Juma,
After playing with the feedback values I think I like 680/150R, your original values, the best. It has a smooth, airy sound I like and the widest soundstage.
I'd like to try a buffer on output. Why? Might get some more low-end dynamics and keep the airy highs without changing the feedback values. Also, might be awesome for headphones.
Do you have any recommendations?
Currently I am thinking about using the output stage from Wayne's Pass DIY headphone amp (attached) since the bias mechanism is convenient.
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Thanks Juma. That is good info. Currently Iam playing around with coupling caps to see if I can get it where I want it without adding another stage. Believe it or not, right now I have a 100uf Silmic cap on output bypassed by a poly cap and it sounds very good. I like the original lower feedback values. But at this point it’s really more tweaking than anything.
Thanks, I'll give it a shot. I'm trying to learn how to use KiCad so it might be a little bit before I get it to work.
So, I started to put together Juma's LSK/CM front end (very nice) + the pass optocoupler output stage. Why? I want to drive headphones AND power amps at home and Juma's front end has the sound I am looking for, spatially.
I am wondering about the feedback values. 680/150R sounds right to me...Best mix of dynamics and soundstage.
Wayne's follower uses 4.75K/1K for feedback. About the same ratio and gain. But the ground resistor loads the BJT's in Juma's stage and 1K might wrong.
Should I stick with 680/150R? Feedback will be as attached, all the way to the output of the follower stage. I am planning on mounting Juma's front end above the mosfet followers like UGS to potentially experiment with other follwers and front ends later. Fun stuff.
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After the first version I made of Juma's Current Mirror preamp was auditioned I decided to go back and try it again with better matched BC transistors (matched to 2% or so) and a little more attention to detail (mundorf silver solder, teflon wire, etc.). This time I used Takman metal films and omitted the pot since I was using a cap on output anyways. Hoping it sounds better than the first version but it's too early to tell. Seems a bit warmer but it's not broken in yet. I really like the sound of these preamps though.
Curiously, both versions seem to raise the DC offset on the power amp 10-20mv although they are cap coupled and they have no measurable DC on preamp output. Strange.
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Are you using a signal generator? If so, place a cap between the sig gen and your preamp and see if that solves the 20mV offset.
I do not. But I have a CD with some tone sweeps...I could use that.
Bu all my sources are cap coupled as well. It really is kind of strange.
Noted. Even so, capacitors leak. It might be worthwhile to use and external cap to rule out the source being a problem.
Hi Juma,
What is the current consumption of the last circuit with bipolar 12V?
I am always tempted to use batteries.
Chul
Juma will probably tell you for sure but there is 10-12ma through the bjt outputs...so maybe 30ma per channel?
Thank you Hikari1,
I have 1400mA Lipo battery packs, and maybe this can be worth trying. I will use the output cap, for (-) (+) voltage may not be the same after a while causing DC offset?
Cheers,
Chul
Hi Chul,
you didn't write the post # of schematic that you are interested in, so I'll guess it's a sch. from post #539.
The consumption will depend on Idss of your JFETs and it can be anything from 15 to 25 mA, per rail, per channel (with current through Q6,Q7 set at 10mA)
Offset will drift mostly with temperature, not with small variations of PS voltage.
Read posts from #485 onwards.http://www.diyaudio.com/forums/pass-labs/244106-lsk-pre-baf-2013-a-post4254886.html
you didn't write the post # of schematic that you are interested in, so I'll guess it's a sch. from post #539.
The consumption will depend on Idss of your JFETs and it can be anything from 15 to 25 mA, per rail, per channel (with current through Q6,Q7 set at 10mA)
Offset will drift mostly with temperature, not with small variations of PS voltage.
Read posts from #485 onwards.http://www.diyaudio.com/forums/pass-labs/244106-lsk-pre-baf-2013-a-post4254886.html
what was the issue with the power amp
Hello Hikari1,
I like the detailed reports about your journey to your optimal solution. (I have a BA-3 as preamp being build(for way too long now), this one will be the next candidate)
In post #534 LSK pre - BAF 2013 you wrote:
Did you change the Power amp or did you fix the issue with the F5T?
When fixing the F5T what did you fix?
Happy listening
Bert
PS, Ah, I see your are stil investigating: F5 dc offset issue
Hello Hikari1,
I like the detailed reports about your journey to your optimal solution. (I have a BA-3 as preamp being build(for way too long now), this one will be the next candidate)
In post #534 LSK pre - BAF 2013 you wrote:
Did you change the Power amp or did you fix the issue with the F5T?
When fixing the F5T what did you fix?
Happy listening
Bert
PS, Ah, I see your are stil investigating: F5 dc offset issue
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Yes. It appears I have some jfet leakage on the F5. It makes the DC offset on the power amp fluctuate depending on the preamp used. So, not the fault of Juma's preamp. Which is excellent and I highly recommend it.
Hello friends! I was reading into this thread. Really interesting and very informative. Thanks Juma and everyone else giving feedback, pointers and help.
I find that the only downside of forum-type threads like this is that there is no way to get the most current info / version in one place. You need to read schematic on page 1, then follow to post 66, then read 500 other posts to know that CXX on page 30 is reversed... it is really, really hard to follow, specially for non-pros like me.
Which is the 'Current Mirror Version'? Is current referring to the most recent or to electric current? Is there one 'final post' where most of the knowledge of the original thread is summarized?
Thank you very much, best regards,
Rafa.