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Old 18th August 2006, 11:03 PM   #1
Sherman is offline Sherman  United States
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Question Input level required- PP KT88

I am thinking about building this circuit from Plitron (though I may use James OPTs). If I'm calculating this correctly it appears the first three stages combine for a gain of about 27.

If my calculations are correct then no preamp would be needed using a CD player as a source. Am I on the right track here? This circuit is described as a power amp and indeed there is no volume pot on the schematic.

If it does require a preamp where did I go wrong in my calculations and how much gain would be needed from the pre?

Thanks!

(Edit- forgot to attach schematic. )
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Old 19th August 2006, 11:15 PM   #2
kevinkr is offline kevinkr  United States
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Actually I don't know how you came up with that gain number of 27 it's from outer space somewhere..

What you need to know is that the closed loop gain is approximately 1 +(39K/470) = 83, however this assumes that the open loop is sufficiently high for this to be true, and it probably isn't. Note also that the 16 ohm tap is used so multiply the overall gain by 0.707 (the sq rt of the winding ratio for 2x the impedance is 1.414) for a gain at the 8 ohm tap of about 53X or slightly over 34dB..

I think in practice this is probably more than enough gain for use without a pre-amplifier with modern digital sources where full output is 2Vrms or greater.
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Old 20th August 2006, 02:57 PM   #3
Sherman is offline Sherman  United States
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Kevin,

Thanks for the reply and help calculating this. What I did was calculate the gain of each stage separately and add them as follows-

* First stage is grounded cathode, gain- about 11

* Second stage is split load phase splitter, gain- about .9

* Third stage is differential amp, gain- about 17

I'll have to revisit the schematic and "Valve Amplifiers" and figure out the correct way to do this!
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Old 20th August 2006, 03:25 PM   #4
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You can add gains expressed in dBs, but you have to multiply gains expressed as ratios. So, if your individual figures are correct, then total gain for the first three stages is 11*0.9*17 = 168.3 which is 44.5 dB.
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Old 20th August 2006, 03:50 PM   #5
Sherman is offline Sherman  United States
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Quote:
Originally posted by Brian Beck
You can add gains expressed in dBs, but you have to multiply gains expressed as ratios. So, if your individual figures are correct, then total gain for the first three stages is 11*0.9*17 = 168.3 which is 44.5 dB.

Thanks! I pulled out Morgan Jones and started working through it again and realized Kevin was talking about dB and my numbers were only voltage gain. Now it comes together (and I wasn't that far off)!
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Old 20th August 2006, 05:04 PM   #6
kevinkr is offline kevinkr  United States
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Hi Sherman,
I was talking about closed loop gain, and you were talking about open loop gain of the driver stage, neglecting the considerable gain in the output stage and the step down ratio of the opt.

Also you need to take into account that the first phase splitter produces a differential output of 2 x ~0.9 so there is an additional 6dB there, plus the differential output from the LTP.

I think you'll find that the actual driver stage gains are actually well over 50dB, and that even with the output transformer included the output stage gain will be >10dB so this would equate to an open loop gain of something like 60dB referred to the 16 ohm tap.. Since this is a Williamson topology that would jibe well with typical amps using this configuration.
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Old 21st August 2006, 04:47 PM   #7
Sherman is offline Sherman  United States
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Kevin,

Thank you again for your explanation and help. I was indeed ignoring the output stage altogether. My goal was simply to estimate the drive voltage delivered to the output stage. In my KT88 SE amps I found that 33V input on the KT88 seemed to give the best performance (combination of output power and distortion).

I was wondering two things-
* first, if the stages feeding the KT88 PP output stage in this schematic would deliver sufficient voltage
* second, if the 33V input I found to work in SE KT88 amps would be sufficient here

I truly appreciate the time you spent looking this over and calculating things. It takes me quite a while and though I know you are much faster it is still great that you took the time.

I'm going to sit down later this week if time permits and work my way not only through this circuit but through a couple other PP circuits as well. I believe I have a pretty solid understanding of the basics of SE designs, a reasonable grasp of parallel SE designs but am lacking when it comes to PP.

It is thanks to contributers like yourself that people like me are able to learn so much through this forum.
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