aleph 1.2 high frequency rool off and restricted bandwidth

[ackcheng]

Hi!,

My friend has recently built an Aleph 1.2 but noticed that there are some high frequency roll off. On testing, there is a significant drop of bandwidth from 1K Hz to 10K HZ.

Any idea on where we should start looking for problems?

Thanks

[Nelson Pass]

The most likely culprit (aside from measurement error) is
passive component values - resistors and capacitors.

Let's ask the questions:

1) Are there any parts deviations from the standard design

2) What is the - 3dB point and what is the slope (or) what is
the gain figure at various frequencies, say 1 Khz, 10 KHz, 20 KHz.

?

[ackcheng]

Dear Mr. Pass,

Thanks very much for your kidn help

This is how we measured it.

1. We connect the signal generator to the input of the amp
2. The input signal is 1kHz square wave
3. The output of the amp is connected to the oscilloscope and a dummy load
4. Then we slowly increase the amplitude of the input signal until the output of the amp is 10V. No problem, very good square waveform
5. Then we change the frequency to 10Kz,
6. The waveform becomes the picture indicated below and the voltage also dropped to about 7V

I will check with my friend to see if there is any change to the original circuit

Thanks very much for taking your precious time and allowing us to use your great design!

[ackcheng]

Further information:

-3dB was noted at around 10Kz.
At 1KHz, the gain was 6x
At 10Kz, the gain was 0.7x
At 20Kz, the gain was not measured

[Nelson Pass]

That's a rolloff, alright.

I have not seen such a case before, and I have to presume
that it's a part value, and less likely, a layout issue.

Assuming that you did not use big power transistors for the
input stage, it is unlikely to be the capacitance of the transistors.

Referring to the Aleph 1.2 service schematic:

Most likely culprits are the values of the small capacitors, C6-8.
In particular, be certain that C7 is not attached to the Base of
Q5. There is no node on the schematic when those lines cross.

[imlulu]

I am posting on behalf of the maker of Aleph 1.2.

Thanks for your advise Mr. Pass. My friend changed C6 and C8 to Silver MICA. The waveform finally looks good now. I can't imagine this 2 little caps can make such a big difference.

Now we also have another question. We observe below when making measurement:

signal gen 1KHz > aleph 1.2 > 10V output 8 ohm

signal gen 10KHz > aleph 1.2 > 17-18V output 8 ohm

Is it normal? The higer input frequence, the higher output and vice versa?

Thanks.

[ackcheng]

Apparently the original caps did not come with the value specified. That was what causing the problem.

Does anyone knows the causes of increase in voltage with higher frequency? Is this normal?

[mlloyd1]

are you sure the scope probe was calibrated?

mlloyd1

[ackcheng]

Don't think there is anyproblem with the probe...