L'Amp: A simple SIT Amp

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bonzo15011978

Member
Joined 2009
Cannot decide

I'm between two builds: version 11 with lightbulbs or inductive one?
From the "rest of the system" point of view the inductive version seems better as I use fullrang speakers that need some aid in the bass region, from the "building" point of view the simple version 11 seems a more straightforward project.
Another thing I can't understand is the alternative bias circuit described here:
http://www.diyaudio.com/forums/pass-labs/201655-lamp-simple-sit-amp-196.html#post3730777
Can I use the same values or I need to change some values? Do I have simple to connect 47k (or better, 10k) input resistor to S2 point and the inductor to S1?
Thank you in advice for your help!
 
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Michael Rothacher

Pass DIY Apprentice
Joined 2001
passdiy.tv
Paid Member
Bonzo,

Whether you choose inductor or light bulbs, you'll want to use an adjustable bias voltage source (e.g. Part Deux). This will afford you more flexibility in terms of transistors, fine-tuning, etc. and everyone seems to prefer no degeneration. The bias in Figure 11 is a self-bias arrangement and the un-bypassed one-ohm resistor provides degenerative feedback. It's sooooo 2010 and rather out of fashion.

I don't know if the supply you referred to will be quiet enough. You'll want to ask someone who has tried it. I haven't had the chance.

The noise for the amp from Part Deux is around 150-200 microvolts, which is nice and quiet.

:)
 
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Antoinel

Member
Joined 2011
bonzo15011978 said:
I'm between two builds: version 11 with lightbulbs or inductive one?
From the "rest of the system" point of view the inductive version seems better as I use fullrang speakers that need some aid in the bass region, from the "building" point of view the simple version 11 seems a more straightforward project.
Another thing I can't understand is the alternative bias circuit described here:
http://www.diyaudio.com/forums/pass-labs/201655-lamp-simple-sit-amp-196.html#post3730777
Can I use the same values or I need to change some values? Do I have simple to connect 47k (or better, 10k) input resistor to S2 point and the inductor to S1?
Thank you in advice for your help!
Click to expand...
Hello bonzo15011978. Is this your birthdate? If yes, you have a very long life ahead of you.
 
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Antoinel

Member
Joined 2011
bonzo15011978 said:
@ Antoniel: Are you suggesting to try both versions? :D
Anyway, thank you both for your advice, hope my limited skills allow me to build at least one version!

Ciao
Click to expand...
Hello bonzo15011978. My apology for distracting you. I was asking whether the bold and underlined set of numbers 15011978 in your name mean your birth date on the fifteenth of January, 1978. The advice of Mr.Rothacher he gave you is the only and best one to follow.

Best regards.
 
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bonzo15011978

Member
Joined 2009
Antoinel said:
Hello bonzo15011978. My apology for distracting you. I was asking whether the bold and underlined set of numbers 15011978 in your name mean your birth date on the fifteenth of January, 1978. The advice of Mr.Rothacher he gave you is the only and best one to follow.

Best regards.
Click to expand...

Yes Antoinel, it's my birhtday date, and you didn't distract me, I was only joking as sometimes I seems always in hurry up to finish my amps, and you (involuntary) made me "stop and think", thank you for this!
I must say it's also a pleasure to be answered by you, by Mr. Rothacher and all the people like you that share their knowledge.
Well, as writen somewhere in this thread: "start build and then listen to yourself"...maybe I'll start building the simple fig.11 version just to begin to listen to, while saving for the rest of the components for the another versions!

Ciao
 
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DIYspaceW

Member
Joined 2009
About the floating Pass bias supply...
I want to give it a try, but before I open up the box,
are some more questions:
Should the bias supply not be ON the same time as the main psu? So there should
be not much inrush current, or am I wrong?
I want to adjust the bias voltage to -12.5 to -13V to find the sweet spot here,
is this even possible?
 
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DIYspaceW

Member
Joined 2009
This one I mean!

386255d1386432204-lamp-simple-sit-amp-neg-bias.jpg


audiong said:
Hi nicoch58,
Before I attempt to answer your question , some details are in order - my 193V SIT differs from Mike's design in several aspects. I tried different biasing schemes and settled on self-bias with a by-passed source resistor, which at the time sounded best to me. After the initial novelty of listening to the SIT wore off, I came to the conclusion that the amp was 'warmer' than I liked. There is really no easy way to adjust a self-bias amp, you have to do more than just twiddle a pot, that was when I switched to fixed-bias.
My previous experience with fixed-biasing (tubes) convinced me that unless I was prepared to regulate B+, it was not a good idea just to regulate the bias voltage - best to let both voltages float along together - so it is with the SIT. I used the attached unregulated negative supply circuit suggested by no other than The Master and in doing so eliminated a whole bunch of components including my least-favored IC, the 79xx. So far I have been pleased with the results.
I added a 0.33 ohm source resistor to limit the power-on current surge , prepared to accept the resulting degeneration in exchange for a longer and happier life for the SIT.
The power supply is a CRCLR configuration and I had to use a 20v transformer (Antec AS-4220) to make up for the voltage drop.
In addition to the 2SK82, both SIT amps have following in common :
Input cap. - ERO MKC 1864 4uF 10% 200v
Output cap - four 2200uF 35v Nichicon SE electros., one 10 uF Rifa PHE420, one 1.5uF Sprague Vitamin Q, all caps in parallel.
There is no simple answer to your question . I could live happily with either. The 193V SIT seems to be 'livelier' (function of voltage gain ?) and has plenty of what the Brits call 'pace and rhythm', the Follower SIT is more laid-back although no less dynamic, it is just not as 'frenetic' as the 193V. Both amps are VERY sensitive to source components, more so than any amps I have encountered and as I mentioned, in some circumstances both amps sounded 'tubebier' than the Yamamoto. Both amps are sensitive to bias adjustments and if you have the equipment, you could adjust the distortion spectra to your taste. The Follower SIT has a more benign clipping characteristic than the 193V when both amps are pushed to the limit. I happen to think that not enough research has been done on amp clipping phenomena , which I feel has more relevance than the fixation on distortion spectra, to good sound. But then what do I know ?
Click to expand...
 
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OllBoll

Member
Joined 2010
I built an inducted L'amp a while ago and compared to a similar amp but with a R100 and found I liked the R100 more. Now I've built a DeLite too and found that I had a problem with my setup which was high source impedance. The R100 didn't care but the R085 in the DeLite did and sounded much better when it was solved.

So... I'm considering revisiting the L'amp =)

I remember higher order distortion starting to appear above 4 W so my question is: If I want to parallel say 2 2sk82, do I need to match more tightly than simply buying 2 of the same rank and then possibly having a separate bias supply for each?
 
indra1

indra1

Member
Joined 2010
Zen Mod said:
everything is already shown on sch you posted

small caps will be filled much faster then big ones . . . .
Click to expand...

Hi, I'm a little concerned about the faster part of the advise. Since time constant T = R x C and the T for the 0.25 ohm x 30.000 uF is 7.500 us, lower than T for 1.000 ohm x 47 uF which is 47.000 us. Of course the load also affects the turn on transient voltage, but a 100 ohm resistor instead of 1.000 ohm at R5 position would be safer, more independent of the load. Don't you think?

Cheers
 
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DIYspaceW

Member
Joined 2009
This was also my thought. So I didnt do it that way.
If I ever try fixed bias I will use separate transformers and simply a switch to turn the bias
first before the main supply. This way its safer.
But anyway, whit current autobias I dont miss anything...why bother...:)
 
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DIYspaceW

Member
Joined 2009
In my experience delay circuits can fail but my four point start up procedure in case of fixed bias never! :)
Right now I use three solid heavey switches on every monoblock.
My power up procedure:
1. Main power to the inrush current limiter circuit and heaters for the driver stage
2. Anode power for tube
3. Power on the SIT whit autobias
Never made a mistake.
The power off procedure is just reverse in action...:)
Sometimes I get the filing I am starting a rocket when I jump from one side to the other...nice workout too...:)
 
S

sensphoto

Member
Joined 2010
negative bias transformer for L'SIT

Hi, my power transformer didn't have extra 15v for negative bias supply but I have one Talema 70050K (dual secondary @7v/7.5VA) which I can connected the secondary in series to get 14vac,

My question is it possible to running negative bias supply with this transformer for stereo module? My plan for negative bias supply is: single secondary-full wave rectifier-electrolytic capacitor and 2 regulators (LM7912) for each channel. I planned to run bias supply is always On

My second question, I still didn't decide what bridge rectifier for b+ supply (18vac) maybe peoples in here have suggestion..
Regards,

handoko
 
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