6V6 line preamp

[Koonw]

I did a FFT comparision of your sch in triode and pentode mod. The FFT of triode mode is missing H2 while the pentode mode is quite normal.

 

[rongon]

Did you run those simulations using 1M ohm loads? If so, maybe try running them into 100k ohms or even 50k ohms. That's more like a real world load. Perhaps also put 500pF or 1nF from out to ground to simulate a long cable run of not-so-great interconnects.

 

[Koonw]

Yes 1Meg load, now change to 100k and 500pf. H2 has came up somewhat but still way below H3, only affect triode mode. Both H3 has dropped somewhat.

 

[Koonw]

Now .options plotwinsize=0, both H3 is below H2, what happened here? So now the difference (almost 30db) is in H2 only?

 

[rongon]

I'm afraid I don't understand what exactly LTspice is doing when it creates the FFT and THD readings.
I use the following SPICE directive for the simulation, and take the FFT from that:

.options plotwinsize=0
.tran 0 100m 60m 1u
.four 1k 10 v(output)
.OPTIONS numdgt=8
.option noopiter

It would be very surprising if a triode stage, even with NFB, made more H3 than H2 at small signal levels.

 

[Koonw]

Some of options are causing the FFT plot to slop a lot, so best to not including them. With the same rules applied to both modes, triode FFT mode appears more deviated from original (Salas's), and as it has less H2 to start with, apply more fb brings it down further (according to normal logic).

 

[kodabmx]

RL =- 10k? A 6V6 should easily drive 10k, right?

 

[Anchan]

Run your sim on the original Salas circuit into a 100k load and see how accurate. I measured my actual circuit, we can compare

 

[Koonw]

Thank you very much for the offer, much obliged.

 

[Anchan]

That’s a slightly different one that came later. Try the 330V-340V 5k sim from post #1

 

[Koonw]

Here they are/

 

[Anchan]

Real life version

some of the spurious stuff below -105 could be the analyzer. That’s about at its resolution.

 

[rongon]

Here's the circuit from post #1 simulated in LTspice with the directive I've been using. One graph is the FFT using an Ayumi N. 6V6 model, the other is using Adrian Immler's model.

THD at 1V rms out into 100k ohm load:
AN model: 0.126% (-58dB)
AI model: 0.085%

Those predictions look a bit rosier than what happens in real life. The real thing looks like it makes H2 at -53dB down. which would be about 0.15% THD, I think. (How does one calculate that...?)

Koonw, I couldn't get your 6V6 model to run, Strange, because there were no error messages.

EDIT TO ADD:
I attached the .asc as well.

 

[Koonw]

I post the entire asc. You use .options plotwinsize=0, I use .options plotwinsize=1

 

[Salas]

Real life version

some of the spurious stuff below -105 could be the analyzer. That’s about at its resolution.

I dug up three charts from the original gain version preamp. Took them in 2007-2008! Not a today's performance measurement rig and wrong Y scale (SPL) but you get the numbers.

 

[rongon]

Thanks Salas.
Any idea what the output signal voltage would have been at half volume?
I usually quote fantastical SPICE predictions using 1V rms output as the reference point.

 

[rongon]

You use .options plotwinsize=0, I use .options plotwinsize=1

Thanks Koonw.

What does the 'plotwinsize' argument do? Any idea?

From a quick search, I found this, where I saw this:

the .options statment sets plotwinsize = 0 which disables compression and numdgt is used to set significant figures used for output data. If numdgt > 6, double precision is used.

 

[Salas]

Thanks Salas.
Any idea what the output signal voltage would have been at half volume?
I usually quote fantastical SPICE predictions using 1V rms output as the reference point.

That FFT was obviously set up for measuring speakers at the time hence the octave smoothing and SPL scale. As far as I can remember I was normally using 1V RMS for full DUT output because the common man's audio cards of old would quickly build up own harmonics around 1.5V and fully saturate at 2V.
Half volume pot must have been the physical knob's mid position. As we see something like -16dB difference between full and half in the charts, that gives 150mV RMS. In other words -1dB below unity (In=Out test signal level) on the pot when we consider the preamp's 15dB gain. One O'clock should have been unity when "half_vol" chart was 12 O'clock.

 

[Koonw]

https://detail--infomation-com.tran...l=auto&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp

 

[Koonw]

I settle for these settings, it's about as much details one can hope to see, with Bohman Windows.

.tran 0 {simtime} {dlytime} {timestep}
.options plotwinsize=0
.options method=gear
.options numdgt=7
.param numcyc=150
.param dlycyc=1
.param FFT=18
.param simtime=numcyc/Freq+dlytime
.param dlytime=dlycyc/Freq
.param timestep=(simtime-dlytime)/2**FFT
.four {Freq}  V(out)
.param Freq=1k