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Old 4th August 2006, 02:07 AM   #1
kopite is offline kopite  New Zealand
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Thumbs up Jan E Veiset Mini EL84 pp

Hi Jan,

I really like your Mini EL84 pp circuit, can you tell me what value of feedback resisitor you settled on. I can't read your website page about it but would like to know more, can you explain the circuit in english please.
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Old 4th August 2006, 07:13 AM   #2
jane is offline jane  Norway
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Hi,
The design is very simple and more or less a class A design. It moves into class AB, but not far into AB, so if you are looking for plenty of power this is not the way to go.
The local shunt feedback is ~9dB and will lower the output resistance of the EL84. Global NFB is approx 10dB set by a 1k2 feedback resistor (I have to check this out, Iím not at home now..)
I achieved a damping factor of 8 (Zo=1 ohm) with 10dB gNFB. The distortion figures are also quite low, 3rd harmonic <0.3% and higher order harmonic less than 0.1% at half power output.
The amplifier sounds very transparent and does not have any dominant sound signature.

After some experimenting I found the best sonic working point for the EL84s to be ~41mA cathode current: Rk=2k2/10W and Vk=90V. The cathode voltage (Vk) is adjusted by trimming the CCS.

(I will upload the updated schematic later today).

Jan E Veiset
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Old 4th August 2006, 08:33 AM   #3
jane is offline jane  Norway
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Here is the current schematic for the miniPP:

Click the image to open in full size.

Jan E Veiset
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Old 4th August 2006, 02:46 PM   #4
corbato is offline corbato  India
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Hi Jan

Cute design!

Do you have any measurements?
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Old 4th August 2006, 03:55 PM   #5
jane is offline jane  Norway
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Key figures:
Max Po: 7V rms into 8 ohm. (40Hz - 25kHz)
Frequency response (1W, -3dB) 22Hz - 40kHz
DF: 8 (Zo = 1 ohm)
THD: Less then 0.3% (5V rms output)

Distortion pattern 400Hz input (5V rms output):
Click the image to open in full size.

But if you are looking for power, take a walk to SY's red light district: EL84 in pentode mode with very stiff cathode bias is the way to go for some real good spanking.

Jan E Veiset
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Old 4th August 2006, 07:26 PM   #6
kopite is offline kopite  New Zealand
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Hello Jan,
Thank you for the updaye on your circuit. I'm not looking for a lot of power. Can you tell me the power out in watts please.

I also like Sy's circuit but I'm not sure if I can fit the led board into my space.

Thanks again for your time.
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Old 5th August 2006, 10:17 AM   #7
jane is offline jane  Norway
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Hi,
Power output = Vo*Vo/R. If Vo = 7V rms, Po = 6.1W
I think that is close to what's possible to achieve in class A in this circuit.
(max theoretical class A Po = (2xIq)^2 x Rl/4 = (2x0,04)^2x8000/4 = 6.4W)

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