Go Back   Home > Forums > Source & Line > Analog Line Level

Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 20th February 2012, 05:59 AM   #21
diyAudio Member
 
wrenchone's Avatar
 
Join Date: Feb 2004
Location: Silicon Valley
I just fired up my HP8903A for the first time with my breadboard shut away in a diecast bud box, and I'm getting results between .01-.05% distortion depending on frequency and amplitude. I find it somewhat suspicious that using a 1kHz center frequency, I get a really significant reduction in distortion when I engage the 80kHz high pass filter. This points to either pickup or oscillation. I probably need to change the input BNCs to ones isolated from the case, and use a single point ground. It would probably be productive as well to ground the box to one of the front panel ground connectors on the 8903A - which one? At any rate. I'll also be looking at my breadboard with a square wave input to try and smoke out any instability. Things are never really straightfoward....

Last edited by wrenchone; 20th February 2012 at 06:10 AM.
  Reply With Quote
Old 20th February 2012, 03:51 PM   #22
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
Usually it's pickup. If the circuit is oscillating you will see quite a bit of
distortion at audio frequencies. If you have a Gate stopper resistor it is
pretty unlikely the circuit will oscillate.

  Reply With Quote
Old 20th February 2012, 10:54 PM   #23
diyAudio Member
 
wrenchone's Avatar
 
Join Date: Feb 2004
Location: Silicon Valley
With the current setup, here's what I get for THD with the 80kHz LPF in place - 1kHz, 0.1V - 0.027%, 1kHz, 0.5V - 0.016% 1kHz, 1V 0.036%

10kHz, 0.1V - 0.03%, 10 kHz, 0.5V, 0.05%, 10 kHz, 1V - 0.154%

Gain is ~ 5.5, so the more realistic level of excitation is 0.1-0.2V.

Obviously, I need to put some more work in the setup. Another useful thing might be to cascode the depeletion mode mosfet current source I'm using for a load in order to cut down on its capacitance - this may improve the 10kHz distortion.
  Reply With Quote
Old 21st February 2012, 12:26 AM   #24
diyAudio Member
 
wrenchone's Avatar
 
Join Date: Feb 2004
Location: Silicon Valley
Here's the circuit I'm messing with for the time being.
Attached Images
File Type: gif Schade_fet2.gif (11.2 KB, 686 views)
  Reply With Quote
Old 21st February 2012, 04:58 AM   #25
BFNY is offline BFNY  United States
diyAudio Member
 
Join Date: May 2004
Location: Saratoga NY
Quote:
Originally Posted by wrenchone View Post
With the current setup, here's what I get for THD with the 80kHz LPF in place - 1kHz, 0.1V - 0.027%, 1kHz, 0.5V - 0.016% 1kHz, 1V 0.036%

10kHz, 0.1V - 0.03%, 10 kHz, 0.5V, 0.05%, 10 kHz, 1V - 0.154%

Gain is ~ 5.5, so the more realistic level of excitation is 0.1-0.2V.

Obviously, I need to put some more work in the setup. Another useful thing might be to cascode the depeletion mode mosfet current source I'm using for a load in order to cut down on its capacitance - this may improve the 10kHz distortion.
Need to also tell us source output impedance (assumed to be 50 ohms) and load input impedance (assumed to be 1M ohm or greater for test gear). Try a load of 1k, 10k, 100k, and see what happens to your numbers.

Tweak the supply voltage , shown as +30V, up and down if possible, and see what happens. This may show a sweet spot. Simulation meets the real world.
  Reply With Quote
Old 27th February 2012, 07:13 AM   #26
diyAudio Member
 
wrenchone's Avatar
 
Join Date: Feb 2004
Location: Silicon Valley
The source impedance of the 8903A is 600ohms, with an input impedance of 100K. I won't tinker with these until I am sure of a proper setup. I modified my test box (it already had isolated in/out jacks) for a single point ground to the case, and connected that ground to one of the ground connectors on the 8903A front panel. The resulting measurements were made with 400 Hz LP and 80 kHz HP filters engaged:

1k/0.1V - 0.021%, 1k/0.2V - 0.012% 1k/0.3V - 0.011% 1k/0.4V - 0.0125% 1k/0.5V - 0.015%

10k/0.1V - 0.024%, 10k/0.2V - 0.022%, 10k/0.3V - 0.03%, 10k/0.4% - 0.039%,
10k/0.5V - 0.049%
  Reply With Quote
Old 28th February 2012, 04:23 AM   #27
diyAudio Member
 
wrenchone's Avatar
 
Join Date: Feb 2004
Location: Silicon Valley
Should have done this a while back. but here are the residuals read on the 8903A with a BNC cable hooked from in to out. I'm not all that impressed - maybe this beast needs a cal session, fresh caps, or a better cable (the ones I have feel dodgy). Anyway, with 400 Hz LP and 80 kHz HP engaged, the results are:

1kHz: 0.1V/0.0081% 0.2V/0.005% 0.3V/0.0047% 0.4V/0.0048% 0.5V/0.005%

10kHz: 0.1V/0.008% 0.2V/0.0047% 0.3V/0.0041% 0.4V/0.0043% 0.5V/0.0045%

Can some one else that also has an 8903A comment on these residuals?
  Reply With Quote
Old 28th February 2012, 01:08 PM   #28
BFNY is offline BFNY  United States
diyAudio Member
 
Join Date: May 2004
Location: Saratoga NY
the spec on the 8903A in the range you are using is -80dB, or .01%

manual here - see page 1-13
http://www.ece.stevens-tech.edu/Boes...sersManual.pdf
also comments here
Differences between HP 8903A and HP 8903B
Looks like yours is OK. Maybe try an FFT method, like ARTA on a PC soundcard? That way you can see the harmonics.
  Reply With Quote
Old 2nd March 2012, 03:01 AM   #29
diyAudio Member
 
wrenchone's Avatar
 
Join Date: Feb 2004
Location: Silicon Valley
OK, so the 8903A is operating near (actually below) its published limits, and there's not a lot of difference between the 1kHz and 10kHz residuals (though the rise at low amplitude is a bit troubling). Anyway, it's established that the 10kHz THD is definitely higher than the 1kHz THD for the basic version of the "FETSchade" as shown.

The circuit depends on high levels of very local feedback for low THD, so that anything that impacts the open loop gain should affect the THD. There are two things that I can think of. The first is that any real current source load will have a finite impedance (albeit high) associated with a shunt capacitance. A single DN2540N5 has ~1M impedance (dependent on bias current?) with some associated shunt capacitance that will result in reduced open loop gain as a function of frequency. Also, the gain mosfet still has Miller capacitance, even though said capacitance is being driven with a source follower.

So, there are three approaches to reducing the 10kHz THD. The first would be to increase the gain mosfet transconductance by boosting the bias current, hoping that the impedance of the depletion mode current source load is not too adversely affected by the increased current. The second approach would be to cascode the current source load somehow. The third approach would be to cascode the gain mosfet. The fourth approach would be to accept the increased distortion and forge ahead.

Anyway, I finally got a chance to drag my test setup in to work and do a gain-phase plot on a HP 4194A gain-phase analyzer (if I ever hang out a shingle as a consultant, I want one). The frequency response is not shabby at all. The first attachment shows the G-P plot with markers at 1 kHz and 10 kHz.
Attached Images
File Type: gif Schade_fet_LFG-P.gif (39.5 KB, 555 views)
  Reply With Quote
Old 2nd March 2012, 03:04 AM   #30
diyAudio Member
 
wrenchone's Avatar
 
Join Date: Feb 2004
Location: Silicon Valley
The plots shown here are the same damn scan with markers at 100kHz and the - 3dB point.
Attached Images
File Type: gif Schade_fet_HFG-P.gif (41.8 KB, 515 views)
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
L'Amp: A simple SIT Amp ra7 Pass Labs 2037 16th April 2014 03:15 PM
SIT Amp Article Out Now Variac Site Announcements 0 2nd December 2011 07:55 AM
SIT-Preamp VladimirK Solid State 36 27th August 2011 08:36 PM
SIT Power Amplifier??? mad_z Solid State 8 26th January 2009 08:31 PM
New SIT - old problem StevenOH Parts 2 13th June 2008 10:46 AM


New To Site? Need Help?

All times are GMT. The time now is 07:08 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2