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Old 23rd May 2008, 09:01 PM   #1
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Question Global Feedback on Mikael's KT88SE

I can already hear the gasps from the purists
I was wondering if its possible to add global feedback to this design? I've been looking at other SE designs with feedback, and I'm not sure how to even attempt to implement it in Mikael's design.
It looks like the feedback is fed into some kind of divider circuit on the cathode of the drive tube. I've also seen at least one design with a potentiometer to (I assume) vary the amount of feedback.
I'm just curious to see how this amp would sound with feedback.
Anyone tried this with Mikael's design yet?
Thanks for any pointers anyone can provide me with.
Glenn
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Old 24th May 2008, 03:33 AM   #2
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Default Re: Global Feedback on Mikael's KT88SE

Quote:
Originally posted by porkchop61
I can already hear the gasps from the purists
I was wondering if its possible to add global feedback to this design? I've been looking at other SE designs with feedback, and I'm not sure how to even attempt to implement it in Mikael's design.
It's a simple, straightfoward design. You can add gNFB and make it variable the same way I'm doing in a design I'm developing:

Wolverine Prelim Schemo. Keep the 100uF cathode bypass capacitor, add at least a 10K pot, and you also won't be needing that coupling capacitor since this design runs the cathode near ground potential, and not off a (-) rail.

All that remains is to determine how much max NFB you might want to include to figure out what the Rf resistor needs to be.
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Old 24th May 2008, 12:03 PM   #3
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Default Re: Re: Global Feedback on Mikael's KT88SE

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Originally posted by Miles Prower
All that remains is to determine how much max NFB you might want to include to figure out what the Rf resistor needs to be.
Thanks for the info.
This was the KT88 that I was looking at that had the feedback:
http://www.bonavolta.ch/hobby/en/audio/kt88_1.htm

I'm not sure how to determine the amount of feedback needed to calculate the resistor required. Is there any rule of thumb as to where to start?

Also, is it best to use the largest value tap off the secondary of the OT to supply the feedback?

Thanks again.
Glenn

Edit:
Here's the schematic for Mikael's amp:
Click the image to open in full size.
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Old 24th May 2008, 07:21 PM   #4
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Default Re: Re: Re: Global Feedback on Mikael's KT88SE

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Originally posted by porkchop61
I'm not sure how to determine the amount of feedback needed to calculate the resistor required. Is there any rule of thumb as to where to start?
The first thing you need to know is the open loop gain from the input to the 8R output. For a project like this, 12db(v) of max gNFB is probably enough. I found that 12db(v) is tending towards a "solid statey" sound, and you won't be needing that much. If you make the gNFB variable, you can dial in as much or as little as you like or think you need..

Quote:
Also, is it best to use the largest value tap off the secondary of the OT to supply the feedback?
No. Take off the gNFB from the same taps that you're using for the speeks.
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Old 24th May 2008, 07:57 PM   #5
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Glenn,

Those who are aghast at the idea of negative feedback should disconnect the governors on their lawnmower engines or on their household generator engines and see what happens. Or they should try driving themselves to work with their eyes closed! Negative feedback is required for output impedance correction in all these instances and tetrode amplifiers are no exception. While the UL feedback will help, the output Z will still be too high for many speakers, at least that has been my experience.

Connect a 30 ohm resistor beneath the 1k and 100uF. Connect one side of the OT secondary to common. Connect the other side to a 10k pot, then to a 470 ohm resistor, then to the top of the 30 ohm resistor; an adjustable voltage divider. If it squeals, reverse the output leads. This will allow you to dial in what sounds best in your application.

Another way to implement feedback is from the anode of the output valve to the anode of the 6N1P. A resistor of several hundred kilo-ohms between the two anodes will accomplish some negative feedback, but experimentation will be required to find out what sounds best. You are working with very high voltages at this point so forget about making adjustable feedback when done this way. When switching the resistor the filter capacitors must be completely discharged. Please use bleeder resistors in your power supply.

I agree with Miles, and especially that you will probably end up with around 10dB of feedback, although the specific number is not particularly important. More important is that the amplifier sounds great to you when you are finished with it and listening to it!

Best regards,

Wade
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Old 25th May 2008, 09:50 PM   #6
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Thanks guys very much.
I was just looking for a starting point. Who knows, I may not even like any feedback. That's the fun part about this hobby, tweaking, tweaking, tweaking

Wade-
I agree with you on the merits of using feedback. I've worked with servo motor designs for 28 years at my job. It's the whole point of feedback, having a closed loop system relying on error correction. Designed a lot of plotters and photoplotters over the years (Gerber).

Glenn
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Old 28th May 2008, 11:14 AM   #7
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How about this approach?
It has the benefit of not changing impedance as the amount of feedback is adjusted. I can size the capacitor as required for roll off.

Click the image to open in full size.
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Old 28th May 2008, 11:29 AM   #8
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Yes. That looks great to me.
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