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All SIT 50W+ Single Ended Amp (beta)
All SIT 50W+ Single Ended Amp (beta)
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Old 11th July 2020, 07:24 PM   #1
plasnu is offline plasnu  United States
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Default All SIT 50W+ Single Ended Amp (beta)

While pursuing Firstwatt's Articles and Mr. Rothacher's threads for years, I've always wanted to build an all SIT amp. I finally started testing one on my workbench, and it seems the amp is working well so far. This amp is similar to Michael's LuminAria + Mofo (L'amp) with some additional ideas.

Feedbacks, critics, ideas and suggestions are very welcome.

--------

All SIT 50W+ Single Ended Amp (beta)

- All SIT. No other active components sans 2xPSUs.
- Sony 2SJ28 gain + Tokin 2SK182ES follower.
- Minimal signal path. Simple but heavy and hot.
- 600:600 transformer is used for testing.
- Water cooling (will be tested soon).
- Gain: 20dB
- Bias current: Sony 95mA / Tokin 4.2A (updated v1.2)
- Max: 50W+ @8ohm
- Power Consumption: 220W (updated v1.2)

---------------
Updated (V1.2):
Updated Schematics and measurement (V1.2)
Attached Images
File Type: png New.png (89.4 KB, 1086 views)

Last edited by plasnu; 2nd August 2020 at 11:54 PM.
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Old 11th July 2020, 07:28 PM   #2
plasnu is offline plasnu  United States
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Default Measurement

Measurement was done with RMAA @96K. 8ohm dummy load. Looks good to me so far.

Frequency Response 1W/2.83V -0.21, +0.03
THD 1W/2.83V 0.04147%
THD 0.1W/0.9V 0.02246%
THD 20W/12V 0.28347%
THD 50W/20V 0.46546%
IMD 1W/2.83V 0.04335%
Attached Images
File Type: png 1Wf.png (18.9 KB, 1082 views)
File Type: png 1WTHD.png (26.6 KB, 1022 views)
File Type: png 0.1WTHD.png (27.1 KB, 1021 views)
File Type: png 20W THD.png (28.7 KB, 1006 views)
File Type: png 50WTHD.png (28.7 KB, 119 views)
File Type: png 1WIMD.png (27.2 KB, 113 views)

Last edited by plasnu; 11th July 2020 at 07:49 PM.
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Old 11th July 2020, 07:31 PM   #3
plasnu is offline plasnu  United States
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I'm also testing a tranformer-less balanced input version of the amp. The measurement of this version is a little different.

- Same output stage with a long tail pair differential input/gain stage.
- High freq compensation filter (C3) to improve freq response.
- THD @1W/2.83V
Attached Images
File Type: png LTPf.png (28.5 KB, 120 views)
File Type: png LTPTHD.png (39.1 KB, 99 views)
File Type: png LTP2.png (100.7 KB, 250 views)

Last edited by plasnu; 11th July 2020 at 07:46 PM.
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Old 11th July 2020, 07:32 PM   #4
plasnu is offline plasnu  United States
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My LTSpice emulations are reasonably close to real amps, thanks to Lynn's 2SK182ES Spice model which is close to my Tokin batch from Watanabe-san (thank you, Lynn & Watanabe-san).

High voltage SONY model below is mine, which was also helpful to predict operational points, but use it at your own risk.

*2SK82 (High Voltage)
*--------------------------------------------------
.SUBCKT 2SK82HV D G S ; Drain Gate Source
+ PARAMS: MU=10.49 X=2.85 K=0.014 N=0.5 VCT=8.57 RG=2MEG
*--------------------------------------------------
B1 D S I=K*PWR(URAMP((V(G,S)+VCT)+(N*LN(V(D,S))+(V(D,S)/MU))),X)
*FOR MULTISIM COMMENT OUT ABOVE LINE (*) AND UNCOMMENT NEXT LINE
*B1 D S I=K*PWR(MAX((V(G,S)+VCT)+(N*LN(V(D,S))+(V(D,S)/MU)),0),X)
R1 G S {RG}
CGS G S 380P
CGD G D 142P
CDS G S 42P
.ENDS 2SK82HV
*--------------------------------------------------

*2SJ28 (High Voltage)
*--------------------------------------------------
.SUBCKT 2SJ28HV D G S ; Drain Gate Source
+ PARAMS: MU=10.49 X=2.85 K=0.014 N=0.5 VCT=8.57 RG=2MEG
*--------------------------------------------------
B1 S D I=K*PWR(URAMP((V(S,G)+VCT)+(N*LN(V(S,D))+(V(S,D)/MU))),X)
*FOR MULTISIM COMMENT OUT ABOVE LINE (*) AND UNCOMMENT NEXT LINE
*B1 S D I=K*PWR(MAX((V(S,G)+VCT)+(N*LN(V(S,D))+(V(S,D)/MU)),0),X)
R1 G S {RG}
CGS G S 380P
CGD G D 142P
CDS G S 42P
.ENDS 2SJ28HV
*--------------------------------------------------


Well, That's it so far.
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Old 11th July 2020, 08:21 PM   #5
s610adam is offline s610adam  United States
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Congrats!!! Very cool and great numbers, especially IMD, looks like that's way below what a tube amp would do

What choke are you using to bias that input pair?
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Old 12th July 2020, 03:34 AM   #6
plasnu is offline plasnu  United States
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Thank you, and yes, the measurement is better than my expectation.

The choke for Sony is an old surplus, TF4SX04HA. The values of the parts on actual amp have been changed a little since LTSpice. The topology has not been changed.

Here is the list of the value from actual amp (measured amp).

Amp #1:

- R6 1.3Kohms This transformer termination resistor determines high frequency response.
- L1 0.12L 2x MOT in series (2x 0.06L according to xrk971)
- TrimPods 1K and 10 ohms
- R4 Many small resistors in series, and I don't measure the value.
- J2 Sony VDS 145V / HV 255V - 90mA. I still need to find a sweet spot.

Amp #2 LTP:

- L2 0.1L/2.25A (TF4SX04HA)
- TrimPods 1K and 10 ohms + small resistors
- R6 R9 1K They work fine with my DAC
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Old 12th July 2020, 06:38 AM   #7
plasnu is offline plasnu  United States
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Temp test result. Now I'm pretty sure that output capacitor can be eliminated, but please do it at your own risk if you want to try it. Inrush voltage is not measured. I will use a speaker DC protection board with delay start anyway.

Temp (Tokin) - Temp (Heatsink) - Speaker terminal DC offset

27C - 27C - -10mV (10 seconds after power on)
40C - 44C - -4mV
40C - 50C - -1mV

*Tokin's top cover is always colder than heatsink.
*The bottom plate of Tokin is about the same temp as heatsink.
*Measured with a heat measurement gun with laser.

I use Arctic MX-4 paste and Alumina sheet from Ebay. Alumina sheet has to be cracked. You can't cut nor make a hole, but works fine with Tokin.

Square ALUMINA CERAMIC TILES / SHEETS - .020" (.5mm) Thick x 4" x 4" - US SELLER | eBay

Last edited by plasnu; 12th July 2020 at 07:06 AM.
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Old 12th July 2020, 07:56 PM   #8
claudio52 is offline claudio52  Italy
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All SIT 50W+ Single Ended Amp (beta)
Great!
Your project confirms that the Sony VFET with low current and high voltage has exceptional linearity.
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Old 12th July 2020, 08:16 PM   #9
plasnu is offline plasnu  United States
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Thank you, but it has already confirmed by Michael's LuminAria.
It requires low impedance source.
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Old 12th July 2020, 10:47 PM   #10
plasnu is offline plasnu  United States
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THD comparison of 2SJ28 high voltage gain stage only (Amp#1).

If the goal is lower THD, CCS would be better, but I think I will stick to resistor for now.

*50K ohm load.
*Drain resistor was increased for lower THD.
*2.83V/20V is a little less than 1W/50W amp output.

1 2.83V VDS-150V-90mA drain-1.5K (current configuration)
2 2.83V VDS-150V-90mA drain-0.5K (lower drain resistor)
3 2.83V VDS-90V-70mA drain-0.5K (lower setting)
4 20V @Setting#1
5 20V @Setting#3
6 20V frequency response @Setting#1 (hmm...?)
Attached Images
File Type: png 140V1W.png (51.9 KB, 178 views)
File Type: png 140V0.5K.png (50.7 KB, 59 views)
File Type: png Lum 1W.png (52.1 KB, 49 views)
File Type: png 140V 50W.png (55.2 KB, 40 views)
File Type: png Lum50W.png (54.8 KB, 40 views)
File Type: png freq.png (40.8 KB, 43 views)

Last edited by plasnu; 12th July 2020 at 11:00 PM.
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