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Old 4th March 2012, 05:55 AM   #11
tomchr is offline tomchr  United States
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Default So... I took some measurements

Setup: JJ 300B in the configurations listed in Post #1 driven by my sand amp. I call it A23 III as it lives in the chassis of a Parasound A23 that suffered water damage. I reused the power supply and built an amp to fit the chassis. The A23 III is documented in This Thread.

The two images show far field and near field measurements using a Dayton EMM6 microphone. Near field is 40 cm from the speaker with the microphone centered vertically between the woofer and tweeter and centered horizontally on the speaker front. Far field is about 20 cm forward of my usual listening position - about 140 cm from the plane intersecting the speakers. The closest wall is 170 cm away and the second closest is 200 cm away from the left speaker - the one used in this measurement. The speakers are 90 cm from the carpeted floor. The room is about 20 m^2 with an opening into the kitchen (15 m^2). The room is relatively live, though, I've never measured the reverb time.

Before listening tests I ran a quick test on my distortion analyzer (HP 8903A). The three 300B setups measure identically.
The Loftin-White and Cathode Bias circuits sound very close, if not identical, to each other. If I was to pick a difference between the two, I'd say that the cathode bias has slightly muddy highs compared to the Loftin-White. I found both the Loftin-White and Cathode Bias somewhat boomy in the bass and kinda tense to listen to. The fixed bias is an improvement over the previous two in my opinion. The difference is not huge, but I find the highs more enjoyable and precise. It seems more precise overall.

So, it seems the bias configuration of the output stage matters a little bit but not greatly. The fixed bias does have the distinct advantage that B+ is lower and the parts cost of the bias circuit is a handful of 10-cent resistors and a small trimpot.

~Tom
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File Type: jpg FarField.jpg (182.7 KB, 403 views)
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Old 4th March 2012, 10:17 AM   #12
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Quote:
Originally Posted by tomchr View Post

Or the speaker, actually...

~Tom
Of course, the choice of the speaker is important if you want to use SE amps in general but most of times, especially with harder-to-drive speakers, the weakness of the driver will come before the full capability of the 300B is reached.

Cheers,
45
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Old 4th March 2012, 11:04 AM   #13
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Quote:
Originally Posted by tomchr View Post
The fixed bias is an improvement over the previous two in my opinion. The difference is not huge, but I find the highs more enjoyable and precise. It seems more precise overall.

So, it seems the bias configuration of the output stage matters a little bit but not greatly. The fixed bias does have the distinct advantage that B+ is lower and the parts cost of the bias circuit is a handful of 10-cent resistors and a small trimpot.

~Tom
Well, I think you need a better bias supply than that. I agree that fixed bias (in my case combination bias) had the clearest sound and by far the best bass compared to cathode bias, but I wasn't satisfied with a slightly dirty treble. A film cap as the last cap seems a requirement for the sound quality - this was audible - and no doubt Schottky diodes. As for what else would improve it I'm open to suggestions because this is potentially a good option.

andy
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Old 4th March 2012, 08:51 PM   #14
tomchr is offline tomchr  United States
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Quote:
Originally Posted by andyjevans View Post
Well, I think you need a better bias supply than that. I agree that fixed bias (in my case combination bias) had the clearest sound and by far the best bass compared to cathode bias, but I wasn't satisfied with a slightly dirty treble. A film cap as the last cap seems a requirement for the sound quality - this was audible - and no doubt Schottky diodes. As for what else would improve it I'm open to suggestions because this is potentially a good option.
I really like the sound of this fixed bias circuit. I agree that the highs could use a little clean-up, but the sound stage is awesome. As is the presentation overall.
Today, I'll add a source follower to it. Probably a 2SK3564 as it has the lowest gate capacitance among the high-voltage devices I've been able to find. That will make the driver's job much easier. Currently, I've cap coupled the grid of the 300B. As my driver isn't capable of driving the 300B into saturation (my 150 W sand amp "only" has a 50-ish Vp output voltage swing) I don't have issues with blocking distortion. But the driver also can't source any grid current as the DC coupling cap gets in the way. I'll build it up and post results.

As far as the quality of the bias circuit goes... I'm not looking to start a religious shouting match about this brand of boutique resistors versus that brand of boutique resistors.
Now, I do agree that the parasitic components present in any component can play a role in the perceived sound quality. This is why I tend to use metal film resistors. I've measured the 10-cent metal films commonly available and found them to be "flat" (resistive) up to 40 MHz (the upper limit on the impedance analyzer). I also tend to use polypropylene caps as they don't suffer (as much if at all) from dielectric absorption and the good ones have extremely low ESR, resulting in an extremely low loss tangent. Again, I have measured these parameters using an HP 4194A impedance analyzer and compared with the manufacturer's specs. The Solen caps I have meet the published specs... 47 uF, 630 V, 5 mOhm ESR confirmed. That's impressive!
So I tend to base my component choices on data... I realize that few people have access to the test equipment needed to confirm the manufacturer's data, but everybody has access to the manufacturer's data... If the manufacturer chooses to publish it, that is. If they don't... Shop elsewhere. That's for me anyway. Others are free to be different.

~Tom
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Old 4th March 2012, 09:29 PM   #15
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Tom,

Reverse transfer capacitance (Crss), not gate capacitance, is the critical parameter for a FET performing voltage follower duty. Select a part whose Crss is both low and stable. The wimpy 12AX7 drives the ZVN0545A with aplomb. For triodes with more "stones" than the 'X7, the IRFBC20 is fine.
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Old 5th March 2012, 02:54 AM   #16
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2 pF low enough?

http://aosmd.com/res/data_sheets/AOT1N60.pdf

785-1184-5-ND 72 cents each at Digikey.
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Old 5th March 2012, 04:13 AM   #17
tomchr is offline tomchr  United States
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Quote:
Originally Posted by Eli Duttman View Post
Reverse transfer capacitance (Crss), not gate capacitance, is the critical parameter for a FET performing voltage follower duty. Select a part whose Crss is both low and stable.
Tru dat. Thanks for being more specific than I was. Yeah... The gate-source cap shouldn't matter as the source voltage follows the gate voltage. And as the drain sits at AC ground, it's the raw drain-gate cap (Crss) that's important. No miller effect as the drain is at AC ground.

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Originally Posted by tubelab.com View Post
2 pF low enough?

http://aosmd.com/res/data_sheets/AOT1N60.pdf

785-1184-5-ND 72 cents each at Digikey.
That's seriously impressive. Thanks, George!

The sound (mainly the highs) has cleaned up quite a bit with the source follower. Still can't measure a difference, though. Neither on the test bench nor in front of the speaker. At least not in the frequency or impulse response. I haven't measured the power response.

~Tom
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Last edited by tomchr; 5th March 2012 at 04:19 AM.
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Old 8th March 2012, 02:28 AM   #18
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Quote:
Originally Posted by tomchr View Post
Do you happen to have measured data for the IMD? I don't... But it would be interesting to see.
I just remembered I was going to post this days ago. Sorry, I was figuring out how to use ARTA so levels aren't the best here and I didn't know how to get it to compute the percentage for IMD. I don't know if the frequencies/ratios I used for this are the best/most standard. I was just playing around at this point.
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Old 31st March 2012, 10:38 PM   #19
tomchr is offline tomchr  United States
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I don't know if there is a formal standard for IMD measurements. There probably is, but I'm not really a standards person... Most measurements I see of IMD use 9 kHz and 10 kHz at -3 dB of full power and measure the 1 kHz IMD component.

I swapped the 2SK3564 for the AOT1N60 that George suggested for the source follower. The jury is still out for the final verdict, but initial impressions are good. With the 2SK... I was getting rather 'tense' highs (for lack of a better word) and lots of fizziness on metallic percussion instruments. All this seems gone now and replaced by a much more balanced sound. I wish I could correlate this with measurements, but no dice (not that I really tried that hard).

~Tom
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Old 2nd April 2012, 10:19 AM   #20
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A friend of mine did what Stereophile does: 19 and 20kHz at 1W and 10W. Then you can find results to compare with. A pic of his PP amp, 1W, 8-ohm, 6550, 6J5GT input tubes, 6dB gNFB attached.
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