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Old 27th May 2010, 06:58 AM   #161
Calvin is offline Calvin  Germany
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Hi,

I tested OPCs circuit structure with an IRF530, 365Ohms Drain-resistor and +40V/-20V well regulated supplies and 50-80mA of idle current.
The PCM1794A in Mono-mode supplies 12.4mA idle centre current and 15.6mApp signal which is more than usual. The Gate of the IRF was very susceptible to HF/RF (better without 220Ohm gate-resistor).
Generating a 1kHz sine 0dBfs signal measured mere -60dBTHD+noise.
Now I don´t think that the over-sensitive Gate spoiled the result so much that it adds up to the 40dB difference between OPC´s and my measurements.
Had simmed the circuit before and got around -80dB THD -which seemed realistic to me, since it is already a good value for non-NFB designs and simmed values had always been better than real values.
The PCM1794A measured with just a small resistor in its output (15-20Ohms) performed exceptionally well with THD at -107dBfs (which is even better than the datasheet value). So I don´t assume the PCM to be the source of the problem but the I/V-circuit, i.e its implementation.
The situation is a bit frustrating and I´m sure others will experience similar probs. The circuit seems to be sensitive to parameter variations.
Maybe OPC just found the golden spot?

jauu
Calvin
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Old 27th May 2010, 01:15 PM   #162
regal is offline regal  United States
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Quote:
Originally Posted by Calvin View Post
Hi,

I tested OPCs circuit structure with an IRF530, 365Ohms Drain-resistor and +40V/-20V well regulated supplies and 50-80mA of idle current.
The PCM1794A in Mono-mode supplies 12.4mA idle centre current and 15.6mApp signal which is more than usual. The Gate of the IRF was very susceptible to HF/RF (better without 220Ohm gate-resistor).
Generating a 1kHz sine 0dBfs signal measured mere -60dBTHD+noise.
Now I don´t think that the over-sensitive Gate spoiled the result so much that it adds up to the 40dB difference between OPC´s and my measurements.
Had simmed the circuit before and got around -80dB THD -which seemed realistic to me, since it is already a good value for non-NFB designs and simmed values had always been better than real values.
The PCM1794A measured with just a small resistor in its output (15-20Ohms) performed exceptionally well with THD at -107dBfs (which is even better than the datasheet value). So I don´t assume the PCM to be the source of the problem but the I/V-circuit, i.e its implementation.
The situation is a bit frustrating and I´m sure others will experience similar probs. The circuit seems to be sensitive to parameter variations.
Maybe OPC just found the golden spot?

jauu
Calvin

When you say well ragulated supply, how well? I know one of the disadvantages of the D1 is it has practically no PSRR. All applications I have seen use elaborate power supplies.
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Old 27th May 2010, 01:49 PM   #163
opc is offline opc  Canada
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Hi Calvin,

There are too many differences between the circuit you built and the one I posted to be making any comparisons. If you want to get the measurements I got, you need to build exactly the same circuit, and use exactly the same DAC. It would appear you've changed everything, and you're expecting the same results which is not reasonable. It sounds like you've omitted the gate stopper resistors (which is causing oscillation) and I'm still not clear on what you've done with the source resistor. What's the voltage at the source with the DAC hooked up? Can you post an actual schematic with all the part values?

Here's what I think is happening:

With a 365 ohm drain resistor, and with the PCM1794s higher output current, you'll be getting roughly 4 times the output voltage that the circuit in post #137 provides, or about 8VRMS at 0dBFS. If you look at your simulation, one side of the balanced circuit at a time, you'll see that the lower portion of each wave is rounded over at that output level. When this happens, the net result at the balanced output looks alright, but distortion increases. You can't get that much output with this circuit and still get low distortion.

If we're going to do this correctly, then you need to go back and build EXACTLY what you see in post #137 with 45-50V rails. You're looking at maybe $10 worth of parts from Digikey to get everything on that schematic. Adjust it for 2.5VDC at the source of the mosfet, hook it up to the PCM1794, and give it a test. If you're output is more than 2VRMS at 0dBFS (and it will be) then we might need to drop the resistor values and lower the rails a little bit to dial it in. If you can give me the RMS output at 0dBFS then I can plug it into my simulation and we can bang out a great schematic that will work perfectly with the 1794. This would be very useful for everyone here, and it would solve your problem. When you build a circuit for the first time, you should never take shortcuts.

Regal:

I'm not sure I agree with your PSRR statement for the D1. Any noise from the PSU will appear equally across both branches of the balanced circuit which means they should cancel at the output providing the next stage has decent CMRR. It's part of the advantage of running a balanced system. Using regulators isn't really all that fancy, and it's good practice for circuits that don't draw much current since it's easily implemented and doesn't waste too much power.

I'll try to measure this sometime today, and I'll report back. The problems Calvin has are more related to the circuit he built, and I doubt even the best power supply would change the results.

Cheers,
Owen
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Old 27th May 2010, 02:11 PM   #164
regal is offline regal  United States
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Quote:
Originally Posted by opc View Post
Regal:

I'm not sure I agree with your PSRR statement for the D1. Any noise from the PSU will appear equally across both branches of the balanced circuit which means they should cancel at the output providing the next stage has decent CMRR. It's part of the advantage of running a balanced system. Using regulators isn't really all that fancy, and it's good practice for circuits that don't draw much current since it's easily implemented and doesn't waste too much power.

I'll try to measure this sometime today, and I'll report back. The problems Calvin has are more related to the circuit he built, and I doubt even the best power supply would change the results.

Cheers,
Owen
You're probably right forgot this was designed for a differiential output DAC chip so should have good CMRR, the original Pass D1 stage didn't have this added benefit.
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Old 27th May 2010, 04:19 PM   #165
opc is offline opc  Canada
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Hi Guys,

I’ve got a quick update with more transconductance values for most of the fets I tested in the previous round.

These measurements were taken at 130mA ID and 25VDS, which matches the way they’ll perform in the D1 with 50V rails and 200/400ohm resistors.

FQA19N20 0.871S
FQA32N20 0.922S
FQA46N15 0.837S
STW75NF20 0.566S
IRFP4310Z 0.795S
IPW50R140 0.669S
IRF1310 0.788S
IRF640 0.548S
IRF740 0.554S
IRF540 0.848S
IRF644 0.581S
IRF510 0.377S

I was curious to see if the higher current and voltage would make some fets stand out more than they had at lower current and voltage, but it turns out that everything just performs better in general, and the two FQA devices are still looking to be the best bet.

If you’re going to build this, then you might as well get a few FQA32N20 parts from Digikey, especially if you consider that they’re only $1.91 if you buy 10 of them. With 50V rails, you’re looking at nearly 1S of transconductance, which will really help to minimize voltage swing at the DAC outputs.

Cheers,
Owen
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Old 27th May 2010, 07:56 PM   #166
qusp is offline qusp  Australia
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FQA32N20 FTW!!! bought a whole rack of them when I saw it leading 'the charts' number 1 with a bullet yes indeed it was very handy that the best part for the job was only 1.91 each. i'll only be running 45v rails for this current PCB though as some of my caps are only 50v. but I guess i'll grab your PCB when they are ready and just move everything over and add a couple parts. should have most everything needed in my parts bin anyway.

I do have to say I was watching from the sidelines waiting for you to mention that Owen, comparing the 2 circuits for performance when there was almost nothing the same
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Old 28th May 2010, 09:29 AM   #167
Calvin is offline Calvin  Germany
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Hi,

I put the schem and sim-reults in the pdf. As You will notice the schematic differences are mainly the drain- and source-resistances due to the different DAC-currents and supply voltages.
Output voltage 0dBFs is ~2Vrms (single output) and ~4Vrms (differential output) and not 8Vrms (OPC: how did You calculate 8Vrms??)

Input impedance: 1.1Ohms, Output impedance: 365Ohms, Transfer function: ~-364mV/mA. (This gain x times the input current of 7.8mApeak gives 2Vrms, not 8Vrms!)
The gate voltage is set so that there is 0V at the source. The PCM doesn´t like an voltage offset at its outputs, especially not one of 2.5V. There are internal protection diodes that start conducting above ~250mV. Besides, measurements showed that THD is best with lowest voltage swing at the DAC-output (meaning the input impedance of the I/V-converter should not be greater than app. 20Ohms).
The idle current of the MOSFET is 42.4mA, the current through the source resistor 54.8mA (=idle current + DAC-centre current of 12.8mA).
The sim shows THD-values no better than app -80dB at 0dBfs and single output (-107dB in differential mode, because of K2-cancellation). As the THD-tables of the sim show -6dBfs reduces the THD-values by -6db and -12dBfs by -12dB. -12dBfs meaning an output voltage of ~500mVrms in single-mode (THD -95dB) and 1Vrms in differential-mode (THD -130dB).
The linear increase in THD-values with rising output level indicates that the output voltage at fullscale is not too high (driving the circuit into saturation or clipping). Sims also doesn´t show a distorted/flattened sine-wave-peak.
In practise the result using a IRF530 was worse than the sim (~-60dB at 0dBfs, single output), because the gate of the MOSFET reacted very sensitive to RF/HF, which implies, that extra care must be taken when layouting a PCB. Controlling the gate´s behaviour would lead to better THD-figures. So I assume the circuit could come close to the simmed values of -80dB THD at 2Vrms, but even better?? At the moment I doubt it.

jauu
Calvin

ps. one thing I wondered about, is the increase in input impedance towards 500kHz and the resulting increase in input voltage the circuit and the DAC-output see.
Attached Files
File Type: pdf D1 simple IRF540 simu.pdf (110.4 KB, 190 views)

Last edited by Calvin; 28th May 2010 at 09:40 AM.
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Old 28th May 2010, 12:46 PM   #168
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Hi

I have replaced gain devices IRF610 from prometheus pcbs with IRF540
I have hoped that I will get much better sound regarding IRF540 higher transconductance
I use It with pcm63
the sound became unealistic, with much more HF harmonics no low bottom
I was not happy at all
after this I increased bias from 10 mA to 25 mA replacing source and drain resistors
again - not any better
I don't have opportunity to do measurements but I think that NP have done his work very well choosing IRF610 and 10 mA bias
Owen you could try to replace source and drain resistors with ccs like I/U stage D1 clone with 2sk389 finetuned...
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Old 28th May 2010, 01:10 PM   #169
regal is offline regal  United States
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Please explain how/why you would use this with a PCM63?



Quote:
Originally Posted by samoloko View Post
Hi

I have replaced gain devices IRF610 from prometheus pcbs with IRF540
I have hoped that I will get much better sound regarding IRF540 higher transconductance
I use It with pcm63
the sound became unealistic, with much more HF harmonics no low bottom
I was not happy at all
after this I increased bias from 10 mA to 25 mA replacing source and drain resistors
again - not any better
I don't have opportunity to do measurements but I think that NP have done his work very well choosing IRF610 and 10 mA bias
Owen you could try to replace source and drain resistors with ccs like I/U stage D1 clone with 2sk389 finetuned...
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Old 28th May 2010, 02:00 PM   #170
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I am talking about replacement of a FET with high gs FET
what Is this
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