting at 47pF, polys much bigger. Should I go for the 47pF styro or pick ceramic caps ? 3) R65 ? R66 ? 4) how to wire a volume control pot to the amp ? 5) under what circumstances should I consider to populate the optional 20k pots for R63 / R64 ?

I'm sure others are more qualified but here goes:

1) yup
2) I think these are the compensating caps? I think they're supposed to be 5 or 10pf mica ones. I left mine unpopulated.
3) for testing. You can stick 10k resistors in there when testing for offset.
4) I did it: rca input -> pot -> input on board.
5) dunno

 

[bg4533]

R65 and R66 go to ground. Should be 5-10 times the value of the pot. I used a 50k pot and 1M resistor since it is what I had.

I used 10k LED for the resistor. Value depends on the LED. I believe Tangent has a calculator on his site.

Wire the pot to the input. RCA signal to pot. RCA ground to pot and board. Pot output to board.

edit:
hmmm...so am I wrong about the use of R65 and R66? If so I will remove it so I don't confuse others.

 

[bg4533]

Someone will ask eventually so...

Info on changing the gain and making a gain switch: link

 

[steinchen]

thanks dudes

for matching transistors take a look at
http://www6.head-fi.org/forums/showthread.php?t=91235
http://www6.head-fi.org/forums/showthread.php?t=93115

if c27 / c28 doesn't need to be a film type the recommendation goes to silvered mica and Kemet Golden Max I guess

R67: when using a low current LED (2V 3mA) a 10k resistor will fit, with a high current blue LED (3V 10mA) you'll need a 3k 0.5W resisitor

R65 / R66: 5-10 times the value of the pot sounds reasonable and meets the recommendation for the PPA amps

 

[bg4533]

I didn't use C27/C28. Everything is fine. No oscillation.

What are these caps for?

Can someone post some part numbers?

 

[steinchen]

10pF 200V 5% Kemet Golden Max digikey #399-1888-ND
10pf 500V 5% Mica digikey #338-1061-ND

 

[Pars]

Quote:

Originally Posted by bg4533 R65 and R66 go to ground. Should be 5-10 times the value of the pot. I used a 50k pot and 1M resistor since it is what I had. edit: hmmm...so am I wrong about the use of R65 and R66? If so I will remove it so I don't confuse others.

I don't think they are necessary if you are using a pot. These pads were provided if you DON'T use a pot. If that's the case, they should have something around 0.5 Meg in there. As you said, putting in low values can be used for testing (like 47-100 ohms), which effectively shorts the inputs. I have 475K in my Rev. B board right now until I get around to casing it up and putting a pot in.

Dan or anyone? Any different opinions?

-Chris

 

[amb]

Quote:

Originally Posted by Pars I don't think they are necessary if you are using a pot. These pads were provided if you DON'T use a pot.

I recommend populating R65/R66 anyway, at least 10x the value of the pot. This is in case the pot's wiper loses contact, the input FET would not lose its bias path to ground and cause a big DC offset.

About C27/C28, these are compensation caps for taming a minor overshoot in the square wave response.

 

[Pars]

Quote:

Originally Posted by amb I recommend populating R65/R66 anyway, at least 10x the value of the pot. This is in case the pot's wiper loses contact, the input FET would not lose its bias path to ground and cause a big DC offset.

Good to know, although I would think that to be unlikely. Nonetheless, cheap insurance. Guess I will populate the equivalent position in the headamp V2 board that I am currently using.

BTW, the compensation caps were suggested by Stackofhay at 33pf for the stock gain, less if you reduce the gain. He used the CDC micas from Mouser.

Thanks for the input Amb.

 

[amb]

Quote:

Originally Posted by Pars BTW, the compensation caps were suggested by Stackofhay at 33pf for the stock gain, less if you reduce the gain.

Actually I think it's just the opposite... you would use less capacitance if the gain is higher. This is because the GBP of the amp is essentially constant wrt. gain, and as you increase the gain, you need less capacitance to achieve the same corner frequency and phase margin.

 

[bg4533]

Has anyone else built a Dynalo in the Hammond case listed in the BOM?

When mine has been in use for many hours it gets pretty warm. Maybe too warm. I need to take some temperature measurements. I just wanted to warn people of this.

 

[skyskraper]

try taking temp measurements of the case, transistors in case, without lid, etc. iirc i measured a rise of like 4c with my dynalo in a 1ru rack case, not a huge rise at all.

how much extra space is there between the transistors and the lid of the case? maybe you could stick a low profile heatsink on top of them if it really is bad?

 

[BrokenEnglish]

Quote:

Originally Posted by steinchen 2) the spec for C27 / C28 is missing. I can't find that small polypropylens, even styroflex types are starting at 47pF, polys much bigger. Should I go for the 47pF styro or pick ceramic caps ?

reichelt.de stocks 10pf, 22pf, 33pf "glimmer" caps, steinchen... specwise they are hard to beat... not quite cheap though...

 

[steinchen]

Glimmer caps are in fact Mica caps. Just wondered if c27/c28 need to be film or ceramic. I think I got some spare 10pF Kemet Golden Max and silvered mica caps left over from my M³ build. I will take a look in my box and let availability decide.