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Old 13th January 2018, 02:22 PM   #31
banat is offline banat  Serbia
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Koonw

many thanks for SIM ! ,
in your opinion why THD is so (2%) high?, is because of to low OLG? , or because OPS consist only from one 12B4A pair?, or something else ?
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Old 13th January 2018, 02:47 PM   #32
Koonw is offline Koonw
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Oh yes I though it could drive 32 ohms at reduced level at 1.5V rms out, thd is 1%
I measured the output Z, 115 Ohms, with this load it is 0.1% at 1.5V rms out, and 2.5% at 21V rms. All depend on load and level out.

Edit: But I just check the MP 1 pair version, measure Z is only 12.5 ohms so maybe something wrong there.

Edit2: Oh ya I found it's amount of NFB is much lower with this amp compared to MP, the gain of 6au6 is much higher than 12ax7, so I guess if use same tube and same NFB level, there should be no differences.

Last edited by Koonw; 13th January 2018 at 03:10 PM.
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Old 13th January 2018, 03:23 PM   #33
banat is offline banat  Serbia
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Join Date: Dec 2009
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I think that one reason for high Z out is to low OLG ,and in the same time to low GNFB ,
possible solution ,that VAS stage which is 1/2 of Ecc83 need to be replaced with one 6AU6 connected and supplied similar as is by MP and Futterman OTL amp ,
second reason maybe is to low current thru 6S4 tube? , which is only 0,53mA .
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Old 13th January 2018, 03:48 PM   #34
Koonw is offline Koonw
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Just change to 6au6, very close THD 0.42% while MP 1 pair is 0.34% @ 1V rms out 32 Ohms, and out Z now same near 15 ohms, you need the hearing test.

Last edited by Koonw; 13th January 2018 at 03:57 PM.
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Old 13th January 2018, 06:02 PM   #35
banat is offline banat  Serbia
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Join Date: Dec 2009
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Yes !

-------------------------

ps,later I will shown one Sincl./Peter. updated and DIY friendly OTL variant , which will be AC coupled to LS ( so no DC protection is needed) .
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Old 14th January 2018, 08:11 AM   #36
banat is offline banat  Serbia
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Here`s updated Sinclair-Peterson based OTL amp, is AC coupled so no any DC protection is needed, V1 can be also EF86 or any other similar pentode, V2 can be also 6SN7 but with both triode section connected in parallel, output power tubes are 6C33C-B now, because of `stacked` PSU design both B2 & B3 voltage values can be in simple way scaled up or down just to get desired or optimum amp performance.
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