DCAudioDIY.com

DC Area Audio DIYer's Community
It is currently March 28th, 2024, 6:45 am

All times are UTC - 5 hours [ DST ]




Post new topic Reply to topic  [ 16 posts ]  Go to page Previous  1, 2
Author Message
PostPosted: July 25th, 2016, 8:43 pm 
Offline

Joined: July 15th, 2016, 10:02 am
Posts: 74
Thanks, David. I wish I could claim credit for the schematic but it's a driver board made for the Dynaco ST-70 by Shannon Parks (http://parksaudiollc.com/).

Your comments are intriguing but I admit a little bit over my head (I'm new to this). I had someone recommend me to try reactive loads before. They suggested a paralleled cap with resistor load (varying sizes of capacitor from .1 uF to 16 uF or so) and a cap only load (varying cap sizes from .01 uF to 16 uF). I assume I should do this test at various frequencies? Are ceramic caps ok? And what am I looking for on my scope? Excessive ringing? And when you say "reduce feedback," how is that done? Do I change the R5/R7 network so that more of the feedback voltage is shunted to ground?

Justin


Top
 Profile  
 
PostPosted: July 26th, 2016, 8:29 am 
Offline
User avatar

Joined: July 24th, 2015, 4:17 pm
Posts: 1701
Location: Parkville, Maryland
David explained it much better than I could -- he lives and breathes this stuff! :clap:

_________________
Walt


Top
 Profile  
 
PostPosted: July 27th, 2016, 5:20 pm 
Offline

Joined: March 2nd, 2013, 2:43 pm
Posts: 216
Location: Potomac, MD
Justinis- The feedback resistor ratio that determines overall feedback is the 100 ohm R6 and the 2.7 k R10. Decreasing R6 or increasing R10 will reduce feedback. The 220 pF C4 is the compensation capacitor. Ideally, you would like to not need C4, but it is unlikely that you will be able to take it all the way to zero with anything but very low feedback. But the real thing I would like to get rid of is the R14 C 5 network. All of this stuff is the way it is because the designer is trying to get the best possible damping factor and lowest measured distortion into a resistive load. This is the old specmanship game, but will not lead to the best sound with most speakers, and may oscillate with some. It is also one of the reasons that we were told never to run a tube amp without a load. Without a load these designs frequently oscillate and arc over the tube sockets and catch fire, and even destroy the output transformer. Fully testing the amp with reactive, especially purely capacitive loads ( you can use 1 kHz square wave), will reveal problems. If I were to probe your amplifier's performance and optimize I would start by removing the R14C5 diddle and C4, and increase R10 to a high value such as 50k. Then measure the square wave at a couple of frequencies. You will likely find the bandwidth somewhat disappointing, but you should be able to put any capacitor as a load and get stability. The damping will not be very good either, but now you will want to begin to reduce R10 in steps to see where the amp misbehaves, like excessive ringing that doesn't damp out or downright oscillates with capacitive loading. Also try with resistive loading to see where things are bandwidth wise. Once you find the limit on where you can go with R10, raise it back up, say by 50 % to make a safe margin. Now this gives what may be an acceptable performance in terms of damping and bandwidth, but it may fall short. If it falls short, then you can start reducing R10 again, but at the same time injecting some C4 compensation back into the design, always testing with those capacitor loads at 1 kHz and bandwidth performance with a resistive load at 10 kHz. You want to determine the output impedance for damping as well--you would like to be under four ohms on the eight-ohm tap. This measurement can be done at 60 Hz by injecting the line frequency though a suitable, say 10-ohm resistor, from a 6-volt filament transformer and measuring the voltage on each side of the resistor to determine the current through the resistor and then applying Ohm's law to determine the impedance of the amplifier output port.

Anyway, I would be happy to have you over with your amplifier to show you the way I do these measurements and optimizing. Other issues may come up when feedback is reduced, like increases in hum and noise that was canceled better with the high feedback.

Good luck, David


Top
 Profile  
 
PostPosted: July 27th, 2016, 5:30 pm 
Offline
Site Admin
User avatar

Joined: February 28th, 2013, 10:38 am
Posts: 1682
It's hard to see what would need to be done w/o the rest of the schematic, but I'd be tempted to take the OPT out of the feedback loop....

Roscoe

_________________
I can explain it to you, but I can’t understand it for you.


Top
 Profile  
 
PostPosted: July 27th, 2016, 6:13 pm 
Offline

Joined: July 15th, 2016, 10:02 am
Posts: 74
David -- Thanks so much for such a detailed response. I'll have to chew on this.

Here is the rest of the schematic (I am using the regulated screens option):

Image

Image


Top
 Profile  
 
PostPosted: July 27th, 2016, 6:34 pm 
Offline
Site Admin
User avatar

Joined: February 28th, 2013, 10:38 am
Posts: 1682
Well, if I'm doing the math right, to take the OPT out of the feedback loop, you'd need to change R9/R10 to 56K, C3/C4 to 10pf and run a 1uF cap between J12/J13 and the blue primary connection on the respective OPT. That ought to get you to the point of having the same amount of feedback you have now. Larger values of R9/R10 will give you less feedback. A dual-gang pot of 100-250K would let you adjust the feedback easily so you can figure out what value works best (10-25K would be a good choice for a dual-gang pot the way things are right now) and then put in fixed resistors when you've got it figured out.

Roscoe

_________________
I can explain it to you, but I can’t understand it for you.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 16 posts ]  Go to page Previous  1, 2

All times are UTC - 5 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 40 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
cron
Powered by phpBB® Forum Software © phpBB Group