5W fet power amplifier with "tube" sound

Tube or solid-state, this section goes to eleven!
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KMG
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Post by KMG »

Amplifier is build on standard tube topology and suitable for home low audio lewel playing.

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Measurements an 1W output power:
Amplitude-frequency response (presence min)
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the influence of presence control with the different values of C13 (according to schematics)
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THD at 1kHz
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Transformer primary coil impedance Ra-a is 360 Ohm.
I used rewinded 15W power transformer for output one.
Only one requirement - the thickness of plates of transformer core must be not more than 0.35 mm.
The sectioning of transformer:
On each half - 1/4 primary - secondary - 1/4 primary.
Primary windings in series, secondary windings in parallel.
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Post by RnFR »

looks very cool, but i'm having some trouble reading the schematic. do you think you could upload a larger version?
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Post by KMG »

to get full size of schematics & photo use links for them.
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Post by RnFR »

ah ok, i was right clicking and going to "view image". thanks for the clarification.
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Post by bajaman »

very nice as usual :wink: - thanks for sharing :thumbsup
any board layout or PCB available?
cheers
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Post by KMG »

Ok, but you have to modify layout for output transformer you will use.
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Post by bajaman »

Thanks :thumbsup
any chance of some more details regarding the output transformer ? - specifically the number of turns on each quarter secondary and the number of turns on each half primary.
Also - what size wire gauges are you using ? what is the turns ratio etc?
finally what is the approximate transformer core cross sectional area? - I intend to build a E and I type transformer, but I will not be mounting it on the circuit board.
many thanks
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Post by KMG »

My transformer:
Core cross section 20x16 mm
Each half bobbin:
120 turns 0.25 mm (1/4) primary
75 turns 0.5 mm secondary (for 8 Ohm load)
120 turns 0.25 mm (1/4) primary
trans.jpg
trans.jpg (11.51 KiB) Viewed 4518 times
For calculation for your core and load impedance you can use transformer calculator for tube PP.
Enter primary impedance Ra-a 360 Ohm, core parameters, power, load impedance etc.
If you wish to design your own fet PA you can use my program to select primary impedance for desired power and supply voltage
http://milas.spb.ru/~kmg/files/projects/fetpa/Pa.exe
pa.jpg
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Ra - impedance of half primary winding (1/4 Ra-a)
Vsupply - power supply voltage
Sig Max % - signal amplitude (100% is maximum signal without distortion)
Vsat - fet saturation voltage at peak current
Id0 - idle current
Average Pd - Average drain power (for one side of PP, halfwave)
Average Pl - Average load power (fullwave, but without taking into account efficiency of the transformer)
Average Id - Average drain current (for one side of PP, halfwave)
Peak Pd - Peak drain power (instantaneous)
Peak Id - Peak drain current (instantaneous)
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Post by bajaman »

Thanks so much for that :thumbsup
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Post by KMG »

I found one problem with the represented layout, he has a tendency to the self-excitation.
It`s not schematic problem because "air wired" prototype doesn`t have this tendency and work fine.
I think that reason it consists in the fact that trace from the transformer to the output connector passes near the gate traces of Q3.
Problem of this layout can be solved by placing 200pF capacitor from Q2 drain to ground.
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Post by Jarno »

Cracking engineering job KMG!
I am amazed how little windings you need for this output transformer, that shouldn't be too hard to make yourself. But output transformers for tube amps need way more windings, right?
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Post by KMG »

But output transformers for tube amps need way more windings, right?
Output transformer coils windings depends on voltages on coils and transformer lowest passing frequency.
Schematics and layout updated to new version.
External connections
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Post by KMG »

Finally assembled in box 170(W)x55(H)x100(D) mm

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Post by DeanM »

Nice work!! i bet your jcm800 preamp plugged into this would sound great!! you should try record a clip of that!

Les paul > JCM800 preamp > 5W FET PA > Greenback/V30 > SM57

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Thanks for posting!

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Post by KMG »

Experiment with drain to gate diodes (1N4148) for tube output stage grid current emulation.
Without diodes on the left, with diodes on the right (feddback on, presence and Depth = 0)

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Post by bajaman »

Go Mike go :thumbsup
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Post by yosemitesam144 »

all these links seem to be down? all images come up as links. a lot of kmg's articles are down too.

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Post by bajaman »

be patient :wink:
he is in the process of moving to another server 8)
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Post by KMG »

If you need now, you can replace beginning of link by http://217.119.30.118/~kmg/ instead of http://milas.spb.ru/~kmg/
All informatiom remains on our server. Changed only internet service provider, so changed IP address.
After moving domain name to new IP all will be ok.
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Post by KMG »

Two samples of fet PA with tube preamps. Presence, Depth and Cut in clockwise position.
Recording path: Ibanez RG3120TW -> tube preamp (JCM800/SLO-100) -> fet PA -> Celestion G10 Vintage 25HR -> capacitor microphone (Octava MK105 in 2 meters from cabinet) -> mic preamp -> DELTA-44 -> Samplitude v8.
http://217.119.30.118/~kmg/files/projec ... jcm800.mp3
http://217.119.30.118/~kmg/files/projec ... pa_slo.mp3
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