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how to remove feedback loops

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Posted on June 11, 2017 at 14:07:35
vinnie2
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Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Found this schem for the UA-1 and would like to try it as a pure circuit without feedback loops or the 6ca4 if possible. Can anyone tell me if that can be done and would it ruin the amp if I did? Supposed to be a great little amp and I would like to breadboard one in it's simplest form.

 

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RE: how to remove feedback loops, posted on June 11, 2017 at 15:04:02
rage
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Posts: 792
Joined: December 17, 2010
 each of the output tubes need their screen tied to the plate through a 100 ohm resistor. currently the screens are tied directly to b+. this will triode strap the output stage and reduce the need for feedback.

next remove the global feedback loop from the secondary of the output transformer to the cathode of the driver tube. you should also remove the switch and combine the 3 resistors into one. you may want to bypass that resistor with a capacitor also - or try it both ways. without a capacitor on the cathode you will introduce degenerative feedback and reduce gain. good luck!

 

RE: how to remove feedback loops, posted on June 11, 2017 at 15:10:01
rage
Audiophile

Posts: 792
Joined: December 17, 2010
 there is other stuff going on there after that driver tube that may be related to the feedback loop.

you might want to try the el cheapo amp or even look at the original st35. both have feedback but simpler circuits. the st35 could be built with a 12ax7 and 12au7 for the cathodyne phase splitter. el cheapo uses a differential pair with a ccs which i prefer.

 

RE: how to remove feedback loops, posted on June 11, 2017 at 16:14:55
vinnie2
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Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Ok, I will take a look at them. Thanks!

 

RE: how to remove feedback loops, posted on June 11, 2017 at 18:31:50
Ejam
Audiophile

Posts: 17
Joined: March 15, 2001



Vinnie

I appreciate the desire to have a no feedback amplifier, they do if well designed sound good, but with the UA-1 you have three main problems to overcome. First, without overall feedback the damping factor will be very low and depending on your speakers this may lead to boomy bass. Second, overall feedback reduces the distortion of the output stage (which generates the most), again you may live with this or not. Third, the gain of the amplifier will increase as the feedback reduces gain, making the amp very sensitivity. The suggestion to bypass the input pentode with a cap will further increase gain and hence sensitivity. Your system may not be able to cope with this. Regarding the vacuum rectifier, it has its own benefits, such as slow warm up and lack of switching artifacts. If you replace with diodes then the voltage secondary will need to be lowered. In essence there are a lot of difficulties to overcome and it maybe be better to design your own amp.

I would suggest given the 6BQ5 if you want to a non-overall feedback design to start with a triode output stage and a single triode up front. Phasing splitting can be done as per Jack Elliano's design (included) or the 6BQ5 Oddwatt (on the net). Very simple to breadboard, no overall feedback and having built both, great sounding. Hope this helps.

PS. Don't skimp on the output transformers and use 10k if trioding

 

RE: how to remove feedback loops, posted on June 12, 2017 at 04:15:36
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Looks like what I am looking for alright, thanks! Would you by any chance have a link to the original schematic? The print on the one in your post is blurry and hard to read. Thanks again.

 

RE: how to remove feedback loops, posted on June 12, 2017 at 14:04:24
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Never mind.... Jack was kind enough to send me a better copy. He said they took it down from his site because it wasn't getting enough interests. Thanks anyway!

 

RE: how to remove feedback loops, posted on June 12, 2017 at 14:25:05
Stuben
Audiophile

Posts: 669
Location: Guber Ohio
Joined: December 30, 2005
 Vinnie,

Remove the feedback loop, put a nice solid voltage on G2, run your cathodes through the opt secondaries to ground for cathode feedback, you still need the resistance up or down stream of your OPT secondaries. Use a good output transformer I've used 5K for my stuff both PPP and SET. The point about a 10K primary is valid, it should help in your quest for lower distortion with no GFB. You will have the bias voltage across your speaker terminals, for these easy to drive Pentodes, it's not a big deal.
Give it a try. I found my build to sound better in PPP than strapping for triodes, if this were a SE application it would be a different story. I used the LL1682 OPT PP with a 5 ohm secondary.

Stuben

 

RE: how to remove feedback loops, posted on June 12, 2017 at 14:39:35
Stuben
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Posts: 669
Location: Guber Ohio
Joined: December 30, 2005
 One thing for got mention, maybe someone else did...removing the loop will negate some of voltage requirements for the front end which is there to accommodate the FB Loop, so you can back down the distortion some by dropping the input some.

Stuben

 

RE: how to remove feedback loops, posted on June 12, 2017 at 16:57:34
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Thanks for both posts, I appreciate the information.

 

It would be best to find an already vetted design, posted on June 14, 2017 at 09:22:39
DAK
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Posts: 2712
Location: PACIFIC
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 than to work the kinks out of the UA1 scheme. It would be hard to remove something like the global negative fb circuit since it has a big part in keeping the amp stable. If you replace the fixed fb resistor with a suitable variable one you can see or hear exactly what will happen as you slowly remove the fb. Or just go with a proven design. I think somebody mentioned the "odd watt" scheme which has been made by many people.

 

RE: how to remove feedback loops, posted on June 14, 2017 at 09:59:25
bare
Audiophile

Posts: 1879
Joined: April 14, 2009
 Seriously? Go here;
http://www.diyaudio.com/forums/pass-labs/
Read up on which Firstwatt features No feedback .. Then Build it.
Simple inexpensive circuits actually
Your el84 amp would not even come close soundwise.

 

RE: how to remove feedback loops, posted on June 14, 2017 at 11:48:36
rage
Audiophile

Posts: 792
Joined: December 17, 2010



I drew this up for you vinny... try it on your breadboard if you like.

lm334 ccs uses 20 ohm rset to ground. negative supply is optional but not needed.

you could use a trimmer instead of 20 ohm resistor... 10 ohms is 10ma I think, 68 ohms is 1mA.

should be class A operating point. 12at7 driver as ran differential pair/LTP.

I've done it with 12at7, 12au7, 12ax7. very simple circuit.

 

RE: how to remove feedback loops, posted on June 14, 2017 at 11:50:53
rage
Audiophile

Posts: 792
Joined: December 17, 2010



here's the ultra simple LM334 CCS.. (current sink)

yes a cascode works better, yes a negative supply is better with a CCS.... but this works and is simple as can be.

 

RE: how to remove feedback loops, posted on June 14, 2017 at 14:30:12
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Did you have something specific in mind, or are you just tossing the whole web site out here?

 

RE: how to remove feedback loops, posted on June 14, 2017 at 14:37:27
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Thanks for the plan and the other info. I will have to see if I have a 334 around or order one. Don't use too many SS parts. Thanks again!

 

RE: how to remove feedback loops, posted on June 15, 2017 at 17:50:55
rage
Audiophile

Posts: 792
Joined: December 17, 2010
 I've heard several builds of the nelson pass first watt projects. I really like the concept....but prefer the sound of the EL84... either SE or PP.

mosfets with feedback are great but I still prefer tubes.

 

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