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Working with Current O/P DAC Chips
Working with Current O/P DAC Chips
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Old 28th April 2007, 12:46 AM   #1
Eli Duttman is offline Eli Duttman  United States
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Default Working with Current O/P DAC Chips

In a thread about 5842 GG gain blocks, Brian Beck posted the following.


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Originally posted by Brian Beck
Thanks Josh,

Well, that’s simple enough. The problem is that the input resistance (the cathode) is still pretty high because he doesn’t use feedback. The cathode gives about 770 ohms, which is in parallel with the 60 ohm resistor, for a combined resistance of 55 ohms. The PCM63, for example, has a current output port resistance of 670 ohms, lower than you’d think. It expects to drive a load much lower than that (a virtual ground) to ensure accuracy of the D-to-A conversion. 55 ohms is still a large portion of 670 ohms, better than 100 ohms, but I would want lower still.

I now notice that Broskie calls out the TDA1541 for the DAC. Its output resistance is not specified in the data sheet, but the data sheet for the “A” version says this: “To ensure no performance losses, permitted output voltage compliance is ±25 mV maximum.” Since it puts out 4mA at peak, this means that the load resistance must be lower than 6.25 ohms for the DAC to work to spec. I expect this is typical for most current-output DACs.

Brian has provided some important info. If a 4.99 Ohm I/V resistor is employed, a 19.96 mV. (peak) signal is produced. A 6922 with its sections in cascode can yield the gain needed to raise that feeble signal to the level usually associated with CDPs.

Recently, Eddie Vaughn was kind enough to provide me with a set of operating conditions for 6922 cascodes.

Is there any interest?
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Old 28th April 2007, 02:30 PM   #2
Brian Beck is offline Brian Beck  United States
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Eli,

A good topic for a thread. I waited for others to chime in, and seeing nothing from overnight, I thought I'd toss out a few thoughts.

If you use a very small resistor as a pseudo- I/V converter at the output of a current-output DAC, then the following amplifier stage starts to share some similarities to a phono stage, without the RIAA equalization of course. The input levels are about the same as a moving-magnet phono stage, and the gain required will be similar (as at 1KHz in the RIAA stage). A typical passive EQ RIAA stage (MM) has a low frequency gain of about 60dB, a mid-frequency gain of about 40 dB, and a high frequency gain of about 20dB. Here we need something like 40 to 50dB across the board, depending on the I/V resistor selected and the DAC chip specifics, of course. So in a general way, we can borrow techniques, particularly as regards low-noise behavior, from existing RIAA stage designs. But we can detune the raw gain from 60dB to maybe 40 to 50dB. This suggests two triode stages, with or without feedback, plus perhaps a CF for low-output resistance.

I must say that we have two conflicting desires here. On the one hand, we want to make the I/V resistor very small to keep the DAC within spec, but on the other hand we want the signal voltage to be high enough to keep noise from being a problem. As we drop the resistor value, the gain must go up, and with it the noise. The noise from a typical RIAA stage will be much higher than that from a DAC. So this technique of a using a very small resistor for I/V sacrifices S/N ratio performance, but perhaps could sound pretty good otherwise. As you know, I'm no fan of FETs, but here we could consider a FET/triode cascode for lowest noise. Since the FET's gate is driven by a 5-ohm source resistance, the effect of the variable gate capacitances is knocked out.

These conflicting goals remind me of an analogy: It's like riding the brakes in a car traveling at a steady highway speed. As we reduce the resistor value to make the DAC happy, it's like pressing the brake pedal harder. To keep the same speed (output voltage), we must press the accelerator pedal harder (gain) to compensate. Fuel efficiency (low noise) suffers.

This is why I suggested considering the tube "opamp" with feedback as a way to allow the DAC to see a virtual ground, while preserving good S/N. So we have a trade-off between S/N ratio and the use of feedback. Which approach might sound better is the big question, and it all would depend on many factors.
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Old 28th April 2007, 02:40 PM   #3
Brian Beck is offline Brian Beck  United States
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To reduce the digital garbage going into the tube stage, and as part of a gentle reconstruction filter, we can add a cap in parallel with the 5-ohm I/V resistor (using your example value). The cap has to be pretty large to make a given time constant with only 5 ohms. If we put a 1uF cap in parallel with the 5-ohm resistor, we'll get a pole (-3dB) at about 32kHz. This cap diverts digital noise current away from the 5-ohm resistor, reducing the resultant I/V voltage that would get into the tube stage and cause IMD. For a sharper filter, we can add a series L followed by a shunt C after the 5-ohm resistor. And we could take the passive filtering even further still. But I would suggest at least a single shunt cap to reduce the slew rate of noise going into the tube stage.
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Old 28th April 2007, 04:47 PM   #4
Sheldon is offline Sheldon  United States
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Since we are talking about a stage with similar amplification requirements as a phono stage, we could use a step up transformer after the I/V resistor, too. Might filter some of the hf. Pros and cons?
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Old 28th April 2007, 05:12 PM   #5
Eli Duttman is offline Eli Duttman  United States
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Brian,

I agree, the low value I/V resistor regime is similar to that of a MM phono cartridge.

I LIKE the low pass cap. at the I/P idea. A 1 muF. RTX MultiCap paralleled by a 100 pF. silvered mica part should do an excellent job of shorting HF crud to ground.

6922s are definitely quiet enough for MM phono section service. I suggested a 6922 cascode, as 100% of the gain needed can be obtained in a single block. A 10 KOhm load resistor in combination with a DC coupled voltage follower seems correct.

FWIW, I have some thoughts about the "hard" sound that's been associated with cascodes. I suspect parasitic oscillation is a good part of the problem. The upper triode in a cascode is, in fact, operating GG. That (IMO) means: no grid stopper resistor, a high quality cap. connection to ground, and RF suppression on the heater leads. The common cathode lower triode needs (sic) both grid and plate stopper resistors. A 1 KOhm Carbon comp. on the grid and a 100 Ohm Riken on the plate "feels" right to me.

Oh yeah, since noise is an issue, expensive Vishay S102s in the 4.99 Ohm I/V positions may be a rational selection.
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Old 28th April 2007, 05:22 PM   #6
Brian Beck is offline Brian Beck  United States
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Quote:
Originally posted by Sheldon
Since we are talking about a stage with similar amplification requirements as a phono stage, we could use a step up transformer after the I/V resistor, too. Might filter some of the hf. Pros and cons?
Sheldon
Yes, that could improve S/N, and provide some filtering, but it would introduce the usual limitations of transformers. Might be worth it though.
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Old 28th April 2007, 05:24 PM   #7
Eli Duttman is offline Eli Duttman  United States
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Quote:
Originally posted by Sheldon
Since we are talking about a stage with similar amplification requirements as a phono stage, we could use a step up transformer after the I/V resistor, too. Might filter some of the hf. Pros and cons?
Sheldon

Sheldon,

A custom trafo might work. In order to avoid caps., the DAC chip has to work directly into the trafo's primary. That means the trafo has to present a 5 Ohm load to the chip. The secondary is going to have specific resistive loading requirements. Can the balance between frequency response and ringing be maintained?
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Old 28th April 2007, 05:41 PM   #8
Brian Beck is offline Brian Beck  United States
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Quote:
Originally posted by Eli Duttman
FWIW, I have some thoughts about the "hard" sound that's been associated with cascodes. I suspect parasitic oscillation is a good part of the problem.

Oh yeah, since noise is an issue, expensive Vishay S102s in the 4.99 Ohm I/V positions may be a rational selection.
I suspect you're right about instability being a factor in the hard sound. Also, the voltage gain equation for the cascode is dependent on the more non-linear parameter gm rather than on the more constant parameter mu, as it is in the case of the common-cathode stage with a large load resistor. In this regard it's similar to a pentode and will make more higher-order distortion components that can sound hard if they're large enough. For our application, the signal levels coming off that 4.99 ohm resistor are pretty small, so a cascode has promise (as Allen Wright would no doubt agree).

I agree that the 4.99-ohm resistor ought to be of good quality, but more because we want a low voltage coefficient of resistance to keep distortion low. This 5-ohm noise resistance will be dwarfed by the first stage triode's noise resistance. A 6922 might have an equivalent noise resistance of 300 ohms if it's biased richly. That will make at least 18dB greater noise than a 5-ohm resistor, not to mention grid stopper noise contributions, 1/f noise, etc.
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Old 28th April 2007, 06:11 PM   #9
Eli Duttman is offline Eli Duttman  United States
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Quote:
I agree that the 4.99-ohm resistor ought to be of good quality, but more because we want a low voltage coefficient of resistance to keep distortion low.
Brian,

The Vishay S102s seem to stand up well to your criterion. I see a < 0.025 muV. of current noise per applied V. specification.
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Old 28th April 2007, 06:48 PM   #10
JoshK is online now JoshK  Canada
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I just noticed this thread. Was working on projects last night. I like your ideas, they make sense. But I still have a question with regards to the grid resistance versus the cathode resistance. Didn't we want the load resistance low as well, or does it not matter?
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