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Old 30th July 2010, 01:21 PM   #4251
Salas is online now Salas  Greece
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Originally Posted by SGregory View Post
I need to read my posts better. That was suppose to say "I wasn't going to eliminate the buffer".
Because you are a tube guy it rung true. Tube guys avoid buffers because many have negative experiences by using the wrong tube. They do the paper highs stiff mids thing. This buffer in your design can pass 200kHz square, another story. My 6V6 trioded configured as a 30mA bootstrapped cathode follower is doing the dynamics and tone thing significantly better than my DCB1 no matter the two needed Teflon caps, and that is what I use my simplistic with.
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Old 30th July 2010, 02:19 PM   #4252
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Ok. Here is my latest shunt layout with suggested changes included. Went back to single brd per shunt to allow better distribution on heat sinks and to improve chassis layout. Would this circuit benefit with a top plane of copper grounded to chassis?

Gate stoppers on Fets.
Grounds brought to center of board in large copper pad.
It worked out that sense components are isolated from power side.


Anything glaringly wrong?
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File Type: png Component View Shunt no trace with remote sense.png (18.8 KB, 238 views)
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Old 30th July 2010, 02:32 PM   #4253
Salas is online now Salas  Greece
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Looks well distributed. Just check that all connections are 100% compliant with your reg schematic for one last time. I don't think that a top ground plane will add anything more than unwanted capacitance. When you will finish it, just connect a dummy load for say 40mA consumption with all 4 wires (twisted per force and sense pairs) and connect the oscope across that load and sweep time and voltage knobs. You got a fine HP oscope recently when debugging your valve amp I remember. If it gives you a thin straight line, you are done.

P.S. Don't forget to mini sink the BC546 that has the Jfet collector load.
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Old 31st July 2010, 08:24 AM   #4254
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Originally Posted by SGregory View Post
Joao, What variant of the sk170 are you using? GR= 3-7 BL = 6-12, and V= 10-?. I was curios, if I could find the V variant it may eliminate the need for a buffer all together and still have a reasoably low impedance.
SG

I use a BL version. Maybe you can use a V variant, but my way is to work with transformers, as well I like to have stable impedance standards: the studio norm 600 ohm output impedance and 10Kohm input impedance.

Other point is to drive cables at the output easely so I like to use medium power transistor on the output stages. My experiences with driving the LCR RIAA showed me that's better to drive them with more ability in delivering current.
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Old 31st July 2010, 08:32 AM   #4255
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Here is what I was commenting. I thought you said using a buffer after first stage too.
That would be a nice idea to use a buffer after every stage to drive ...

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A buffer although sometimes beneficial, I use it in the simplistic too, its a stage on its own utilizing 100% local feedback in my eyes. Zo with the Jfet buffer here is around 40 Ohm. Else my DCB1 unity pre isn't a stage, its a glowing wire for connecting people's CDPs.
What's DCB1 and with which JFET you get the 40 ohms?
Of course you have 100% feedback, but local feedback. Remember in triode, which so many music lovers prefer, you have local feedback as well because of it's internal electrical structure!

I agree that you can use paralleled FET / BJT for getting more current, and then dynamic, but see my response to SGregory because I like the idea of using a buffer.

Another topic regarding your answer with BJT as input device of the phono stage. How would you solve the problem with tha base voltage - I mean that the base voltage is 0V - and not using an input capacitor?
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Old 31st July 2010, 10:29 AM   #4256
Salas is online now Salas  Greece
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DCB1.

Short answer for 0V in a BJT headamp is...never. Complementary symmetrical beta matched emitter follower input with trimmed current source can go a long way though. Still, in case of failure, happens to carts what happens to speakers if a complementary amp fails.

This is a wide discussion about general electronics ideas for phono non the less, and not about a certain concept that the simplistic is...
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Old 31st July 2010, 09:07 PM   #4257
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DCB1.

Short answer for 0V in a BJT headamp is...never. Complementary symmetrical beta matched emitter follower input with trimmed current source can go a long way though. Still, in case of failure, happens to carts what happens to speakers if a complementary amp fails.

This is a wide discussion about general electronics ideas for phono non the less, and not about a certain concept that the simplistic is...
Thanks for the explanation! May I have one other question - not direct related with the RIAA amp

I exoerimented with the SK170 in a linestage as well but I think this is not the best device for this application. Any suggetion what transistor is better suited for a line stage?

Last edited by Joao@AltheaMusica; 31st July 2010 at 09:08 PM. Reason: typing correction
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Old 31st July 2010, 09:18 PM   #4258
Salas is online now Salas  Greece
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2SKs they are not the best if you want gain in line applications indeed. See about BF862 if you don't mind handling surface mount components. I prefer tubes at that signal level in general.
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Old 31st July 2010, 09:54 PM   #4259
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Quote:
Originally Posted by AndrewT View Post

LOOP AREA is critical.
I just replaced power lines from psu to shunts with coax mic cable and did the same in the sense wires from shunt to riaa (another type of coax).

Things got better but I still hear hum at full power and if I remove the lid, hum is all over the place.

I am sure there is no ground loop (as it would be aparent even with the v1 shunts)

I am inclined to the Loop Area idea. What can we do in a pcb to reduce its antena effect ?

I can modify the layout so Force runs near GND lines inside the pcb area but can it help ?

Ricardo
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Old 31st July 2010, 11:00 PM   #4260
Salas is online now Salas  Greece
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The lid thing says it must mainly be loop area pick up. It would be great if you could see with an oscope that your power lines are free of any oscillation too. Can you shield the force cable runs also?

By tightening and shortening the gnd runs near power runs everywhere is certainly beneficial. Also, can your big film psu caps change orientation for a test, or just move regs in angles a bit and see if something changes?
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