ProtoDAC TDA1387 X8 project

[hifinet]

The caps I have are consistent with the data sheet. 

10uf 63V:  L = 31.5, H = 20.5, W = 11.5

6.8uf 100V:  L = 31.5, H = 23.5, W = 13.5

I'm curious, if you moved the row of holes/pads for the output caps 1-1.5mm inward toward the DAC chip on the side of the GPIO connector, would the assymetry vs the other side lead to capacitance or inductance issues? that offset would make it easier to make sure the 'red box' caps would have the little nibs on the corners sit on the pcb surface with that cap being more stable. Potentially it would make mounting the MKP equivalents difficult. I'm probably over-thinking this.

In any case, thanks for the new Gerbers.

Skip

It's good to know that the dimensions match up with the data sheet. But, looking at this MKT data sheet, the 10uF 63V polyester is the same size. I guess the markings on the cap. Ultimately, it's how it sounds. I think the sonic signatures of PETP, PC and PP are fairly distinctive.

I think you are correct about the PCB design. I designed the PCB before I had access to the WIMA 6.8uF red caps, and it used dimensions based on a similar value PP cap in the EasyEDA library. It does look that it would barely fit and clear the inner GPIO pins with 0.5-1mm to spare. Another factor was that the dimensions of the module were slightly bigger (by 0.5-1mm in length) than the DIP28 dimensions in the program. I needed to adjust the space for the C3/C4 caps. You can fiddle with EasyEDA for hours, with refinement after refinement. This was my first design with EasyEDA. At some point I just had enough. I doubt anyone could hear the asymmetry.

[hifinet]

I have been asked about a balanced output ProtoDAC.

Two ways:

1. Use a line level transformer to convert single ended to a balanced output. The transformer needs to be of high quality, for example Lundahl, Sowter, Jensen, Hammond, UTC, Peerless. The resulting performance can be very high quality, and an added benefit of using a transformer is isolation. This is probably the best method, but most expensive. This is a good article on how to do it.
   
2. Use an active device, a single ended to balanced converter. Here is one on eBay using the DRV134. No personal experience. 
   

The other method would involve a redesign. Use two TDA1387 X8 modules and a 74ACT86 logic IC. One module gets DATA (from I2S) and the other inverted DATA. The analog outputs will be 180 degrees out of phase. [Please see correction in post #198.]

[Skip Pack]

I have used Lundahl 1544A transformers in the past on a dac output, balanced to se in that case, and they worked beautifully. They aren't cheap, but not crushingly expensive either. I imagine there are some nice Cinemags with nickel cores that would do a good job.

[hifinet]

The Lundahl 1544a seems like a do it all, general purpose transformer. It can probably do want you want, you just need to figure out how to connect the windings properly.

I found this in Jensen AN-003, 2.4 - A Simple Alternative. A transformerless method of converting single ended to true balanced. From a transformer company. Very generous of them. I guess Bill is an indispensable engineer at Jensen.

Can this be applied to ProtoDAC? I am not sure.  The key phrase is "adds an impedance matched passive network to ground for the - output." Since it is balanced, the impedence of this path needs to match the output impedence of the ProtoDAC. It may take some trial and error.

[Skip Pack]

When I got the 1544As (2008) they came with small pc boards and very clear documentation. Don't know the story now -- there's a new US importer.

When you insert a transformer, you could replace the output cap with the transformer primary, and the matching in that case would probably more demanding then if the transformer was after the output cap. In the tube world I've seen several well-regarded designs where center-tapped autoformers are used as phase splitters before PP output stages following single ended voltage gain stages. I have two Magnequest autoformers designed for just such use. You need the cap before because the autoformers will not take DC, and the output stage, tubes or SS, will often need a cap after the autoformer depending on the output stage bias situation.

[Edzacc]

Hello 
I am new to this forum though I have been using the Moode player for many years with a RPi 3b.
Recently I came accross the ProtoDac project, looks just right,  
a ladder type dac, not many good quality components and affordable so I have ordered one. 
The aim to put together a good sounding standalone system.
(small for factor, low consumption, integrated)
What would put the icing on the cake to the Rpi + ProtoDac + Moode player would be a i2c hat amplifier?.
Any ideas, recommendation?

Hope this is a suitable thread to put this post in

Many thanks EZ

[hifinet]

The ones that I am aware of in a HAT format are the JustBoom Amp, HiFiBerry Amp2, Allo Volt Amp using TI TPA3118. HiFiBerry AAMP60 using TPA3128 and capable of 60W. HiFiBerry Amp3 using the MA12070 also 60W capable. There are probably others. Of course, they require an external power supply up to the task to get the rated output (75W+).

I just picked up a JustBoom Amp for $20 at the PiShop.us . The layout is a little off to integrate as a HAT with ProtoDAC, when oriented with the GPIO cutout on the Amp. The analog signal input is on the opposite end of the PCB, near the USB/Ethernet end. It was designed to integrate with their DAC HAT. You would probably orient the other way so the ProtoDAC outputs are near the analog input of the Amp. It does have 5V/GND pads near the GPIO cutout,  so it can back power ProtoDAC and the RPi. I was planning to use it stand alone mounted on a thick piece of aluminum plate as a base for additional cooling.

[hifinet]

Hello 
I am new to this forum though I have been using the Moode player for many years with a RPi 3b.
Recently I came accross the ProtoDac project, looks just right,  
a ladder type dac, not many good quality components and affordable so I have ordered one. 
The aim to put together a good sounding standalone system.
(small for factor, low consumption, integrated)
What would put the icing on the cake to the Rpi + ProtoDac + Moode player would be a i2c hat amplifier?.
Any ideas, recommendation?

Hope this is a suitable thread to put this post in

Many thanks EZ

The TDA1387 is not a ladder type DAC, like most other multibit DACs. It is a continuous calibration DAC. There is more information on this unique technique in the data sheet, and also described on page 2-3 in Philips Designer's Guide August 1997. The TDA1387 was one of the most advanced multibit DACs produced by Philips.

[TheOldPresbyope]

I'm confused.

What would put the icing on the cake to the Rpi + ProtoDac + Moode player would be a i2c hat amplifier?.

The ProtoDac takes in digitally encoded audio via I2S and outputs analog audio via RCA jacks. 

The hat amps I'm familiar with either have a built-in I2S DAC (examples: Justboom and HiFiBerry) or stack on a vendor-specific DAC (the Allo Volt stacks on the Allo Piano1.1/2.1.

The Allo Volt+ is a standalone analog audio amp with input RCA jacks so it could suit but so could other standalone amps.

Did I miss something?

Regards,
Kent

[hifinet]

There is a JustBoom Amp HAT, which is a DAC plus amp on one HAT sized PCB using a TAS5756M Amp / DAC chip. I was referring to the JustBoom Amp, which is a standalone amp that takes analog input and uses the TPA3118D2DAP.

The Allo Volt Amp doesn't have RCA (analog) inputs. The Allo Volt+ is the standalone amp with RCA inputs.

HiFiBerry Amp2 is a DAC plus AMP, Amp3 takes input from the GPIO, so also a DAC plus Amp combo. The HiFiBerry AAmp60 is the one that takes analog input.

Thank you Kent for pointing out my errors.