my ongoing amp project since 2004 or so, ver. 13.29.b1.xx.n - diyAudio
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Old 22nd September 2009, 01:03 PM   #1
diyAudio Member
Join Date: Feb 2005
Location: Gelderland
Default my ongoing amp project since 2004 or so, ver. 13.29.b1.xx.n

this is the current state of the amplifiing part. made of some new, some very old, and very heavy parts ;p

the input capacitor is 4u7 and 1u foil (mkp and mkt iirc), the j-fet a 2sk55 from a old antenna amp iirc (idss some 6 mA, biased at about -0,5 V), the pnp folded cascode a 2sa733, the npn a 2sc4883a, which carries the idle current, and drives the kathode of the el84 tube. however since the tube AC
signal path is closed, the current changes are very low, and the cathode is effectively a very high impedance, as if driven from the grid (lots of juice there ;p). the parafeed choke is a sowter 8982 (100H 50mA), the parafeed caps audyn 47 uf, and 0u47 wima mp3-x2. the output transformer a vanderveen pat 1070 uc (i love it, wide band, low coupling capacity if wired correctly, humongous inductance (1500H primary iirc )), 4k to 5 ohm, driving a 8 ohm speaker (reflecting 6k4 thus).

amp completely silent ear to speaker idling, i tried measuring distortion once with my soundcard, and came to about 0,25% THD, of mainly 2, very little 3, and some 4th harmonic. i did not spice it on comp.

the points F are 6.3V glow, point I is a bias point where some 7 mA flows in,
biased at some 48ma of plate and grid 2 (sometimes i run it harder, or leaner).
H is HV.

there are many details i thought of, please ask anything ;p

the next thing i will do about it is build a decent case.

i will make some pics in a while, in the photo gallery thread.
i use it day to day for anything i need to hear.
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Last edited by jechentau; 22nd September 2009 at 01:28 PM. Reason: corr.
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Old 22nd September 2009, 03:04 PM   #2
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Join Date: Feb 2005
Location: Gelderland
some pics
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File Type: jpg P9220003.JPG (126.1 KB, 196 views)
File Type: jpg P9220004.JPG (121.7 KB, 187 views)
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Old 24th September 2009, 08:39 AM   #3
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Join Date: Feb 2005
Location: Gelderland
i would like to add some things.

about the sound; my ears decide.
i put in such large input caps, because i did not like the low rolloff otherwise.
with this it has power and clarity from infra to ultrasound although there is slight rolloff on the highs (or my ears ;p). i also perceive it as very fast.
at times, when there is glitch or noise, in electronic music, it sounds as if the actual amplifier is breaking down, also micro detail is very clear. and reverberations. old recordings sound old, clear ones clear, bad ones bad etc.
it is at this stage i feel that i need to build a proper case, as this amp with these parts, is as good as done, soundwise.

i may decide to make a modular solid state output as well, to be switched (then on the point where now the cathode is at, thinking of a source follower +ccs with 2x5 2sk135 i still have, on 24 volt, with a few glue transistors.)

i would like a duplicate, for stereo must be nice as well, given clarity of highs.

another fun bit is that the green led, on the pics, goes from off to very bright, within a few mA of idle current variation (shunted by a trimmer).
since this is a class a amplifier, with good psu ripple filter, the led is on constantly, unless there is "rumble" on the source material, or on the mains, it responds as well, below a few Hz.

i have a ersa switching solder station, and i could see the switching, on that green led of the amp by slightly fluctuating intensity, there was quite some resistance on the way to my dwelling, and inside. but where i am now, with the transformer just down the road, i can't see it anymore.

i can see peak hour fluctuations very well.

i may regulate the HV, but i don't mind it, and sometimes like it ;p

the EL84 is a Miniwatt from 1956, the 2sc4883a from sanken, that gray horizontal cap a old elna. the parts come from telephone switches, computer psu's, mig welder, commodore 1541 drive, old kit amp, sunn bass amp, a aquarium heater, old amplifiers, model accu charger, and a bunch of new parts.

it is on nearly 24-7 since years, i run it with this miniwatt, a telefunken el84 <> with oval plate, and box fins for grid 1, and a sovtek el84.

momentary i like this miniwatt best.

i can't decide what kind of case yet. ofcourse i will resloder everything, trim wires, pay extra attention to grounding (apparently well enough as is).

the soundcard noises clearly audible btw, but the amp selve inaudible.

yes, i'm very satisfied with it.

i may dub the output circuit "hemitron" ;p

Last edited by jechentau; 24th September 2009 at 08:41 AM.
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Old 24th September 2009, 10:46 AM   #4
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Join Date: Feb 2005
Location: Gelderland

or the whole amp, "semitron" for that matter.
yep, will do ;p
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Old 24th September 2009, 06:28 PM   #5
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Join Date: Feb 2005
Location: Gelderland
i changed/added a couple minor things, namely replaced the mkc from the pnp's base to ground, with mkp. (hear no diff though ;p)

and added a 18v zener parallel to the caps from the grid to ground, to be a little more secure against further failure if the tube shorts internally somehow, or gets gassy or such.

tips/ideas/suggestions or any questions? plz do type ;p
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Old 25th September 2009, 08:24 AM   #6
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Join Date: Feb 2005
Location: Gelderland
i ordered better foil caps for various points for psu HF bypassing input coupling, and also speakerbox crossover upgrading.

in general, could one use bi-opt, one for hi freqs, other for low freqs, for a huge but multi-way compound bandwith? with 2 not very expensive opt's (one very small ;p) seems like a nice idea.

for the case; i think i will house the entire SS section in a own small module (have a mini hammond alu box), and maybe the el84 too (to keep high speed wiring short), protruding on the top side of it. some psu caps from the the bottom side, and that box suspended in the amp frame. and such done that the lid can be taken off, to work and measure, while it remains working.

actual psu in a own shielded compartiment, and the opt and anode choke far from the psu, but close to the amplifying box. terminals for choke, Anode and Kathode to outside, so that easily the parafeed assembly can be changed.
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