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Old 30th March 2012, 08:26 PM   #1
BRSHiFi is offline BRSHiFi  United States
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Default Revision 2. Balanced

I recently completed my KT88 PPP amp. I am planning a revision 2. The new plan is to:

- Make the amp naturally more stable.

- Simplify the overall design.

- Introduced a balanced topology (with XLR inputs) to lower the noise floor.

What do you think of my overall new design? Anything you recommend I do different?

Spice projects a distortion figure of about 0.75% THD. This sound accurate?

Also, a 1:1 transformer on the input I heard helps reject noise better. What PC mount unit do you recommend?
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Old 30th March 2012, 10:33 PM   #2
tomchr is online now tomchr  United States
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Maybe I'm just misunderstanding your question, but your circuit is already balanced. Hence, I don't get your line item of making it balanced.

You could just cap couple the two inputs to the (+) and (-) of the XLR connector and be in business (I think).

For input transformers, I like the Jensen Transformers. Specifically the JT-11P-1HPC would do the job nicely.

A differential or balanced input will not lower the noise floor of the amp, though. It will help in reducing the effects of EMI (electromagnetic interference) provided that the induced EMI is common mode on the input.

~Tom
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Old 30th March 2012, 10:50 PM   #3
BRSHiFi is offline BRSHiFi  United States
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This post is about my new balanced design, my old design in not (old design is below).

Stubborn Oscillation

Does that clear it up?
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Old 31st March 2012, 12:12 AM   #4
tomchr is online now tomchr  United States
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Not really... Your schematics in Post #1 appear to be of Rev. 2. Is this not the case? At least the schematics in Post #1 show an amp with a balanced input.

~Tom
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Old 31st March 2012, 12:51 AM   #5
BRSHiFi is offline BRSHiFi  United States
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The schematics in post 1 are my Rev. 2.
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Old 31st March 2012, 05:20 PM   #6
BRSHiFi is offline BRSHiFi  United States
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What do you think of my overall balanced design? Anything you recommend I do different?

Spice projects a distortion figure of about 0.75% THD. This sound accurate?
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Old 31st March 2012, 07:14 PM   #7
HpW is offline HpW  Switzerland
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Quote:
Originally Posted by kingneb View Post
What do you think of my overall balanced design? Anything you recommend I do different?

Spice projects a distortion figure of about 0.75% THD. This sound accurate?
Well, I did also rebuild my old Cary SLM-100 (auto bias to variable bias) who is similar to your project.

I planed for the second phase to go for XLR input while my Wadia 27 has XLR outputs.

I found some schematics on the internet who just use a differential input circuit, IMHO other words the simplest to have both symmetric or asymmetric and a phase inversion outputs. Then would use the driver (as in your current schematic) for the 2 * 2 KT88 or use a the phase shifter with symmetric connections. The question rises about the NFB ... what I would like to skip or connect them to phase shifter.

just my 2 cents to my planned project, what is in some regard very similar.
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Old 31st March 2012, 07:40 PM   #8
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Quote:
Originally Posted by kingneb View Post
Spice projects a distortion figure of about 0.75% THD. This sound accurate?
What SPICE program do you use? LTSpice or Simetrix ?
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Old 31st March 2012, 07:45 PM   #9
BRSHiFi is offline BRSHiFi  United States
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LTSpice and PSpice
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Old 31st March 2012, 07:56 PM   #10
tomchr is online now tomchr  United States
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The accuracy of the spice models will determine how accurate your THD figure is. I use the MATLAB script that Norman Koren has on his website. It seems to produce models that are accurate for DC and AC operation below clipping - provided I take the time and care to read the numbers off the plate curves accurately. The THD behavior at clipping doesn't seem to be accurately modeled. So my spice sims tend to give slightly optimistic figures for max power out and such.

Your schematic looks like a brute-force approach. It looks like two half-circuits rather than a differential circuit now that I look closer. I would suggest using a differential pair (long-tailed pair) for the input stage. This would give you a differential input, common-mode rejection, and such. Of course an input transformer would do the same. The CMRR of the Jensen JT11 is hard to beat.

~Tom
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