6CA7 UL Push Pull Amp Design - Page 3

scott17 · 3rd April 2012, 03:13 AM

I "fixed" the feedback circuit that I so royally screwed up previously.

I finally got the equipment needed to do testing of the amp with Rightmark Audio Analyzer. I have attached the results below.

The test setup is an HP TX1000, 64-bit Windows 7, with a Behringer UCA222 USB sound card. I used a Behringer DI100 direct box for connection of the speaker terminals back to the sound card input. I have also included the loopback test baseline results in the attachment. That is, UCA222 output to DI100 input, DI100 output to sound card input.

RMAA wants the signal level for testing at -1.0dB. I was able to get the signal level adjusted to -1.1dB for the tests. I did not have the compensation circuit connected for the tests. After testing, the compensation doesn't seem to be necessary. The amount of feedback used for these tests was calculated to be about -4 to -5dB.

I measured right at 1mV AC on the speaker terminals with no input signal. The amp is dead quiet.

I ran the tests several times with precisely the same results. The amp was on for about 1 hour before I tested.

I'm not sure at this time why the left channel is so much different than the right. Maybe a difference in the output transformers. I'll swap the tubes around and see if that makes a difference.

I'm very pleased with these results, although I could use a better soundcard for testing than the UCA222. The Realtek HD Audio built-in card on my TX1000 is actually worse than the UCA222 when tested with RMAA.

Bottom line though, the amp sounds fantastic.

DF96 · 3rd April 2012, 09:10 AM

The channel difference could be due to a slight mismatch in output valves. If you can separate into odd and even order distortion this might help.

scott17 · 7th April 2012, 10:15 PM

Quote:

Originally Posted by DF96

The channel difference could be due to a slight mismatch in output valves. If you can separate into odd and even order distortion this might help.

Here are the THD plots that correspond to columns B & D from the previous post of the RMAA test. The left channel of the soundcard was used for testing both channels of the amplifier. The right channel shown on the THD plots was just looped through the soundcard.

DF96 · 8th April 2012, 10:28 PM

The left channel has slightly higher second order distortion. A small imbalance could do this. Nothing to worry about.

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Differential push-pull design question	Tyimo	Tubes / Valves	4	15th January 2010 07:05 PM
push-pull design	nukaidee	Multi-Way	31	9th May 2009 09:18 AM
push pull power amp design question (a lot of actually :) )	krusty75	Tubes / Valves	6	25th September 2008 05:07 PM
modify parallel push-pull EL84 to single push-pull	chungtat	Tubes / Valves	12	3rd November 2005 11:25 PM
833 push pull design help	sickss	Tubes / Valves	19	7th March 2005 03:25 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	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

3rd April 2012, 09:10 AM	#22
DF96 diyAudio Member Join Date: May 2007	The channel difference could be due to a slight mismatch in output valves. If you can separate into odd and even order distortion this might help.

8th April 2012, 10:28 PM	#24
DF96 diyAudio Member Join Date: May 2007	The left channel has slightly higher second order distortion. A small imbalance could do this. Nothing to worry about.