Preamp ideas for F5

juma · 2012-06-22 4:42 pm

buzzforb said:
I see that the source of the bottom BS is grounded, so how does the 12K resistors affect bias

Here you are talking sch from post #206. BS170 is a small enhanced mode MOSFET and R8/R4 voltage divider sets the Vgs of Q2 i.e. the Id of Q2. The second voltage divider R8/R10 is a feedback loop that sets the gain ratio and puts the Q2 in "Schade"/triode mode. So, the R8 has double duty.

Your question from post #214 (about sch. in post #53) mentions the 100k resistor. Yet, that sch. contains no 100k resistor...

Also you mention 1014D SRPP which is JFET and can not substitute BS170 (which is enhanced mode MOSFET).

In post #216 you talk again about sch. from post #206 and 100k resistor - it doesn't set the Zin, it's there just to provide discharge of the coupling cap. The Zin is set by R10 (because of specific feedback loop).

In post #220 you raise again the sch. from post #53 and cascoded buffer, and yes, when the Gate and the Source of the JFET are at same potential (Vgs=0) the Id=Idss, which is not the case in either sch. since the Id of the preamp (sch. #53) is less than Idss (set by P1 for both JFETs) and the Id of cascoded buffer if also less than Idss (also set by pot).

I hope this answers your questions. If not, rephrase them and ask again (but with more clarity and exact references).

All in all, I declare you a sole champion of MQ (messy questions) ! :rofl:

Zen Mod · 2012-06-22 5:21 pm

well

he's my man Buzz !

:clown:

buzzforb · 2012-06-22 7:08 pm

I would like to thank everyone who has helped me get to this point. It is only because of their generous contribution that I am able to accept this award.

You think it is bad on your end, imagine what is happening in my head. I personally think I deserve an award for powers of observation as well. (BS170 Enhancement mode...DUH!) NO worries anymore, Ill just let you know how they sound!

zgregz · 2012-06-25 7:08 pm

2SK170 LTP and CCS

Hello,

I was wondering if modifying the schematic from post #29 to use 2SK170 for both the LTP and the CCS would be a good idea.
It seems to work on LTSpice as far as I understand what I am simulating but I would like to know your thought about it.

Thanks for helping another newbie

Greg.

zgregz · 2012-06-25 8:26 pm

Here is what I am thinking about :

Feel free to comment.
Thx

juma · 2012-06-25 10:13 pm

Zgregz, if you want to keep those resistor values you'll have to use 2 paralleled JFETs in CCS (J3) to produce 20mA. J1 and J2 should have Idss of about 12mA each and J3a and J3b should have Idss=10mA each.
Some JFETs are made symmetric but still I'd put Gate and Source (not the Drain) together on the -U rail.
Watch the Pd of J3a and J3b (18V x 10mA = 180mW). I'd use a 680R resistor in the Drain (upper) branch to lower the dissipation.
Anyway, the CCS from post #29 will work better than 2 paralleled JFETs....

tinitus · 2012-06-25 10:27 pm

juma said:
Another idea for a nice preamp suitable to drive F5 (post #55 and onwards) :
http://www.diyaudio.com/forums/analog-line-level/206532-if-you-dont-have-sit-fake-2.html#post2950967
When used to drive F5, C4can be 1uF and R9=1Meg. For C3 (10uF) I successfully used SMD (0603) COG cap.

how about R6(input to ground), can it be high impedance, like 1Mohm ?
or what would be max ?

a bit ot, sorry

well, I see you say 'fair zin', so I guess not so easy to change much there

juma · 2012-06-25 11:34 pm

Tinitus, read the post #221. R6 is a part of voltage divider that sets the Vgs and it can't be 1M because the other part of that divider (R5) sets the gain with R7(which determines the Zin).
Go 22 posts back in this thread and read from there on...
Anyway I made a better version of that preamp:
MOSFET-trioda Mju folover verzija - Solid State - diyaudio.rs

tinitus · 2012-06-25 11:43 pm

mosfet-trioda

hmm, bs170 should be easy enough
Im tempted

zgregz · 2012-06-26 5:54 am

juma said:
Zgregz, if you want to keep those resistor values you'll have to use 2 paralleled JFETs in CCS (J3) to produce 20mA. J1 and J2 should have Idss of about 12mA each and J3a and J3b should have Idss=10mA each.
Some JFETs are made symmetric but still I'd put Gate and Source (not the Drain) together on the -U rail.
Watch the Pd of J3a and J3b (18V x 10mA = 180mW). I'd use a 680R resistor in the Drain (upper) branch to lower the dissipation.
Anyway, the CCS from post #29 will work better than 2 paralleled JFETs....

Thanks Juma!
I didn't think about power dissipation!

It may be a better idea for a newbie like me to use the CCS from post #29.

I have a bunch of 2SK170BL and some ZTX, but no BD139, is there a suitable alternative to the BD139 in the CCS? I have some D44h11, can I use these instead of the BD139?

Thanks again,
Greg.

juma · 2012-06-26 10:05 am

D44 is in higher power class but you can use it. BD139 is a useful BJT you should get some:
BD139 - STMICROELECTRONICS - TRANSISTOR NPN TO-126 | Farnell France

zgregz · 2012-06-27 6:43 pm

Thanks for your comments and help Juma!
I will try with the following schematic in a first time (and get some BD139):

It seems to work on LTSpice, I will try it in real world!
All comments are welcome!

Greg.

Jay · 2012-06-28 5:36 am

May be there are some interesting ideas for a fet preamp from this blog (V/I, current output amplifiers): V-to-I Amplifers

wengtech · 2012-07-17 3:55 am

hi,
i plan to build aleph p1.7
my question is : can i use single channel for stereo unbalanced?

An externally hosted image should be here but it was not working when we last tested it.

is the schematic possible?

thanks.

AudioSan · 2012-07-17 9:03 am

the boards are for 2ch balanced. when you use it un-balanced, -in goes to signal ground. and same with -out.

Zen Mod · 2012-07-17 9:34 am

wengtech said:
hi,
i plan to build aleph p1.7
my question is : can i use single channel for stereo unbalanced?

......

is the schematic possible?

thanks.

no

you'll have huge crosstalk between channels , due to nature of circuit ;

do what Audiosan wrote , but do not ground - output leg

Samuel Jayaraj · 2012-07-17 9:52 am

Only ground -input and wire both + and -outputs separately. You can test and know for yourself which output sounds better in your setup.

AudioSan · 2012-07-17 10:13 am

yes of course

too early in the day here

Zen Mod · 2012-07-17 10:22 am

yup

for me , even if thinking per se is extremely risky operation , thinking before second mornin' coffee is even more dangerous

:clown:

wengtech · 2012-07-17 10:23 am

hi all,
thanks for info, so i can not use them for stereo unbalanced. ok got it.

basically i really wish to hear the balanced audio system, with aleph 30, aleph p1.7, and a biwire spk, but i can not found a balanced cdplayer (or at least the way to tweak my cdplayer becoming balanced).

so i decide to use unbalanced, because there is no point to be balanced without all system balanced, am i right?

anyway:
how about to wire the input with RCA (- to gnd), and the output with XLR.
so, preamp to cdplayer (RCA), preamp to poweramp (XLR).

Preamp ideas for F5

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