Technical help requested on using Aikido Line stage as output stage for dac

[Gordon Macfarlane]

Hi guys , now that I have a new pre amp the Aikido line stage I was building has become redundant.

I am wondering if I can use the line stage as an output stage for my TDA1541A based dac.

I spoke to John Broksie a couple of years ago and he suggested using his 9 pin Aikido with passive I/V via a 50ohm resistor between input and ground.

I have an assembled OCTAL Aikido board.If I use 6sl7 input valves and 6sn7's outputs this should give me enough gain but can I just use a simple resistor IV as with the 9pin circuit?

Thanks in advance,

Gordon

9 pin circuit :

Edited March 18, 2014 by Ozcall

[svenr]

I'm a bit at a loss - what does IV stand for?  Perhaps mark in the schematic which part you are concerned about.  Thing that concerns me is the low input impedance of the stage at only 51 Ohms - that could have implications on the voltage drop at the DAC output buffer.  Also, the 1uF coupling cap is critical depending on the input impedance of the preamp.  I'd expect that this value gives you too high a lower boundary frequency to be satisfying.

[Zaphod Beeblebrox]

'IV' means: Current to Voltage.

 

The IV stage converts the current output from the DAC to a Voltage. Of necessity, a current input stage has a low input impedance.

[Gordon Macfarlane]

51 OHM RESITOR near dac output is the i/v converter.

[svenr]

Makes all sense suddenly...

[RoHo]

Yes, Ozcall, it will work fine as an I/V gain stage as you have shown.  The type of tubes don't matter, 9 pin or octal.  If you have plenty of gain you could experiment with a lower I/V value, should reduce distortion.

[Gordon Macfarlane]

Thanks Roho.

[HdB]

The output pins of the 1541A dac chip require a current injection - there's a couple of different approaches to this but possibly the Thorsten L system that's used in the AMR dac is possibly 'the best' - John Brown from 'EC Design' thread on diyAudio has also shown a couple of simple quite effective similar circuits

 

Sorry for to 'rain on your parade' for a simple adaptation

 

Aikido makes a really good voltage gain stage for a hybrid power amp and an even better drive stage for a top line headamp, provided the power supply is a good quality unit.

[Gordon Macfarlane]

Hi J , thanks for the info.

Are you saying that the Aikido line stage specifically is unsuitable for this job unless 'current injection is used or that all valve line stage require CI to work well ?

Cheers,

Gordon

Edited April 6, 2014 by Ozcall

[HdB]

Sorry, missed your reply

 

I'm not sure what sort of sound you would get if you didn't add the current injection network as it seems to be part of the dac's requirements  so can't really add anything relevant here

 

The dac chip also requires a maximum voltage on the output pins of 25mV and as the dac outputs max 4ma (+/-2ma_ this is where the theoretical maximum 12 Ohms loading resistance comes from - there are plenty of people that have used much higher values and 39 Ohms has been a popular value over the years - on the other hand, using an OPA 844 provides a load of about 10 Ohms if done in the Pedja cct or Georgehifi method, etc so would suggest it's an aiming point.

 

It seems that the best place to provide the current injection is from the dac chips +5volt and -15 volt supply lines but unfortunately, as the injects directly into the output circuit, you do need to have very good quality power supplies - I use Salas Shunt regs but there are quite a few others - I would suggest that the simple 3 pin regs, even if done well (ie with added Cmx filters, for example) just won't be good enough.

 

There are a couple of threads over on diyAudio that have been going for a long time and has recently come back to life - John Brown's circuits are regarded as rather good and it's just about getting the pcb and power supplies up to scratch

 

A few decisions need to be made such as O/s or NOS, the type of DEM used, reclocking, the master clock, and so on

 

Over the years there have been plenty attempts to get really good results from this chip mainly by concentrating on the output stage, the clock, changing to NOS and the power supplies, but have ignored the decoupling caps, the DEM, the pcb, the reclocking, etc and for the most part, rather unsatisfactory results -  the use of the Red Baron pcbs is a good start and just reading up on the design behind them gives a good idea about how good it can be.

 

I don't know if ryanj (his diyAudio name) is a member here but he has a very highly developed 1541A and he also uses one of those SD card readers as the source instead of a computer.

 

I hope this info is of some use ....  

[Gordon Macfarlane]

Hi Jh , thank you for taking the time to supply me with all the info. My diy skills and electronics understanding is still fairly rudimentary and so most of the above is beyond my current ability to implement however I will save the info for future reference.

My tda1541a dac started life as Thorsten's Satch dac but has been paired back to just the digital board with input direct to the CS8412 chip and new psu for Lampinzator. The output stage was also designed by Thorsten for DIYHIFISUPPLY . i/v is passive with 50 ohm AB resistors.

Might ask around and see if I can get one of the tech savvy guys to build me the transimpedance stage that George and Mick have discussed.

Once again , thanks for the info.

Gordon

ps :Have read through the threads you mention on DIY audio a few times and still only understand a small amount