Amp Camp Amp - ACA

[kikko]

Hello, while waiting for the kits, i'm assembling a dual mono chassis using hifi2000 heatsinks and hifi2000 galaxy front/back plates.
I'm crating a "mini dissipante" case for the camp amp.

If someone is intrested i can post some images tomorrow morning.

[zany]

would love to see what you have in mind kikko... I am looking at something similar to slomatt's mini-aleph chassis - similar size heatsinks.

[tinitus]

and take it apart for adjustments ?

[zany]

still in the planning stage in my head...

[kikko]

As promised i'm posting some images of my heatsinked case.



Ok actually the case is composed by parts and mask tape... but the project is quite simple:



Panel: _____
Heatsink: HHH

______
H H
H H
H H
______

The heatsinks will be drilled and the holes tapped.

Last part will be creating an upper and a lower panel:



Panels are from modushop (like everything else in this little project..) and needs to be cutted to fit (a simple jigsaw is ok).



Here you can see the heatsinked case near another case (for the pre).

I will post more info soon: now i have to drill a couple of heatsinks

[kikko]

Some images in my previous post aren't showing..
No problem, i have almost completed the case:




Drilling heatsinks











Now i need to fix the front and back panels to the heatsinks.
Thank you for watching

[LBHajdu]

I though it might be quite a tease to simulate the circuit and measure the output resistance on the simulation. Disclaimer these simulations are mostly accurate but sometimes like a republican campaign speeche it can differ from reality by a vast margin. The actual measurement by Mr. Pass and or others on the actual amp supersedes these simulations. I am using a method described by Rod Elliott of Australia on his Elliott Sound Products - The Audio Pages web site where the output voltage is measured across a known load and with no load attached. Then the following formula is applied:

Vu=Voltage out un-loaded
VL= Voltage out loaded
RL=Load resistance
Zo = Output impedance
Zo = (Vu - VL) / (VL / RL)

I am assuming most people are building the amp camp to drive full range drivers or at least high efficiency drivers. The multi-way speaker crowd usually want an output impedance as low as possible (zero point something proceeded by several zeros). For full range drivers we want a little higher output impedance something more like a 300B amp which I have seen quoted with damping factors between 2 and 6. Converted to output impedance it is between 4 and 1.3 ohms. I personally use an 8ohm power resistor on my class-d amp to make it sound more decent on my Fostex 6” drivers. My observation is that the value is not super critical as long as it's a few ohm, though I don't have the power resistors to try values higher then ten ohms.

OK, lets get to it:

The value is it is drawn by Mr. Pass is:
Feedback NetWork 10K/68.1K
Vin = 0.5v @ 1KHz
Vout@8ohm load = 2.49509V
Vout@1Kohm load = 2.88103V
Zo = (Vu - VL) / (VL / RL)
1.2374385 = (2.88103 - 2.49509)/(2.49509 / 8)
6.4649677 damping factor = 8/1.2374385

The value given by Mr. Pass is a damping factor of 3 which would be 2.66 ohms. The value I simulate is off by 1.4ohms not that much, but I don't think I could hear the difference between one ohm more or less. If we assume the simulation works perfectly the difference can be explained by the model picking slightly different transconductances for there parts compared to the lot of parts used in the true build (just speculation).

[Loudthud]

What do you get for a quiescent voltage at the drain of M3 with R26 at 4K?

[Loudthud]

Figure 3 on page 2 of the A40 article shows the incremental output impedance for different classes of amps. Is there any way to measure this on an exsisting amp? Could you measure it by using small signals and adjusting the DC level to different points? Would the output capacitor interfere with operating conditions (ie no load current flowing) at different DC points?

Link: http://www.passdiy.com/pdf/a40.pdf

[flg]

Take an Output Voltage and Output Current measurement at 2 (widely spaced) points. Subtract the 2 Voltagte and the 2 Current measurements. Divide the Voltage difference by the Current difference and thats the Output Z under those conditions.