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TDA1541A Diy Pcb - "Distinction-1541 v2"
TDA1541A Diy Pcb - "Distinction-1541 v2"
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Old 28th July 2015, 12:39 PM   #11
ryanj is offline ryanj  Australia
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TDA1541A Diy Pcb - "Distinction-1541 v2"
Default Resistive attenuator simulation.

Hi All,

It has come to my attention in a discussion that; the diode attenuators may not be the best option for attenuation. Due to the large current pulse - Ground bounce will likely be an issue. Originally I was attracted to the fast rising edge that the diode attenuator produced in the hope that this will reduce trigger uncertainty within the 1541 thus less jitter.

So the compromise going from the diode attenuator to a resistive attenuator will be a slower rise time, but there will be less harmonics entering the 1541. Also there will only be a ~5mA pulse through the resistive attenuator as opposed to ~100mA pulse in the diode attenuator, so we will also be getting a lot less/zero bounce on the digital ground plane.

Attached is a simulation of what ill try in the next few days. I should be able to use the diode attenuator pcb instead of making a new one.

Ill report back with a few scope shots.

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File Type: jpg resistive attenuator sim.jpg (140.1 KB, 1100 views)

Last edited by ryanj; 28th July 2015 at 12:44 PM.
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Old 29th July 2015, 01:07 PM   #12
ryanj is offline ryanj  Australia
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TDA1541A Diy Pcb - "Distinction-1541 v2"
Default Resistive attenuator test.

Attached is a scope shot of the resistive attenuators in action. I didn't have all the exact values on hand thus why the DC bias is at about 1.3V with a total amplitude of about 2V. Probably should be aiming for a range not too different anyway as the input threshold for the 1541 is about 1.6V if i remember correctly.

If you look at the image you may notice that the trace is not too sharp compared to the diode attenuators, I think this might be due to having thick film resistors in the signal path contributing their large noise spectrum - which will translate to jitter. I'll replace them with some MELF and take another look.
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File Type: jpg resistive attenuator test.jpg (958.4 KB, 1031 views)
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Old 30th July 2015, 12:56 PM   #13
ryanj is offline ryanj  Australia
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TDA1541A Diy Pcb - "Distinction-1541 v2"
Default MELF vs Thick Film

Hi Guys,

Attached is a comparison of MELF and thick film resistors in the resistive attenuator circuit posted above.

The shot on the left of each image is the Circuit with all melf resistors, and the right in both images is the shot I took yesterday with the thick film resistors.

Notice the width of the rising edge. Amazing what good quality resistors can do. Makes sense though - less noise, less jitter.

I haven't had a chance to have a good listen with this attenuator, but there are already advantages using this setup.
-I can drop the 5V reg on the distinction PCB (Originally there to power the diode attenuators and isolate the flip flop and buffer from the large current pulse the diode attenuator was producing) and run all the digital side off the one 3.3V shunt reg.
-Lower part count
-Smaller PCB (maybe)
-No ground bounce to worry about.
-Less high frequency harmonics entering the 1541.

Seems like a better option.

Last edited by ryanj; 30th July 2015 at 12:59 PM.
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Old 9th September 2015, 03:23 PM   #14
ryanj is offline ryanj  Australia
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TDA1541A Diy Pcb - "Distinction-1541 v2"
Default Update

Preliminary design. Not yet tested or fabricated.

Hi Guys,

Time for a bit of an update.

Attached is a preliminary design layout of layers 1-4 which I have been working on.

I've added the whole voltage reg as indicated in the schematics in a previous post - so all you'll need is a pre regulated, pre filtered 24V supply, and a 6V supply for the digital side. More details once I get this all tested.

I've also added 2220 Pads to try out those Rubycon ST 22uF film caps - they look the goods, and i'm keen on trying these without any Electrolytic caps even though this may cause an impedance hump at 100 - 200k.

On the digital side - I've taken a lot of care in shielding - hence all the via stitching. All stiplines have been properly calculated for imdedance. Ground fill surrounding stiplines from layers 2 and 4. Power planes on layer 1 with Ground on layer 2 giving extra distributed PSU decoupling capacitance. Split Analog and Digital ground planes connected at pin 14.

I've designed in the resistive attenuators - definitely a better solution in terms of current flow/ground bounce compared to the diode attenuators - not that they didn't seem to perform well, just all that current flow will likely have caused issues.

Thermal pads have been calculated to around 25 degrees celsius above ambient (-15V) Once tested I'll know for sure how far the analog supplies can be pushed. With extra current shunted to gnd, performance is improved. I've added trimmers in parallel to the CCS current setting element to tune this.

A separate return path has been kept for the 14 decoupling caps.

A 3.3mH choke with a DC resistance of 9 ohms has been added in place of the CCS resistor on the analog side - as this gives extra ripple rejection from the source (3dB). If this was increased to 10mH - according to the simulated you would get around 9dB of extra ripple rejection.

I'm starting to feel comfortable with this design. Shouldn't be too long for a prototype to be ordered soon.

Regards,

Ryan
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File Type: jpg UpdateSep.jpg (285.6 KB, 774 views)
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Old 15th September 2015, 03:12 PM   #15
ryanj is offline ryanj  Australia
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TDA1541A Diy Pcb - "Distinction-1541 v2"
Default Close up of digital side.

Hi Guys,

Been doing a few final modifications before I get a prototype made.

Attached is what I have so far. Might be worth throwing in a bit more ground stitching.

The Image is looking from the bottom of the board, layer 2 is the goldish colour - Continuous ground plane (split for digital and analog and connected at pin 14).

Layer 3 is in green and contains the striplines.

Layer 4 is in blue - ground fill, components.

To the right of the image you will see the MCK input stripline sweep past the reclocker and continue to the MCK output buffer to maintain 50ohm impedance.

Ryan
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Old 17th September 2015, 09:06 AM   #16
Extreme_Boky is offline Extreme_Boky  Australia
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Very nice! I am looking forward to trying one myself, if you make it available... Still have few 1541's lying around.

Regards,
Nick
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Old 17th September 2015, 11:02 AM   #17
ryanj is offline ryanj  Australia
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TDA1541A Diy Pcb - "Distinction-1541 v2"
Thanks Nick,

I'm really looking forward to it myself. It will be very interesting to see how these shunt regs perform so close to the pins compared to the salas shunt regs roughly 70mm away (in my setup). If all goes well in the prototype I have every intention to make it available.
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Old 17th September 2015, 01:01 PM   #18
Extreme_Boky is offline Extreme_Boky  Australia
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Quote:
Originally Posted by ryanj View Post
Thanks Nick,

I'm really looking forward to it myself. It will be very interesting to see how these shunt regs perform so close to the pins compared to the salas shunt regs roughly 70mm away (in my setup). If all goes well in the prototype I have every intention to make it available.
The shunt regs can only sound good. Judging by your attempt to simulate trace inductance by inserting series inductors into power supply rails few posts back - I don't see any reason not to expect an implementation capable of producing excellent results.

Regards,
Nick
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Old 17th September 2015, 01:36 PM   #19
ryanj is offline ryanj  Australia
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TDA1541A Diy Pcb - "Distinction-1541 v2"
Hi Nick,

Yeah I have a feeling they will sound better than the salas shunts (great regs, hands down) purely based on the fact that they will be 3mm away from the pins, so the current loop is kept as small as possible. The power planes will also provide extra distributed capacitance - Every little contribution to performance will help. The simulations can only show half the story though, especially when It comes to audio. It shall be very interesting to hear the results.

Ryan
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Old 17th September 2015, 03:40 PM   #20
wlowes is offline wlowes  Canada
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Great initiative Ryanj,
You have to keep the 1.0 build for comparison. Quite a few variables changing, but will be really interesting to get your opinion of the impact of all this excellent work between rev 1.0 and 2.0.
Walter
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