Building the ultimate NOS DAC using TDA1541A

[tessier]

Hi -ecdesigns-

How about this ECC82 IV amp buffer for the tda1541a ?

Thanx

Paul

[-ecdesigns-]

Hi tessier,

How about this ECC82 IV amp buffer for the tda1541a ?

Here are some basic properties for a TDA1541A I/V stage :

- Output compliance of +25 / -25mV at the DAC output -must- be met, these are absolute maximum values.
- Bandwidth of the TDA1541A output signal must be limited prior to feeding it into an I/V stage.
- All bit currents -must- be returned to +5V
- The I/V stage should be perfectly able to handle the presented bandwidth.
- Full DC-coupling is -required-
- I/V stage should be able to resolve signals down to 61 nano amperes / 30 microvolts.

Your circuit generates -0.004 * 35 = -140mV at the DAC output, this grossly exceeds output compliance. Since no +2mA bias current is used, signal does not swing around 0V, this increases the problem.

When using a suitable +2mA bias current source, max. I/V resistor value would be: 0.05 / 0.004 = 12.5 Ohms. If the +2mA bias current is not used, the I/V resistor value has to be reduced to 6.25 Ohms.

The bandwidth of the TDA1541A output signal is too large to directly drive a passive I/V resistor or circuit. So the output signal must be bandlimited prior to feeding it to an I/V resistor or active circuit. This can be done by connecting a suitable capacitor between TDA1541A output and GND or connecting a suitable capacitor at the output of a step-up transformer.

Step up transformers can limit bandwidth, provide sufficient signal amplitude while meeting output compliance, and also handle low level signals quite well.

Suppose we use a Sowther 9762 1:12.8 step-up transformer. When using a 1K I/V resistor at the transformer secondary, the reflected secondary load will be 1000 / 12.8^2 = 6.1 Ohm. This will meet output compliance (-0.004 * 6.1 = -24.4mV). The output signal amplitude will then be 312mVpp. If an output signal of say 6.65V is required, tube amplifier gain needs to be 18.

It is also possible to use both secondary windings of the 9762 to drive a differential tube amplifier stage. In this case we have 2 I/V resistors of 500 Ohms each.

This however doesn't solve the issue with bit return currents. The selected bit currents are routed through the reflected load of 6.1 Ohm to GND. The unselected bit currents are routed to +5V (inside the TDA1541A). This means that the load current on the +5V varies with the signal, this results in a ripple voltage that is extremely difficult to reduce to required low levels in the microvolt range. The ripple on the +5V (that also powers logic circuits) will introduce trigger uncertainty, introducing deterministic jitter (jitter related to music content).

The use of a transformer no longer provides full DC-coupling, full DC-coupling is required for highest transparency.

[regal]

Originally Posted by -ecdesigns-

Hi tessier,





This however doesn't solve the issue with bit return currents. The selected bit currents are routed through the reflected load of 6.1 Ohm to GND. The unselected bit currents are routed to +5V (inside the TDA1541A). This means that the load current on the +5V varies with the signal, this results in a ripple voltage that is extremely difficult to reduce to required low levels in the microvolt range. The ripple on the +5V (that also powers logic circuits) will introduce trigger uncertainty, introducing deterministic jitter (jitter related to music content).

The use of a transformer no longer provides full DC-coupling, full DC-coupling is required for highest transparency.

So are you saying that the only sufficient I/V stage for a TDA1541 is power by the same +5V for the chip? That would severely limit any potential I/V stage design. This just doesn't sound reasonable, basically our only option would be an IC opamp or a terribly high distorting jfet buffer.

[JOSI1]

[QUOTE=-ecdesigns-;3338517]Hi tessier,


This however doesn't solve the issue with bit return currents. The selected bit currents are routed through the reflected load of 6.1 Ohm to GND. The unselected bit currents are routed to +5V (inside the TDA1541A). This means that the load current on the +5V varies with the signal, this results in a ripple voltage that is extremely difficult to reduce to required low levels in the microvolt range. The ripple on the +5V (that also powers logic circuits) will introduce trigger uncertainty, introducing deterministic jitter (jitter related to music content).
QUOTE]

Hello John,

could You please explain how the issue with bit return currents is solved in the MK14 I/V circuit shown in post #4489?

Thank You for your support

[tessier]

Originally Posted by -ecdesigns-

Hi tessier,

Here are some basic properties for a TDA1541A I/V stage :

- Output compliance of +25 / -25mV at the DAC output -must- be met, these are absolute maximum values.
- Bandwidth of the TDA1541A output signal must be limited prior to feeding it into an I/V stage.
- All bit currents -must- be returned to +5V
- The I/V stage should be perfectly able to handle the presented bandwidth.
- Full DC-coupling is -required-
- I/V stage should be able to resolve signals down to 61 nano amperes / 30 microvolts...

Hi

I've seen in the thread that some guy's suggested to use the Aikido tube preamp as a IV amp.

Is it a good ideas and could you tell me where I can find a IV amp circuit using the Aikido ?

I have few 12AU7 to made a IV amp.

Thanx a lot.

Paul

[guido]

Originally Posted by regal

So are you saying that the only sufficient I/V stage for a TDA1541 is power by the same +5V for the chip? That would severely limit any potential I/V stage design. This just doesn't sound reasonable, basically our only option would be an IC opamp or a terribly high distorting jfet buffer.

Check the datasheet of the TDA1542 and how the opamps are powered...

[Zoran]

ECC82 is good in general for amplifying the low signal after the R-IV.
iF You dont mind the higher output resistance of 6K cca. caused with 6.5K-7K Internal resistance of the tube? In a simple basic topology.
And phases will be good too because R-iv shifts of 180deg and Tube stage shifts too of 180 deg...
But this circuit from post 4701 is complicated, no additional phase shift,
tooo many C on signal path, AND very low anode voltages 40V cca per tube section...
Tis is tube deserves higher voltage, not a transistor.
there is no need for that, but just for plain circuit.
cheers

[Zoran]

Q for EC
will the low values of R-IV like 6 to 12 ohms overheat the TDA1541A chip
from Your knowledge?

[Zoran]

guido please could You point the link for TDA1542
I try to find, but without success
thanks

[abraxalito]

No need to disturb EC for this one - no, the TDA1541A is current-out so even shorting its output will do no damage at all.