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Old 22nd March 2013, 09:55 PM   #2331
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Default First impressions

Well,

itīs still very early but sound wise nothing bad has happened. The only negative thing is some hum in the servoed channel. Only heard with one ear next to the (101dB) speaker. Here it probably would be good to filter the opamp output a bit.

Maybe it would be better to see if I can get some really nice and constant 230V out of the wall.........has anybody tried USVīs? I found some from AEG (Protect type D) which make there own sinus and cost a lot less than a PS Audio Powerplant.

William
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Old 24th March 2013, 05:34 PM   #2332
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The servo implementation sounds promising so far, William. Impressive that you have so little hum into 101dB speakers. Mine are probably somewhere in the 87-90dB range and I can hear hum within about 1 foot of the speaker. I still need to fiddle around a bit to remove this. I don't get the mains voltage swings quite to the same extent that you do, thus I was thinking of using the servo just to bring down the absolute offset more quickly. I'm wondering how a servo will be affected by summer (19c) vs winter (13c) temperatures in my basement.

I think I still need to play around with values for C9/C10 in my JFET amp to better control oscillation. I think I'll just order a wide variety of caps in the pF range and seeing which ones seem to work better. Right now, I have 680pF cap in place and behavior is clearly better than with no cap here, but still have some collapse in power output beyond clipping, but only at the top end of the audio spectrum (20kHz) and only into 4R loads.

Directly comparing amps with JFETs and 9610's, I am getting more stable relative offset over time, greater power output, and wider bandwidth with the 9610's. With JFETs, maximum power is a bit reduced and waveform deterioration with the JFETs gets nastier beyond clipping. 10kHz square waves at 10Vrms are identical between the 9610s and the JFETs.

With 9610's and ~22v rails I can get 4mV relative offset and:
8.8A bias provides 120w/4R and 78w/8R
9.25A bias provides 144w/4R and 90w/8R
9.6A bias provides 156w/4R and 98w/8R

I don't quite understand why output into 8R increases with bias- I thought 8R output was voltage limited and 4R output was current limited...

With JEFTs and ~22v rails, I can get ~30mV relative offset cold and ~100mV of relative offset when warm and:
8.8A bias provides 90w/4R and 80w/8R
9.25A bias provides 115w/4R and 83w/8R
9.6A bias provides 119w/4R and 83w/8R

Putting the amp back down in my theater room, the obvious difference with the new JFET front end is that I get more treble and more balanced midrange. I find these to be welcome improvements, though the magnitude of the change is small. At first, I wondered if this is worth it *for me* since my theater setup makes use of EQ in the preamp - thus, this is something I could largely duplicate with EQ instead of changes to the amp. For someone who uses the amp in a more "purist" form (source, volume control, amp, speaker, no EQ), the change to JFETs is clearly valuable.

But after a day of listening to music (in pure direct mode, which bypasses any digital EQ in my preamp), I am detecting another interesting improvement: It's hard to describe it well, but the impression I have is that the speaker driven by the 9610 amp seems to "call more attention to itself." Music seems to be coming more directly from the speaker, so I'd call that speaker more localizable during listening. The speaker driven by the JFET amp seems to have "disappeared" more into the room. I can't as easily identify the physical location of the speaker while listening.

At first, I thought this was an SPL imbalance between the amps due to the change, but a few minutes with my meter proves this theory wrong.

With the JFETs, the speaker seems to have completely disappeared, leaving behind a wider soundstage and more focused imaging. This is clear while listening to both music and movies.

I will have to spend some time with higher SPL movie playback to see if I can detect any signs of earlier clipping due to the reduced power output with JFETs. Movie nights with friends and family often "redlines" the audio chain, so power output is also important to me.

I think I need to spend some more time listening before I change anything else...

Edit: the little "kink" in the sine wave trace posted just above is present with BOTH the 9610 and the JEFT differential just prior to clipping into 8 ohms...
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Last edited by Eric; 24th March 2013 at 05:38 PM.
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Old 24th March 2013, 06:21 PM   #2333
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Old 24th March 2013, 07:05 PM   #2334
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Hi Eric,

servo works fine. I made a second one with a lot more filtering on in- and outputs and the power supply. This is more silent now but I can still hear a slight hum with my ear to the speaker, the other side is completely silent.
Soundwise I canīt hear any difference.

I watched the other channel a bit longer, it varies between +3 and -1volt absolute offset depending on the mains voltage (with 27k matched McMillans).
If I keep the mains voltage constant it varies less than a volt from startup. (starts at 11V and then goes down to 1V within 15 seconds)

So if your mains is reasonably constant I would go for the PTC without the dc-servo. This will also manage your temperature differences without a problem.

I will try to remove the McMillans and see what happens with the sound. This will take a while though as I'm away for work almost constantly for the next 3 weeks and need to build a second one for the other channel. Then I can probably also post a schematic and some pics.

Not sure whatīs wrong with your relative offset. Mine is around 3mV and stays there independent of temperature.

Don't know what causes the power raise into 8R and difference between 9610 and Jfets. I would try measuring the output voltage without a load and see where you land. With 8R and your bias the max voltage should be very close to that without load (you just loose some voltage over the output and source resistors)
I remember having 100 watts into 8R and 4R with my 7.5A bias and around 23V rails. Did you calculate the power and compared it to your results?
I didn't see any difference in power between 9610 and JFets, only bandwidth changed.

If you calculate the power difference in dB I don't think you will hear any difference in clipping behavior.
The little kink belongs to the Aleph-X

William
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Old 25th March 2013, 01:52 AM   #2335
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Quote:
Originally Posted by Zen Mod View Post
looking at pointed schm , I can think of few things :

- for voltage bias of cascodes - put them to 12V , changing D3 to 12V ; nice cap - 2u2 will do , is fine to put across diode

that's where , at least to my experience , Jfets are somewhat happier than on under 9


DZ - you can put plain 1N4148 in series , cathode pointing down of course - it will somewhat null tempco of DZ itself , but dunno for overall CCS tempco ..... I'm used to use two-bjt- CCS ( Widlar ) which is having somewhat better Tempco than sole mosfet CCS
Choky, I think I'm beginning to understand what you've been telling me... Thanks for the additional link, I'll have to read it a few more times for it to sink in more. DZ in Graeme's schematic is just just a single 9.1v Zener in my amp right now. I'll add another one in series along with a small cap and see if things improve.

When cold, my JFET amp is quieter at the speaker than the 9610, but when warm they are the same.
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Old 25th March 2013, 01:33 PM   #2336
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Choky: The zener in Newby's nice writeup is a 5.6v. I presume the point is to control the swing of the input signal to keep it from overwhelming the current source. Thus, are "normal" 9.1v zeners too high for this task? My amp starts to clip with an input signal of about 2.1Vrms.

I have a pile of 9.1v zeners in my box. I would need to order 5.6v ones...

Thanks,
Eric
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Old 25th March 2013, 08:14 PM   #2337
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Hi Eric,

the 5.6V Zener has nothing to do with the input voltage to the diff pair. It keeps the current source fets from switching completely on by limiting the gate-source voltage to a value just above the normal "clipping" gate voltage.

I have tried those "clamping" zeners in my amps but in my case 5,6V didnīt help against the kink. Probably because the voltage depends on the source resistors used. I never looked deeper into this but it probably is worth another try.

William
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Old 27th March 2013, 01:48 PM   #2338
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I've been playing around with various CCS configurations (I copied Graeme's directly) for the front end differential and haven't really had any more "success" in restoring the lost power (yes, probably only 1.0-1.5dB as William points out) when I left the 9610's behind.

I am now getting more stable output into 4R loads, but still get some voltage collapse beyond 20kHz clipping. Output into 8R shows more "regular" clipping patterns and power output continues to increase right up until full rail voltage hits the outputs - though this is heavily clipped. As frequency increases, power output falls, so I need to play around with a greater variety of compensation caps for C9/C10 and C2/C4.

I tried Zen's back to back zener trick and this didn't really change the little "kink" on the downside of the sinewave at clipping.

Relative offset still creeps up slowly, starting about -13mV and seems to settle around 84mV or so. This seems closely tied to the ZTX550 parts. I have four of them all in a row and if I pinch them together with my fingers, relative offset drops to single digits of mV - which made me very happy. So, I put some thermal goo between them and pulled them tightly together with a plastic zip tie. This didn't do as much as I had hoped to keep relative offset from drifting upward... After a little while, it still climbed to ~100mV. I wonder if I should try to wrap a little copper band around them and tie this off as well.

Still playing around. After a few weeks the frustration is wearing off and being replaced by a spirit of adventure

What are the (dis)advantages of removing the 550's and just going with the 2SJ74's? In the Aleph-J schematic, Nelson just used 109's directly... This looks more like the approach that William used...
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Old 27th March 2013, 02:58 PM   #2339
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what value Rs you're using for input Jfets ?
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Old 27th March 2013, 04:28 PM   #2340
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I forgot to include in above, when I moved to Graeme's CSS for the input differential, I had to change VR2 from a 200R to a 1K pot in order to adjust absolute offset to 0v. The 200R pot didn't give me enough room to bring offset down to zero.

Using Graeme's schematic for labels, I have the following:

R18/R28 (only resistor between input signal and jfets) = 10k
R19/R29 (input to ground) = 10k - this seems to be a decade higher than input resistor in most other designs... Why is it 10k here?
R16/R30 (feedback) = 100k
R47/R47 (McMillan) = 10k
R23/R25 (connects ZTX550 collector to -Rail) = 392R
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