PP amp crossover distortion - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

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 1st January 2012, 08:49 PM   #1
db! is offline db!  Canada
diyAudio Member
 
Join Date: Nov 2009
Location: Ontario
Default PP amp crossover distortion

I recently (re)built one of my amps and I've come across a small problem. When I initially listened to the amp, it sounded like crap; loads of distortion. Upon scoping it, there was a large amount of crossover distortion.

The design and schematics can be seen here: audio diy

The circuit was built exactly as seen in the schematic with the exception of the 270r cathode resistor and the inclusion of CRC filter in the power supply. The cathode resistor was replaced with a simple LM317 CCS.

The test signal is a 70hz sine wave 350mv P-P. I'm not sure of the exact quiescent current but the voltage from cathode to ground is approx 28v.

Does anyone have any ideas where/what the problem might be?
Attached Images
File Type: jpg distortion.jpg (98.5 KB, 321 views)
  Reply With Quote
Old 1st January 2012, 11:47 PM   #2
diyAudio Member
 
Join Date: Jun 2011
the cathode is not low enough impedance. divide the push pull cathode and use 2 ccs (one per cathode circuit) or use the resistor-grouped cathode method like you had originally.

I don't like using ccs in the final stage since it effects the dynamic headroom of the output stage.
  Reply With Quote
Old 2nd January 2012, 12:39 AM   #3
diyAudio Member
 
nigelwright7557's Avatar
 
Join Date: Apr 2008
Location: Carlisle, England
You need a way of adjusting the cathode current to get rid of cross over distortion.
Too little current and you get cross over distortion.
Too much current and you lose headroom.
__________________
PCBCAD50 software. http://www.murtonpikesystems.co.uk
  Reply With Quote
Old 2nd January 2012, 02:38 AM   #4
db! is offline db!  Canada
diyAudio Member
 
Join Date: Nov 2009
Location: Ontario
Quote:
Originally Posted by DavesNotHere View Post
the cathode is not low enough impedance. divide the push pull cathode and use 2 ccs (one per cathode circuit) or use the resistor-grouped cathode method like you had originally.

I don't like using ccs in the final stage since it effects the dynamic headroom of the output stage.
The 832A is a dual beam tetrode with a common cathode within one envelope. There is no way to separate the cathodes.

Quote:
You need a way of adjusting the cathode current to get rid of cross over distortion.
I am using a small trimpot with the LM317 CCS. Increasing and reducing current has no real affect on distortion.

Is it possible that the distortion is coming from the driver stage? I haven't had time to scope this section yet.
  Reply With Quote
Old 2nd January 2012, 01:42 PM   #5
diyAudio Member
 
artosalo's Avatar
 
Join Date: Feb 2010
In the picture of your first post, there is no cross over distortion, which should appear symmetrically at rising and declinig side of the half cycle.
Instead this is unsymmetrical and possibly caused by saturation of the OPT's core.

Quote:
The test signal is a 70hz sine wave 350mv P-P.
At input ? What is output level ?

Quote:
I haven't had time to scope this section yet.
Take that time. With a scope you will see immediately where the problem comes from.
  Reply With Quote
Old 2nd January 2012, 02:01 PM   #6
diyAudio Member
 
Join Date: Jun 2011
the only way I see is to get the impedance down in the cathode (if it was ok, then substituting the ccs caused the xover distortion).

use a mosfet based ccs instead of a bjt based like the lm317 voltage reg.

otherwise, I would directly ground the cathodes and run grid bias, but I'm kinda of a brute force guy.

grid leak resistor method- tieing the grid resistor 390k to the top of the ccs-cathode might give you that extra bit of electrons from grid leakage, but the mosfet based would be more of an academically sound choice.

Last edited by DavesNotHere; 2nd January 2012 at 02:08 PM.
  Reply With Quote
Old 2nd January 2012, 02:51 PM   #7
Jebem is offline Jebem  Portugal
diyAudio Member
 
Jebem's Avatar
 
Join Date: Dec 2011
Location: Lisbon
What kind of scope are You using?
Also, where are You checking the output signal?
Is the problem happening on both channels?

As a general troubleshooting steps, I would:
- Replace the output tube just in case.
- Check the components for wrong values or connections, restoring the original cathode resistor and cap to start with.
- Then check all the the voltage points.
- Then disconnect the feedback loop for a while to make further tests.
- Use a sine wave and a square ware at 1Khz (one at a time), 100mV at the input, so one can compare the results.
- Then use a scope to read the signals at anode of the amplifier section 12AT7 to see how clean is the signal.
- Then look for the signals at the inverter outputs (anode and cathode) of the second section of the 12AT7and compare them; it should be of the same amplitude but in opposite phase.

Good luck!
  Reply With Quote
Old 2nd January 2012, 07:32 PM   #8
tricomp is offline tricomp  Denmark
diyAudio Member
 
Join Date: May 2004
Location: Jutland
Quote:
Originally Posted by artosalo View Post
In the picture of your first post, there is no cross over distortion
Exactly; this is not cross-over distortion.
What OP-transformer are you using?

A simple way to check if the CCS is to blame is of course to just replace it with the original cathode RC-complex.

With the cathode at 28V at present it seems to me that you're running the tube with too high bias getting into the non-linear region; check curves. It won't cause the distortion you're experiencing but it also won't sound very good. Probably somewhere around 20-25V is suitable, but don't go above 25V.
Pushing this tube into AB2-operation with a fixed-bias, grounded cathode and a suitable CF/CCS-drive seems more rewarding, judging from the curves. You're getting (should be getting) around 7-8 Watts from your design, but this tube holds potential for a lot more imho.

rgds,

/tri-comp

Last edited by tricomp; 2nd January 2012 at 07:36 PM.
  Reply With Quote
Old 2nd January 2012, 07:43 PM   #9
diyAudio Member
 
Join Date: Jan 2002
Location: nowhere
I agree with Artsolo, that is not x-over dist. The first thing I would do is check the testsignal at input tube, then grids of the 832. If all is well there, I'd disconnect the feedback, then send in a testsignal 500-1kHz, 100mV like Jebem said, and go from there.
At 70Hz the output transformer may be either giving you a phase shift (which may explain the non-symmetric waveform) or even be saturating (not likely), so using a testsignal that rules out the transformer is a better idea. Once you've figured out the amp at midfrequency, you can check out the low- and high extremes.
  Reply With Quote
Old 2nd January 2012, 07:54 PM   #10
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
A CCS in this circuit maintains a constant mean current. What you actually need is a constant quiescent current. Not the same thing at all. You either need to run the valves hot with no signal, or accept crossover distortion with large signal as the bias shifts due to second-order distortion. Simple cathode resistor bias is better than CCS bias, as it partly compensates for bias shift with signal. Best of all is voltage bias.
  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
Crossover distortion - be gone! Circlotron Solid State 4 24th August 2009 12:09 PM
7th harmonic distortion from PP pentodes depends on bias? ray_moth Tubes / Valves 7 1st April 2009 09:04 PM
A question about distortion in SE vs. PP designs: BHD Tubes / Valves 8 18th November 2005 06:36 AM
Crossover distortion lumanauw Solid State 100 14th January 2005 12:58 AM
Crossover Distortion? Killjoy99 Multi-Way 8 30th March 2004 05:10 AM


New To Site? Need Help?

All times are GMT. The time now is 10:39 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