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Old 15th May 2015, 02:12 AM   #1
cyto is offline cyto  United States
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Default AK phonostage

I just built an "AK phonostage" , it started on a thread over at AK and now there are boards (and kits I believe) available for the stage and PSU. I only got the phonostage board and did the rest myself.

I built a stand alone 15VDC dual power supply that uses XLR for the power out. I did this so I could use it to power more than one device and also because I prefer to have the PSU separate from the signal electronics, used XLR because it can handle low voltage and looks good. I basically copied Jim Hagerman's Bugle 2 power supply, here is a link to that build:

Dual Power Supply Photos by cytovette | Photobucket

I already have a Cornet 2 phonostage that I built myself and had been tweaking for 2 years, just had the itch to build something and after looking at the schematic I realized that I already had 95% of the parts. My build looks different because of that, in order to fit the big capacitors (like orange drops). I had to use both sides and be inventive. I really like that the gain and input impedance are adjustable design, I actually built adjustable input impedance into my Cornet because I like that feature. Here is a link to that build:

AK Phonostage Photos by cytovette | Photobucket

Where I got the board: https://hackaday.io/project/4788-muf...no-preamp-pp-2

As far as the sound? My 2 channel system consists of; Yaqin MS-20L integrated amp, Technics SL-1200M3D with cardas tonarm wire going directly to RCA jacks, KAB fluid damper and AT120E cartridge with ATN150 stylus, 1981 Klipsch La Scalas with ALK crossovers and two Emotiva Ultra 10 subs.

The AK phonostage is dead quiet, something I miss with a tube phonostage. The sound is clean and detailed. I used to have a Musical Fidelity V-LPS and this beats it, however it does not beat the Cornet 2. Comparing it to the Cornet 2 may not be quite fair as I spent $500+ building that not counting tubes. The key difference is that the Cornet has a wider soundstage and slightly better imaging, but then it cost 10 times as much to build.

In my system it sounded bright and lacking a little bass at first, Klipsch speakers tend to do that, I raised the gain from 40 to 44 and cut the input impedance from 47k to 33k and pretty much did the trick.

Overall this is not the best phonostage out there but it sounds very good and sounds better than anything in it's price range. More than worth the cost and should be a relatively easy build for less experienced builders. I was a licensed electronics tech in the 70s tube days and spend my freee time tweaking my equipment now.

BTW I have no affiliation with the designer or seller, just reporting my findings.
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Old 28th May 2015, 03:32 PM   #2
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Hi cyto, and thanks for reviewing my phono stage. I'm glad that you like it, and I really enjoyed seeing the pictures.

I've run the circuit through Spice, and it shows that the (simulated) RIAA accuracy is +/-0.023 dB from 1-33.000 Hz. For 1-100.000 Hz, it's +/-0.4 dB.

Click the image to open in full size.

I have written quite a lot of information about the phono stage on hackaday.io, if anyone's interested.
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Old 28th May 2015, 03:42 PM   #3
rayma is offline rayma  United States
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Quote:
Originally Posted by skrodahl View Post
I've run the circuit through Spice, and it shows that the (simulated) RIAA accuracy is +/-0.023 dB from 1-33.000 Hz. For 1-100.000 Hz, it's +/-0.4 dB.
Did you get this curve by subtracting the exact RIAA curve from the Spice output,
or by using an inverse RIAA network at the input and simulating the response?
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Old 28th May 2015, 03:49 PM   #4
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Quote:
Originally Posted by rayma View Post
Did you get this curve by subtracting the exact RIAA curve from the Spice output,
or by using an inverse RIAA network at the input and simulating the response?
I used this solution to simulate the RIAA signal:
SPICE transfer function for RIAA testing
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Old 28th May 2015, 04:00 PM   #5
rayma is offline rayma  United States
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Quote:
Originally Posted by skrodahl View Post
I used this solution to simulate the RIAA signal:
SPICE transfer function for RIAA testing
If you used an inverse RIAA network at the input for simulation, the hf rise is due to that network,
not your phono circuit. The inverse RIAA network cannot have an accurate high frequency response,
and its output into the phono stage levels off at a certain point, instead of continuing to increase.
This causes your simulated hf rise. A primary advantage of the passive equalization approach is the
accurate hf response.

Last edited by rayma; 28th May 2015 at 04:27 PM.
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Old 28th May 2015, 04:07 PM   #6
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Thanks for the explanation rayma.
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Old 28th May 2015, 04:18 PM   #7
rayma is offline rayma  United States
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Quote:
Originally Posted by skrodahl View Post
Thanks for the explanation rayma.
If you do the simulation in a different way, without the inverse RIAA network,
the results will be even better, especially at hf.
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Old 29th May 2015, 08:12 AM   #8
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I traced the strange hf simulation to a misplaced output capacitor on the output. (blush)

This looks better:
Click the image to open in full size.

It's still within +/-0.025 dB from 1 to 100.000 Hz, with easy to find standard components. Capacitors are 68 and 47 nF, and all resistors are from the E24-series.
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Old 8th September 2015, 10:48 AM   #9
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Now for some noise measurements of the Muffsy Phono Preamp.

The noise floor of the Muffsy Phono Preamp PP-3, powered by the Muffsy Hifi Dual Power Supply, is well below -100 dB. The average of all measured points, from 20-20.000 Hz is -107.74 dB and the highest single harmonic is at 92.11 dB (2 kHz).

The measurements are taken with a Rigol DG1022 function generator feeding a 1kHz/5mVpp signal into the phono stage. The PP-3's output signal is recorded using a Behringer UCA202 soundcard, and the FFT is generated from that recording. 16.384 measurement points have been recorded, almost 1 measurement per Hz.

The measurements are done without any calibration, on a relatively cheap USB sound card with a whole range of error sources that will provide less than optimal values. It's also recorded using the standard Windows mixer.(A QuantAsylum QA400 is on its way though...)

Click the image to open in full size.

Notice that there is a 50 Hz distortion in the graph (with harmonics). That's unfortunately from the power cable that is running across the input of the PP-3 inside the B-0509 cabinet.

There is also a slope from 10-200 Hz. This slope is identical when I use the Behringer sound card to measure the Rigol DG1022 directly. It probably stems from the DC blocking high-pass filter in the sound card.

Finally, the abrupt roll-off at 20 kHz is the bandwidth of the sound card. It can only measure up to that level.

Here are some numbers:
]The average level of all measured points is -107.75 dB (20-20.000 Hz), including the 50 Hz distortion and the less than ideal measurements up to 200 Hz. This does not include the steep roll-off at frequencies above 20 kHz.

The highest peak is at 100 Hz, coming in at 88.6 dB. That value can be disregarded as it comes from my power cabling running to close to the phono stage input

Harmonics:
1st (2 kHz): -92.11 dB
2nd (3 kHz): -100.38 dB
3rd (4 kHz): -98.85 dB

The highest harmonic peak comes in at -92.11 dB, again disregarding the 50 Hz noise and its harmonics.

All the noise that's measured lies well below -100 dB
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Old 29th September 2015, 01:05 PM   #10
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Default Measurements, for real

I just got a QuantAsylum QA400 audio analyzer and it just takes the guesswork out of it all. I thought I'd share my findings with you.

The measurements are with a 1 kHz sine in and an output of 1 Vrms from the CNC, with a bandwidth from 10 to 20.000 Hz. As it stands, I'd need a decent inverse RIAA circuit to measure through the whole bandwidth.

The phono stage is equipped with OPA2134 and +/- 15V PSU (LM317/337). No out of the ordinary components, just standard 1% metal film resistors and Wima MKS2 caps.

THD: 0.00190%
THD+N: 0.01013%
SNR: 80,6 dB
IMD: 0.0195% (ITU-R 19+20 kHz)
Dynamic Range: >100 dB (THD+N: 1%)
RIAA Compliance: +/- 0.025 dB

The dynamic range can't be determined without an attenuator, as the QA400 goes into clipping long before the THD+N increases by more than 0.01%.

I'll measure crosstalk as well, when I find time for it.

Click the image to open in full size.

-skrodahl

Audiobah TPA3116 || ESP P88 || ProJect Debut Carbon Espirit DC || Muffsy Phono Preamp PP-3 || RuneAudio / RaspBerry Pi / HifiBerry DAC || Dynaco A25
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