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PostPosted: 27 Jan 2013, 09:24 
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Joined: 19 Mar 2012, 19:22
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Location: Wales, UK
Hi,

I am trying to calculate how much global NFB I have in my amp. Is there an equation that I can use to do this?

Thanks

Phil


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PostPosted: 27 Jan 2013, 12:00 
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Joined: 20 Sep 2008, 19:49
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Location: Montreal, Canada
Hi Phil,

I believe you would need to inject lets say 1KHz sine wave in your amp and then measure the voltage across lets say an 8 ohm load. This would need to be measured with the NFB in place and then measured again with the NFB loop disconnected. These 2 values will correspond to a feedback ratio.

I normally use a scope and measure the amp output voltage without the NFB loop connected, this will give you the highest output voltage. Once the NFB loop is connected your output voltage will drop and this drop will be proportionnal to the NFB gain.

Ex: I always try to aim for 4Vpk on the scope and then I increase the loop gain (by reducing Rfeedback) until the 4Vpeak at the output goes down to about 2.8Vpk. This voltage ration of 2.8/4 gives you 0.707 which correspond to a gain of 3db. Therefore the overall NFB gain is 3db.
IIRC, a voltage ratio of 0.5 (Ex 2Vpk/4Vpk) gives a NFB of 6db.

There are other method, but this one works quite well.

p.s. If you apply NFB and the output voltage increases instead of decreasing then you are using positive feedback...not good.

Hope this helps,

Rgds,
Iso

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DIY SS Amp : LM3875 GC | TA2024 | TA2020 (AMP-6) | TA2022 | Amp Camp Amp | JLH
DIY Tube Amp : RH84 SE | 6V6 SET | 13FD7 SET (Chinook) | 807 UL SE | 6BQ5 Class A PP UL | 6LR8 SE
DIY Spkrs : Cyburgs Needle | MLTL (FE206E) | CSS EL70 BR | Open Baffle (Alpha 15A + FE206En) (in process...)
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To come : MLTL (CSS enabled EL70) + more tube amp...


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PostPosted: 27 Jan 2013, 16:46 
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Joined: 08 Aug 2009, 03:11
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Location: Chilliwack, BC
Isophon wrote:
I believe you would need to inject lets say 1KHz sine wave in your amp and then measure the voltage across lets say an 8 ohm load. This would need to be measured with the NFB in place and then measured again with the NFB loop disconnected. These 2 values will correspond to a feedback ratio.

Yup!

There's no calculation, you just have to "do it" on a live amp.

Even in Spice, tube models are hardly real world devices and the program will be inaccurate by as much as 5dB, IME.

Cheers!

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-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
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PostPosted: 27 Jan 2013, 17:10 
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Joined: 16 Oct 2009, 09:39
Posts: 7
Hi Geek...I was puzzling about about this today -- how do I connect the feedback resistor? For instance, assume I have a 6EJ7 triode connected with a 400 ohm unbypassed cathode resistor. I'm going to apply feedback from the output transformer secondary with a resistor "globally" to this cathode resistor. How do I know whether to apply this feedback directly to the 6EJ7 cathode (thus impressing the entire feedback voltage across the entire 400 ohm resistor to ground) or do I make a voltage divider, say a 100 ohm and 300 ohm in series and apply the GNFB signal to the 100 ohm to ground resistor? Obviously the value of the feedback resistor will change due to where on the voltage divider it is connected for a given feedback.

See pic below. And hints on getting a ball park value for the feedback cap also appreciated!
Attachment:
Capture.JPG


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PostPosted: 27 Jan 2013, 21:34 
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Joined: 20 Sep 2008, 19:49
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Location: Montreal, Canada
Looking forward for a response from Geek or others regarding the GNFB.

For the small cap across the feedback resistor, it's value is normally between 50-500pF, simply inject around 8-10 KHz square wave into your amp and connect your scope at the output. If the rising edge of the square wave has overshoot (small oscillation) you need to reduce it, simply increase the cap value until that rising edge goes away.

See attached pic, the top wave = no cap, bottom wave looks like adequate feedback cap. was added but a value slightly bigger would be even better to really remove any form of oscillation. :)
Attachment:
Feedback cap.jpg

Rgds,
Iso


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_________________
DIY SS Amp : LM3875 GC | TA2024 | TA2020 (AMP-6) | TA2022 | Amp Camp Amp | JLH
DIY Tube Amp : RH84 SE | 6V6 SET | 13FD7 SET (Chinook) | 807 UL SE | 6BQ5 Class A PP UL | 6LR8 SE
DIY Spkrs : Cyburgs Needle | MLTL (FE206E) | CSS EL70 BR | Open Baffle (Alpha 15A + FE206En) (in process...)
DIY Pre-Amp : MooseFET.
To come : MLTL (CSS enabled EL70) + more tube amp...


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PostPosted: 27 Jan 2013, 23:57 
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Joined: 08 Aug 2009, 03:11
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Location: Chilliwack, BC
Hi Tom,

I'd put it straight to the cathode and let your gNFB resistor + the cathode bias resistor serve as the divider.

If you don't have enough gain, do as you've shown and bypass the 300 ohm resistor with an electrolytic.

To calculate the capacitor, you really need a scope, as Isophon says. The capacitor value will change with the gNFB resistor as well as placement (divider ratio) on the cathode.

If you have an unstable amplifier (the cap can make for a phase-shift oscillator in some circumstances), a resistor in series with the capacitor roughly 1/10 the gNFB resistor can help stabilize things.

Cheers!

_________________
-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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PostPosted: 29 Jan 2013, 18:41 
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Joined: 19 Mar 2012, 19:22
Posts: 107
Location: Wales, UK
Thanks everyone for your help. It is much appreciated :up:

I have learnt something interesting from this topic to which is I need to install a capacitor across my gNFB resistor. I never thought about doing this. How much of an effect does it have on sound quality? Also should this capacitor be a good quality type?

Many Thanks

Phil


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PostPosted: 29 Jan 2013, 21:14 
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Joined: 08 Aug 2009, 03:11
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Location: Chilliwack, BC
Sound quality should actually improve.

I always use a silver mica capacitor.

Cheers!

_________________
-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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PostPosted: 30 Jan 2013, 08:35 
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Joined: 27 Jun 2011, 10:13
Posts: 125
Location: PA
Sound does inprove and I also use silver mica. It realy does make a difference.


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PostPosted: 30 Jan 2013, 18:23 
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Joined: 19 Mar 2012, 19:22
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Location: Wales, UK
Hi,

Thanks again for your help and for suggesting a suitable capacitor :cop: . I will be giving them a try in my amp over the weekend.

Phil


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