John Curl's Blowtorch preamplifier part III

Status
Not open for further replies.
Jump to Latest
#18,585
> Apparently someone is taking our designs and making something for themselves and others. Go for it!

For Scott's phono I made the changes to the circuit only after discussion with Scott.
There is only one private example now (mine).
Scott is the only other person who is in procession of those PCBs and schematics.

It was just an offer to make them available in case anyone is interested, including all matched FETs (from a pool >>200).
Assuming of course Scott has no objections.
I don't think many people around here appreciate those circuits, judging from the responses of the original thread by Scott.


Patrick
 
#18,587
So I should send you a set of PCBs and FETs, and you want to Beta test, without waiting for Scott ?
(He might dream up some more changes, you never know ......)

Send me your email and postal address by PM.
I need to arrange someone to sort out the FETs.
May take a few weeks.

May also take some time to finish the documentation.
Scott does not need them. 🙂


Cheers,
Patrick
 
#18,588
Surface mount? Current project has one IC with .025" lead spacing, wasn't a problem soldering it. Half a dozen .050" lead spaced opamps, but I do try to keep all the resistors and COG capacitors to 1205 size. However the 402 diodes are a bit harder.

The secret is in putting vias as the end of their pads to allow for a landing spot for the tip of the soldering iron.

The PCB is limited in size to 2.55" x 1.6".

The first one of the moderately complex design turned out to be a disaster. A number of shorts that did not show up in the layout software but seem to have crept into the physical reality.

Looks like the next version may use a jumper or two to avoid traces that weave between sides, or a four layer board.

Right now it is three stacked boards. I suspect it will go to just two.

Three days in on debugging it now!

Did find a few component value errors ( ex. 4.7 uF not .047!) and can lose two resistors but will know today if I have to add one.
 
#18,589
EUVL said:
I don't think many people around here appreciate those circuits, judging from the responses of the original thread by Scott.
Click to expand...
The question is that phono is a little archaeological for most of the people, but the tread is passionate for a designer or a DIYer. Un example of good engineering OMHO.

Specifications cleverly studied and explained, a simple and elegant solution, in a perfect technical approach (no surprise from Scott), with just the right amount of audiophile approach (that surprised me from Scott), with an open mind for a community work and suggestions.

No doubt that this circuit and the J.C. one are the bests that some can get or build on the market. As I said before, hats on to all the participants, and specially you and Dimitri.
 
#18,593
I regularly put down 28 pin chips with 0.65 c2c pin spacing. The trick is to secure 1 corner pin centrally on its pad, then the diagonally opposite pad, making sure they are aligned. Press down with a small screw driver to make sure the chip is flat on the board. Then you do the unsoldered corners. After that I run the solder quickly down each side so there's just a line of solid solder on both sides (all pins now shorted).


Next step is to use solder wick to remove the excess solder. Spray with solder flux to clean and the jobs done, finish off by examining with USB microscope.

Sounds like a lot of steps, but its actually very quick.

One other thing, you have to use very fine solder - I think mine is 0.5mm or 0.8mm diameter.

1206, 0805, SOT23 and SOD323 all a piece of cake.
 
#18,598
EUVL said:
Assuming of course Scott has no objections.
Patrick
Click to expand...

Certainly not. As I originally mentioned I was playing around with Bob's VinylTrak front end and decided to try and do an all in one stage version. I traded it off a little light on gain at 1kHz, this only shows up in the last octave of SNR and A weighted it's fairly insignificant (needle down not any issue at all).

The no-feedback, all FET, all PS caps was just a fun aspect, but the performance even by measured standards is pretty good. An open loop V to I into the RIAA network as the only forward gain is not the same as a G = 1000 amp in front of a passive RIAA.

I disagree with several popular ideas on phono issues. Ticks and pop's are not music and reproducing their full amplitude is pointless. If doing RIAA you get some spread due to the impulse response, clipping a pop off cleanly reduces its total energy and is a good thing. An open-loop FET pair has a very soft overload characteristic and because JFET's are majority carrier devices there are no charge storage or sticking issues.
 
#18,600
scott wurcer said:
Certainly not. As I originally mentioned I was playing around with Bob's VinylTrak front end and decided to try and do an all in one stage version. I traded it off a little light on gain at 1kHz, this only shows up in the last octave of SNR and A weighted it's fairly insignificant (needle down not any issue at all).

The no-feedback, all FET, all PS caps was just a fun aspect, but the performance even by measured standards is pretty good. An open loop V to I into the RIAA network as the only forward gain is not the same as a G = 1000 amp in front of a passive RIAA.

I disagree with several popular ideas on phono issues. Ticks and pop's are not music and reproducing their full amplitude is pointless. If doing RIAA you get some spread due to the impulse response, clipping a pop off cleanly reduces its total energy and is a good thing. An open-loop FET pair has a very soft overload characteristic and because JFET's are majority carrier devices there are no charge storage or sticking issues.
Click to expand...

The real issue here are phono stages that have very limited overload capability.

John Atkinson has tested a few with low overload and one IIRC had only c. 9dB and I think that is too little - and especially so when its is just very easy to engineer phono amps that are quiet and have reasonable overload.

WRT clicks and pops, surely the better approach is to just be able to deal with it as it comes. Does clipping it make it any less obtrusive? If it clips, it generates more harmonics and I'd say that's 'spreading' it as well.
 
Status
Not open for further replies.
Top Bottom