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Old 8th March 2013, 02:14 AM   #2031
Salas is offline Salas  Greece
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Quote:
Originally Posted by bnorrish View Post
Quick eyeball from the experts. I took Salas suggestion to increase the resistance of R21-R24 to increase the voltage around Q3. My differed quite a bit from Wayne's. A few of you said this was probably due to different values for ldss on the FETs I had vs what Wayne had.

I swapped in 20R resistors for the R21-R24 and measured - see attached. Original values for 10R in in parentheses. Voltages are pretty close, still a little under.

Close enough? How important is to be spot on with these?

Also, my voltage across R6 is higher, at 1.8 vs 1.5. Is this acceptable, close enough?

It sounds really good on the bench with my cheap test system - no issues. As I put a fair bit of expense and time into this project, I would like to tune it to its full potential, within reason.
It looks much better for voltage bias now. ~23mA total draw by the JFETs when the original annotation suggests 20mA (VR9/R9). Also 0.7V Vbe for Q3 looks normal at the current it passes. How is the current sharing between JFETS? I.e. is the mV drop across each 20R fairly close? Should be about 115mV? 1.8V across R6 means 1.2mA VS 1mA which is easily done if the LED is happier for forward voltage drop than in the prototype. No big deal.
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Old 8th March 2013, 02:37 AM   #2032
6L6 is offline 6L6  United States
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Salas has the proper analysis... It looks great.

The 1.8v across the resistor could be trimmed with a different LED, but honestly, don't bother.

Set it up on your main system and play a bunch or records. If it sounds good, play a bunch more. After 30-40 discs open it up and re-measure, just to see if it's stable. If it is (which it will be in all probability) then you now have permission to buy a new cartridge or start a new project.
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Old 8th March 2013, 02:39 AM   #2033
Salas is offline Salas  Greece
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As for the spot on importance question, look at this Toshiba chart. Its the transconductance VS drain current. As you see 5 to 5.6 is practically the same just above the 30 millisiemens mark. So your input stage gain is very close to the original (23mA/4JFETS=5.75mA each on average VS 5mA in original). The larger resistors promote better sharing also. Wayne knew that there is an optimal region at about 5-6mA for the type no matter if adding a bit of nVsqrtHz from the net Rs resistance (Value/4). Its a thermal trade off for each one's dissipation at IDSS and noise most probably. 8.5V or 9.4V VDS at about 0.1V VGS look linear alike. Gate leakage current should be very close too. In a nutshell you should be sorted as you are.
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File Type: png Snap 2013-03-08 at 05.23.50.png (84.0 KB, 159 views)
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Old 8th March 2013, 05:46 AM   #2034
kamis is offline kamis  Serbia
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I have the same problem with CCS red led in my X-Ono. It is much easier to trim R28 and R28 to get current you want, than to try to find 1.5V leds.
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Old 8th March 2013, 10:20 AM   #2035
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Quote:
Originally Posted by Salas View Post
How is the current sharing between JFETS? I.e. is the mV drop across each 20R fairly close? Should be about 115mV? .
The drop across R21-R24 is 114.7 to 116 mv.

I will catch up the other posts later. Snowstorm last night and I have to get out they door and get to the city. Cannot work from home today.
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Old 8th March 2013, 11:36 AM   #2036
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Originally Posted by 6L6 View Post
Salas has the proper analysis... It looks great.

The 1.8v across the resistor could be trimmed with a different LED, but honestly, don't bother.

Set it up on your main system and play a bunch or records. If it sounds good, play a bunch more. After 30-40 discs open it up and re-measure, just to see if it's stable. If it is (which it will be in all probability) then you now have permission to buy a new cartridge or start a new project.
Thanks. My audio budget in Quicken has been red flagged by the wife. May have to cool it for a bit. I do need to get a chassis as I have most of the parts for a B1. Buying the chassis is the hardest part on the wallet.
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Old 8th March 2013, 11:42 AM   #2037
Zen Mod is offline Zen Mod  Serbia
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go to Ikea and buy wooden drawer

then wax it and voila!

if screening is needed - use either copper or Al foil laid on inside walls

if good for Stratocaster , it's good for phono gadget
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Old 8th March 2013, 11:43 AM   #2038
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Quote:
Originally Posted by Salas View Post
As for the spot on importance question, look at this Toshiba chart. Its the transconductance VS drain current. As you see 5 to 5.6 is practically the same just above the 30 millisiemens mark. So your input stage gain is very close to the original (23mA/4JFETS=5.75mA each on average VS 5mA in original). The larger resistors promote better sharing also. Wayne knew that there is an optimal region at about 5-6mA for the type no matter if adding a bit of nVsqrtHz from the net Rs resistance (Value/4). Its a thermal trade off for each one's dissipation at IDSS and noise most probably. 8.5V or 9.4V VDS at about 0.1V VGS look linear alike. Gate leakage current should be very close too. In a nutshell you should be sorted as you are.
I appreciate the explanation. I get the gist of what you are saying, but I still have fair bit to learn. Thanks for reviewing with me.
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Old 8th March 2013, 11:53 AM   #2039
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Originally Posted by Zen Mod View Post
go to Ikea and buy wooden drawer

then wax it and voila!

if screening is needed - use either copper or Al foil laid on inside walls

if good for Stratocaster , it's good for phono gadget
I have the Parmetal cases for the Pearl 2. I may need to be resourceful for the B1.

My F5 is sitting on the floor in the Living Room. Very low WAF. My wife made me custom order a cherry end table to put it on. Also got the quote from the landscaper yesterday to replace a retaining wall.
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Old 8th March 2013, 12:02 PM   #2040
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ZM

You play guitar? I thought of building DIY guitar amp. I sold my Strat. I have a Taylor koa 314ce with a Fender Acousitronic amp. My playing has suffered - been building too many gadgets!

I had checked out Magma in your signature. Crazy Serbian Prog Rock! I like.
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