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Old 6th May 2010, 08:14 AM   #111
lauret is offline lauret  Netherlands
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Quote:
Originally Posted by opc View Post

As I mentioned above, the lateral BUZ fets were some of the best I've tried, and there was no rhyme or reason to it so far as I could see. Keep in mind that things like gate charge and the associated capacitances don't seem to carry much importance here since the gate and source don't actually swing by any appreciable amount. For that same reason, I don't think that "faster" devices (lower gate charges and capacitances) would provide any significant benefit in bandwidth. I will check this though.
Check the distortion at lower frequecies as hinted by Nelson Pass:

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Originally Posted by Nelson Pass View Post
I spent some time with the R550 last weekend. As an output
device it has more bandwidth than the other power parts such
as the R100's or IRFP240's, and this is due to lower capacitance.
With that comes greater distortion at lower frequencies. For
this it would be natural if your source impedance was high or
you were building a tweeter amp.

The IRF610 still has lower capacitance, but also much lower
transconductance, about 1/5 as much.

I don't know what a D1B1 is, but if it's the IV for a DAC, then
you are better off with JFETs like the 2SK170's, maybe a
few in parallel.

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Old 6th May 2010, 11:31 AM   #112
qusp is offline qusp  Australia
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Quote:
Originally Posted by lauret View Post
Please let us know what works best for you so we all can learn!
absolutely, i'm not one to hoard information if I think its meaningful; as long as it doesnt fall outside the bounds of my NDA and owen, I think you know why I think high resolution is very attainable

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Originally Posted by opc View Post
Hi Guys,

I certainly didn't mean to discourage you qusp, you should, by all means, try every possible device you can and see what works best. I was just generally trying to convey the trends I've seen in the 8 devices I've tried it with.
oh of course Owen, its all good mate, I was simply explaining my motives and thought the fact I was flexible in my direction was a point worth mentioning; particularly to Lauret. happy to take onboard any practical experience I may lack, didnt mean to sound cross

Quote:
As I mentioned above, the lateral BUZ fets were some of the best I've tried, and there was no rhyme or reason to it so far as I could see. Keep in mind that things like gate charge and the associated capacitances don't seem to carry much importance here since the gate and source don't actually swing by any appreciable amount. For that same reason, I don't think that "faster" devices (lower gate charges and capacitances) would provide any significant benefit in bandwidth. I will check this though.
think we are entering a new area here mate and there may well be a n unexpected hero in all this, due to a new paradigm, previously it was quite appealing to have an almost infinitely high input impedance, but this has changed with the sabre, making buffers of many types unusable

Quote:
As for the capacitor values, I came upon mine by simulation, but I'll confirm with a few measurements tomorrow. As qusp mentioned, it's a tradeoff between how much HF garbage you're willing to let through, and how high you want your response to extend. I'll work out the values for -1dB at 50k, 100k, 200k, and 400k providing the circuit makes it that high.
great mate! thanks, yeah I ordered some 500R for R27/34 and some 0.0027 and 0.001 for C15/C16. got this from mouser, who have a pretty limited range of films, these are the MKP and FKP I got, but will use the evox/Rifa PPS for the final build, even thought about socketing the positions for these, but worry about the added inductance and capacitance effecting any results I get.

I guess youve made your parts choices for the BOM already owen, but wondered if you are aware of the newish expended range of panasonic SMD PPS film caps available with higher voltage ranges than previously and in sizes as small as 0805. would be perfect for this design I think mate and wouldnt require any changes to the layout as they come in 0805, 0603 even I think (nut not sure on that one), 1210, 1206, right up to 2824. also the sanyo oxicap and poscap are pretty special, though probably too low voltage.

you are still going mostly SMD here hey mate?? with options in some positions for through-hole. hope you stuch to your guns there mate, I love SMD these days. as long as there is those 2 positions for larger more exotic films i'm happy with every other part being SMD and i'm sure I can just jerry-rig the pin headers to fit the ackodac or just use short molex

Cheers,
Owen[/QUOTE]
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Old 6th May 2010, 04:26 PM   #113
anbello is offline anbello  Italy
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Quote:
Originally Posted by qusp View Post
... i'm using transformers on the output of my buff II at the moment and enjoy it very much, as far as the above its far from ideal, but its very pleasing all the same. good numbers please me a great deal, but they do not rule me
Hi qusp,
I wolud like to know which kind of transformers are you using with the buffalo DAC and in which way they are connected (with or without resistor/capacitor on primary and/or secondary)

Thanks
Andrea
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Old 6th May 2010, 05:07 PM   #114
qusp is offline qusp  Australia
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O-netics from Bud Purvine who is a member here under the name BudP. they are 600:600, with dual primaries and dual secondaries. I have simply tied both primaries and secondaries together in series and connected the output if the buff II directly to the primary and the output directly to the secondary (balanced output to my active monitors) I tried with trimpots from each leg to ground on the primary, but didnt find any improvement I could put my finger on, so removed them. the impedance of the transformer itself is doing the I/V (as well as the dac outputting mostly voltage at this point anyway)
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Old 6th May 2010, 11:59 PM   #115
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Default Understanding D1 I/V

Just thought I would summarize my understanding of the filters in the D1 I/V circuit for you folks to review and correct me if I am wrong.

C15 and R27 in parallel with the gate impedance of Q2 form a low pass filter. The MOSFET gate impedance is high enough to be negligible. The -3 dB cutoff frequency of the filter is thus approximately 1 / (2 pi R27 C15), which for the stock D1 schematic gives about 40 kHz. Same frequency for the modified I/V circuit values of Post #81 (higher C15 making up for lower R27).

The role and size of C40 are a bit more nebulous. I think C40 and R26 in parallel with the input resistance of Q3 form another low pass filter to reject RF transients from the DAC’s digital circuitry. The input resistance Rs of a MOSFET in common gate configuration is approximately the inverse of its transconductance (cf. “common gate” in Wikipedia). Mr. Pass states here Pass Labs D1 that the transconductance of an IRF610 at 10mA is about .1S, which translates into a resistance Rs of 10 ohms. 10 mA is close to the operating condition of the stock D1. Hence here the contribution of R26 is negligible and the -3 dB cutoff frequency is approximately given by 1 / (2 pi Rs C40), or about 1.6 MHz. For the circuit values of Post #81 the -3 dB frequency is approximately 500 kHz, perhaps a bit less because of the slightly higher transconductance of the MOSFET due to the 30 mA bias current.

I have not measured the above. So if this is wrong please feel free to jump in. What I am sure of, is that it sounds good .
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Old 7th May 2010, 12:44 AM   #116
qusp is offline qusp  Australia
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yeah it was mainly the LPF at R27/C15 and co that was of interest to me, because I cant live with a LPF of 40khz, so I set the values there to give either ~110khz or ~220khz I changed the R down by 1/3 from 1.5K to 500R and have bought 0.0027, 0.0015 and for interest 0.001uf as well. these are not the final caps anyway as I will be using PPS for them, but I figured this was a good place to start. I just used a quick RC calc rather than plotting it out, because I was low on time and it wasnt crucial for this round.

thanks for the comprehensive rundown tho, that will come in handy; provided its correct lol
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Old 7th May 2010, 01:24 AM   #117
opc is offline opc  Canada
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Hi Guys,

If there isn't already a saying like "The simplest things in life are often the most complicated", then I'm going to claim it, and apply it here.

I spent the better part of the day trying a few different things with the buffered D1B1, and I found out a few really neat things.

1. Everything in this circuit affects everything else!!!!!! There is not a single part that you can change without tossing the balance of the circuit out the window.

2. The transconductance of the mosfets is important, and affects both the filter values, distortion, and voltage swing at the output of the DAC.

I measured four different devices today, and I can say that higher transconductance fets are going to perform better in this circuit. I'll post THD+N graphs tomorrow, but the difference was more than 10dB from the lowest transconductance to highest transconductance devices.

I also measured the voltage swing at the DAC output with both an IRF610 and a IRF1324. It was 42mV P-P with the 610 and 12mV P-P with the 1324. Higher TC devices present a lower impedance to the DAC.

The filters are very messy to deal with. The value of the upper caps running from +18V to the drain, works in concert with the value of the lower caps running from source to ground. Making the lower caps large, causes the response to peak before rolloff, making them small causes a lower order rolloff with no peaking. Getting the ratio just right causes the rolloff to be steep, while maintaining flat response in the passband. I'll post some measurements tomorrow to better illustrate this.

The mosfet chosen will change the FR, so every mosfet will need it's own values of caps to get a desired rolloff. I did have some luck with simulation, but when I put the values in the actual circuit I had to tweak the values a little to get a flat response.

With the IRF1324's, a 400ohm source and 182ohm drain resistor, the cap values were 27nF up top and 220nF at the bottom. This cause a 0.05dB bump upwards at 15kHz, and the response was down 0.1dB at 20kHz falling steeply thereafter. I don't remember the -3 and -6 points, but I'll check tomorrow.

Anyhow, I've got a lot more work to do, and I'm going to spend some time trying to sort out why everything does what it does.

Measurements to follow tomorrow with the un-buffered D1 using the IRF1324's and properly tuned C values for a few different -1db points.

Cheers,
Owen
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Old 7th May 2010, 01:41 AM   #118
qusp is offline qusp  Australia
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oh joy!! :S maybe i'll be omitting the buffer section afterall. burt lucky I have craploads of the normal metal films just because I do, I bought numbers of different cap values and with more on the way, I suppose the dielectric will also effect the circuit in something like this by the sounds of it. also bought several devices with plans for more. so lucky I dont have my heart set on anything but the best sound I can find for my ears
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Old 7th May 2010, 10:23 AM   #119
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Owen - Thank you for your measurements, analysis and post!

Quote:
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2. The transconductance of the mosfets is important, and affects both the filter values, distortion, and voltage swing at the output of the DAC.
Oh yes! Of course! transconductance varies with frequency... So different FET types have difference transconductance curves and thus require different value of C40. I had missed this completely till now.
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Old 7th May 2010, 10:32 AM   #120
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Quote:
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The role and size of C40 are a bit more nebulous... Hence here the contribution of R26 is negligible and the -3 dB cutoff frequency is approximately given by 1 / (2 pi Rs C40), or about 1.6 MHz.
Had a hunch I was wrong. Indeed I was. Filtering is provided by the FET's transconductance too!
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