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Old 30th December 2012, 09:39 PM   #21
generg is offline generg  Germany
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You should really look for a sound card solution to measure distortion.... No money!

Look if the sound card does 24bit and 48KHz.....

Use ARTA for windows

If you are Mac user use a thirty day demo of Electroacoustic toolbox....

To have prober wiring from amp to sound card PM me.....

More fun ....... And I am afraid nobody will know much more than Nelson, ZM and all the others said here.....
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Old 30th December 2012, 11:43 PM   #22
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Default Wow

Thanks everyone for the tremendous response!

Nelson,
Thank you. Yes I went through the Idss exercise. The jfets are both 1.027 Vgs and I am currently using a 1.5ohm resistor for R3 (ZV9 schematic)
I am embarrassed to say that these jfets are from the original GRollins group buy Wow, I'm slow... The lot code from the jfets are all 0540G. I imagine that there are a lot of people with this same lot code jfet in functioning amps. FYI, all of my IRFP244 devices that I am using have a Vgs of approximately 4.44V

Zen Mod and Dzius both suggest that increasing bias should improve things, so I guess it won't hurt to change R8 (F3 schematic) back to 1ohm (from 2ohm - F3 schematic) to increase the bias to 2A and see what happens to the voltage across the jfet and the gain. I'll report back tomorrow and let you know what happens.

Generg,
I will send you a PM about using ARTA to measure distortion. I have actually been looking at this since I have been thinking about an active crossover to use with these new F3's and my Aleph-X. If I can use it to help with this then that would be great!
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Old 31st December 2012, 02:48 AM   #23
BDP is offline BDP  United States
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Maybe this will help

Pass JFET Amp- I'm making one!
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Old 31st December 2012, 01:08 PM   #24
yup is offline yup
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Quote:
Originally Posted by BDP View Post
Maybe this will help

Pass JFET Amp- I'm making one!
Thanks BDP, that helps me feel a little better about the voltages I am seeing. Looks like performance will be good within a fairly wide range.

Can you hear a difference between the ZV9 and the F3 with the different bias currents?
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Old 31st December 2012, 06:59 PM   #25
yup is offline yup
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Default Updated voltages at 2A bias current

I increased the bias current to 2A and let the amp warm up for several hours. I have attached the schematic with the new voltages. Interestingly, the jfet now has about 2.6v across it. What a big difference a little more bias current makes. The hottest spot on my heat sink is 125F directly underneath Q3. Overall, the heat sinks are about 100F, but my shop is a little cooler than my living room will be.

Click the image to open in full size.

I was able to increase the gain to 14.5db within the range of P2
I put dip switches on this board for both feedback and the Aleph current source so I can play around with turning those on and off. This should come in handy for setting the ACS.
With a 1Vpp sine wave input, I measure 2.65Vpp output with the ACS off (8.5db). Just to confirm, I should adjust P2 to give 5.3Vpp at 1Vpp input (14.5db) to achieve 50% gain via the ACS? Or to 4.3Vpp to achieve 12.75db (commercial F3)?

I will continue to try to try to find a way to measure the distortion at some point to do some final adjustments. And a lot of listening

Now on to the next issue, the square wave response...
On the scope the bottom trace is the input at 0.5v/d and the top trace is the output across an 8ohm resistor at 2.0v/d.
The first image is at 1kHz and the second image is at 10kHz

Anyone care to guess what is going on or what I should look at? I tried a 50ohm resistor across the amp input and without - no significant change.

Click the image to open in full size.
Click the image to open in full size.
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Old 31st December 2012, 07:02 PM   #26
Zen Mod is offline Zen Mod  Serbia
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fiddle with C9
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Old 1st January 2013, 08:26 PM   #27
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Well, I fiddled with C9 and then removed it completely, but it didn't improve the square wave response.

I tested a different amp I have to be sure something wasn't wrong with my test setup and it worked fine at both 1kHz & 10kHz (actually it had a slight ringing on the leading edge).

I tested the amp with the feedback dip switch turned off completely. The bottom trace is the sine wave input at 0.5v/d and the top trace is the output across an 8ohm resistor at 5.0v/d (clipping with signal generator at minimum output)

I have C7 (F3 schematic) in circuit, but I don't currently have R22 or LED1 in circuit. Could this cause this type of issue with the square wave?

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Old 1st January 2013, 09:17 PM   #28
Zen Mod is offline Zen Mod  Serbia
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gate resistors ditto at gate's pins ?
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Old 1st January 2013, 09:28 PM   #29
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Quote:
Originally Posted by Zen Mod View Post
gate resistors ditto at gate's pins ?
Zen Mod,
Yes, I moved the gate resistors off board for Q2 & Q3. They are actually soldered directly to the transistors since they are mounted on the side heat sinks about 8 or 9 inches away from the board.
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Old 1st January 2013, 09:42 PM   #30
Zen Mod is offline Zen Mod  Serbia
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increasing C9?
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