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Old 21st January 2019, 11:22 AM   #541
albertNL is offline albertNL  Australia
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Pass Aleph P 1.7 preamp builders thread
Thanks for your explanarion Claas.
I had a look with the scope at the output from my old usb soundcard using the signal generator included in REW that I downloaded. On the scope it looks like a sine wave but not perfect and not as smooth as I had liked. I assume the old soundcard is not that great.
I then tried playing a 1kHz sine from a test cd I had ripped but the output from my 16 bit squeezebox classic was not that good either.
The new Focusrite 2i2 I bought for testing does not work on my old workshop test pc that runs XP. Arrgh!

So, I hoped to get some work done today but everything I touched went wrong. I even managed to get a cable stuck around my wheelchair and had to call someone to free me :-(

Tomorrow is another day. I hope to get the scope hooked up and do some testing.
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Old 21st January 2019, 10:25 PM   #542
chede is offline chede  Germany
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Don't give up !

In the past, I used my old (bought used) little Tektronix signal generator. My Rigol scope using its FFT function showed about -40 dB second harmonic distortion on my 2A3 SET amps. So I thought - maybe that's the distortion of the tube amps ...

Distortion spectra for my M2 amps were similarly in the - 40 dB range ... hmm ... maybe the FFT that's built into the Rigol doesn't have higher resolution ?
When I used the Focusrite 2i2 I got, I measured distortion easily down to -80 dB and lower for the M2. So I found out, my old Tektronix signal generator did have harmonics in the -40 dB range for a 1 kHz sine wave ...

What did I draw from this ?
- the not-very-good signal generator still shows a very clean sine wave and very clean rectangle on the scope. It also proved perfectly adequate for oscillation hunting.
- the Focusrite 2i2 does not only have a very good ADC for measuring, but also a very low-distortion DAC / signal generator. I think it would pay off to make work in your system.

For oscillation measurement, you don't need a very good signal generator. The output from your Squeezebox should work for a 1 kHz sine wave. If you get a very un-clean signal, chances are that you have noise or bad connections somewhere between Squeezebox and scope.

You can also start by looking for signs of oscillation on the pre-amp output without a signal input; inputs of the preamp grounded, scope probe to the output and scanning through the frequency range ... in my case with the Sony amp, oscillation shows without a signal ...

Best of luck,
Claas
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Old 23rd January 2019, 07:59 AM   #543
albertNL is offline albertNL  Australia
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Things are getting weird. Remember that the amp works and sounds fine (although I admit I only tested it on some cheap speakers so far).

I attached the Focusrite 2i2 balanced to the amp input and generated a 1 kHz sine wave. I attached a Rigol probe to one of the inputs (+ or -). The amplifier was turned off and I got this on the scope
DS1Z_QuickPrint1.png
It is not perfect, but it is as I expected a 1 kHz sine wave.

I then switched the amplifier on and the scope screen changed to
DS1Z_QuickPrint2.png
Where did the fuzziness come from?
I interpret that the input is somehow changed by switching on the amplifier. I assume this should not happen. What could cause this?

To make sure the input circuitry is correct, I have taken high-res photo's to check all resistor values (at the time I bent the leads so that the value could be read after soldering ). Everything (all resistors on the board) check out. All resistors are Dale that I bought from Mouser or Digikey and I am pretty sure I checked every one of them before soldering because I tried to match them as close as possible.

I also tried to get some idea of the distortion but the results are so strange that I cannot interpret them. Changing the fft parameter from 64k to 16k had a huge effect on the fft analysis, with dominance of k2/k3 reversed! I expected that changing parameters would affect the results, but had not expected such a fundamental difference. Or could this be due to a problem in the amp? Is there a preferred set of parameters to use?

Thanks,

Albert
More confused than before I started testing
Attached Images
File Type: png DS1Z_QuickPrint1.png (41.7 KB, 162 views)
File Type: png DS1Z_QuickPrint2.png (45.6 KB, 160 views)
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Old 23rd January 2019, 09:46 AM   #544
Bfpca is offline Bfpca  Canada
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For a start you should not be concerned about the FFT spectrum when doing basic functional testing. It will be much easier if you just leave that feature turned off for now. What immediately comes to mind when looking at the input waveforms is where do you have the scope probe ground lead connected and also how are you connecting the balanced input signal to the 1.7 board. If the focusrite is truly balanced you only need to connect the + and - signals - (Pin 2 and 3 on an xlr connector) You do not need pin 1 at all. If however, like some low priced balanced pro audio gear, the output of the focusrite is not truly balanced then you need to connect pin 1 to the 1.7 to get a quasi balanced signal.
If you connect your scope probe directly across the output of the focusrite with the 1.7 disconnected what does the waveform look like?
What does the 1.7 output look like on the scope with the input signals you are measuring above. Measure each phase of balanced output with respect to the circuit ground.
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Old 23rd January 2019, 10:21 AM   #545
albertNL is offline albertNL  Australia
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Hi Brian,

The Focusrite 2i2 is supposed to be balanced.
Scarlett 2i2 | Focusrite
I assumed it would be properly balanced and had not thought about the possibility that it was not.

I connected one line output of the 2i2 to an XLR jack on the back panel. It is connected directly to IN+, IN- and GND on the AP1.7 board (no input selection switch involved). The probe was connected to one of the inputs (can't remember if it was + or -) and GND at the same point on the AP1.7 board.

Tomorrow I will see what happens when I disconnect pin 1 (GND) from the XLR jack to the Ap1.7 board (thus running only + and - from 2i2 to AP1.7).
I will also try running + and GND (grounding - on the AP1.7 board) and will measure 2i2 output unconnected to AP1.7.

I did have a look at the output. The output of the AP1.7 looked reasonable. Not as good as the first sine, but not as bad as the second either. There was a differences in amplitude of the + and - outputs. If I remember corrrectly about 10% at about 700mV output (note that output is measured after onboard volume relays, with volume at about mid-level) which seemed much too high.
I did not notice the imbalance when I listened to the amp because it was connected single ended to a DIY F4. It sounded OK on cheap old speakers.

Thanks and I will report back.

Albert
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Old 24th January 2019, 10:38 AM   #546
albertNL is offline albertNL  Australia
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Some interesting measurements today!

1000Hz sine wave, -3dBFS, REW signal generator through Focusrite 2i2.
No difference whether laptop was powered by battery or AC.

2i2 to Rigol, probe connected at pins 1 and 3 of 3m XLR cable. No connection to amplifier.
DS1Z_QuickPrint3.png

Noise (5 ns horizontal division)
DS1Z_QuickPrint5.png

For comparison, Rigol probe shorted. Same 5 ns horizontal division.
DS1Z_QuickPrint6.png

Is the XLR cable noisy? Probe connected directly to 2i2 to eliminate cable. Same output.
DS1Z_QuickPrint7.png

Same but using different time/division (100 us).
DS1Z_QuickPrint8.png

2i2 seems to have proper balanced output. Images of pins 1+2 and 1+3 look the same. Output of pins 2+3 looks the same but double the amplitude of 1+2 or 1+3.

Next the AP1.7 meaurements.
Attached Images
File Type: png DS1Z_QuickPrint3.png (46.5 KB, 110 views)
File Type: png DS1Z_QuickPrint5.png (53.4 KB, 93 views)
File Type: png DS1Z_QuickPrint6.png (48.4 KB, 93 views)
File Type: png DS1Z_QuickPrint7.png (56.9 KB, 7 views)
File Type: png DS1Z_QuickPrint8.png (112.4 KB, 9 views)
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Old 24th January 2019, 10:50 AM   #547
albertNL is offline albertNL  Australia
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AP1.7 left channel. Volume at max.
Input + and input - connected to GND.
Rigol probe (yellow) to input + and GND.
DS1Z_QuickPrint34.png
DS1Z_QuickPrint25.png
WTF???

Rigol probe (blue) to output + and GND.
DS1Z_QuickPrint28.png
DS1Z_QuickPrint33.png

Rigol probe (pink) to output - and GND.
DS1Z_QuickPrint22.png
DS1Z_QuickPrint32.png

Just to reiterate, inputs grounded. And the amp sounds fine?!?
Attached Images
File Type: png DS1Z_QuickPrint34.png (52.6 KB, 233 views)
File Type: png DS1Z_QuickPrint25.png (68.6 KB, 14 views)
File Type: png DS1Z_QuickPrint28.png (107.9 KB, 12 views)
File Type: png DS1Z_QuickPrint33.png (57.3 KB, 10 views)
File Type: png DS1Z_QuickPrint22.png (94.2 KB, 8 views)
File Type: png DS1Z_QuickPrint32.png (53.5 KB, 9 views)
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Old 24th January 2019, 11:04 AM   #548
albertNL is offline albertNL  Australia
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I had been thinking before that it seemed as if the input (measured at the input!) was distorted by the amp circuit. I had checked all resistor values between the input and the active part of the circuit and found nothing wrong (or any other resistor for that matter). Then I realised that I had listened to the amp connected to a diy F4 and had set the amp for maximum gain, including bypassing the 10k input resistor.
aleph-17 switches.gif

After switching the 10k back into the circuit, the shorted input looked a lot better!
DS1Z_QuickPrint35.png

The outputs with shorted inputs looked better too!
DS1Z_QuickPrint36.png

So, back to the signal generator. 1000Hz sine wave as before. This is what the input and output look like with the 10k resistors in the circuit.
DS1Z_QuickPrint40.png

I tested the other channel as well and it is also fine.

What did I learn? Well, besides a lot about my scope and other test and measuring equipment an important lesson was to trust my ears because the amp sounded ok.
Attached Images
File Type: gif aleph-17 switches.gif (55.1 KB, 42 views)
File Type: png DS1Z_QuickPrint35.png (61.2 KB, 42 views)
File Type: png DS1Z_QuickPrint36.png (95.5 KB, 17 views)
File Type: png DS1Z_QuickPrint40.png (45.6 KB, 15 views)
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Old 24th January 2019, 11:07 AM   #549
chede is offline chede  Germany
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With regard to #547:
That looks like oscillation to me ...
Inputs grounded, no signal in, right ?
The amp could still sound OK because the 400-500 mV of multi-MHz oscillation is just riding on top of the audio signal. But it is an additional stress on the amp and the components, and maybe on devices downstream.
Now, the interesting question: how to tame the oscillation ? Here the people who have experience with this circuit need to chime in ...

With regard to #548:
That looks good !
What I don't understand at the moment: Why was the amplifier oscillating with the 10k Ohm resistors bypassed but the inputs shorted to ground ?

Best regards,
Claas

Last edited by chede; 24th January 2019 at 11:14 AM. Reason: Answered to #547 - follow-on post #548 as cross-post - changed the picture :-)
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Old 24th January 2019, 12:01 PM   #550
albertNL is offline albertNL  Australia
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Quote:
Originally Posted by chede View Post
With regard to #548:
That looks good !
What I don't understand at the moment: Why was the amplifier oscillating with the 10k Ohm resistors bypassed but the inputs shorted to ground ?

Best regards,
Claas
Hi Claas,

I don't understand it either. As a guess I think that the oscillation originates at the gain 610's but that is mainly because they are closest (their gates connect to the input through the 10k resistors). Could be something else entirely haha.

If I understand the user manual correctly, bypassing the 10k resistor is the default setting.
What I find interesting is that the user manual does not specify that the input impedance is changed (which I assume it is) by the switch.

More measuring I think to see if I can locate the source of the oscillation. No idea what I am looking for but that should not deter me.

Albert
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